Nephrolithiasis — Viva Defence

Long case viva — enteric hyperoxaluria from Crohn disease

Candidate's opening statement (SASPOP)

"Doctor, my patient is a 48-year-old man with Crohn disease and a prior terminal ileal resection, who has had four episodes of calcium oxalate nephrolithiasis in six years. His problems are: enteric hyperoxaluria from fat malabsorption (24-hour urine oxalate 0.9 mmol per day), hypocitraturia from chronic diarrhoea (citrate 1.2 mmol per day), low urine volume (1.2 L per day), and a counterproductive low-calcium diet that is worsening his oxalate absorption. The priority is to correct the metabolic abnormalities with targeted dietary and pharmacological therapy to prevent further stone recurrence, which can be reduced by 50 to 80 per cent with sustained adherence." [1]

Problem list

1. Recurrent calcium oxalate nephrolithiasis — four episodes in six years
2. Enteric hyperoxaluria from Crohn disease with ileal resection (fat malabsorption)
3. Hypocitraturia from chronic diarrhoea and metabolic acidosis
4. Low urine volume from chronic diarrhoea
5. Low-calcium diet (500 mg per day) — actively harmful, increasing oxalate absorption
6. Crohn disease — the underlying driver, requiring optimisation [1]

Integrated management plan

Dietary modification. Increase fluid intake to produce above 2.5 L of urine per day (given his diarrhoeal losses, this will require 3 to 4 L intake). Correct the low-calcium diet — increase to 1000 to 1200 mg per day, taken with meals so that calcium binds intestinal oxalate. The Borghi trial proved that normal calcium with reduced sodium and protein halves stone recurrence compared with low calcium [2]. Add a low-oxalate diet (restrict spinach, nuts, chocolate, tea) and a low-fat diet to reduce the fatty acids that saponify calcium.

Pharmacotherapy. Potassium citrate 10 mEq twice daily, titrated to urine pH 6.5 to 7.0 — this addresses both the hypocitraturia and the metabolic acidosis from chronic diarrhoea [5]. Oral calcium carbonate 500 to 1000 mg with each main meal to bind intestinal oxalate. Cholestyramine for refractory hyperoxaluria (binds bile acids, reducing colonic oxalate permeability).

Not indicated. Thiazide diuretics — his urinary calcium is normal. Thiazides are for hypercalciuria, not hyperoxaluria. [1]

Underlying disease. Optimise the Crohn disease management to reduce diarrhoea and fat malabsorption. [1]

Follow-up. Annual 24-hour urine re-evaluation to confirm the biochemistry has normalised. The goal is not just prescribing therapy but confirming it works. [1]

Examiner probing questions

Examiner: "Why does a low-calcium diet increase stone risk in this patient?" [1]

Dietary calcium normally binds intestinal oxalate in the gut lumen, forming insoluble calcium oxalate that is excreted in the stool. When calcium intake is low, or when fat malabsorption saponifies the available calcium (as in Crohn disease), free oxalate is absorbed from the colon into the portal circulation and excreted by the kidney, producing hyperoxaluria. A low-calcium diet therefore increases oxalate absorption and urinary oxalate, which is the primary driver of calcium oxalate stone formation. The Borghi 2002 NEJM trial proved this: men with hypercalciuria on a normal-calcium, low-sodium, low-protein diet had half the recurrence rate of those on a low-calcium diet (relative risk 0.49) [2].

Examiner: "How would you manage this patient's acute renal colic if he presents to the emergency department?" [1]

NSAIDs are first-line — diclofenac 75 mg intramuscularly or intravenously, which reduces prostaglandin-mediated ureteric inflammation and pain. I would check his renal function first, as patients with chronic diarrhoea may be volume-depleted and NSAIDs can worsen AKI. If NSAIDs are contraindicated, I would use morphine. Non-contrast CT KUB confirms the stone. For a stone below 5 mm, expectant management is appropriate. For 5 to 10 mm distal ureteric stones, I would consider tamsulosin — the SUSPEND trial showed no overall benefit [4], but the Hollingsworth meta-analysis suggested modest benefit for larger distal stones [6]. For stones above 10 mm, intervention is needed (ESWL, ureteroscopy, or PCNL depending on size and location). The critical emergency to recognise is obstructive pyelonephritis — fever with an obstructing stone requires urgent decompression, not antibiotics alone.

Examiner: "What imaging would you use in this patient, and when would you use ultrasound?" [1]

Non-contrast CT KUB is the gold standard — it detects all stone types except indinavir, measures size and density, identifies hydronephrosis, and excludes alternative diagnoses. The STONE trial showed ultrasound is a safe first-line alternative with no significant difference in adverse events [3]. I would use ultrasound as the first imaging in pregnancy, children, and recurrent stone formers to limit cumulative radiation. In this patient with recurrent stones, I would use ultrasound for known recurrences where the diagnosis is clear, reserving CT for atypical presentations or pre-operative planning.

Branching scenario — primary hyperparathyroidism

Examiner: "Now consider a 55-year-old woman with three episodes of calcium phosphate stones in two years. Her serum calcium is 2.85 mmol per litre (albumin 42), phosphate is 0.7 mmol per litre, and PTH is 12 pmol per litre. 24-hour urine shows hypercalciuria. What is the diagnosis and management?" [1]

This is primary hyperparathyroidism causing hypercalcaemia-driven hypercalciuria with calcium phosphate stones. The elevated serum calcium (2.85), low phosphate (0.7), and inappropriately high PTH (12) confirm the diagnosis — the PTH should be suppressed in the setting of hypercalcaemia if the parathyroids were functioning normally. The stones are calcium phosphate (not oxalate) because the alkaline urine from hypercalciuria and the higher urine pH favour phosphate precipitation. The definitive treatment is parathyroidectomy — this resolves hypercalcaemia, reduces hypercalciuria, and decreases stone recurrence in the majority of patients. The patient should be referred to an endocrine surgeon. Medical therapy with cinacalcet lowers serum calcium but does not fully correct the metabolic abnormality and is not first-line for stone prevention. I would also check a DEXA scan (hyperparathyroidism causes osteoporosis), renal function, and 25-hydroxyvitamin D (deficiency can stimulate PTH). All first-time stone formers should have serum calcium checked — hyperparathyroidism is a surgically curable cause of recurrent stones that must not be missed. [1]

Branching scenario — cystinuria

Examiner: "What if the patient were a 19-year-old man presenting with his first kidney stone, with a family history of stones in both parents, and urine microscopy shows hexagonal crystals?" [1]

This is cystinuria, an autosomal recessive defect in the proximal tubular transporter for cystine and dibasic amino acids (SLC3A1 or SLC7A9 genes). Cystine is poorly soluble at physiological pH (solubility increases only above pH 7.5), so the management is: massive fluid intake (above 4 L per day) to keep cystine concentration below its solubility product, urinary alkalinisation with potassium citrate to pH above 7.5, and chelation therapy with tiopronin or D-penicillamine for recurrent stone formers despite conservative measures. Cystine stones are faintly radiopaque (sulfur content) and very hard, making ESWL less effective — ureteroscopy with laser or PCNL is often needed for stone removal. A cyanide-nitroprusside screen of the urine confirms cystinuria, and genetic testing can identify the specific gene. In children or young adults presenting with stones, a thorough metabolic evaluation is mandatory on the first episode (unlike in adults, where it is typically done after recurrence), because inherited metabolic abnormalities are much more common in this age group [1].

Branching scenario — struvite staghorn calculus

Examiner: "And what if the patient were a 60-year-old woman with recurrent Proteus urinary tract infections and a large branching staghorn calculus on CT, with a urine pH of 7.8?" [1]

This is a struvite (magnesium ammonium phosphate) staghorn calculus — an infection stone caused by urease-producing organisms, classically Proteus mirabilis. Urease hydrolyses urea to ammonia, raising urine pH above 7.2 and precipitating magnesium ammonium phosphate. The key management principles are: complete surgical removal, typically by percutaneous nephrolithotomy (PCNL), because residual fragments harbour infection and recurrence is inevitable; culture-directed antibiotics as an adjunct but NOT as sole therapy (antibiotics cannot penetrate the stone matrix adequately); and treatment of any underlying urinary tract obstruction or foreign body that predisposes to infection. Leaving a staghorn calculus in place risks pyonephrosis, xanthogranulomatous pyelonephritis, progressive renal destruction, and potentially loss of the kidney. If the patient presents with obstructive pyelonephritis (fever, rigors, sepsis), urgent decompression with a nephrostomy or stent is needed before definitive stone removal once the sepsis has resolved. Potassium citrate should NOT be used in struvite stones — alkalinising an already alkaline urine promotes further struvite formation. [1]

Short-case discussion — abdominal examination

Examiner: "A 45-year-old man presents with acute right flank pain. Examine his abdomen." [1]

My routine: general inspection — is the patient writhing (suggesting renal colic) or lying still (suggesting peritonitis); vital signs — fever suggests infection, hypotension suggests sepsis or AAA. Hands and face for signs of systemic disease associated with stones (tophi for gout, Cushingoid features). Abdomen — inspect for scars and distension; palpate for tenderness (costovertebral angle tenderness suggests renal pathology, peritoneal signs suggest a surgical abdomen), a pulsatile mass in the older patient (AAA), and organomegaly; auscultate bowel sounds and renal artery bruits. Perform fist percussion at the costovertebral angle for renal angle tenderness. [1]

Presentation: "The patient is writhing in pain, unable to find a comfortable position. Blood pressure is 145 over 90, pulse 98, afebrile. The abdomen is soft with right costovertebral angle tenderness but no peritoneal signs. There is no palpable abdominal mass or organomegaly. These findings are consistent with renal colic. I would perform urinalysis for haematuria and arrange non-contrast CT KUB." [1]

Examiner: "What would make you concerned about a more dangerous diagnosis?" Fever with obstruction (obstructive pyelonephritis), haemodynamic instability or a pulsatile mass in the older patient (AAA), peritoneal signs (surgical abdomen), or anuria (bilateral obstruction). These are red flags requiring urgent investigation and intervention. [1]

Examiner: "How would you distinguish the pain of renal colic from peritonitis?" The patient with renal colic writhes and moves — the pain is visceral, from ureteric spasm, and no position is comfortable. The patient with peritonitis lies still and resists movement — the pain is somatic, from parietal peritoneal inflammation, and movement worsens it. This single bedside observation is one of the most valuable discriminating signs in the acute abdomen. [1]