USEFULNESS OF SINGLE-FIBER EMG FOR DISTINGUISHING NEUROMUSCULAR FROM OTHER CAUSES OF OCULAR MUSCLE WEAKNESS

Consecutive patients (n = 114), who had single fiber electromyography of the frontalis muscles for symptoms suggestive of ocular myasthenia gravis, were followed up for a mean of 14 months (3-64 mos). At follow up, based on strict criteria, 23 patients were classified as having o cular myasthenia gravis, 8 patients were diagnosed as having mitochond rial myopathy or oculopharyngeal dystrophy, 18 patients were found to have other diseases and 65 patients remained without a definite diagno sis. The single fiber electromyography data of these patients were the n reviewed. The patients with ocular myasthenia gravis had, on average , more than 7/20 single fiber pairs with jitter > 45 mu s and mean jit ter of 56 mu s. The 8 patients with mitochondrial myopathy or oculopha ryngeal dystrophy had an average of 5/20 single fiber pairs with jitte r > 45 mu s and a mean jitter of 52 mu s and could not be separated fr om the group with ocular myasthenia gravis on the basis of the single fiber electromyography results. The 18 patients with definite other di agnosis had an average of less than 1/20 single fiber pair with jitter > 45 mu s and a mean jitter of 25 mu s. This group could be clearly s eparated from the group with ocular myasthenia gravis. We conclude tha t single fiber electromyelography is useful in the separation of ocula r myasthenia gravis from other causes of oculomotor weakness except mi tochondrial myopathy and oculopharyngeal dystrophy.