Dual-energy X-ray absorptiometry versus single photon absorptiometry of the radius.

Radial diaphyseal bone mineral density (BMD) was measured at the standard one-third site by dual-energy X-ray absorptiometry (DEXA) and by 125I single photon absorptiometry (SPA) in 70 consecutive subjects, aged 12-86 years, with metabolic disorders of the skeleton. Each patient was measured once by the DEXA (Hologic QDR-1000) instrument and four times by the SPA (Norland 2780) instrument on the same day by one or the other of 2 technicians. The DEXA and SPA measurements were linearly related and highly correlated (r = 0.975, P less than 0.0001) over a range from severe osteopenia to high normal BMD. Ninety-five percent of the variation in the BMD determined by SPA was accounted for by DEXA, so that the BMD(SPA) = 1.035 +/- 0.027 (SEM) x BMD(DEXA)-0.007 +/- 0.019 (SEM). This permits continued use of previously accumulated SPA data-bases. The coefficient of variation for repeat measurements by DEXA was 1.2% and by SPA 1.6%. Examination time by DEXA was 6-7 minutes, about 45% shorter than the corresponding SPA determinations. DEXA is the superior method for evaluation of the radius, as it provides faster and more precise measurements in clinical practice.

Decreased serum ionized calcium and normal vitamin D metabolite levels with anticonvulsant drug treatment.

Alterations in vitamin D metabolism are generally thought to account for the hypocalcemia and osteopenia caused by long term treatment with anticonvulsant drugs. Regional variation in the incidence and severity of anticonvulsant drug-induced bone disease has been attributed to differences in sunlight exposure, with most reports coming from areas with limited sunshine or from institutionalized patients. Serum ionized calcium levels in 109 ambulatory adult epileptic outpatients receiving chronic anticonvulsant drug therapy in Georgia were decreased [4.73 +/- 0.02 (+/-SE) vs. 4.97 +/- 0.01 mg/dl; P less than 0.001). Immunoreactive PTH concentrations were increased (5.5 +/- 0.4 vs. 4.0 +/- 0.3 microliterEq /ml; P less than 0.005), while bone mineral content was reduced, averaging only 88.8% of the predicted normal values. Hypocalcemia and osteopenia occurred in spite of normal mean levels of serum 25-hydroxyvitamin D and 1,25-dihydroxy-vitamin D. The indirect relationship between serum concentrations of antiepileptic drugs and the serum ionized calcium level, and the lack of correlation with vitamin D metabolite levels suggested that hypocalcemia was independent of the effect of the drugs on vitamin D metabolism. Bone biopsies revealed increased osteoid but normal calcification front formation, accelerated mineralization rate, and decreased mineralization lag time indicative of increased skeletal turnover, rather than osteomalacia.

Qualitative bone defect in uremic osteosclerosis.

Osteosclerosis, an increased volume of trabecular bone, is a common but often misinterpreted feature of uremic osteodystrophy. Despite the apparent radiographic density of osteosclerotic bone, pain and fracture may be associated. If accumulated osteoid and woven bone exceed the volume of lamellar bone removed in chronic renal insufficiency, bone density may be reduced despite increased trabecular volume. Concomitant histomorphometric and photon absorption determinations of transileal bone biopsies were done to investigate the relationship between quantity and quality of bone in uremic and non-uremic osteopenic patients. In osteopenic patients with uremia, bone core density had no significant relationship to trabecular bone volume or mineralized bone volume whereas in non-uremic osteopenic patients, these parameters were directly related (r = 0.867 and r = 0.921, respectively, p less than 0.001). The bone core density in the uremic patients was negatively correlated with the total osteoid volume (r = -0.764, p less than 0.05) and positively related to the serum phosphorus concentration (r = 0.739, p less than 0.05). Serum levels of immunoreactive parathyroid hormone (iPTH) and alkaline phosphatase activity were higher in the patients with radiographic osteosclerosis than in the other uremic patients. The lack of correlation between bone volume and density indicates a qualitative defect in uremic bone. It appears that in uremia, elevated iPTH and serum phosphorus levels may augment bone formation, albeit poorly mineralized with woven architecture. While radiographic density paradoxically increases, the amount of normally mineralized bone may be reduced.