Osteomalacia/rickets
Summary
Osteomalacia and rickets result from defective mineralization of bone matrix (osteoid), most commonly due to vitamin D deficiency. Rickets occurs in children with open growth plates, causing skeletal deformities, while osteomalacia occurs in adults after growth plate closure, causing bone pain and increased fracture risk.
Detail
Pathophysiology: Vitamin D is essential for intestinal calcium and phosphate absorption. Deficiency (from poor sun exposure, malabsorption, renal disease, or dietary insufficiency) leads to hypocalcemia, which triggers secondary hyperparathyroidism, further increasing phosphate wasting and worsening mineralization defects. The result is accumulation of unmineralized osteoid matrix.
Rickets (children, open growth plates): Presents with bowing of legs (genu varum), craniotabes, rachitic rosary (costochondral junction widening), Harrison groove, frontal bossing, delayed fontanelle closure, and growth retardation. Growth plate cartilage fails to mineralize properly, causing widened, irregular epiphyseal plates.
Osteomalacia (adults, closed growth plates): Presents with diffuse bone pain, proximal muscle weakness, and increased risk of fractures (especially pseudofractures/Looser zones on imaging). Waddling gait may occur due to proximal myopathy.
Lab findings: Low/normal serum calcium, low phosphate, elevated alkaline phosphatase, elevated PTH (secondary hyperparathyroidism), low 25-hydroxyvitamin D.
Causes: Vitamin D deficiency (most common - poor intake, malabsorption, limited sun exposure), chronic kidney disease (impaired 1-alpha hydroxylation), liver disease, anticonvulsant use (induces vitamin D catabolism), hypophosphatemic disorders (X-linked hypophosphatemic rickets - FGF23 excess), and renal tubular acidosis.
Treatment: Vitamin D and calcium supplementation; treat underlying cause (e.g., phosphate replacement in hypophosphatemic rickets).
Distinguish from osteoporosis: Osteoporosis has normal mineralization but decreased bone mass/density with normal calcium, phosphate, and PTH levels.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Harrison's Principles of Internal Medicine
- Goljan Rapid Review Pathology
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