Procainamide can cause respiratory failure [1-3] that has been attributed to diaphragmatic weakness from impaired neuromuscular junction transmission; in previously reported cases, all but one patient had preexisting myasthenia gravis [3]. We report a patient who, while taking procainamide, developed respiratory failure caused by necrotizing myopathy with diaphragm involvement. Electrodiagnostic evaluation showed no neuromuscular junction abnormality.

A 74-year-old man developed supraventricular arrhythmias after coronary artery bypass grafting and was treated with procainamide. Two weeks later arm weakness and back and shoulder pain developed. Physical examination showed muscle tenderness and symmetric proximal weakness. Reflexes and sensation were normal. He had a sterile, exudative pleural effusion. The creatine kinase level was 18 000 IU/L with a normal MB fraction. The urine myoglobin level was elevated. Renal function, Westergren sedimentation rate, and antinuclear antibody levels were normal. Anti-double-stranded DNA and anti-histone antibodies were positive. Procainamide was withdrawn.

After 7 days, he had difficulty moving and speaking. He was alert and could move his eyes but had no other voluntary movement. Sensory and reflex examination results were normal. The bicarbonate level was 41 mmol/L, increased from 25 mmol/L at admission. Arterial blood gas analyses showed a pH of 7.24, a PCO₂ of 85 mm Hg, and a PO₂ of 110 mm Hg. He was mechanically ventilated for 3 days.

Electromyographic evaluation of the limbs and both diaphragms showed profuse fibrillation with myopathic motor unit action potentials. Repetitive motor nerve stimulation showed no decrement. Single-fiber electromyogram of the biceps was normal. Examination of the muscle biopsy specimen showed a necrotizing myopathy.

Respiratory indices improved, and 1 month later muscle strength was normal except for minimal proximal lower extremity weakness; an electromyogram showed marked improvement.

The patient had necrotizing myopathy shown by clinical examination, an elevated creatine kinase level, electromyogram, and muscle biopsy. His myopathy was probably caused by procainamide, which has been associated with acute myopathy and polymyositis in the setting of a lupus-like syndrome [4, 5]. Antinuclear antibody was absent, and the biopsy specimen did not show inflammatory infiltrates. Nevertheless, he did have autoantibodies that have been seen in drug-induced lupus and probably had a reaction to procainamide that primarily affected muscle. We found no other explanation for the muscle fiber necrosis and regeneration on the biopsy or test results. We did not find evidence of impaired neuromuscular junction transmission through either repetitive nerve stimulation or single-fiber electromyogram and believe that necrotizing myopathy with diaphragm involvement caused his respiratory failure.