Loss of Seizure Control in a Patient With Vitamin D Deficiency and Phenytoin-Induced Hypocalcemia

Phenytoin is a widely used antiseizure drug with well-documented side effects, including hypocalcemia, particularly in patients with concomitant vitamin D deficiency. Decreased serum calcium levels can induce seizures. In stabilized patients under long-term anticonvulsant treatment with phenytoin, loss of seizure control is rare but has been reported. This report illustrates a case of a 69-year-old woman under treatment with phenytoin for more than 10 years, who presented persistent hypocalcemia despite calcium correction, and seizures refractory to treatment with four combined antiepileptic drugs. She also presented with low vitamin D and elevated parathyroid hormone levels. Only when phenytoin administration was stopped it was possible to correct hypocalcemia and achieve seizure control. This case illustrates the need for regular monitoring and supplementation with calcium and vitamin D for patients under prolonged treatment with phenytoin. The proposed mechanism for phenytoin-induced hypocalcemia is reviewed. When installed, hypocalcemia can be resistant to supplementation until phenytoin is stopped, and in rare cases may lead to loss of seizure control.