Do anticonvulsant drugs commonly induce osteomalacia?

Drug-vitamin D interactions: a systematic review of the literature

Extensive media coverage of the potential health benefits of vitamin D supplementation has translated into substantial increases in supplement sales over recent years. Yet, the potential for drug-vitamin D interactions is rarely considered. This systematic review of the literature was conducted to evaluate the extent to which drugs affect vitamin D status or supplementation alters drug effectiveness or toxicity in humans. Electronic databases were used to identify eligible peer-reviewed studies published through September 1, 2010. Study characteristics and findings were abstracted, and quality was assessed for each study. A total of 109 unique reports met the inclusion criteria. The majority of eligible studies were classified as class C (nonrandomized trials, case-control studies, or time series) or D (cross-sectional, trend, case report/series, or before-and-after studies). Only 2 class C and 3 class D studies were of positive quality. Insufficient evidence was available to determine whether lipase inhibitors, antimicrobial agents, antiepileptic drugs, highly active antiretroviral agents, or H2 receptor antagonists alter serum 25(OH)D concentrations. Atorvastatin appears to increase 25(OH)D concentrations, whereas concurrent vitamin D supplementation decreases concentrations of atorvastatin. Use of thiazide diuretics in combination with calcium and vitamin D supplements may cause hypercalcemia in the elderly or those with compromised renal function or hyperparathyroidism. Larger studies with stronger study designs are needed to clarify potential drug-vitamin D interactions, especially for drugs metabolized by cytochrome P450 3A4 (CYP3A4). Healthcare providers should be aware of the potential for drug-vitamin D interactions.

Pathophysiology of bone loss in patients receiving anticonvulsant therapy

Many studies have shown that patients taking antiepileptic drugs (AEDs) are at increased risk for metabolic bone disease and low bone mineral density. Although early reports of bone disease in patients with epilepsy often involved institutionalized patients, who may be at risk because of lack of physical activity, reduced sunlight exposure, and poor nutrition, low bone density has also been reported in well-nourished, ambulatory outpatients with epilepsy. Traditionally, attention to the problem of AED-induced bone loss has been focused on those drugs that induce the hepatic cytochrome P450 enzyme system, thereby increasing the metabolism of vitamin D. However, the mechanisms of AED-induced bone loss appear to be multiple, and all types of AEDs are potentially implicated. Besides hepatic enzyme induction, mechanisms may include direct effects of AEDs on bone cells, resistance to parathyroid hormone, inhibition of calcitonin secretion, and impaired calcium absorption. An understanding of bone biology and the pathophysiology of bone loss can aid in the identification and monitoring of patients at risk and in the planning of appropriate prophylactic and therapeutic measures, by which most of the morbidity associated with AED-induced bone loss can be prevented.

Risk of age-related fractures in patients with unprovoked seizures

The incidence of age-related fractures was determined in a cohort of 467 Rochester, Minnesota residents with unprovoked seizures. The 30 initial hip fractures observed were significantly more than the 13.19 expected [standardized morbidity ratio (SMR) = 2.3; 95% confidence interval (CI) 1.5-3.3]. The overall incidence of distal forearm (Colles) fractures was not significantly increased (SMR = 1.6; 95% CI 0.9-2.5), based on 17 cases. Hip fracture incidence was increased during the first 10 years after seizure diagnosis but was not related to anti-epileptic drug (AED) use. In contrast, the increase in Colles' fracture incidence was associated with duration of therapy, as the SMR for greater than or equal to 10 years of AED use was 2.4 (95% CI 1.0-5.0). The incidence of vertebral fractures was slightly higher among treated patients but was not associated with duration of AED use. Thus, there was no consistent pattern to support the contention that fracture incidence is greatly increased by long-term AED use.

Hormone binding globulins and anticonvulsant therapy

Serum sex hormone binding globulin, thyroxine binding globulin, cortisol binding globulin and vitamin D binding globulin, together with total levels of the appropriate bound hormones, were determined in 21 epileptic subjects who had been stabilised by long-term anticonvulsant therapy. Serum sex hormone binding globulin capacity was higher in these patients than in appropriate control groups (men p less than 0.05; women p less than 0.01), and values correlated with serum phenytoin levels in the female subjects (p less than 0.01). Thyroxine binding globulin levels were unaffected by anticonvulsants, but significant reductions in serum thyroxine (men p less than 0.05; women p less than 0.001) and triiodothyronine (men p less than 0.05; women p less than 0.01) were observed. Cortisol binding globulin capacity was appreciably elevated in patients of either sex (p less than 0.001), and in the women this was accompanied by a reduction in serum cortisol (p less than 0.001) and a significant correlation with the serum phenytoin concentration (p less than 0.01). Neither vitamin D binding globulin capacity nor serum 25-hydroxycholecalciferol levels were influenced by anticonvulsants in this study. It is concluded that anticonvulsant drug therapy causes widespread alterations in the normal homeostasis between hormones and their serum binding globulins. Such alterations may well have clinical significance.