Differential Diagnosis: 2-Year-Old with Recurrent Limb Tightness (Tetany) + Response to IV Calcium

Most Likely Diagnosis: Hypocalcemic Tetany

PRIMARY DIFFERENTIALS (Causes of Hypocalcemia)

1. Vitamin D Deficiency Rickets (MOST COMMON in this age group)

• The most common cause of hypocalcemic tetany in toddlers aged 1-3 years in developing countries
• Results from nutritional vitamin D and calcium deficiency
• Low 25(OH)D, low calcium, low phosphate, elevated PTH (secondary hyperparathyroidism)
• Associated features: bowing of legs, rachitic rosary, widened wrists, delayed dentition, craniotabes
• Look for: exclusive breastfeeding without supplementation, dark skin, limited sun exposure, poor dietary intake

2. Vitamin D-Dependent Rickets Type 1 (VDDR-1 / Pseudovitamin D Deficiency Rickets)

• Autosomal recessive mutation in 1α-hydroxylase (CYP27B1) - cannot convert 25(OH)D to active 1,25(OH)2D
• Presents in infancy-early childhood with severe hypocalcemia and rickets despite adequate vitamin D intake
• Normal 25(OH)D, very low 1,25(OH)2D, high PTH
• Responds only to calcitriol (active vitamin D), not regular vitamin D

3. Vitamin D-Dependent Rickets Type 2 (VDDR-2)

• Mutation in the Vitamin D Receptor (VDR) - end-organ resistance to active vitamin D
• Similar presentation but does not respond to calcitriol
• Classic clue: alopecia totalis (seen in ~50% of cases)

4. Hypoparathyroidism

• Isolated congenital hypoparathyroidism (PTH gene mutations)
• DiGeorge Syndrome (22q11.2 deletion) - developmental failure of 3rd/4th pharyngeal pouch; associated with cardiac defects, immune deficiency, dysmorphic facies
• Autoimmune hypoparathyroidism - isolated or as part of Polyglandular Autoimmune Syndrome Type 1 (APS-1 / APECED) - look for mucocutaneous candidiasis, adrenal insufficiency
• Kenny-Caffey syndrome / Sanjad-Sakati syndrome - hypoparathyroidism with short stature and dysmorphic features
• Lab: low Ca, high phosphate, low/absent PTH, low urinary calcium

5. Pseudohypoparathyroidism (PHP)

• End-organ resistance to PTH (PTH is high, but target organs don't respond)
• Lab: low Ca, high phosphate, high PTH
• PHP Type 1a: Albright hereditary osteodystrophy (short 4th metacarpal, round face, short stature, subcutaneous ossifications)
• PHP Type 1b: normal phenotype, methylation defect at GNAS locus
• Intracranial (basal ganglia) calcifications on CT are a feature

6. Hypomagnesemia

• Magnesium deficiency impairs both PTH secretion and PTH action at target organs
• Can cause hypocalcemia refractory to calcium replacement unless magnesium is repleted
• Causes: malabsorption, chronic diarrhea, Gitelman syndrome, diuretic use, familial hypomagnesemia syndromes
• Key clue: hypocalcemia not responding adequately to calcium alone

7. Malabsorption Syndromes

• Causes secondary vitamin D and calcium deficiency
• Conditions: celiac disease, short bowel syndrome, inflammatory bowel disease
• Low albumin, fat-soluble vitamin deficiencies

8. Chronic Kidney Disease (CKD)

• Impaired 1α-hydroxylation in the proximal tubule → low 1,25(OH)2D → hypocalcemia
• Phosphate retention → hyperphosphatemia → binds Ca and suppresses 1,25(OH)2D
• Lab: elevated creatinine, high phosphate, high PTH (secondary hyperparathyroidism)
• Often co-existing renal osteodystrophy

9. Alkalosis (Respiratory or Metabolic)

• Alkalosis increases protein binding of calcium → reduces ionized (free) calcium
• Total serum calcium may be normal, but ionized calcium is low
• Common in hyperventilation, prolonged vomiting, sepsis
• Tetany without a structural calcium deficit - resolves with correction of pH

SECONDARY / LESS COMMON DIFFERENTIALS

Diagnostic Algorithm (from Brenner & Rector's The Kidney)

Recommended Initial Workup

Quick Summary: Most Likely by Age 2

1. Nutritional Vitamin D deficiency rickets - most common worldwide
2. Hypoparathyroidism (congenital/DiGeorge) - especially if phosphate is high and PTH is low
3. Pseudohypoparathyroidism - if PTH is high with dysmorphic features
4. VDDR Type 1 - if rickets present but not responding to regular Vit D
5. Hypomagnesemia - especially if refractory to IV calcium