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Post-Polio Syndrome: Pathophysiology and Clinical Management
Anne Carrington Gawne and Lauro S. Halstead

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VIII. ELECTRODIAGNOSTIC FEATURES.

The EMG in acute poliomyelitis is characterized by poor MUAP recruitment at the onset of weakness, followed by the development of fibrillation potentials in widespread muscle groups after 3 to 4 weeks.[79] The fibrillation potentials subside as reinnervation by surviving motor units proceeds during recovery. In muscles with too few motor units to have adequate reinnervation, fibrillation potentials can persist indefinitely; however, in both partially and completely denervated muscle the fibrillation potential amplitude decays over time. Kraft found that in subjects with partial traumatic peripheral nerve lesions, initially the amplitude of the fibrillation potentials were as high as 1600 µV, with most greater than 700 µV.[80] One year later, all potentials were less than 100 µV.

The same is true in chronic polio. Some investigators report that there is an increase in muscle membrane instability between those patients that are clinically stable (no new weakness) and those who are clinically unstable (new weakness). Nelson found that in those patients who were seen for unrelated medical problems, three of 27 had fibrillation potentials, while in those with delayed weakness, 14 of 29 had fibrillations (p <0.01).[43] Martinez studied 34 patients, 17 with stable polio and 17 with new weakness. He found that 41% of those with unstable polio had fibrillations, while none with stable strength did.[37] However, other investigators, including Cashman, Weichers, Hayward, and Seaton, examined groups of patients with and without new weakness and found evidence of ongoing enervation in the form of fibrillations in both groups.[23,26,27,81,82] Some electromyographers have found that muscle membrane instability is more pronounced in the more severely affected muscles.[83,84] The muscles that become extremely weak and atrophic have few to no MUAPs, but virtually all, including clinically normal muscles, will have large, polyphasic MUAPs and abnormal, decreased recruitment.[79] In a study comparing the size of MUAPs in control subjects and polio patients with and without new weakness, Agre found that the size of MUAPs was significantly larger in all polio patients, and that those with new weakness had larger units than those without. He found no significant correlation between strength and MUAP size.[42]

Single fiber studies have shown abnormalities, including increased jitter and blocking and increased fiber density and motor unit territory.[85] When comparing electrophysiological and biopsy findings, a significant correlation between the percentage of fibers exhibiting jitter and increased fiber density has been found, suggesting that muscles with the most enlarged motor units as a result of sprouting are more likely to exhibit instability later in life.[85]

Defects at the neuromuscular junction have been studied using repetitive stimulation techniques by Trojan et al. Single fiber EMG was performed, and jitter was measured during both low- and high-frequency stimulation in 17 PPS patients and nine normal controls. In five of 17 PPS patients and one control, jitter was significantly higher at high-frequency stimulation. The remaining patients were not significantly different from controls. Those PPS patients with abnormal jitter had a significantly longer time since their acute polio. The authors concluded that the neuromuscular junction defect in PPS patients is probably due to ineffective conduction along immature nerve sprouts and exhaustion of acetylcholine stores, and that this may be dependent on time after acute poliomyelitis.[86]

Findings on macro-EMG include increased macro-EMG amplitude, up to 2000% above the mean. Lange et al. found that polio-affected muscles with normal strength had large-amplitude macro-EMG signals (1000 to 4110 µV), weak but stable muscles also had increased large macro-EMG amplitude, while muscles with new weakness had smaller than expected macro-EMG amplitude (130 to 450 µV). Fiber density and jitter were increased in all three groups, but percent blocking doubled in weak muscles.[87]

In summary, EMG abnormalities seen in chronic polio include large polyphasic MUAPs in both conventional and macro-EMG. Although reports differ, it is a fair consensus that there can be evidence of muscle membrane instability in the form of fibrillations in both stable and unstable muscles, although generally these fibrillation potentials are small and sparse. Findings on single fiber EMG include increased jitter, blocking, and fiber density. High-frequency stimulation accentuates this increased jitter, especially in those muscles with new weakness, reflective of defects in the neuromuscular junction. At this time, however, there is no single EMG finding that can distinguish muscles that are developing new weakness.

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Document preparation: Chris Salter, Original Think-tank, Cornwall, United Kingdom.
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Created: 5th June 2000
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