Intractable muscle pain syndrome, osteomalacia, and axonopathy in long-term use of phenytoin

Muscle pain syndrome is a complicated situation in that even an extensive work-up may overlook the primary etiology. A patient with multiple medical problems, including a history of partial gastrectomy due to trauma, a below-knee amputation from arteriosclerotic disease, and chronic phenytoin therapy for a seizure disorder, developed intractable, generalized muscle pain, with total dependency in mobility and self care. He was admitted to an acute care hospital and was eventually discharged with a diagnosis of occult malignancy. After being referred to our amputee clinic for prosthetic management, the patient was admitted for inpatient rehabilitation. Electrodiagnostic studies and muscle biopsy revealed typical neuropathic findings and nonspecific myopathic changes. Laboratory studies showed a vitamin D deficiency with secondary hyperparathyroidism. Discontinuation of phenytoin and treatment with calcium and vitamin D resulted in rapid relief of muscle pain and marked improvement in mobility and self care. This case illustrates an unusual combination of phenytoin-induced axonopathy and osteomalacia that incapacitated an otherwise functional person with below-knee amputation.