X-linked hypophosphataemia

familial vitamin D-resistant rickets classically transmitted genetically as an X-linked dominant trait. This disorder is the most common form of renal tubular rickets and osteomalacia. Patients manifest lifelong hypophosphataemia secondary to renal tubular phosphate loss as well as decreased intestinal absorption of calcium and normal serum levels of calcium. Remission of rickets usually follows closure of the growth plates, but recurrence of symptoms is common later in life.

Radiographic features may be diagnostic. Children may exhibit mild and nonspecific rachitic changes at the growth plates. Osteopenia is not prominent. Although some patients may have bowing of long bones, deformity is frequently minimal. Loosers zones are more prevalent with age and can be complicated by complete fractures. Adults tend to have a generalized increase in bone density, especially in the axial skeleton. Enthesopathic calcification and ossification develop in the paravertebral ligaments, anulus fibrosus and capsules of apophyseal and appendicular joints. In the spine, changes may resemble those of ankylosing spondylitis or diffuse idiopathic skeletal hyperostosis. However, the sacroiliac joints in X-linked hypophosphataemia show no bone erosions. Narrowing of the spinal canal is common. The pelvis may exhibit multiple sites of calcification in the acetabulum, the iliolumbar ligaments and the sacroiliac ligaments. The appendicular skeleton also shows abnormalities, including multiple sites of new bone formation at various muscle and ligament attachments. Separate small ossicles may develop around various joints, particularly in the carpus.

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