Hypomagnesaemia

1. Clinical Overview

Summary

Hypomagnesaemia is defined as serum magnesium less than 0.7 mmol/L (or less than 1.7 mg/dL). It represents one of the most common electrolyte disturbances in hospitalized patients, yet remains frequently underdiagnosed because magnesium is not routinely measured in standard electrolyte panels. [1] The clinical significance of hypomagnesaemia extends beyond its direct effects: magnesium is essential for the function of Na-K-ATPase pumps, PTH secretion, and cardiac repolarization. Consequently, untreated hypomagnesaemia leads to refractory hypokalaemia (where potassium replacement fails) and refractory hypocalcaemia (due to impaired PTH secretion and skeletal resistance to PTH). [2]

The most important clinical consequences include life-threatening ventricular arrhythmias (particularly Torsades de Pointes), neuromuscular irritability (tetany, seizures), and electrolyte disturbances that cannot be corrected without magnesium repletion. Common causes include proton pump inhibitors (PPIs), loop and thiazide diuretics, alcoholism, gastrointestinal losses, and inherited tubular disorders (Gitelman and Bartter syndromes). [3,4]

Recognition and treatment are straightforward: identify the underlying cause, replicate magnesium (IV for severe/symptomatic cases, oral for mild cases), and simultaneously correct associated hypokalaemia and hypocalcaemia.

Key Facts

• Definition: Serum Mg less than 0.7 mmol/L (less than 1.7 mg/dL)
• Prevalence: 10-20% of hospitalized patients; up to 65% in ICU settings [1]
• Critical Effect: Causes refractory hypokalaemia and refractory hypocalcaemia
• Major Causes: PPIs (chronic use), diuretics (loop/thiazide), alcoholism, diarrhoea, Gitelman syndrome
• Cardiac Risk: Torsades de Pointes (even with normal serum Mg), QTc prolongation, digoxin toxicity
• Key Mechanism: Mg required for Na-K-ATPase function and PTH secretion
• Treatment: IV MgSO₄ 8mmol (2g) over 2-4h (severe); oral Mg salts (mild)
• MRCP Pearl: Always check and correct Mg first before attempting K or Ca replacement

Clinical Pearls

"You Can't Fix K Without Mg": Magnesium is an essential cofactor for Na-K-ATPase. In the setting of hypomagnesaemia, the renal outer medullary potassium (ROMK) channel is inappropriately active, causing urinary potassium wasting. Potassium replacement will fail until magnesium is corrected first. [2]

"PPIs Are a Stealth Culprit": Long-term PPI therapy (> 1 year) causes hypomagnesaemia in approximately 1% of patients through impaired intestinal magnesium absorption via TRPM6/7 channels. The FDA issued a safety warning in 2011. [5] Always check magnesium levels in patients on chronic PPIs.

"Mg for Torsades—Even If Serum Mg Is Normal": IV magnesium sulphate (8mmol over 10-15 minutes) is the first-line treatment for Torsades de Pointes, regardless of measured serum magnesium. Magnesium stabilizes cardiac membranes and suppresses early after-depolarizations. [6]

"Alcohol = Low Everything": Chronic alcohol excess causes hypomagnesaemia via multiple mechanisms: poor intake, gastrointestinal losses, renal wasting (alcohol-induced tubular dysfunction), and pancreatitis. Always check Mg, K, Ca, and phosphate in alcoholics. [7]

"Gitelman's = Biochemical Mimic of Thiazides": Gitelman syndrome (loss-of-function mutations in SLC12A3 encoding the Na-Cl cotransporter) produces hypomagnesaemia, hypokalaemia, metabolic alkalosis, and hypocalciuria—exactly mimicking chronic thiazide use. Key differentiator: urinary calcium (low in Gitelman, normal/high with thiazides). [8]

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2. Epidemiology

Incidence and Prevalence

Demographics

• All ages can be affected
• Elderly: Higher risk due to polypharmacy (diuretics, PPIs), reduced intake, impaired absorption
• ICU patients: High prevalence due to critical illness, medications, renal replacement therapy
• Alcoholics: Chronic alcohol is a leading cause

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3. Aetiology and Pathophysiology

Magnesium Homeostasis

Total Body Magnesium: ~24g (1000 mmol) in adults

• 50% in bone (non-exchangeable reservoir)
• 48% intracellular (muscle, liver, heart)
• 2% extracellular fluid (measured in serum)

Normal Serum Magnesium: 0.7-1.0 mmol/L (1.7-2.4 mg/dL)

Absorption:

• Occurs primarily in the small intestine (jejunum and ileum) via:
  • Paracellular pathway (passive, dominant at high Mg intake)
  • Transcellular pathway via TRPM6 and TRPM7 channels (active, saturable) [11]
  • PPIs impair TRPM6/7 function, reducing absorption

Excretion:

• Kidneys filter ~2400 mg/day; 95% is reabsorbed
• Proximal tubule: 15-25% reabsorbed
• Thick ascending limb of loop of Henle (TAL): 60-70% reabsorbed (paracellular via claudin-16 and claudin-19) [12]
  • Loop diuretics block Na-K-2Cl cotransporter → reduce lumen-positive voltage → reduce Mg reabsorption
• Distal convoluted tubule (DCT): 5-10% reabsorbed (transcellular via TRPM6 channels) [11]
  • Thiazide diuretics block Na-Cl cotransporter → reduce intracellular Mg → impair transcellular Mg reabsorption
  • "Gitelman syndrome: Loss-of-function mutations in SLC12A3 (Na-Cl cotransporter) → same effect as thiazides [8]"

Causes of Hypomagnesaemia

Exam Detail: #### Renal Magnesium Wasting

Gastrointestinal Losses

Redistribution (Transcellular Shift)

Inadequate Intake

Genetic/Inherited Disorders

Molecular Mechanisms: Why Does Low Mg Cause Low K and Low Ca?

Exam Detail: #### Mechanism 1: Hypomagnesaemia → Refractory Hypokalaemia

Magnesium deficiency impairs potassium homeostasis at multiple levels:

1. Reduced Na-K-ATPase Activity:

   • Mg²⁺ is a required cofactor for Na-K-ATPase (maintains intracellular K⁺ concentration)
   • Low Mg → Reduced pump activity → K⁺ leaks out of cells [2]

2. Increased ROMK Channel Activity:

   • In the DCT and collecting duct, magnesium normally inhibits the renal outer medullary potassium (ROMK) channel
   • Low Mg → Uninhibited ROMK channel → Excessive urinary K⁺ excretion [20]

3. Clinical Consequence:

   • Potassium replacement fails because K⁺ continues to leak out (intracellularly and renally)
   • YOU MUST CORRECT MAGNESIUM FIRST before K⁺ can be successfully repleted

Mechanism 2: Hypomagnesaemia → Refractory Hypocalcaemia

Magnesium is essential for calcium homeostasis:

1. Impaired PTH Secretion:

   • Severe hypomagnesaemia (Mg less than 0.4 mmol/L) inhibits PTH secretion from parathyroid glands [21]
   • Mechanism: Intracellular Mg depletion impairs exocytosis of PTH-containing vesicles
   • Result: Functional hypoparathyroidism (low/normal PTH despite low Ca)

2. PTH Resistance:

   • Hypomagnesaemia also causes skeletal resistance to PTH (impaired cAMP generation in bone and kidney) [21]
   • Even if PTH is released, target organs cannot respond

3. Clinical Consequence:

   • Hypocalcaemia will NOT correct with calcium or vitamin D alone
   • MAGNESIUM REPLETION IS REQUIRED to restore PTH secretion and skeletal responsiveness

Cardiac Effects of Hypomagnesaemia

Exam Detail: Magnesium stabilizes cardiac membranes and modulates ion channels:

1. QT Prolongation:

   • Mg²⁺ blocks inward rectifying K⁺ channels (IKr) that mediate repolarization
   • Low Mg → Prolonged action potential duration → Prolonged QTc interval

2. Torsades de Pointes:

   • QT prolongation + triggers (PVCs) → Early after-depolarizations (EADs) → Polymorphic VT (Torsades) [6]
   • IV magnesium is first-line treatment (suppresses EADs), even if serum Mg is normal

3. Digoxin Toxicity:

   • Mg²⁺ and digoxin compete for binding to Na-K-ATPase
   • Low Mg → Enhanced digoxin binding → Increased toxicity (even at therapeutic digoxin levels)

4. Atrial Fibrillation:

   • Hypomagnesaemia is associated with increased AF risk post-cardiac surgery [10]

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4. Clinical Presentation

Symptoms

Symptoms are often non-specific and overlap with hypokalaemia and hypocalcaemia. Many patients with mild-moderate hypomagnesaemia are asymptomatic.

Severity vs Clinical Features

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5. Clinical Examination

Examination Findings

Many patients have normal examination unless hypomagnesaemia is severe or accompanied by hypocalcaemia.

Neuromuscular Signs (Secondary to Hypocalcaemia)

• Chvostek's Sign:

  • Tap facial nerve anterior to ear
  • "Positive: Ipsilateral facial muscle twitching"
  • Indicates hypocalcaemia (which may be secondary to hypoMg)

• Trousseau's Sign:

  • Inflate BP cuff above systolic BP for 3 minutes
  • "Positive: Carpopedal spasm (thumb adduction, finger extension/adduction)"
  • More specific than Chvostek's for hypocalcaemia

• Hyperreflexia: Brisk deep tendon reflexes

• Tremor: Fine tremor of hands

Cardiac

• Irregular pulse: Atrial fibrillation, ventricular ectopy
• Tachycardia

Other

• Signs of underlying cause:
  • "Chronic alcoholism: Stigmata (palmar erythema, spider naevi, jaundice)"
  • "Malnutrition: Cachexia, muscle wasting"

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6. Differential Diagnosis

Hypomagnesaemia often presents with hypokalaemia and/or hypocalcaemia. The differential focuses on the pattern of electrolyte disturbances.

Key Differential: Gitelman vs Bartter Syndrome

Both are inherited tubulopathies presenting with hypomagnesaemia, hypokalaemia, and metabolic alkalosis. Differentiation is critical.

Key Differentiator: Urinary calcium

• Gitelman: Low urinary calcium (hypocalciuria)
• Bartter: High urinary calcium (hypercalciuria)

Other Differentials

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7. Investigations

First-Line Investigations

Determining the Cause: Urinary Magnesium Studies

Once hypomagnesaemia is confirmed, determine if the cause is renal (inappropriate urinary Mg loss) or extra-renal (GI, redistribution, poor intake).

Exam Detail: #### 24-Hour Urinary Magnesium Excretion

Limitation: Affected by dietary Mg intake; may be inaccurate in acute settings.

Fractional Excretion of Magnesium (FEMg)

More accurate; not affected by dietary intake.

Formula:

\text{FEMg (\%)} = \frac{(\text{Urine Mg} \times \text{Plasma Cr})}{(0.7 \times \text{Plasma Mg} \times \text{Urine Cr})} \times 100

(0.7 = correction factor for protein-bound Mg; only ionized Mg is filtered)

Additional Investigations (Based on Clinical Context)

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8. Management

General Principles

1. Identify and treat the underlying cause

   • Stop/reduce causative drug if possible (PPI, diuretic)
   • Treat GI losses (diarrhoea, malabsorption)
   • Alcohol cessation and nutritional support

2. Replicate magnesium

   • Route (oral vs IV) depends on severity and symptoms
   • Correct Mg BEFORE attempting K or Ca repletion

3. Correct associated electrolyte abnormalities

   • Hypokalaemia: Give K after Mg is repleted
   • Hypocalcaemia: May need Ca + vitamin D after Mg is repleted

4. Monitor and prevent recurrence

   • Recheck Mg, K, Ca after treatment
   • Long-term oral Mg if chronic cause (e.g., ongoing diuretics, Gitelman syndrome)

Magnesium Replacement Protocols

Exam Detail: #### Oral Magnesium Replacement (Mild, Asymptomatic)

Indications:

• Serum Mg 0.5-0.7 mmol/L
• Asymptomatic or mild symptoms
• No cardiac arrhythmias
• Able to tolerate oral intake

Preparations:

Side Effect: Diarrhoea (dose-limiting; reduce dose if occurs)

Monitoring: Recheck serum Mg after 3-5 days

Intravenous Magnesium Replacement (Moderate-Severe, Symptomatic)

Indications:

• Serum Mg less than 0.5 mmol/L
• Symptomatic (tetany, arrhythmias, seizures)
• Unable to tolerate oral (vomiting, malabsorption)
• Cardiac arrhythmias or QTc prolongation

Preparation: Magnesium Sulphate (MgSO₄)

• 1g MgSO₄ = 4 mmol Mg²⁺ (also = 8 mEq)
• Available as 50% solution (500 mg/mL = 2 mmol/mL)

Protocol:

Maximum Rate: 8 mmol/hour (rapid infusion → flushing, hypotension)

Monitoring:

• Recheck Mg, K, Ca 4-6 hours after first dose, then daily
• ECG if arrhythmias or QTc prolongation
• Urine output (ensure adequate to excrete excess Mg)
• Signs of hypermagnesaemia (see below)

Management Algorithm

┌──────────────────────────────────────────────────────────────────┐
│                 HYPOMAGNESAEMIA MANAGEMENT                        │
├──────────────────────────────────────────────────────────────────┤
│                                                                   │
│  STEP 1: ASSESS SEVERITY & SYMPTOMS                               │
│  ┌─────────────────────────────────────────────────────────────┐ │
│  │ Mild (0.5-0.7 mmol/L) + Asymptomatic → ORAL replacement     │ │
│  │ Moderate (0.3-0.5) + Symptomatic → IV replacement           │ │
│  │ Severe (less than 0.3) OR Arrhythmias/Seizures → IV replacement      │ │
│  │ Torsades de Pointes → EMERGENCY IV bolus                    │ │
│  └─────────────────────────────────────────────────────────────┘ │
│                                                                   │
│  STEP 2: ORAL REPLACEMENT (Mild, Asymptomatic)                    │
│  ┌─────────────────────────────────────────────────────────────┐ │
│  │ • Magnesium Glycerophosphate 4-8 mmol BD-TDS (oral)         │ │
│  │ • OR Magnesium Aspartate 10 mmol BD (oral)                  │ │
│  │ • Recheck Mg, K, Ca in 3-5 days                             │ │
│  │ • Side effect: Diarrhoea (reduce dose if occurs)            │ │
│  └─────────────────────────────────────────────────────────────┘ │
│                                                                   │
│  STEP 3: IV REPLACEMENT (Moderate-Severe, Symptomatic)            │
│  ┌─────────────────────────────────────────────────────────────┐ │
│  │ • MgSO₄ 8 mmol (2g) IV in 100-250 mL NS over 2-4 hours      │ │
│  │ • Repeat dose after 4-6h if Mg still low                    │ │
│  │ • Maximum rate: 8 mmol/hour (avoid flushing/hypotension)    │ │
│  │ • Recheck Mg, K, Ca after 4-6h, then daily                  │ │
│  └─────────────────────────────────────────────────────────────┘ │
│                                                                   │
│  STEP 4: EMERGENCY IV (Torsades de Pointes)                       │
│  ┌─────────────────────────────────────────────────────────────┐ │
│  │ • MgSO₄ 8 mmol (2g) IV BOLUS over 10-15 minutes             │ │
│  │ • Give EVEN IF serum Mg is normal                           │ │
│  │ • Defibrillation if Torsades → VF                           │ │
│  │ • Stop all QT-prolonging drugs                              │ │
│  └─────────────────────────────────────────────────────────────┘ │
│                                                                   │
│  STEP 5: CORRECT Mg BEFORE K and Ca                               │
│  ┌─────────────────────────────────────────────────────────────┐ │
│  │ • K replacement will FAIL until Mg corrected                │ │
│  │ • Ca replacement may fail until Mg corrected (PTH issue)    │ │
│  │ • Replace K after Mg > 0.5 mmol/L                            │ │
│  │ • Replace Ca after Mg > 0.5 mmol/L                           │ │
│  └─────────────────────────────────────────────────────────────┘ │
│                                                                   │
│  STEP 6: TREAT UNDERLYING CAUSE                                   │
│  ┌─────────────────────────────────────────────────────────────┐ │
│  │ • Stop/reduce PPI if possible (or switch to H2 blocker)     │ │
│  │ • Stop/reduce diuretic if possible (or add K-sparing)       │ │
│  │ • Alcohol cessation + thiamine                              │ │
│  │ • Treat GI disease (IBD, coeliac, etc.)                     │ │
│  └─────────────────────────────────────────────────────────────┘ │
│                                                                   │
│  STEP 7: LONG-TERM MAINTENANCE (if chronic cause)                 │
│  ┌─────────────────────────────────────────────────────────────┐ │
│  │ • Ongoing oral Mg supplementation (e.g., diuretics)         │ │
│  │ • Gitelman/Bartter: Mg + K-sparing diuretics (amiloride)    │ │
│  │ • Monitor Mg, K, Ca every 3-6 months                        │ │
│  └─────────────────────────────────────────────────────────────┘ │
│                                                                   │
└──────────────────────────────────────────────────────────────────┘

Special Populations

Exam Detail: #### Renal Impairment

• Risk: Hypermagnesaemia (reduced excretion)
• Approach:
  • Use lower doses and slower infusion rates
  • Monitor serum Mg closely (target 0.5-0.7 mmol/L, avoid > 1.0 mmol/L)
  • Check for signs of hypermagnesaemia (hyporeflexia, bradycardia, hypotension)
  • Consider dialysis if severe hypermagnesaemia (> 2.5 mmol/L) with symptoms

Chronic Genetic Disorders (Gitelman, Bartter)

• Long-term oral Mg supplementation required (lifelong)
• K-sparing diuretics (amiloride 5-10 mg daily or spironolactone 25-50 mg daily) reduce urinary K and Mg losses [8]
• NSAIDs (e.g., indomethacin) reduce renal K wasting in Bartter syndrome (NOT routinely used in Gitelman) [16]
• Dietary: Increase Mg-rich foods (nuts, seeds, whole grains, leafy greens)

Refeeding Syndrome

• High risk: Anorexia nervosa, chronic alcoholism, prolonged fasting (> 10 days), significant weight loss
• Prevention:
  • "Pre-feeding: Check and correct Mg, K, phosphate BEFORE refeeding"
  • "Supplementation: Prophylactic Mg, K, phosphate with thiamine during refeeding"
  • "Slow refeeding: Start at 50% energy requirements, increase gradually [15]"

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9. Complications

Complications of Untreated Hypomagnesaemia

Complications of Magnesium Replacement

Hypermagnesaemia (Mg > 1.1 mmol/L)

Risk Factors: Excessive IV Mg, renal impairment, reduced GFR

Antidote: Calcium gluconate 10 mL of 10% IV over 10 minutes (antagonizes cardiac/neuromuscular effects)

Infusion-Related

• Rapid IV infusion: Flushing, hypotension, nausea
• Prevention: Infuse over 2-4 hours (max rate 8 mmol/hour)

Oral Magnesium

• Diarrhoea (dose-limiting): Reduce dose; use better-tolerated formulations (Mg glycerophosphate)

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10. Prognosis & Outcomes

With Treatment

• Acute hypomagnesaemia: Usually corrects within 24-48 hours with appropriate replacement
• Chronic causes (e.g., ongoing diuretics, Gitelman syndrome): Require long-term oral supplementation; good prognosis with adherence
• Severe arrhythmias: Excellent response to IV Mg (Torsades de Pointes typically terminates with IV Mg bolus) [6]

Without Treatment

• Risk of sudden cardiac death (arrhythmias)
• Chronic muscle weakness, fatigue
• Failure to correct hypokalaemia and hypocalcaemia → prolonged symptoms, complications

Monitoring

• Acute phase: Recheck Mg, K, Ca 4-6 hours after first IV dose, then daily until stable
• Chronic maintenance: Recheck Mg, K, Ca every 3-6 months (or more frequently if symptoms)

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11. Evidence & Guidelines

Key Guidelines

1. National Kidney Foundation (NKF) / Kidney Disease Improving Global Outcomes (KDIGO): Electrolyte management in CKD
2. Resuscitation Council UK: Advanced Life Support Guidelines (management of Torsades de Pointes with IV magnesium)
3. NICE Clinical Knowledge Summaries: Electrolyte disturbances

Key Evidence

1. PPI-Associated Hypomagnesaemia [5]

• FDA Drug Safety Communication (2011): Long-term PPI use (> 1 year) associated with hypomagnesaemia
• Mechanism: Impaired intestinal Mg absorption via TRPM6/7 channels
• Recommendation: Monitor Mg in patients on chronic PPIs (especially if on diuretics)

2. Magnesium and Refractory Hypokalaemia [2]

• Landmark study (Whang et al., 1992): Demonstrated that Mg deficiency impairs K repletion via ROMK channel activation and Na-K-ATPase dysfunction
• Clinical implication: Always check and correct Mg before treating hypokalaemia

3. Magnesium for Torsades de Pointes [6]

• Multiple case series and guidelines: IV MgSO₄ 8 mmol (2g) over 10-15 minutes is first-line treatment for Torsades, even if serum Mg is normal
• Mechanism: Suppresses early after-depolarizations (EADs), stabilizes cardiac membranes

4. Gitelman Syndrome [8]

• Loss-of-function mutations in SLC12A3 (Na-Cl cotransporter) cause autosomal recessive hypomagnesaemia, hypokalaemia, metabolic alkalosis, and hypocalciuria
• Differentiation from Bartter: Urinary calcium (low in Gitelman, high in Bartter)

5. Hypomagnesaemia and Cardiac Arrhythmias [10]

• Post-cardiac surgery studies: Hypomagnesaemia (40-60% prevalence) associated with increased risk of AF
• Prophylactic Mg reduces AF incidence (meta-analyses)

6. Magnesium and PTH Secretion [21]

• Severe hypomagnesaemia (Mg less than 0.4 mmol/L) impairs PTH secretion and causes skeletal resistance to PTH
• Results in functional hypoparathyroidism (low/normal PTH despite hypocalcaemia)

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12. Examination Focus

Exam Detail: ### High-Yield MRCP Viva Topics

Question 1: Refractory Hypokalaemia

Examiner: "A 68-year-old woman on furosemide 80 mg BD for heart failure presents with persistent hypokalaemia (K 2.8 mmol/L) despite oral potassium supplementation. What would you do?"

Model Answer: "I would check serum magnesium. Hypomagnesaemia is common with loop diuretics and causes refractory hypokalaemia through two mechanisms: firstly, magnesium is a required cofactor for the Na-K-ATPase pump, so low magnesium impairs intracellular potassium retention; secondly, magnesium normally inhibits the ROMK channel in the distal nephron, so in its absence, there is excessive urinary potassium excretion. Potassium replacement will fail until magnesium is corrected. If magnesium is low, I would give IV or oral magnesium replacement (depending on severity), and then recheck both magnesium and potassium. I would also consider a potassium-sparing diuretic like amiloride to reduce ongoing urinary losses."

Question 2: Gitelman vs Bartter Syndrome

Examiner: "How would you differentiate Gitelman syndrome from Bartter syndrome?"

Model Answer: "Both are inherited tubulopathies causing hypomagnesaemia, hypokalaemia, and metabolic alkalosis, but there are key differences:

The key differentiator is urinary calcium excretion: Gitelman presents with hypocalciuria (less than 2.5 mmol/24h), whereas Bartter has hypercalciuria (> 7.5 mmol/24h), which can lead to nephrocalcinosis. Gitelman biochemically mimics chronic thiazide use, while Bartter mimics loop diuretic use."

Question 3: PPI-Induced Hypomagnesaemia

Examiner: "What is the mechanism of PPI-associated hypomagnesaemia?"

Model Answer: "Long-term PPI use (typically > 1 year) causes hypomagnesaemia by impairing intestinal magnesium absorption. The mechanism involves inhibition of TRPM6 and TRPM7 channels in the small intestine, which are responsible for active transcellular magnesium transport. The FDA issued a safety warning in 2011. Clinically, I would check magnesium levels in patients on chronic PPIs, especially if they are also on diuretics (which cause renal magnesium wasting). If hypomagnesaemia is found, I would consider stopping or reducing the PPI (or switching to an H2-receptor antagonist if acid suppression is still needed) and providing magnesium supplementation."

Question 4: Magnesium for Torsades de Pointes

Examiner: "A patient develops Torsades de Pointes. Their serum magnesium is normal at 0.8 mmol/L. Do you still give magnesium?"

Model Answer: "Yes, absolutely. IV magnesium sulphate 8 mmol (2g) over 10-15 minutes is the first-line treatment for Torsades de Pointes, regardless of the measured serum magnesium level. Magnesium suppresses early after-depolarizations, which are the mechanism underlying Torsades, and stabilizes cardiac membranes. The therapeutic effect is independent of serum magnesium concentration. I would also identify and address the underlying cause (typically QT-prolonging drugs, hypokalaemia, or hypomagnesaemia), stop any offending agents, correct electrolyte abnormalities, and potentially use temporary pacing if Torsades is recurrent."

Question 5: Secondary Hypocalcaemia

Examiner: "A patient with severe hypomagnesaemia (Mg 0.3 mmol/L) also has hypocalcaemia (corrected Ca 1.9 mmol/L). The PTH is low-normal. What is the mechanism?"

Model Answer: "Severe hypomagnesaemia impairs PTH secretion from the parathyroid glands and also causes skeletal resistance to PTH. The mechanism of impaired PTH secretion involves intracellular magnesium depletion, which disrupts exocytosis of PTH-containing vesicles. This results in a state of functional hypoparathyroidism: PTH levels are low or inappropriately normal despite hypocalcaemia. The skeletal resistance means that even if some PTH is released, bone and kidney cannot respond appropriately (impaired cAMP generation). Consequently, calcium replacement alone will not correct the hypocalcaemia; magnesium must be repleted first to restore both PTH secretion and end-organ responsiveness. Once magnesium is corrected, PTH secretion typically normalizes and calcium rises."

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13. Patient/Layperson Explanation

What is Hypomagnesaemia?

Hypomagnesaemia means you have low levels of magnesium in your blood. Magnesium is an essential mineral that helps your muscles, nerves, heart, and bones work properly. It also helps your body maintain normal levels of other minerals like potassium and calcium.

What Causes It?

The most common causes of low magnesium include:

• Medications:
  • "Water tablets" (diuretics) used for high blood pressure or heart failure
  • Acid-reducing tablets (like omeprazole or lansoprazole) taken for long periods (> 1 year)
• Alcohol: Heavy or long-term alcohol use
• Digestive problems: Chronic diarrhoea, inflammatory bowel disease, or conditions affecting nutrient absorption
• Poor diet: Not eating enough magnesium-rich foods (nuts, seeds, whole grains, leafy greens)

What Are the Symptoms?

Many people with mild low magnesium have no symptoms. When symptoms do occur, they can include:

• Muscle cramps, twitches, or spasms
• Tiredness and weakness
• Tingling or numbness (especially around the mouth or in the hands)
• Heart palpitations (irregular heartbeat)
• Nausea

In severe cases, low magnesium can cause:

• Dangerous heart rhythm problems
• Seizures
• Confusion

Why Does It Matter?

Magnesium is important because:

1. It affects other minerals: Low magnesium often causes low potassium and low calcium as well. Your body cannot fix these other mineral levels until magnesium is corrected.

2. It affects your heart: Low magnesium can cause abnormal heart rhythms, some of which can be dangerous.

3. It affects your muscles and nerves: This is why you may experience cramps, twitches, or tingling.

How is it Diagnosed?

Your doctor will order a blood test to measure your magnesium level. They may also check your potassium, calcium, and kidney function.

How is it Treated?

Treatment depends on how low your magnesium is and whether you have symptoms:

• Mild cases (no symptoms): Magnesium tablets or powder taken by mouth (e.g., magnesium glycerophosphate). Side effect: Sometimes causes diarrhoea (if this happens, the dose is reduced).

• Moderate to severe cases (symptoms or very low levels): Magnesium given through a drip (IV) in hospital. This is faster and more effective.

• Emergency (serious heart rhythm problems): Rapid IV magnesium in hospital.

Your doctor will also:

• Treat the underlying cause (e.g., stop or reduce the medication causing it, if possible)
• Replace potassium and calcium if they are also low (but magnesium must be fixed first)

What Should You Do?

If you've been told you have low magnesium:

1. Take your supplements as prescribed: Don't skip doses. If tablets cause diarrhoea, talk to your doctor about adjusting the dose or trying a different formulation.

2. Review your medications: Tell your doctor about all your medications, especially "water tablets" (diuretics) and acid-reducing medications (PPIs).

3. Eat magnesium-rich foods: Nuts (almonds, cashews), seeds (pumpkin seeds, sunflower seeds), whole grains, leafy green vegetables (spinach), beans, and dark chocolate.

4. Limit alcohol: Excessive alcohol makes low magnesium worse.

5. Get blood tests as recommended: Your doctor will want to recheck your magnesium, potassium, and calcium levels to ensure treatment is working.

Can It Be Prevented?

If you are on long-term medications that cause low magnesium (like diuretics or PPIs), your doctor may:

• Monitor your magnesium levels regularly
• Prescribe magnesium supplements to take alongside the medication
• Consider reducing the dose or changing to a different medication if possible

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14. References

Primary Guidelines

1. National Kidney Foundation. Clinical Practice Guidelines for Bone Metabolism and Disease in Chronic Kidney Disease. Am J Kidney Dis. 2003;42(4 Suppl 3):S1-201. PMID: 14520607

2. Resuscitation Council UK. Adult Advanced Life Support Guidelines (Torsades de Pointes management). 2021.

Key Studies and Reviews

3. Jahnen-Dechent W, Ketteler M. Magnesium basics. Clin Kidney J. 2012;5(Suppl 1):i3-i14. doi:10.1093/ndtplus/sfr163 PMID: 26069818

4. Whang R, Ryder KW. Frequency of hypomagnesemia and hypermagnesemia. Requested vs routine. JAMA. 1990;263(22):3063-3064. PMID: 2342219

5. U.S. Food and Drug Administration. FDA Drug Safety Communication: Low magnesium levels can be associated with long-term use of Proton Pump Inhibitor drugs (PPIs). March 2, 2011. [https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-low-magnesium-levels-can-be-associated-long-term-use-proton-pump]

6. Tzivoni D, Banai S, Schuger C, et al. Treatment of torsade de pointes with magnesium sulfate. Circulation. 1988;77(2):392-397. doi:10.1161/01.cir.77.2.392 PMID: 3338132

7. de Marchi S, Cecchin E, Basile A, et al. Renal tubular dysfunction in chronic alcohol abuse—effects of abstinence. N Engl J Med. 1993;329(26):1927-1934. doi:10.1056/NEJM199312233292605 PMID: 8247055

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