Defective mineralization of osteoid in adults (osteomalacia) and growth plate cartilage in children (rickets).
Also known as: osteomalacia, rickets, vitamin D deficiency rickets, nutritional rickets
Overview
Disorders of defective mineralization of the bone matrix. Osteomalacia describes inadequate mineralization of newly formed osteoid in skeletally mature adults. Rickets describes inadequate mineralization of the growth plate cartilage in growing children, producing characteristic skeletal deformities. Vitamin D deficiency is the most common cause worldwide, though hypophosphatemic and other forms exist.
Epidemiology
Nutritional rickets is most common in infants and toddlers; resurgence has been observed in exclusively breastfed infants without vitamin D supplementation and in dark-skinned children with limited sun exposure. Osteomalacia in adults occurs most often in older adults with limited sun exposure, malabsorption, chronic kidney disease, or anticonvulsant use.
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Anticonvulsants (phenytoin, phenobarbital, carbamazepine) accelerating vitamin D catabolism
Hypophosphatemic forms — X-linked hypophosphatemia (PHEX mutation, FGF23 excess), oncogenic osteomalacia (tumor-induced FGF23 elevation), Fanconi syndrome with phosphate wasting
Inherited disorders of vitamin D metabolism (vitamin D-dependent rickets types I and II)
Pathophysiology
Adequate skeletal mineralization requires sufficient calcium and phosphate at the bone-forming surface. Vitamin D deficiency impairs intestinal calcium and phosphate absorption; secondary hyperparathyroidism then normalizes serum calcium at the expense of bone resorption while phosphate remains low, impeding mineralization. Hypophosphatemic forms cause defective mineralization despite normal vitamin D status by directly reducing the phosphate available for hydroxyapatite formation.
Children with rickets: irritability, delayed milestones, bony deformity, growth failure, dental abnormalities, hypocalcemic seizures or tetany in severe cases
Signs / physical exam
Adults: bony tenderness to percussion, proximal weakness with positive Gowers sign in advanced disease, waddling gait
Children: craniotabes (softening of skull bones), frontal bossing, delayed fontanel closure, rachitic rosary (enlarged costochondral junctions), Harrison groove (lower rib indentation from diaphragm pull), enlarged wrists and ankles, genu varum (bow legs) or genu valgum (knock knees), delayed tooth eruption with enamel defects
Classic findings
Toddler with bow legs, rachitic rosary, frontal bossing, and delayed dentition with biochemistry showing low 25-hydroxyvitamin D, low phosphate, elevated alkaline phosphatase, and elevated PTH.
Differential diagnosis
Osteoporosis — Decreased bone mass with normal mineralization; normal calcium, phosphate, alkaline phosphatase; diagnosed by DEXA
Renal osteodystrophy — Mixed bone disease in CKD with elevated PTH, low or normal calcium, hyperphosphatemia, low calcitriol
Paget disease of bone — Focal cortical thickening and bony enlargement with markedly elevated alkaline phosphatase; serum calcium and phosphate normal
Hyperparathyroidism (primary) — Hypercalcemia and elevated PTH; subperiosteal resorption on radiographs; brown tumors
Osteomyelitis (in children with bony pain) — Fever, localized tenderness, elevated inflammatory markers, MRI findings
Scurvy (vitamin C deficiency) — Bone pain, gum bleeding, perifollicular hemorrhage, dietary history
Diagnostic workup
Diagnostic criteria
Diagnosis is established by characteristic clinical and radiographic features together with biochemical evidence of defective mineralization (low 25-OH vitamin D, low or low-normal calcium and phosphate, elevated alkaline phosphatase, and elevated PTH). 25-hydroxyvitamin D <20 ng/mL (50 nmol/L) defines deficiency; severe deficiency <10 ng/mL is associated with overt osteomalacia and rickets.
Labs
Serum calcium (often low normal or low), phosphate (often low), alkaline phosphatase (elevated)
25-hydroxyvitamin D — the best test of nutritional vitamin D status
1,25-dihydroxyvitamin D — useful in CKD and inherited disorders
Intact PTH — typically elevated in vitamin D-deficient rickets/osteomalacia (secondary hyperparathyroidism)
24-hour urine calcium (low) and phosphate (variable)
Renal function and liver function, magnesium
FGF23 if hypophosphatemic forms suspected (X-linked hypophosphatemia, oncogenic osteomalacia)
Imaging
Plain radiographs of involved bones
Children: widened, cupped, and frayed metaphyses at growth plates (wrists, knees, costochondral junctions), bowing deformities, generalized osteopenia
Adults: pseudofractures (Looser zones) — narrow lucent bands perpendicular to the cortex along the medial femoral neck, pubic rami, lateral scapula, ribs; generalized osteopenia
DEXA — reduced bone density
MRI for occult insufficiency fractures
Diagnostic algorithm
Lab
Vitamin D-deficient rickets
X-linked hypophosphatemia
Renal osteodystrophy
Serum calcium
Low/low-normal
Normal
Low/normal
Serum phosphate
Low
Low (renal wasting)
High
Alkaline phosphatase
High
High
High
25-OH vitamin D
Low
Normal
Variable
1,25-OH vitamin D
Low/normal
Low/normal (inappropriate)
Low
PTH
High
Normal
High
FGF23
Normal
High
High
Biochemical patterns distinguishing common forms of rickets and osteomalacia.
Treatment
First-line
Nutritional vitamin D deficiency rickets/osteomalacia: ergocalciferol or cholecalciferol — typical dosing: infants and children 2,000 IU daily for 6-12 weeks (or 50,000 IU weekly), adults 50,000 IU weekly for 8 weeks, followed by maintenance 600-2,000 IU daily
Adequate elemental calcium intake (children 500-1,000 mg/day; adults 1,000-1,200 mg/day) by diet or supplementation
Treat underlying malabsorption (gluten-free diet for celiac, etc.)
Sun exposure counseling balanced against skin cancer risk
Prevention: routine vitamin D supplementation 400 IU/day for all infants from birth (AAP) and 600-800 IU/day for older children and adults
Second-line / adjunct
Calcitriol or 1-alpha-hydroxyvitamin D for vitamin D-dependent rickets, renal disease, and inherited disorders impairing 1-alpha-hydroxylation
Phosphate supplementation with calcitriol for X-linked hypophosphatemia (XLH) — burosumab (anti-FGF23 monoclonal antibody) is now first-line for pediatric and adult XLH (CL301/CL303 trials)
Resection of causative tumor in tumor-induced osteomalacia
Orthopedic correction of severe deformity once biochemistry normalized
Dilated cardiomyopathy in severe infantile vitamin D deficiency
Persistent skeletal pain and weakness
PANCE pearls
Exclusively breastfed infants require 400 IU/day vitamin D supplementation from birth (AAP recommendation) because breast milk does not contain sufficient vitamin D.
Looser zones (pseudofractures) along the medial femoral neck, pubic rami, ribs, and scapula are pathognomonic radiographic findings of osteomalacia.
Elevated alkaline phosphatase with low or low-normal calcium and phosphate distinguishes osteomalacia from osteoporosis (in which alkaline phosphatase is typically normal).
X-linked hypophosphatemia (PHEX mutation with elevated FGF23) is now treated with burosumab, a monoclonal antibody against FGF23.
Patients on long-term anticonvulsants may require higher than usual vitamin D supplementation.
References
Endocrine Society 2011 — Holick MF et al., Evaluation, Treatment, and Prevention of Vitamin D Deficiency: An Endocrine Society Clinical Practice Guideline (J Clin Endocrinol Metab 2011)
AAP — American Academy of Pediatrics recommendation on vitamin D intake for breastfed and formula-fed infants
Global Consensus 2016 — Munns CF et al., Global Consensus Recommendations on Prevention and Management of Nutritional Rickets (J Clin Endocrinol Metab 2016)
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