Osteomalacia

From Academic Kids

Osteomalacia is a disease characterized clinically by a painful softening of the bones.

Osteomalacia is characterized histologically by an abnormally high ratio of osteoid (unmineralized bone matrix) to mineralized bone. Osteomalacia is a complex and protean disorder, with over 30 causes or associated diseases identified. However, most cases of osteomalacia are caused by problems with vitamin D metabolism or renal tubular phosphate loss. If one of these causes of osteomalacia becomes operant during childhood while the bones are still growing, the resulting abnormalities in the growing bones are called rickets.

Humans produce vitamin D in their skin following exposure to ultraviolet light. We also obtain it from dietary sources or vitamin supplements. It is then hydroxylated by the liver to form 25 OH D, which circulates, bound to a specific binding protein. This circulating pool of 25 OH D is then further hydroxylated by the kidney as needed, to form the physiologically active form of the hormone, 1,25 (OH)2 D. Problems may occur at various stages of this process and thereby lead to osteomalacia or rickets.

When considering renal tubular disorders, think of just two things: X-linked hypophosphatemia and Fanconi's syndrome. X-linked hypophosphatemia (a.k.a. familial vitamin D-resistant rickets) is the most common form of renal tubular rickets and osteomalacia.

Diagnosing osteomalacia by X-ray can be tricky. Some findings, such as osteopenia or coarsening of the trabeculae, are very nonspecific, and not helpful for diagnosis. Generally, all that one sees is a diffuse osteopenia, which looks just like that seen in osteoporotic patients. In rare cases of osteomalacia, collections of osteoid may build up to the point that these "seams" of osteoid may be seen on plain radiographs as linear lucencies oriented perpendicular to the cortical margin. If large enough, these "Looser's zones" or pseudofractures may help lead one to the diagnosis of osteomalacia. Occasionally, one may see bowing of the long bones in an adult. In general, though, bone biopsy is far more helpful for diagnosing osteomalacia than any radiographic test.

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