What is hereditary hypomagnesemia with secondary hypocalcemia?

Hereditary Hypomagnesemia with Secondary Hypocalcemia (HSH)

Hereditary hypomagnesemia with secondary hypocalcemia (HSH) is a rare autosomal recessive disorder caused by biallelic mutations in the TRPM6 gene, typically presenting in infancy with severe hypomagnesemia, moderate-to-severe hypocalcemia, and neuromuscular symptoms including seizures and tetany that respond to magnesium supplementation but not calcium alone.

Genetic Basis and Pathophysiology

HSH results from mutations in the transient receptor potential melastatin 6 (TRPM6) gene located on chromosome 9q22 1, 2, 3. This gene encodes a critical epithelial magnesium channel responsible for intestinal magnesium absorption and renal magnesium reabsorption 4, 5. When TRPM6 is defective, severe magnesium deficiency develops, which subsequently impairs PTH secretion and action, leading to hypocalcemia 6.

The guideline evidence clearly establishes that severe hypomagnesemia can impair both PTH secretion and PTH action, with patients demonstrating low serum calcium and PTH levels—these are inborn errors of magnesium metabolism that masquerade as primary parathyroid deficiency 6.

Clinical Presentation

Typical Onset and Symptoms

• Age of onset: Most commonly between 9 days and 4 months of life 7, though the majority present before 12 months 3
• Cardinal manifestations:
  • Generalized seizures (most common presenting symptom) 2, 4
  • Tetany and muscle spasms 1, 7
  • Increased neuromuscular excitability 2
  • Axial hypotonia 2

Atypical Presentations

Recent case reports document adult-onset HSH, expanding the recognized phenotypic spectrum. One case described a 51-year-old male presenting with limb weakness and muscle spasms 1, and twin brothers with onset at 18 and 26 months 3, demonstrating that certain TRPM6 mutations (specifically c.704c-c.705(exon7)insT and c.260(exon4)C>A) may cause later and less severe presentations 3.

Critical pitfall: HSH can be misdiagnosed as primary hypoparathyroidism if serum magnesium is not measured 4. The diagnosis is easily made when magnesium levels are checked 4.

Diagnostic Criteria

The diagnosis requires demonstration of the following combination 7:

1. Severe hypomagnesemia (often <0.3 mmol/L) 1, 2
2. Moderate-to-severe hypocalcemia (more pronounced than other infantile hypocalcemic disorders) 7
3. Hypocalcemia refractory to calcium supplementation but responsive to magnesium 7
4. Continuous requirement for magnesium supplementation to maintain normocalcemia 7
5. Lack of hypermagnesiuria and/or impaired intestinal magnesium absorption 7
6. Genetic confirmation: Biallelic pathogenic variants in TRPM6 1, 2, 3

Laboratory Findings

• Serum magnesium: Severely low (0.07-0.28 mmol/L reported) 1, 8
• Serum calcium: Low (1.65 mmol/L reported) 8
• PTH levels: Low or inappropriately normal 6
• Serum phosphorus: Should be monitored 9

Genetic Testing

Whole-exome sequencing or targeted TRPM6 gene sequencing confirms the diagnosis 1, 2, 3. Novel mutations continue to be identified, including:

• c.4914del, p.E1638Dfs*8 1
• c.5571-3C>G 2
• Compound heterozygous mutations (p.T87K and c.705dupT) 3
• c.5616G>A (p.Trp1872Ter) with deletion of exons 20-23 8

Management

Immediate treatment consists of high-dose oral magnesium supplementation, which is life-saving and prevents irreversible neurological damage 2, 4, 8.

Treatment Protocol

• Oral magnesium supplementation is the cornerstone of therapy 4, 8
• Dosing: High doses are required to maintain normocalcemia 4
• Response: Symptoms significantly improve with magnesium replacement 1
• Calcium supplementation alone is ineffective and distinguishes HSH from primary hypoparathyroidism 7

Monitoring

• Regular serum magnesium and calcium levels
• Serum phosphorus and 25(OH)D 9
• Long-term follow-up for persistent hypomagnesemia 8

Prognosis and Complications

When treated appropriately with high-dose oral magnesium supplementation, severe hypomagnesemia becomes uncommon and long-term prognosis is good 4. However, untreated hypomagnesemia can be potentially fatal and severely impair cognitive function 2.

Critical Complications if Untreated

• Irreversible neurological impairment 8
• Severe cognitive dysfunction 2
• Death in infancy (family histories often reveal siblings who died in infancy) 4

Family Considerations

• Inheritance pattern: Autosomal recessive 1, 2
• Gender distribution: Both males and females affected (male:female ratio 15:6 in one review) 7
• Family screening: Siblings may be affected; parents are typically asymptomatic carriers 7

Key Clinical Pearls

• Always measure serum magnesium in infants with unexplained seizures and hypocalcemia 4
• HSH should be suspected when hypocalcemia fails to respond to calcium supplementation 7
• Early recognition and magnesium supplementation are crucial to prevent neurological damage 1, 2, 8
• Genetic testing confirms diagnosis and enables family counseling 1, 8
• The condition requires lifelong magnesium supplementation 7