25-Hydroxyvitamin D levels in chronic anticonvulsant therapy. Factors influencing serum concentration

In recent years, the consideration of long-term tolerability has become markedly more important in the treatment of epilepsies due to the introduction of more antiepileptic drugs (AEDs) and a greater variety of treatment choices. Ongoing discussions and various studies suggest that chronic treatment with enzyme-inducing AEDs may induce vitamin D deficiency and, hereby, reduced bone density. Especially in patients with active epilepsies, this would suggest an increased risk of fractures that might be avoided or reduced by the use of nonenzyme-inducing AEDs. We determined the serum concentration of 25(OH)-vitamin D in the serum of 187 adults patients who were on medication with enzyme-inducing (EAEDs, 57%) or nonenzyme-inducing AEDs (NEAEDs, 43%) in mono- or combination therapy without external substitution of vitamin D at the end of summer (EoS) and winter (EoW). At the EoW, only 3% of the EAED patients and no NEAED patient had 25(OH)-vitamin D levels above 25 ng/ml, which was recently proposed as the lower limit of the normal range. There was no statistically significant difference between the two groups. At the EoS, 23% of the EAED group and 39% of the NEAED group had serum levels above 25 ng/ml. The difference was statistically significant (p=0.02). Our results suggest that there is some influence of enzyme-inducing properties on vitamin D levels which was apparent only at end of summer in this study. Thus, the results indicate that most 25(OH)-vitamin D levels were below the generally accepted normal range independent of the anticonvulsant medication at the end of summer and were even more pronounced at the end of winter. This suggests that the low vitamin D levels of patients mainly reflected geographical and sociological but not medication- or disease-related reasons.