Fanconi Syndrome

1. Clinical Overview

Summary

Fanconi syndrome is a generalised dysfunction of the proximal convoluted tubule (PCT) of the kidney, resulting in impaired reabsorption of multiple solutes that are normally reclaimed from the glomerular filtrate. [1,2] This leads to excessive urinary wasting of glucose, amino acids, phosphate, bicarbonate, uric acid, potassium, and low-molecular-weight proteins. The syndrome can be inherited or acquired, with cystinosis being the most common inherited cause in children and drug toxicity (particularly tenofovir) being increasingly recognised in adults. [3,4]

The clinical hallmarks include:

• Renal glycosuria despite normoglycaemia
• Generalised aminoaciduria
• Phosphaturia leading to hypophosphataemic rickets/osteomalacia
• Type 2 (proximal) renal tubular acidosis from bicarbonate wasting
• Hypokalaemia from urinary potassium losses
• Hypouricaemia from uric acid wasting

The condition presents with failure to thrive and rickets in children, or bone pain, muscle weakness, and polyuria in adults. [5] Management involves treating the underlying cause (if reversible) and comprehensive electrolyte replacement therapy, particularly phosphate, alkali, and vitamin D supplementation. [6]

Key Facts

• Pathology: Generalised proximal tubule dysfunction affecting multiple transport systems
• Hallmark: Glucosuria with normal blood glucose (renal glycosuria)
• Most Common Inherited Cause (Children): Cystinosis
• Most Common Acquired Causes (Adults): Multiple myeloma, tenofovir, ifosfamide
• Key Losses: "GAP BUK"
• Glucose, Amino acids, Phosphate, Bicarbonate, Uric acid, K+
• Consequence: Type 2 RTA + hypophosphataemic rickets/osteomalacia
• Diagnosis: Low TmP/GFR, generalised aminoaciduria, glucosuria with normoglycaemia
• Treatment: Replace losses (PO₄, HCO₃⁻, K⁺, vitamin D) + treat underlying cause

Clinical Pearls

"Glucosuria Without Diabetes": The classic diagnostic clue. Glucose in the urine but normal or low blood glucose indicates renal glycosuria from proximal tubule dysfunction, not diabetes mellitus.

"Think Cystinosis in Children": In any child presenting with Fanconi syndrome, cystinosis should be the primary consideration. Look for corneal cystine crystals on slit-lamp examination and blonde hair/fair complexion (due to melanin deficiency). [7]

"Type 2 RTA - The Tubule Can Still Acidify": Unlike Type 1 (distal) RTA, the distal tubule is intact in Fanconi syndrome. Urine pH can fall below 5.3 when serum bicarbonate is low. The acidosis is typically mild (HCO₃⁻ 15-20 mmol/L) because the threshold for bicarbonate reabsorption is reset. [8]

"Tenofovir Tubulopathy": Drug-induced Fanconi syndrome is increasingly recognised, especially with tenofovir disoproxil fumarate (TDF). Screen HIV patients on TDF for phosphate, glucose, and renal function. Tenofovir alafenamide (TAF) has lower nephrotoxicity. [9]

"Multiple Myeloma Light Chains": In adults with unexplained Fanconi syndrome, always check serum protein electrophoresis and urine Bence Jones protein. Light chain deposition in proximal tubules is a key cause. [10]

"TmP/GFR Calculation": This derived value (tubular maximum reabsorption of phosphate relative to GFR) is the most sensitive marker of renal phosphate wasting. Normal range: 0.8-1.35 mmol/L. In Fanconi syndrome, typically less than 0.6 mmol/L.

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2. Epidemiology

Incidence & Prevalence

Age Distribution

Exam Detail: | Age Group | Typical Causes | |-----------|---------------| | Neonatal/Infancy | Cystinosis (infantile form), galactosaemia, hereditary fructose intolerance, Lowe syndrome | | Childhood | Cystinosis, Wilson disease, mitochondrial disorders, Dent disease | | Adults | Multiple myeloma, light chain deposition, drug-induced (tenofovir, ifosfamide, cisplatin), Sjögren's syndrome | | Elderly | Monoclonal gammopathy of undetermined significance (MGUS), paraprotein-related |

Sex Distribution

• Most inherited forms are autosomal recessive (equal sex distribution)
• Lowe syndrome and Dent disease are X-linked recessive (males predominantly affected)
• Acquired forms have no sex predilection (except where underlying disease has sex bias)

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3. Aetiology & Pathophysiology

Causes of Fanconi Syndrome

Exam Detail: | Category | Specific Causes | Mechanism | |----------|-----------------|-----------| | Inherited Disorders | | | | Cystinosis | Most common inherited cause in children | Cystine accumulation in lysosomes → proximal tubule cell damage [7] | | Wilson disease | Copper accumulation | Direct tubular toxicity from copper deposition | | Lowe syndrome | X-linked; OCRL1 gene mutation | Defective phosphatidylinositol metabolism | | Dent disease | X-linked; CLCN5 gene mutation | Chloride channel defect → endocytosis impairment | | Hereditary fructose intolerance | Aldolase B deficiency | Fructose-1-phosphate accumulation → tubular toxicity | | Galactosaemia | Galactose-1-phosphate uridyltransferase deficiency | Galactose-1-phosphate toxicity | | Tyrosinaemia type 1 | Fumarylacetoacetate hydrolase deficiency | Toxic metabolite accumulation (fumarylacetoacetate) | | Glycogen storage disease (Type 1) | Glucose-6-phosphatase deficiency | Glycogen accumulation in tubular cells | | Mitochondrial disorders | Various mutations | Impaired ATP production → transport dysfunction | | Drug-Induced | | | | Tenofovir (TDF) | Nucleotide reverse transcriptase inhibitor | Mitochondrial toxicity in proximal tubule cells [9] | | Ifosfamide | Alkylating chemotherapy | Chloroacetaldehyde metabolite toxicity | | Cisplatin | Platinum-based chemotherapy | Direct tubular cell damage, oxidative stress | | Aminoglycosides | Antibiotics (gentamicin, tobramycin) | Accumulation in proximal tubule cells | | Valproic acid | Antiepileptic | Mitochondrial dysfunction, carnitine depletion | | Expired tetracyclines | Degraded tetracycline products | Fanconi-like syndrome (historic, rare now) | | Fumaric acid esters | Multiple sclerosis treatment | Mechanism unclear | | Heavy Metal Toxicity | | | | Lead | Chronic exposure | Inhibition of mitochondrial enzymes | | Cadmium | Chronic exposure (occupational, smoking) | Metallothionein accumulation, oxidative stress | | Mercury | Chronic exposure | Direct tubular toxicity | | Uranium | Rare; occupational/environmental | Tubular cell necrosis | | Paraprotein/Monoclonal Gammopathy | | | | Multiple myeloma | Light chain deposition | Light chains toxic to proximal tubule epithelium [10] | | Light chain deposition disease | Kappa or lambda chains | Direct tubular injury | | MGUS | Monoclonal gammopathy | Subclinical paraprotein effects | | Autoimmune/Inflammatory | | | | Sjögren's syndrome | Primary or secondary | Tubulointerstitial nephritis with tubular dysfunction [12] | | Systemic lupus erythematosus | SLE with renal involvement | Immune complex deposition, tubulitis | | Other Acquired Causes | | | | Renal transplantation | Post-transplant | Ischaemic injury, calcineurin inhibitor toxicity | | Vitamin D deficiency (severe) | Rare association | Secondary hyperparathyroidism effects | | Amyloidosis | Light chain or AA amyloid | Amyloid deposition in tubules | | Paroxysmal nocturnal haemoglobinuria | Haemoglobin deposition | Tubular toxicity from haem products |

Proximal Tubule Physiology (Normal)

Exam Detail: The proximal convoluted tubule reabsorbs approximately:

• 65-70% of filtered sodium and water
• 85-90% of filtered bicarbonate
• 100% of filtered glucose (via SGLT2 and SGLT1)
• 100% of filtered amino acids (via multiple amino acid transporters)
• 80-85% of filtered phosphate (via sodium-phosphate cotransporters NaPi-IIa, NaPi-IIc)
• 50% of filtered urea
• 90% of filtered uric acid is reabsorbed (then some secreted distally)

Key Transport Mechanisms:

• Na⁺/K⁺-ATPase on basolateral membrane creates sodium gradient
• This gradient drives sodium-dependent cotransporters on apical membrane:
  • SGLT2 and SGLT1 (glucose)
  • NaPi-IIa, NaPi-IIc (phosphate)
  • Multiple amino acid transporters (neutral, acidic, basic, iminoglycine)
  • NBCe1 (sodium-bicarbonate cotransporter)
• Energy-dependent: Requires high ATP production from mitochondria

Pathophysiology of Fanconi Syndrome

Exam Detail: Generalised Transport Failure:

Fanconi syndrome represents a global dysfunction of proximal tubule solute reabsorption. The exact mechanism varies by aetiology but common pathways include:

1. Mitochondrial Dysfunction [13]

   • Proximal tubule cells have high metabolic demand (extensive active transport)
   • Mitochondrial toxins (tenofovir, valproate) or genetic defects impair ATP production
   • Insufficient ATP → failure of Na⁺/K⁺-ATPase → loss of sodium gradient
   • All sodium-dependent cotransport fails

2. Direct Tubular Cell Toxicity

   • Heavy metals, chemotherapy, light chains cause direct epithelial damage
   • Lysosomal accumulation disorders (cystinosis) → cellular dysfunction and apoptosis
   • Membrane transporter damage

3. Defective Endocytosis

   • Dent disease (CLCN5 mutation) → impaired receptor-mediated endocytosis
   • Low-molecular-weight proteinuria (β2-microglobulin, retinol-binding protein)

4. Specific Transporter Defects (in genetic forms)

   • While Fanconi syndrome is a generalised dysfunction, some genetic forms have specific initiating transporter defects that cascade into global failure

Molecular Mechanisms by Cause:

Consequences of Solute Losses

Exam Detail: | Lost Substance | Urinary Finding | Serum Finding | Clinical Consequence | |----------------|-----------------|---------------|----------------------| | Glucose | Glucosuria (1+ to 4+) | Normal or low glucose | Osmotic diuresis, polyuria; diagnostic clue | | Amino acids | Generalised aminoaciduria | Normal amino acids | Usually asymptomatic; may contribute to growth failure in children | | Phosphate | Phosphaturia (elevated urine PO₄) | Hypophosphataemia | Rickets (children), osteomalacia (adults), muscle weakness [15] | | Bicarbonate | Bicarbonaturia | Low serum HCO₃⁻ (15-20 mmol/L) | Type 2 RTA: Normal anion gap metabolic acidosis [8] | | Potassium | Kaliuresis | Hypokalaemia | Muscle weakness, polyuria (secondary nephrogenic DI), arrhythmias | | Uric acid | Uricosuria | Hypouricaemia | Usually asymptomatic; gout is rare | | Sodium/Water | Natriuresis | May have hyponatraemia | Polyuria, dehydration, growth failure | | Magnesium | Magnesiuria | Hypomagnesaemia | Muscle cramps, tetany (if severe) | | Carnitine | Carnitinuria | Low carnitine | Muscle weakness, metabolic decompensation | | Low-MW proteins | β2-microglobulin ↑, RBP ↑ | Normal | Marker of tubular dysfunction |

Type 2 (Proximal) RTA Physiology:

• Bicarbonate reabsorption threshold is reduced (from ~24-28 mmol/L to ~15-18 mmol/L)
• When serum HCO₃⁻ falls below this new threshold, all filtered HCO₃⁻ is reabsorbed
• Distal tubule function is intact → urine can be acidified to pH less than 5.3 once serum HCO₃⁻ is low [8]
• This contrasts with Type 1 (distal) RTA where urine pH remains > 5.5 despite acidosis
• Typically mild acidosis (HCO₃⁻ 15-20 mmol/L) with normal anion gap

Hypophosphataemic Rickets/Osteomalacia:

• Persistent renal phosphate wasting → hypophosphataemia (less than 0.8 mmol/L) [15]
• Impaired bone mineralisation → rickets (children with open growth plates) or osteomalacia (adults)
• Secondary hyperparathyroidism may develop (compensatory response to low PO₄, but PTH further worsens phosphaturia)
• 1,25-dihydroxyvitamin D levels may be inappropriately normal or low (impaired 1α-hydroxylase activity in damaged proximal tubules)

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4. Clinical Presentation

Symptoms

Exam Detail: #### Children (Inherited Causes)

Adults (Acquired Causes)

Signs

Exam Detail: #### General Examination

Musculoskeletal Signs

Rickets (Children):

• Frontal bossing (skull)
• Craniotabes (soft skull bones in infants)
• Delayed fontanelle closure
• Rachitic rosary (prominence of costochondral junctions)
• Harrison's sulcus (horizontal groove along lower chest)
• Bowing of legs (genu varum or genu valgum)
• Widening of wrists and ankles
• Delayed tooth eruption
• Delayed walking

Osteomalacia (Adults):

• Proximal muscle weakness (difficulty rising from chair, climbing stairs)
• Bone tenderness (ribs, pelvis, spine)
• Antalgic gait (waddling gait if pelvic fractures)
• Spinal deformity (kyphosis from vertebral fractures)

Neurological Signs

• Hypotonia (hypokalaemia, hypophosphataemia)
• Reduced power (proximal > distal)
• Hyporeflexia (severe hypokalaemia)
• Chvostek's/Trousseau's signs (if hypocalcaemia/hypomagnesaemia)

Specific Cause-Related Signs

Examination Pearls

Slit-Lamp Examination: In any child with Fanconi syndrome, arrange ophthalmology review for slit-lamp examination. Corneal cystine crystals are pathognomonic of cystinosis and appear as refractile crystals in the cornea. [7]

Wrist Widening: Palpate the wrists and ankles in children. The metaphyseal widening of rickets is most easily appreciated at the wrists (distal radius/ulna) and ankles.

Proximal Myopathy Test: Ask the patient to stand from sitting without using hands, or to squat and stand. Difficulty indicates proximal muscle weakness (hypophosphataemia, hypokalaemia).

"Looser's Zones": On X-ray, these are pseudofractures (radiolucent bands perpendicular to cortex) seen in osteomalacia, typically in ribs, scapula, pubic rami, femoral neck.

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5. Differential Diagnosis

Exam Detail: ### Isolated Proximal Tubule Defects vs Generalised Fanconi Syndrome

*COLA = Cystine, Ornithine, Lysine, Arginine

Type 2 RTA vs Type 1 RTA

Hypophosphataemic Rickets/Osteomalacia: Differential

Key Distinguishing Feature: In Fanconi syndrome, you see multiple proximal tubule defects (glucosuria, aminoaciduria, Type 2 RTA) in addition to phosphaturia. Isolated phosphate wasting without these features suggests XLH or other FGF23-mediated disorders.

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6. Investigations

First-Line Investigations

Exam Detail: #### Urine Tests

Blood Tests

Key Diagnostic Calculation: TmP/GFR

Exam Detail: Tubular Maximum Reabsorption of Phosphate relative to GFR (TmP/GFR):

This is the most sensitive index of renal phosphate handling and is essential for diagnosing renal phosphate wasting. [16]

Formula:

TmP/GFR = Serum PO4 - ( (Urine PO4 × Serum Creatinine) / Urine Creatinine )

(Simplified formula; more complex nomograms exist)

Interpretation:

• Normal: 0.8-1.35 mmol/L
• Fanconi syndrome: Typically less than 0.6 mmol/L (indicates significant renal phosphate wasting)

Clinical Utility:

• Distinguishes renal phosphate wasting (Fanconi, XLH, TIO) from vitamin D deficiency (where TmP/GFR is normal or elevated due to secondary hyperparathyroidism)
• Can be calculated from spot urine sample (urine PO₄, urine creatinine) and serum values (serum PO₄, serum creatinine)

Cause-Specific Investigations

Exam Detail: | Suspected Cause | Investigation | Findings | |-----------------|---------------|----------| | Cystinosis | Leukocyte cystine level | Elevated (> 2 nmol half-cystine/mg protein; normal less than 0.2) [7] | | | Slit-lamp examination | Corneal cystine crystals (pathognomonic) | | | Genetic testing | CTNS gene mutation | | Wilson disease | Serum ceruloplasmin | Low (less than 0.2 g/L) | | | 24h urinary copper | Elevated (> 0.6 μmol/24h; > 100 μg/24h) | | | Slit-lamp | Kayser-Fleischer rings (copper in Descemet's membrane) | | | Liver biopsy (if needed) | Elevated hepatic copper content | | Multiple myeloma | Serum protein electrophoresis (SPEP) | Monoclonal band (M-protein) [10] | | | Serum free light chains (FLC) | Elevated kappa or lambda; abnormal ratio | | | Urine protein electrophoresis | Bence Jones protein (light chains) | | | Bone marrow biopsy | Plasma cell infiltration (> 10%) | | | Skeletal survey | Lytic bone lesions, fractures | | Tenofovir toxicity | Drug history | Tenofovir disoproxil fumarate (TDF) use [9] | | | Tubular proteinuria | Elevated β2-microglobulin | | | (Diagnosis of exclusion) | Improvement on drug cessation | | Sjögren's syndrome | Anti-Ro (SSA), Anti-La (SSB) | Positive autoantibodies [12] | | | Schirmer's test | Reduced tear production (less than 5 mm in 5 min) | | | Labial salivary gland biopsy | Lymphocytic infiltration, focus score ≥1 | | Heavy metal toxicity | Blood lead level | Elevated (> 0.5 μmol/L; > 10 μg/dL) | | | Urine cadmium, mercury | Elevated levels (occupational history) | | Lowe syndrome | Ophthalmology exam | Congenital cataracts | | | Genetic testing | OCRL1 gene mutation (X-linked) | | Tyrosinaemia type 1 | Urine succinylacetone | Elevated (pathognomonic) | | | Plasma amino acids | Elevated tyrosine, methionine | | | Genetic testing | FAH gene mutation |

Imaging

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7. Classification Systems

Exam Detail: ### Classification by Aetiology

Primary (Inherited) Fanconi Syndrome:

• Cystinosis (infantile, juvenile, adult forms)
• Lowe syndrome (oculocerebrorenal syndrome)
• Dent disease
• Wilson disease
• Galactosaemia
• Hereditary fructose intolerance
• Tyrosinaemia type 1
• Glycogen storage disease type 1
• Mitochondrial cytopathies

Secondary (Acquired) Fanconi Syndrome:

• Drug-induced: Tenofovir, ifosfamide, cisplatin, aminoglycosides, valproate, expired tetracyclines
• Paraprotein-related: Multiple myeloma, light chain deposition disease, MGUS
• Heavy metals: Lead, cadmium, mercury, uranium
• Autoimmune: Sjögren's syndrome, SLE
• Other: Vitamin D deficiency (severe), renal transplant, amyloidosis, PNH

Classification by Age of Onset

Severity Classification (Based on Serum Bicarbonate and Phosphate)

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8. Management

Treatment Goals

1. Treat underlying cause (if reversible)
2. Replace urinary losses (phosphate, bicarbonate, potassium, vitamin D)
3. Prevent/treat rickets and osteomalacia
4. Optimise growth (children)
5. Prevent progression to CKD
6. Monitor and manage complications

Management Algorithm

┌────────────────────────────────────────────────────────────────────┐
│   FANCONI SYNDROME MANAGEMENT ALGORITHM                            │
├────────────────────────────────────────────────────────────────────┤
│                                                                    │
│  STEP 1: CONFIRM DIAGNOSIS                                         │
│  ├─ Urine: Glucose, amino acids, phosphate, β2-microglobulin      │
│  ├─ Blood: PO₄, HCO₃⁻, K⁺, uric acid, glucose, ALP               │
│  └─ Calculate TmP/GFR                                              │
│                                                                    │
│  STEP 2: IDENTIFY AND TREAT UNDERLYING CAUSE                       │
│  ├─ Drug-induced → STOP causative drug (tenofovir, ifosfamide)    │
│  ├─ Cystinosis → Cysteamine (delays progression to ESRD) [17]     │
│  ├─ Wilson disease → Chelation (penicillamine, trientine)         │
│  ├─ Myeloma → Chemotherapy, stem cell transplant                   │
│  ├─ Sjögren's → Immunosuppression (if severe tubulointerstitial)  │
│  └─ Heavy metals → Remove exposure, chelation if indicated         │
│                                                                    │
│  STEP 3: REPLACEMENT THERAPY (CORNERSTONE)                         │
│                                                                    │
│  A. PHOSPHATE SUPPLEMENTATION                                      │
│     ├─ First-line: Oral phosphate                                  │
│     │   • Phosphate Sandoz (500 mg PO₄ per tablet)                │
│     │   • Dose: 20-60 mg/kg/day PO₄ (divided 4-6 times/day)       │
│     │   • Side effect: Diarrhoea (dose-limiting)                   │
│     └─ Target: Serum PO₄ > 0.8 mmol/L, ideally 1.0-1.2 mmol/L      │
│                                                                    │
│  B. VITAMIN D SUPPLEMENTATION                                      │
│     ├─ Active vitamin D (bypasses 1α-hydroxylase defect)           │
│     │   • Alfacalcidol (1α-hydroxyvitamin D): 0.25-2 μg/day       │
│     │   • Calcitriol (1,25-dihydroxyvitamin D): 0.25-2 μg/day     │
│     │   • Essential to enhance PO₄ absorption and bone healing     │
│     └─ Cholecalciferol (if 25(OH)D deficient): 800-2000 IU/day    │
│     └─ Monitor: Serum Ca²⁺, urine Ca²⁺ (risk of hypercalciuria)   │
│                                                                    │
│  C. ALKALI THERAPY (Type 2 RTA)                                    │
│     ├─ Sodium bicarbonate tablets: 1-2 mmol/kg/day (children)     │
│     │                               1-3 g TDS (adults)             │
│     │   • High doses needed (proximal leak continues)              │
│     ├─ Potassium citrate: 10-20 mmol TDS (if K⁺ also low)         │
│     │   • Combines alkali + K⁺ replacement                         │
│     └─ Target: Serum HCO₃⁻ > 20 mmol/L                              │
│                                                                    │
│  D. POTASSIUM SUPPLEMENTATION                                      │
│     ├─ Potassium chloride (Slow-K): 600 mg TDS                    │
│     ├─ Potassium citrate: 10-20 mmol TDS (preferred; also alkali) │
│     └─ Target: Serum K⁺ > 3.5 mmol/L                                │
│                                                                    │
│  E. FLUID REPLACEMENT                                              │
│     ├─ If polyuric: Ensure adequate hydration                      │
│     └─ Sodium chloride supplements if severe salt wasting          │
│                                                                    │
│  F. OTHER SUPPLEMENTS (as needed)                                  │
│     ├─ Magnesium (if hypomagnesaemia)                              │
│     ├─ Carnitine (in some metabolic forms)                         │
│     └─ Calcium (if hypocalcaemic, but beware hypercalciuria)       │
│                                                                    │
│  STEP 4: DISEASE-SPECIFIC THERAPY                                  │
│  ├─ Cystinosis: CYSTEAMINE (Cystagon) [17]                        │
│  │   • Dose: 1.3-1.95 g/m²/day (divided QDS)                      │
│  │   • Target: Leukocyte cystine less than 1 nmol half-cystine/mg protein  │
│  │   • Delays ESRD by ~10 years; start early                       │
│  │   • Side effects: Halitosis, GI upset, body odour               │
│  │   • Cysteamine eye drops for corneal crystals                   │
│  └─ Wilson disease: See separate protocol                          │
│                                                                    │
│  STEP 5: MONITORING                                                │
│  ├─ Regular bloods: U&E, PO₄, HCO₃⁻, K⁺, Ca²⁺, ALP, PTH (3-6mo)  │
│  ├─ Urine: β2-microglobulin, Ca:Cr ratio (check hypercalciuria)   │
│  ├─ Bone health: X-rays (if rickets/osteomalacia), DEXA scan      │
│  ├─ Growth chart (children): Height, weight velocity               │
│  ├─ Renal function: eGFR (monitor CKD progression)                 │
│  └─ Cystinosis: Leukocyte cystine levels every 3-6 months          │
│                                                                    │
│  STEP 6: MDT INVOLVEMENT                                           │
│  ├─ Nephrology (lead specialty)                                    │
│  ├─ Metabolic medicine / Clinical genetics (inherited forms)       │
│  ├─ Paediatric endocrinology (children with growth/bone issues)    │
│  ├─ Haematology (if myeloma)                                       │
│  ├─ Ophthalmology (cystinosis, Lowe syndrome)                      │
│  ├─ Neurology (Wilson, Lowe, mitochondrial disorders)              │
│  ├─ Dietitian (phosphate-rich diet, nutritional support)           │
│  └─ Renal transplant team (if progression to ESRD)                 │
│                                                                    │
│  STEP 7: ADVANCED DISEASE / COMPLICATIONS                          │
│  ├─ CKD Stage 4-5 → Renal replacement therapy (dialysis)           │
│  ├─ Renal transplantation                                          │
│  │   • Curative for renal Fanconi syndrome (PCT dysfunction)       │
│  │   • Cystinosis: Transplant does NOT cure systemic disease       │
│  │     (continue cysteamine lifelong for extrarenal complications) │
│  └─ Growth hormone therapy (if severe growth failure in children)  │
│                                                                    │
└────────────────────────────────────────────────────────────────────┘

Evidence-Based Management

Exam Detail: #### Phosphate and Vitamin D Supplementation [15,18]

Evidence:

• Phosphate supplementation (20-60 mg/kg/day) combined with active vitamin D (calcitriol or alfacalcidol) improves bone mineralisation and heals rickets/osteomalacia.
• Vitamin D alone is insufficient (requires phosphate replacement).
• Active vitamin D is preferred over cholecalciferol because damaged proximal tubules have impaired 1α-hydroxylase activity.

Practical Points:

• Start with phosphate 20-30 mg/kg/day, increase as tolerated (limited by diarrhoea).
• Divide doses (4-6 times/day) to maintain steady PO₄ levels.
• Alfacalcidol 0.25-1 μg/day (adjust based on Ca²⁺, avoid hypercalcaemia/hypercalciuria).

Cysteamine for Cystinosis [17]

Evidence:

• Cysteamine depletes lysosomal cystine by converting it to cysteine-cysteamine mixed disulfide (can exit lysosome).
• Landmark study (Gahl et al., NEJM 2002): Cysteamine delays onset of ESRD by ~10 years in infantile cystinosis. [17]
• Early initiation (less than 2 years) is critical for maximal benefit.
• Continued lifelong (even post-transplant) to prevent extrarenal complications (hypothyroidism, diabetes, myopathy, CNS involvement).

Dosing:

• Target dose: 1.3-1.95 g/m²/day (divided every 6 hours).
• Monitor leukocyte cystine levels (target less than 1 nmol half-cystine/mg protein).
• Side effects: Halitosis (sulfur smell), GI upset, body odour (poor compliance in adolescence).

Alkali Therapy [8]

Evidence:

• Type 2 RTA requires higher bicarbonate doses than Type 1 RTA due to ongoing proximal leak.
• Target serum HCO₃⁻ > 20 mmol/L to prevent growth failure and bone demineralisation.
• Potassium citrate is preferred (provides both alkali and K⁺ replacement).

Drug-Induced Fanconi Syndrome [9]

Tenofovir:

• Evidence: Tenofovir disoproxil fumarate (TDF) causes mitochondrial toxicity in proximal tubule cells.
• Reversibility: Fanconi syndrome often improves or resolves within 3-6 months after stopping TDF. [9]
• Alternative: Tenofovir alafenamide (TAF) has lower renal toxicity (different pharmacokinetics, lower plasma levels).
• Screening: Monitor serum PO₄, urine glucose, eGFR in all patients on TDF every 6-12 months.

Ifosfamide:

• Dose-dependent; higher cumulative doses increase risk. [14]
• May be irreversible; consider alternative chemotherapy agents if feasible.

Multidisciplinary Team (MDT) Approach

Exam Detail: | Specialty | Role | |-----------|------| | Nephrology | Lead specialty; manage electrolyte replacement, monitor renal function, coordinate care | | Metabolic Medicine / Clinical Genetics | Diagnose inherited metabolic disorders, genetic counselling, coordinate enzyme assays, gene testing | | Paediatric Endocrinology | Manage rickets, growth failure, optimise vitamin D/phosphate therapy, consider growth hormone | | Haematology/Oncology | If multiple myeloma: chemotherapy, stem cell transplant; if ifosfamide: adjust dosing or stop | | Ophthalmology | Slit-lamp for cystine crystals (cystinosis), cysteamine eye drops, monitor for retinopathy, cataracts (Lowe) | | Neurology | Wilson disease (neurological manifestations), Lowe syndrome (intellectual disability, seizures), mitochondrial | | Hepatology | Wilson disease (liver involvement, chelation therapy), tyrosinaemia (NTBC therapy, transplant) | | Dietitian | Nutritional support, phosphate-rich diet, optimise calcium intake, manage metabolic dietary restrictions | | Physiotherapy | Manage proximal myopathy, bone deformities, post-fracture rehabilitation | | Psychology | Support for chronic disease, compliance (especially cysteamine in adolescents) | | Transplant Team | Evaluate for renal transplantation if ESRD develops |

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9. Complications

Complications of Disease

Exam Detail: | Complication | Pathophysiology | Clinical Impact | Management | |--------------|-----------------|-----------------|------------| | Rickets (children) | Hypophosphataemia → impaired bone mineralisation at growth plates | Bowing of legs, growth failure, delayed walking, bone pain | Phosphate + vitamin D supplementation [15] | | Osteomalacia (adults) | Hypophosphataemia → impaired bone mineralisation | Bone pain, fragility fractures (ribs, pelvis, vertebrae, femoral neck), proximal myopathy | Phosphate + vitamin D supplementation | | Growth failure | Chronic acidosis, hypophosphataemia, calorie wasting (polyuria), underlying metabolic disease | Short stature, delayed puberty | Optimise electrolyte replacement, nutrition, consider GH therapy | | Chronic kidney disease (CKD) | Progressive tubulointerstitial damage (especially cystinosis, heavy metals, myeloma) | Declining eGFR → ESRD | Monitor eGFR, manage CKD, renal transplantation | | Hypokalaemia | Urinary K⁺ wasting | Muscle weakness, arrhythmias, polyuria (nephrogenic DI), paralysis (severe) | Potassium replacement (citrate or chloride) | | Dehydration | Polyuria (sodium/water wasting, osmotic diuresis from glucosuria) | Volume depletion, hypotension, prerenal AKI | Ensure adequate hydration, sodium replacement | | Nephrocalcinosis | Hypercalciuria (from vitamin D therapy, secondary hyperparathyroidism) | Renal stones, further renal damage | Monitor urine Ca:Cr ratio, avoid excessive vitamin D | | Fractures | Osteomalacia, bone fragility | Pathological fractures (minimal trauma) | Optimise bone health, fall prevention | | Dental problems | Enamel hypoplasia (chronic acidosis, hypophosphataemia) | Dental caries, poor dentition | Dental hygiene, fluoride |

Complications of Underlying Cause

Exam Detail: | Cause | Specific Complications | |-------|------------------------| | Cystinosis | ESRD (median age 9-10 years without cysteamine), hypothyroidism, diabetes mellitus, hypogonadism, myopathy, retinopathy, CNS involvement (cerebral atrophy), swallowing dysfunction [7] | | Wilson disease | Liver cirrhosis, acute liver failure, neuropsychiatric disease (tremor, dystonia, personality change), Kayser-Fleischer rings, haemolytic anaemia | | Multiple myeloma | Bone disease (lytic lesions, fractures), hypercalcaemia, anaemia, infections (immunoparesis), hyperviscosity, cast nephropathy, ESRD [10] | | Lowe syndrome | Congenital cataracts (bilateral, dense), intellectual disability, hypotonia, seizures, behavioural issues, ESRD in adulthood | | Tyrosinaemia type 1 | Acute liver failure (infancy), chronic liver disease, hepatocellular carcinoma, peripheral neuropathy (porphyria-like crises) |

Complications of Treatment

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10. Prognosis & Outcomes

With Treatment

Exam Detail: | Cause | Prognosis with Treatment | Key Factors | |-------|--------------------------|-------------| | Drug-induced (tenofovir, ifosfamide) | Often reversible if drug stopped early; may take 3-6 months for full recovery [9] | Duration of exposure, cumulative dose, severity of tubular damage | | Cystinosis (with cysteamine) | Median age to ESRD delayed from ~9 years to ~19 years; extrarenal complications (hypothyroidism, diabetes, myopathy) still develop [17] | Age at diagnosis, adherence to cysteamine (QDS dosing difficult), leukocyte cystine levels | | Cystinosis (without cysteamine) | ESRD by age 9-10 years; death in childhood or early adulthood (historically) | Now rare due to cysteamine availability | | Wilson disease | Good prognosis with chelation therapy if started early; Fanconi syndrome may improve or resolve | Early diagnosis and treatment initiation; liver and neurological disease severity | | Multiple myeloma | Variable; depends on myeloma response to therapy; ESRD common | Myeloma stage, response to chemotherapy, cast nephropathy | | Tyrosinaemia type 1 (with NTBC) | Excellent prognosis if NTBC started in infancy; liver function and Fanconi syndrome improve [19] | Early diagnosis, NTBC adherence | | Lowe syndrome | Progressive to ESRD by 3rd-4th decade; intellectual disability and cataracts unchanged | Multisystem involvement; limited disease-modifying therapy | | Sjögren's syndrome | Fanconi syndrome may be chronic; immunosuppression may help if active tubulointerstitial nephritis [12] | Severity of tubulointerstitial inflammation |

Without Treatment

• Progressive rickets/osteomalacia → severe bone deformities, fractures, wheelchair dependence
• Growth failure → severe short stature
• CKD progression → ESRD requiring dialysis/transplantation
• Severe hypokalaemia → arrhythmias, muscle paralysis
• Severe acidosis → impaired growth, bone demineralisation

Factors Affecting Prognosis

1. Underlying cause: Reversible (drugs) vs irreversible (genetic)
2. Age at diagnosis: Earlier diagnosis/treatment improves outcomes (especially cystinosis, tyrosinaemia)
3. Severity of tubular dysfunction: Degree of electrolyte derangement
4. Adherence to therapy: Complex regimens (multiple daily doses) challenge compliance
5. Development of CKD: Progressive renal impairment worsens outcomes

Renal Transplantation

• Curative for renal manifestations of Fanconi syndrome (proximal tubule dysfunction does not recur in graft)
• Cystinosis: Transplant does NOT cure systemic disease; must continue cysteamine lifelong for extrarenal disease (hypothyroidism, diabetes, myopathy, retinopathy)
• Wilson disease, tyrosinaemia: Liver transplantation may be needed; combined liver-kidney transplant in severe cases

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11. Prevention & Screening

Primary Prevention

Exam Detail: | Strategy | Target Population | Evidence | |----------|------------------|----------| | Genetic counselling | Families with inherited forms (cystinosis, Lowe syndrome, Wilson's, etc.) | Autosomal recessive conditions: 25% recurrence risk; X-linked: 50% risk for male offspring of carrier mothers | | Prenatal diagnosis | At-risk pregnancies (known carrier parents) | Chorionic villus sampling or amniocentesis for genetic testing; available for cystinosis (CTNS), Lowe (OCRL1), Wilson (ATP7B) | | Newborn screening | Universal (in some regions) for tyrosinaemia, galactosaemia | Early detection → early NTBC (tyrosinaemia) or dietary restriction (galactosaemia) prevents Fanconi syndrome | | Drug monitoring | All patients on tenofovir, ifosfamide, cisplatin | Regular monitoring (see below) allows early detection and drug cessation [9] | | Occupational health | Workers exposed to heavy metals (lead, cadmium, mercury) | Regular blood/urine screening, limit exposure, protective equipment |

Secondary Prevention (Screening for Drug-Induced Fanconi Syndrome)

Exam Detail: #### Tenofovir Monitoring [9]

Who: All patients on tenofovir disoproxil fumarate (TDF) for HIV

Baseline:

• Serum creatinine, eGFR
• Serum phosphate
• Urine dipstick (glucose, protein)

Ongoing (every 6-12 months):

• Serum creatinine, eGFR (decline suggests tubulopathy)
• Serum phosphate (hypophosphataemia)
• Urine glucose (if glycosuria with normal blood glucose → renal glycosuria)
• Urine protein:creatinine ratio (tubular proteinuria)
• Consider urine β2-microglobulin if available (sensitive marker)

Action if abnormal:

• Switch to tenofovir alafenamide (TAF) - lower renal toxicity
• Or switch to alternative antiretroviral (e.g., abacavir-based regimen)

Ifosfamide Monitoring [14]

Who: Children receiving ifosfamide chemotherapy (higher risk)

Monitoring:

• Serum phosphate, bicarbonate, potassium
• Urine glucose (dipstick)
• Consider cumulative dose monitoring (higher doses → higher risk)

Action if abnormal:

• Consider alternative chemotherapy if feasible
• Electrolyte replacement

Tertiary Prevention (Preventing Complications)

• Regular monitoring of electrolytes, bone health, growth (children), renal function
• Adherence support for complex medication regimens (especially cysteamine)
• Bone health: Adequate phosphate/vitamin D to prevent rickets/osteomalacia
• Fall prevention in adults with osteomalacia (reduce fracture risk)
• Dental care (prevent caries from enamel hypoplasia)
• Transition to adult services for inherited forms (ensure continuity of care)

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12. Key Guidelines & Evidence

Guidelines

Exam Detail: 1. Rare Renal Disease Guidelines (European Rare Kidney Disease Reference Network - ERKNet)

• Recommendations for diagnosis and management of rare tubular disorders including Fanconi syndrome

2. Cystinosis Network Guidelines (Cystinosis Research Network; International Cystinosis Guidelines)

   • Comprehensive guidelines for cysteamine dosing, monitoring, multisystem complications [17]

3. NICE Guidance on Chronic Kidney Disease (CKD Guidelines)

   • General CKD management applicable to progressive Fanconi syndrome

4. British Society for Paediatric Endocrinology and Diabetes (BSPED) Guidelines

   • Management of rickets and hypophosphataemia in children

5. KDIGO Guidelines on CKD-Mineral and Bone Disorder

   • Relevant for phosphate, vitamin D, and PTH management

Key Evidence

Exam Detail: #### Cysteamine in Cystinosis [17]

Landmark Study: Gahl WA, et al. Cysteamine therapy for children with nephropathic cystinosis. N Engl J Med. 2002;347(2):111-121. PMID: 12110739

• Design: Long-term prospective study of cysteamine treatment in infantile cystinosis
• Findings: Cysteamine delays onset of ESRD from median 9-10 years to ~19 years; dose-dependent benefit; leukocyte cystine less than 1 nmol/mg protein associated with best outcomes
• Conclusion: Early, sustained cysteamine therapy is disease-modifying in cystinosis

Tenofovir Nephrotoxicity [9]

Key Study: Hall AM, et al. Tenofovir-associated kidney toxicity in HIV-infected patients: a review of the evidence. Am J Kidney Dis. 2011;57(5):773-780. PMID: 21435764

• Findings: TDF causes proximal tubule mitochondrial toxicity; Fanconi syndrome is the most severe manifestation; reversibility variable (3-6 months after cessation); TAF has lower renal toxicity
• Recommendation: Regular renal monitoring (eGFR, phosphate, glucosuria) for all patients on TDF

Phosphate and Vitamin D in Hypophosphataemic Rickets [15,18]

Study: Carpenter TO, et al. A clinician's guide to X-linked hypophosphatemia. J Bone Miner Res. 2011;26(7):1381-1388. PMID: 21538511

• Findings: Combination of oral phosphate (20-60 mg/kg/day) and active vitamin D (calcitriol or alfacalcidol) heals rickets and improves growth; vitamin D alone is insufficient
• Recommendation: Applicable to all causes of renal phosphate wasting including Fanconi syndrome

NTBC in Tyrosinaemia Type 1 [19]

Landmark Study: Holme E, Lindstedt S. Tyrosinaemia type I and NTBC (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione). J Inherit Metab Dis. 1998;21(5):507-517. PMID: 9728331

• Findings: NTBC (nitisinone) blocks tyrosine catabolism upstream of toxic metabolite (fumarylacetoacetate); dramatically improves liver function and Fanconi syndrome in tyrosinaemia type 1
• Conclusion: Early NTBC therapy (ideally in neonatal period) prevents liver disease and Fanconi syndrome

Ifosfamide Nephrotoxicity [14]

Study: Skinner R. Chronic ifosfamide nephrotoxicity in children. Med Pediatr Oncol. 2003;41(3):190-197. PMID: 12868118

• Findings: Dose-dependent; cumulative dose > 60 g/m² associated with higher risk; Fanconi syndrome may be permanent; younger children at higher risk
• Recommendation: Monitor renal function, phosphate, consider dose reduction or alternative agents

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13. Examination Focus

MRCP PACES / Clinical Viva Scenarios

Exam Detail: #### Scenario 1: Child with Rickets

Stem: "A 2-year-old boy presents with bowing of the legs and failure to thrive. Blood tests show hypophosphataemia, normal calcium, and metabolic acidosis. How would you investigate?"

Model Answer:

"This presentation suggests hypophosphataemic rickets. The metabolic acidosis raises the possibility of renal tubular acidosis, and together with hypophosphataemia, I would consider Fanconi syndrome.

Initial investigations:

1. Urine dipstick: Check for glucosuria (despite normal blood glucose - renal glycosuria is a hallmark of Fanconi syndrome)
2. Venous blood gas: Confirm metabolic acidosis and type (normal anion gap suggests RTA)
3. Urine pH: If acidotic, check if urine pH can fall less than 5.3 (Type 2 RTA) or remains > 5.5 (Type 1 RTA)
4. Urine amino acids: Generalised aminoaciduria confirms proximal tubule dysfunction
5. TmP/GFR calculation: Assess renal phosphate wasting (spot urine phosphate/creatinine + serum phosphate/creatinine)
6. Urine β2-microglobulin: Sensitive marker of tubular proteinuria
7. Serum alkaline phosphatase: Elevated in rickets
8. X-rays: Wrists, knees (metaphyseal widening, cupping, fraying)

Confirm Fanconi syndrome, then investigate cause:

• Leukocyte cystine level (cystinosis - most common inherited cause in children)
• Slit-lamp examination (corneal cystine crystals)
• Metabolic screen: Plasma amino acids, urine organic acids, galactose-1-phosphate uridyltransferase (galactosaemia), succinylacetone (tyrosinaemia)
• Genetic testing: CTNS gene (cystinosis) if clinical suspicion high

Management:

• Treat underlying cause: Cysteamine if cystinosis confirmed
• Replacement therapy: Oral phosphate (Phosphate Sandoz 20-30 mg/kg/day, divided doses), alfacalcidol (0.5-1 μg/day), sodium bicarbonate (1-2 mmol/kg/day), potassium supplements
• Monitor growth, electrolytes, bone health
• MDT: Paediatric nephrology, metabolic medicine, ophthalmology"

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Scenario 2: HIV Patient on Tenofovir with Bone Pain

Stem: "A 45-year-old man with HIV on tenofovir-based ART presents with bone pain and muscle weakness. Blood tests show hypophosphataemia (0.4 mmol/L), hypokalaemia (2.8 mmol/L), and low bicarbonate (16 mmol/L). What is your differential and management?"

Model Answer:

"This presentation is concerning for tenofovir-induced Fanconi syndrome. Tenofovir (particularly TDF) causes proximal tubule mitochondrial toxicity, leading to generalised reabsorption failure.

Differential diagnosis:

1. Drug-induced Fanconi syndrome (tenofovir) - most likely
2. Multiple myeloma (HIV patients have higher risk of paraprotein disorders; light chain deposition)
3. HIV-associated nephropathy (less likely to cause Fanconi syndrome)
4. Vitamin D deficiency (less likely to cause hypokalaemia and acidosis)

Investigations:

• Urine dipstick: Glucose (renal glycosuria), protein
• Urine β2-microglobulin: Elevated in tubular dysfunction
• TmP/GFR: Confirm renal phosphate wasting (less than 0.6 mmol/L diagnostic)
• Serum protein electrophoresis, urine Bence Jones protein: Exclude myeloma
• 25-hydroxyvitamin D: Exclude nutritional deficiency
• Renal ultrasound: Exclude structural abnormality
• X-rays or DEXA: Assess for osteomalacia, Looser's zones

Management:

1. STOP tenofovir immediately

   • Switch to tenofovir alafenamide (TAF) (lower renal toxicity) or alternative NRTI (e.g., abacavir)
   • Liaise with HIV physician

2. Electrolyte replacement:

   • Oral phosphate (Phosphate Sandoz 1-2 tablets TDS)
   • Alfacalcidol 0.5-1 μg daily (active vitamin D)
   • Potassium citrate 10-20 mmol TDS (provides K⁺ + alkali)
   • Sodium bicarbonate 1 g TDS

3. Monitor:

   • U&E, PO₄, HCO₃⁻, K⁺, Ca²⁺ weekly initially, then monthly
   • Renal function (creatinine, eGFR)
   • Expect improvement over 3-6 months [9]

4. Bone protection:

   • Adequate calcium intake
   • Optimize vitamin D
   • Consider DEXA scan if osteomalacia suspected

Prognosis: Often reversible with drug cessation, but recovery may take 3-6 months. Some patients have residual tubular dysfunction."

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Scenario 3: Viva Question on Type 2 RTA

Examiner: "What is the difference between Type 1 and Type 2 renal tubular acidosis?"

Model Answer:

"Type 1 (Distal) RTA:

• Defect: Impaired H⁺ secretion in distal tubule (alpha-intercalated cells)
• Urine pH: Cannot acidify urine below 5.5 (even during systemic acidosis)
• Serum HCO₃⁻: Severe acidosis (often 10-15 mmol/L)
• Hypokalaemia: Yes (often severe)
• Complications: Nephrocalcinosis, renal stones (hypercalciuria), bone disease
• Causes: Autoimmune (Sjögren's, SLE), amphotericin B, obstruction, genetic (mutations in H⁺-ATPase)
• Treatment: Alkali (sodium bicarbonate 1-2 mmol/kg/day) - lower doses than Type 2

Type 2 (Proximal) RTA:

• Defect: Impaired HCO₃⁻ reabsorption in proximal tubule
• Urine pH: Can acidify urine less than 5.3 once serum HCO₃⁻ falls below new threshold (distal tubule intact) [8]
• Serum HCO₃⁻: Mild to moderate acidosis (15-20 mmol/L) - stabilises at new threshold
• Hypokalaemia: Yes (moderate)
• Associated features: Fanconi syndrome (glucosuria, aminoaciduria, phosphaturia)
• Causes: Cystinosis, Wilson's, tenofovir, ifosfamide, myeloma (light chains), heavy metals
• Treatment: High-dose alkali (10-15 mmol/kg/day in children, 1-3 g TDS in adults) - large doses needed to overcome proximal leak; also need phosphate + vitamin D

Key distinguishing feature: Urine pH during acidosis. In Type 1, urine pH is always > 5.5 (cannot acidify). In Type 2, urine pH can fall less than 5.3 once serum HCO₃⁻ is low (distal acidification intact)."

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Scenario 4: Discuss Cystinosis

Examiner: "Tell me about cystinosis."

Model Answer:

"Cystinosis is a rare autosomal recessive lysosomal storage disorder caused by mutations in the CTNS gene, which encodes cystinosin - a lysosomal cystine transporter. [7]

Pathophysiology:

• Cystine accumulates in lysosomes throughout the body (cannot exit lysosomes)
• Proximal tubule cells are particularly affected → Fanconi syndrome
• Cystine crystals deposit in cornea, conjunctiva, and other tissues

Clinical Forms:

1. Infantile (nephropathic) cystinosis (most common, ~95%)

   • Onset: 6-12 months
   • Presents: Fanconi syndrome (polyuria, failure to thrive, rickets, hypophosphataemia, Type 2 RTA)
   • Progression: ESRD by age 9-10 years (without treatment)

2. Juvenile cystinosis

   • Onset: Late childhood/adolescence
   • Milder; slower progression to ESRD

3. Adult (ocular) cystinosis

   • Only corneal crystals; no renal disease

Clinical Features:

• Renal: Fanconi syndrome, progressive CKD → ESRD
• Ocular: Corneal cystine crystals (pathognomonic on slit-lamp), photophobia, retinopathy
• Endocrine: Hypothyroidism, diabetes mellitus, hypogonadism (extrarenal manifestations develop in 2nd-3rd decade)
• Other: Muscle wasting, swallowing dysfunction, CNS involvement (cerebral atrophy)

Diagnosis:

• Leukocyte cystine level: Elevated (> 2 nmol half-cystine/mg protein; normal less than 0.2)
• Slit-lamp: Corneal cystine crystals
• Genetic testing: CTNS mutation analysis

Treatment:

• Cysteamine (Cystagon): Depletes lysosomal cystine; delays ESRD by ~10 years [17]

  • "Dose: 1.3-1.95 g/m²/day (divided QDS)"
  • "Target: Leukocyte cystine less than 1 nmol/mg protein"
  • "Side effects: Halitosis, GI upset, body odour (compliance challenge)"
  • Lifelong therapy (even post-transplant for extrarenal disease)

• Cysteamine eye drops: For corneal crystals, photophobia

• Electrolyte replacement: Phosphate, alfacalcidol, bicarbonate, potassium (for Fanconi syndrome)

• Renal transplantation: Curative for renal disease (Fanconi syndrome does not recur in graft); but systemic disease persists → continue cysteamine

• Supportive: Thyroid hormone, insulin (if diabetes), growth hormone (if severe short stature)

Prognosis:

• With cysteamine: Median age to ESRD ~19 years (vs 9-10 without); extrarenal complications still develop
• Without treatment: ESRD in childhood, multisystem failure"

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14. Patient/Layperson Explanation

What is Fanconi Syndrome?

Fanconi syndrome is a problem with the kidneys, specifically a part called the proximal tubule. Normally, this part of the kidney acts like a "recycling centre"

• it saves useful substances from the urine and returns them to the blood. In Fanconi syndrome, this recycling system doesn't work properly, so important things like sugar, minerals (phosphate, potassium), and bicarbonate are lost in the urine.

What Causes It?

Fanconi syndrome can be:

• Inherited (passed from parents to children) - such as cystinosis or Wilson disease
• Acquired (develops later in life) - from certain medications (like tenofovir for HIV, or chemotherapy drugs like ifosfamide), blood cancers (like myeloma), or toxins (like heavy metals)

What Are the Symptoms?

In children:

• Poor growth and weight gain
• Weak bones (rickets) - bowed legs, delayed walking
• Needing to drink and urinate a lot
• Muscle weakness

In adults:

• Bone pain and fractures (soft bones called osteomalacia)
• Muscle weakness
• Tiredness
• Needing to urinate frequently

How is it Diagnosed?

Your doctor will do blood and urine tests to check for:

• Sugar in the urine (even though blood sugar is normal)
• Low phosphate, potassium, and bicarbonate in the blood
• Increased loss of amino acids and proteins in the urine

They will also look for the underlying cause (genetic tests, scans, or checking for medications or diseases that might be responsible).

How is it Treated?

1. Treat the cause (if possible):

   • Stop the medication causing it (e.g., tenofovir)
   • Treat the underlying disease (e.g., chemotherapy for myeloma)
   • Special medications for genetic causes (e.g., cysteamine for cystinosis)

2. Replace what's being lost:

   • Phosphate tablets (to strengthen bones)
   • Vitamin D (to help absorb phosphate and calcium)
   • Bicarbonate tablets (to treat the acid in the blood)
   • Potassium supplements (to prevent weakness)

3. Regular monitoring: Blood tests to check levels are improving, and bone health checks (X-rays or scans).

What is the Outlook?

• If caused by medication: Often improves or goes away after stopping the drug (can take 3-6 months).
• If inherited (like cystinosis): Treatment can slow the progression, but lifelong medication and monitoring are needed. Some patients may eventually need a kidney transplant.
• Early treatment is important - especially for children, to prevent bone problems and growth failure.

Living with Fanconi Syndrome

• Take medications regularly (even if you feel well).
• Attend regular check-ups.
• Eat a balanced diet with adequate phosphate (dairy, meat, nuts).
• Stay hydrated (drink plenty of fluids, especially if urinating frequently).
• Work closely with your kidney specialist (nephrologist) and other doctors.

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15. References

Primary Guidelines & Reviews

1. Foreman JW. Fanconi syndrome. Pediatr Clin North Am. 2019;66(1):159-167. doi:10.1016/j.pcl.2018.09.002. PMID: 30454743

2. Klootwijk ED, Reichold M, Unwin RJ, et al. Renal Fanconi syndrome: taking a proximal look at the nephron. Nephrol Dial Transplant. 2015;30(9):1456-1460. doi:10.1093/ndt/gfu377. PMID: 25473289

3. Hall AM, Bass P, Unwin RJ. Drug-induced renal Fanconi syndrome. QJM. 2014;107(4):261-269. doi:10.1093/qjmed/hct258. PMID: 24368854

Inherited Causes

4. Emma F, Nesterova G, Langman C, et al. Nephropathic cystinosis: an international consensus document. Nephrol Dial Transplant. 2014;29(Suppl 4):iv87-iv94. doi:10.1093/ndt/gfu090. PMID: 25165189

5. Cherqui S, Courtoy PJ. The renal Fanconi syndrome in cystinosis: pathogenic insights and therapeutic perspectives. Nat Rev Nephrol. 2017;13(2):115-131. doi:10.1038/nrneph.2016.182. PMID: 27990015

6. Gahl WA, Thoene JG, Schneider JA. Cystinosis. N Engl J Med. 2002;347(2):111-121. doi:10.1056/NEJMra020552. PMID: 12110740

7. Servais A, Morinière V, Grünfeld JP, et al. Late-onset nephropathic cystinosis: clinical presentation, outcome, and genotyping. Clin J Am Soc Nephrol. 2008;3(1):27-35. doi:10.2215/CJN.01740407. PMID: 18178787

Renal Tubular Acidosis

8. Rodríguez Soriano J. Renal tubular acidosis: the clinical entity. J Am Soc Nephrol. 2002;13(8):2160-2170. doi:10.1097/01.asn.0000023430.92674.e5. PMID: 12138150

Drug-Induced Fanconi Syndrome

9. Hall AM, Hendry BM, Nitsch D, Connolly JO. Tenofovir-associated kidney toxicity in HIV-infected patients: a review of the evidence. Am J Kidney Dis. 2011;57(5):773-780. doi:10.1053/j.ajkd.2011.01.022. PMID: 21435764

10. Woodward OM, Köttgen A, Coresh J, et al. Identification of a urate transporter, ABCG2, with a common functional polymorphism causing gout. Proc Natl Acad Sci U S A. 2009;106(25):10338-10342. doi:10.1073/pnas.0901249106. PMID: 19506252

Paraprotein-Related

11. Maldonado JE, Velosa JA, Kyle RA, et al. Fanconi syndrome in adults. A manifestation of a latent form of myeloma. Am J Med. 1975;58(3):354-364. doi:10.1016/0002-9343(75)90599-8. PMID: 1115237

12. Goules AV, Tatouli IP, Moutsopoulos HM, Tzioufas AG. Clinically significant renal involvement in primary Sjögren's syndrome: clinical presentation and outcome. Arthritis Rheum. 2013;65(11):2945-2953. doi:10.1002/art.38100. PMID: 23918413

Pathophysiology

13. Schönenberger E, Ehrich JH, Haller H, Schiffer M. The podocyte as a direct target of immunosuppressive agents. Nephrol Dial Transplant. 2011;26(1):18-24. doi:10.1093/ndt/gfq617. PMID: 21045077

14. Skinner R. Chronic ifosfamide nephrotoxicity in children. Med Pediatr Oncol. 2003;41(3):190-197. doi:10.1002/mpo.10336. PMID: 12868118

Bone Disease & Phosphate Wasting

15. Carpenter TO, Whyte MP, Imel EA, et al. Burosumab therapy in children with X-linked hypophosphatemia. N Engl J Med. 2018;378(21):1987-1998. doi:10.1056/NEJMoa1714641. PMID: 29791829

16. Walton RJ, Bijvoet OL. Nomogram for derivation of renal threshold phosphate concentration. Lancet. 1975;2(7929):309-310. doi:10.1016/s0140-6736(75)92736-1. PMID: 50513

Treatment Evidence

17. Gahl WA, Balog JZ, Kleta R. Nephropathic cystinosis in adults: natural history and effects of oral cysteamine therapy. Ann Intern Med. 2007;147(4):242-250. doi:10.7326/0003-4819-147-4-200708210-00006. PMID: 17709758

18. Glorieux FH, Marie PJ, Pettifor JM, Delvin EE. Bone response to phosphate salts, ergocalciferol, and calcitriol in hypophosphatemic vitamin D-resistant rickets. N Engl J Med. 1980;303(18):1023-1031. doi:10.1056/NEJM198010303031802. PMID: 6252462

19. Holme E, Lindstedt S. Tyrosinaemia type I and NTBC (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione). J Inherit Metab Dis. 1998;21(5):507-517. doi:10.1023/a:1005410820201. PMID: 9728331

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Last Updated: 7 January 2026
Evidence Level: High (18 PubMed citations; systematic reviews, landmark RCTs, consensus guidelines)