Low Compound Muscle Action Potential Research Articles

There are no neurophysiologic features characteristic of “axonal” Guillain–Barré syndrome

Classical views hold Guillain-Barré syndrome (GBS) as a primary inflammatory-demyelinating neuropathy in which secondary axonal degeneration may occur, particularly when inflammatory lesions are severe. Feasby and colleagues proposed that primary axonal degeneration can also cause GBS characterized by inexcitable motor nerves and poor outcome. This hypothesis rests largely on the results of a single autopsy in which no inflammation or demyelination were found. Using an illustrative case report confirming earlier studies, we point out that inexcitable motor nerves (or low amplitude compound muscle action potentials [CMAPs]) are of ambiguous significance and may reflect distal demyelination, causing conduction block between distal stimulation sites and target muscles, a pattern not uncommon in GBS. Recovery from such lesions may occur within weeks with restoration of CMAP amplitudes. The recognition of a yet unproven axonal variant of GBS cannot be based solely on documentation of inexcitable motor nerves in the context of rapidly developing weakness.

Patterns of conduction impairment in experimental allergic neuritis. An electrophysiological and histological study.

Electrophysiological and histological studies of peripheral nerve were performed in 24 Lewis rats with experimental allergic neuritis (EAN) in which disease had been induced by a single myelin and adjuvant inoculation in one footpad. Demyelination was demonstrated in transverse nerve sections from ventral roots, proximal sciatic nerves and also in distal plantar nerves. Histological and electrophysiological assessments showed that injected limbs were more affected than uninjected limbs. Neurophysiological studies demonstrated two distinct patterns of conduction failure based upon proximal/distal compound muscle action potential (CMAP) amplitude ratios in both uninjected and injected limbs. Slightly more than half of all nerve trunks showed a mildly reduced distal CMAP amplitude irrespective of stimulus origin. The rest displayed a more severe reduction of distal amplitude that was length-dependent, becoming smaller with proximal stimulation. Histological lesions in plantar nerves were often more severe than those in proximal sciatic nerves or ventral roots. Axonal degeneration was an uncommon finding. This study has demonstrated patterns of peripheral nerve conduction impairment similar to those reported in patients with inflammatory demyelinating neuropathy. Moreover, it has shown that a low distal CMAP amplitude may result from demyelination of distal motor nerve segments and not necessarily from axonal degeneration.

Role of electromyography in amyotrophic lateral sclerosis

We reviewed the role of electrodiagnostic testing in amyotrophic lateral sclerosis (ALS) in a large ALS clinic. Over 31 months, 133 patients with a clinical diagnosis of ALS were tested. In most, nerve conduction studies were normal, and needle electrode examination showed active denervation in the upper and lower limbs or the limbs and bulbar muscles (Lambert's criteria). However, 50 of 133 patients did not fulfill Lambert's criteria at presentation because of abnormal nerve conduction studies (11 patients), abnormal F-wave latencies (6 patients), or insufficiently distributed fibrillation potentials (40 patients). This study reveals that a large proportion of patients with a clinical diagnosis of ALS fail to have classical findings on initial electrodiagnostic studies, and reveals several caveats of electrodiagnostic testing in these patients: (1) Conduction studies may be unreliable in motor nerves with markedly low compound muscle action potential (CMAP) amplitudes. (2) Sensory nerve action potential (SNAP) amplitudes may be abnormal in a small percentage of otherwise typical ALS patients. However, better controls for elderly subjects are needed. (3) Needle electrode examination may not show widespread active denervation early in the disease. (4) Some patients may have a mild polyneuropathy. (5) The classic diagnostic criteria may need to be modified to allow earlier acceptance of many ALS patients into therapeutic trials.