Jože Trontelj
Institute of Clinical Neurophysiology, University Medical Centre Ljubljana, Slovenia
At the onset of MG, a fully blown clinical picture may appear within a few days. However, clinicians are increasingly aware that mild weakness and fatigue, e.g. isolated in a few extraocular muscles, manifested as fleeting diplopia or ptosis, may be a hallmark of gradually developing MG. Such patients with minimal symptoms and signs of a few weeks duration are increasingly often referred to EMG laboratories for diagnostic evaluation.
The standard technique of jitter study with axonal microstimulation, known to be highly sensitive in demonstrating dysfunction of neuromuscular transmission, was used as the initial diagnostic test in 249 consecutive MG patients upon their first presentation. A group of 37 patients was studied within 12 weeks (1-12, mean 5.2 weeks) of the onset of their first symptoms and signs.
Two criteria of SFEMG abnormality were used. One was the proportion of neuromuscular junctions (NMJs) showing abnormal jitter with or without blocking (percentage of outliers), and the other was mean jitter of all measured NMJs. A sample of 40-60 NMJs, in a few very abnormal cases about 30 NMJs, was evaluated. The study was performed in the orbicularis oculi, with the addition of the frontalis in a minority of cases. For the sake of consistency, patients with predominantly limb symptoms and SFEMG in a limb muscle were not included. The upper reference limits used were 31 µs for MCD of individual NMJs and 21 µs for mean MCD of all NMJs studied. Up to 2 NMJs having larger jitter with or without blocking in a sample of 60 were accepted as normal.
There was no significant difference in the degree of abnormalities between this and the larger group of 212 patients seen at later times after the onset. Similarly, there was no correlation between duration of symptoms and the degree of SFEMG abnormality within the small group. Rather, the degree of abnormality was correlated with the severity of symptoms and signs.
According to the outlier criterion, none of the patients had a normal jitter study, and none of the 130 patients who turned out not to have MG had an abnormal study. On the other hand, when using the criterion of mean MCD per study there was a small overlap of the MG and non-MG populations.
In conclusion, neuromuscular jitter of orbicularis oculi and the frontalis muscles show high sensitivity and specificity even in the earliest stages of MG, when symptoms are mild and even non-persistent. The degree of abnormality is not correlated to duration of disease. It is more pronounced when symptoms and signs are severe, but quite abnormal jitter study may coexist with clinically mild disease.