ADULT OSTEOMALACIA

Clinical Manifestations. The diagnosis of adult osteomalacia (see Plate 3-14) may be difficult to establish because the changes may be considerably more subtle than those seen in childhood rickets. In early stages, the patients may be asymptomatic and the changes are biochemical—the most sensitive being an elevated serum total alkaline phosphatase. If the total alkaline phosphatase is elevated, then a bone-specific alkaline phosphatase (BSAP) should be obtained. If the BSAP is elevated, then there is a differential diagnosis of elevated BSAP (see Plate 3-14). By exclusion, osteomalacia can be strongly suspected but the gold standard is quantitative bone histomorphometry. A bone biopsy is diagnostic, and there are very specific histomorphometric criteria for the diagnosis of osteomalacia (see Plate 3-14). Once a specific diagnosis is established, then adult osteomalacia has a very narrow group of causes (see Plate 3-14). By biochemical testing, the etiology can be determined; and, by correcting the biochemical abnormalities and keeping them corrected, the symptoms of osteomalacia can be eliminated and the histomorphometry normalized.

Patients with adult and advanced osteomalacia may complain of generalized weakness, especially proximal muscle weakness, bone pain, easy fatigability, and malaise. The physical findings are minimal: tenderness of bony prominences or, in more serious cases, muscle weakness that is severe enough to cause an abductorlurch type of gait (the gluteal, or Trendelenburg, gait). In long-standing cases, a bone deformity such as bowleg, coxa vara, or kyphosis may be common.

Radiographic signs are equally subtle, showing for the most part only a diffuse osteopenia, similar to that seen in other metabolic bone diseases such as postmenopausal or senile osteoporosis, hyperparathyroidism, hyperthyroidism, and diffuse skeletal metastatic tumors such as those seen in multiple myeloma. One distinctive feature seen in more advanced osteomalacia, present in about 25% of cases, is virtually pathognomonic of osteomalacia. Focal collections of osteoid produce localized, narrow, ribbon-like zones of decreased density in the cortices. These zones are almost always symmetric and are located at right angles to the long axes of the bones. On radiography, they resemble partial fractures. These usually painless pseudofractures—called Looser’s zones, Umbauzonen, or milkman syndrome—are usually seen on the concave sides of a long bone, the medial side of the femoral neck, the ischial and pubic rami, the clavicle, the ribs, and the axillary border of the scapula. They may serve as stress risers, thus leading to a true fracture (particularly in the femoral neck or in the pubis).

NUTRITIONAL-DEFICIENCY RICKETS AND OSTEOMALACIA

The classic and most clearly understood cause of nutritional-deficiency rickets and osteomalacia (see Plate 3-15) is a severe chronic deficiency of vitamin D. Deficiency of this fat-soluble sterol vitamin can be dietary, inadequate exposure to sunlight, or malabsorption. Malabsorption can be due to a variety of gastrointestinal conditions, including asymptomatic celiac disease. Because fat-soluble vitamins need to be bound to bile salts to be absorbed in the terminal ileum, deficiency of bile salts may also lead to low vitamin D levels. Although histomorphometric changes of defective mineralization can be seen with 25(OH)D levels less than 10 ng/mL, clinical osteomalacia is usually not seen unless the serum vitamin D levels are even lower and consistently low for months before recognizable fractures or muscle weakness is seen.