[Single fiber electromyography in the diagnosis of myasthenia gravis]

The electrophysiological exam complements the clinical diagnosis in patients with suspected myasthenia gravis. The most common used electrophysiological test are the repetitive nerve stimulation test (RNST) and the single fiber electromyography (SFEMG); this last one is more sensitive than the first one.

To describe the usefulness of SFEMG in a group of myasthenic patients.

We studied 52 patients with a clinical diagnosis of myasthenia gravis (female: 35, male: 17; age range: 10-48 years). They were classified according to Osserman (type I, IIa, IIb, III and IV) and correlated to the electrophysiological findings. RNST was carried out in abductor digiti minimi, trapezius and orbicularis oculi; and SFEMG was carried out in the extensor digiturum communis and frontalis muscles. It was analyzed the sensitiveness of each technique detecting neuromuscular transmission abnormalities, and the correspondence between the results of both techniques.

The most frequent clinical form observed was the type I (22 patients), followed by 18 as type IIa, 9 as type IIb, and 3 as type IV; no one as type III. RNST with positive decrement was observed in 48% of patients, but neuromuscular jitter calculated by means of SFEMG was abnormal in the 100% of them, being statistically significant the difference (chi 2 = 79.72; p = 0.000). The highest positivity index in the SFEMG was observed in the frontalis muscle. Patients from groups IV, IIb and I were the most electrophysiologically affected (t = 8.23211; p < 0.05).

SFEMG is a very sensitive electrophysiological tool, detecting abnormalities in neuromuscular jitter in the 100% of myasthenic patients, with an excellent clinical correlation.