X-linked hypophosphatemia (XLH) is the classical bone disorder caused by renal wasting of the phosphate. Formerly called vitamin D-resistant rickets, it is the most frequently observed condition among the heritable forms of rickets.

Clinical Features

Bowing

XLH may manifest with a variety of phenotypic manifestations. Some patients have hypophosphatemia alone. Meanwhile, others develop severe bowing of the legs after the first year of life, when the child begins to walk and the legs have to bear the weight of the body. This eventually leads to the twisting of the tibial shaft to the front and inwardly, and ultimately, failure to attain normal adult height. Even in treated patients, the improvement from the condition is still incomplete.

In a subset of patients, this bowing may be delayed until adult life, when XLH is detected while evaluating the person for short stature. Both types of bowing, varum (outward bowing) or valgus (inward bowing), could observed in XLH patients.

Short Stature

Short stature is a characteristic of X-Linked Hypophosphatemia, with the length of the lower limbs being markedly reduced compared to that of the arms. This occurs because of the bowing of the bones of the lower limb. The sitting height is relatively spared compared to the standing height. Studies show that this difference only increases when children are treated to improve the rate of bone growth.

Other Features

Other presenting features of XLH include calcification of the joint capsule and other soft tissues around the joints—known as enthesopathy—which results in joint pain and loss of joint mobility to varying extents. In rare cases, this may affect the spinal ligaments and cause compression of the spinal cord. Osteophytes may also be present and may combine with hyperostosis of the vertebrae, sacroiliac fusion, or arthritis, increasing pain and disability.

In some cases, dental abscesses that occur without obvious cause and impairment of sensorineural hearing also appear. The dentine is thought to be weaker than normal because of failure of proper tooth mineralization. This leads to larger than normal pulp chambers and pulp horns that promote abscess formation.

Hearing loss is considered to be a rare occurrence among patients with XLH. Its appearance is associated with a complication of osteosclerosis of the petrous bone.

Imaging

In imaging XLH, r-rays of the distal femur would show metaphyseal accumulation of nonmineralized osteoid, called osteomalacia. Several other radiologic findings are possible. This includes changes in the pelvic bones or the talus of the ankle joint, cranial synostosis, or frontal bossing. Imaging results for XLH patients would usually show that the head is longer, the occiput is less prominent, and the cephalic index is lower than usual.