Phenytoin induced vitamin D deficiency presenting as proximal muscle weakness.

A 6-year-old girl presented with proximal muscle weakness of lower limbs. She was receiving phenytoin for epilepsy for 2 years. Serum phenytoin level was within therapeutic range. Serum 25(OH) vitamin D was low (5 ng/mL) and serum parathyroid hormone level was high. After administration of oral vitamin D, muscle weakness improved and vitamin D level increased to 39.11 ng/mL. Proximal muscle weakness due to vitamin D deficiency following phenytoin intake is rare in children.