PEDBASE.org - The Pediatric Database - Detailed information of FAMILIAL HYPOPHOSPHATEMIA

FAMILIAL HYPOPHOSPHATEMIA

DEFINITION:

A rare x-linked dominant disorder characterized by failure to thrive, short stature, and rickets.

EPIDEMIOLOGY:

incidence: 1-10/1,000,000 live births
(most common form of rickets)
age of onset:
- early infancy -> 15 months
risk factors:
- familial - x-linked dominant (also autosomal recessive and sporadic forms)
  - chrom.#: Xp22.2-p22.1
  - gene: ?

PATHOGENESIS:

1. Gentic Defect

genetic defect -> 2 effects on the kidney
- interferes with proximal tubular reabsorption of phosphate
- interferes with conversion of 25(OH)D to 1,25 (OH)2D
there may be a coexistent deficiency of transepithelial absorp-tion of phosphate in the intestine
growth is normal in early infancy until the serum phosphate levels fall to low values at about 6 months of age

CLINICAL FEATURES:

1. Musculoskeletal Manifestations

smooth bowing of lower extremities when begin to weight bear
waddling gait
coxa vara
genu varus and valgum
short stature
dolichocephaly
no Harrison groove

2. Dental Manifestations

pulp deformities
"intraglobular dentin" tooth anomaly
occasional enamal defects
periapical infections

INVESTIGATIONS:

1. Serum

normal calcium
hypophosphatemia
elevated alkaline phosphatase
low to normal serum 1,25 (OH2)D levels
normal PTH

2. Urine

hyperphosphaturia
normal amino acids

3. Imaging Studies

1. Skeletal X-Rays

evidence of rickets or osteomalacia
coarse-appearing trabecular bone
metaphyseal widening and fraying
cupping of metaphysis at proximal and distal tibia and at distal femur, radius, and ulna

MANAGEMENT:

1. Medical

1. Oral Phosphate Supplements

dibasic sodium phosphate
phosphoric acid
side effects: diarrhea

2. Vitamin D Analog

dihydrotachysterol

2. Surgery

osteotomy for existing limb deformities

3. Prognosis

adult height: 130-165 cm (untreated); 170 cm (treated)
normal life span

Pediatric Database - FAMILIAL HYPOPHOSPHATEMIA

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