Axial osteomalacia--a rare osteosclerotic bone disorder characterized by axial skeleton
pain, coarsening of the trabecular bone pattern on radiographs of the axial but not appendicular skeleton, and
osteomalacia on biopsy of a rib or iliac crest--has been reported in 10 apparently sporadic cases, all of which were in middle-aged or elderly Caucasian men. The etiology is unknown but has been postulated to be a bone cell defect. We describe the clinical, laboratory, pathologic and family study of a black mother and son with
axial osteomalacia associated with polycystic liver and
kidney disease. Investigation of the son suggested that radiographic
osteosclerosis can be detected in early adulthood. Limited skeletal survey of his three children revealed no abnormalities. Examination of undecalcified iliac crest bone after in vivo
tetracycline labeling revealed severe
osteomalacia in the son despite normal circulating
calcium,
inorganic phosphate and
vitamin D metabolite levels and persistently elevated
alkaline phosphatase activity. Although osteoblasts appeared flat and inactive, histochemical studies showed intense
alkaline phosphatase activity in the osteoblasts along most trabecular bone surfaces. Electron microscopy revealed intact matrix vesicles within unmineralized osteoid. The presence also of unexplained
myopathy in the son--characterized by proximal
muscle weakness, persistently elevated circulating
creatine phosphokinase levels and pathogenic changes of
myopathy on biopsy of quadriceps muscle--together with impaired bone mineralization, suggests that a disorder of
vitamin D action may be involved in the pathogenesis of this unusual condition.
Axial osteomalacia affects blacks as well as Caucasians, women as well as men, may be familial, and may perhaps be a developmental abnormality inherited in association with
polycystic kidney and
liver disease.