Brief Alerts
Practice Parameter: Repetitive Nerve Stimulation and Single Fiber EMG
Sources: AAEM Quality Assurance Committee. Muscle Nerve. 2001;24:1236-1238; Muscle Nerve. 2001;24:1239-1247.
What electrodiagnostic study protocol is recommended to confirm the presence of neuromuscular junctionopathy? Searching the Medline English database through 1998 using keywords including myasthenia gravis, Lambert Eaton myasthenic syndrome, botulism, and neuromuscular transmission and junction, 545 relevant articles were identified. Review of these articles and their references revealed 34 articles which met more than 2 of 6 criteria. Specifically, 1) the study was prospective; 2) a clinical diagnosis was made independent of the electrodiagnostic evaluation; 3) had detailed descriptions of the electrodiagnostic evaluation and 4) temperature were stated; and 5) proper reference values; and 6) criteria for abnormal findings were clearly noted. Based on these reports, the following recommendations are offered.
Repetitive nerve stimulation (RNS) recording from an affected muscle is the first step. For myasthenia, ensure that anticholinesterase medication has not been taken for at least 12 hours, stimulate at 2-5 Hz, perform baseline and immediate postexercise repetitive stimulation and repeat at 1 minute intervals up to 5 minutes. Maintain skin at 35°C and look for a reproducible decrement of 10% comparing first to fourth or fifth response in at least 1 muscle. For Lambert Eaton myasthenic syndrome, although there is no agreement on the minimum degree of increment needed, 100% increase in amplitude following exercise or tetanic stimulation is preferred.
If RNS is normal, perform single fiber EMG (SFEMG) in at least 1 affected muscle. If normal, repeat in a second affected muscle. SFEMG is abnormal if more than 10% of fiber pairs have abnormal jitter or blocking, or if mean jitter exceeds published norms.
In purely ocular myasthenia where RNS will likely be normal, SFEMG may be the initial study performed. —Michael Rubin
Predicting Nonrecovery in Traumatic Sixth Nerve Palsy
Source: Holmes JM, et al. Ophthalmology. 2001;108:1457-1460.
Ninety-nine patients with head trauma followed by diplopia and sixth nerve palsy, in the absence of oculomotor weakness, were enrolled in this study to determine which factors predicted persistence of diplopia in primary gaze at 6 months. Fifteen patients did not complete the follow-up period. Among the remaining 84, failure to recover was associated with bilaterality, complete paralysis (inability to abduct past the midline on initial examination), and female gender. Though the last may be chance, the former appear biologically plausible. —Michael Rubin
Dr. Rubin, Associate Professor, Clinical Neurology, New York Presbyterian Hospital-Cornell Campus, is Assistant Editor of Neurology Alert.
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