Single fiber electromyography in normal subjects: reproducibility, variability, and technical considerations.

We performed 59 single fiber electromyography (SFEMG) examinations on 33 patients with spasmodic torticollis, but no other medical or neurological disorder. 14 patients were studied serially (3 examinations over 12 weeks). There was excessive jitter (greater than 55 microseconds) in 14 of 1180 fiber pairs (1.1%). 7 of the 14 abnormal fiber pairs were found at single sites (1 in 20 sites studied). Abnormal jitter occurred at more than 2 of 20 sites in only two patients, both of whom head increased fiber density and were more than 70 years of age. Of the 14 abnormal fibers, subtle technical problems were discovered in 4: bimodal jitter (1); borderline amplitude of single fiber potential (1); and borderline rise time (2). 6 of the 14 jittering potentials were part of a triplet or quadruplet. 4 fibers with increased jitter occurred in pairs and these were seen only in patients over 70. One patient with quiescent non-tropical sprue was excluded from consideration because of increased mean jitter before injection and confirmed during 2 subsequent studies. Therefore, SFEMG is a valid and reliable technique that must be strictly interpreted according to existing guidelines, with special consideration to studies in older patients. Abnormal jitter is rarely found in normal muscle, however, when found, it usually occurs as part of a triplet or multiplet.