SAPP Saskatchewan Awareness of Post Polio Society Inc.


EMG in the Differential Diagnosis

(reprinted from Electromyogr. clin. Neurophysiol., 1988, 28, 269-272)


Post-Polio Syndrome (PPS) is characterized by the presence of recurrent weakness, fatigue and muscle pain in muscles that had previously been paralyzed or weakened as a result of acute poliomyelitis. These recurrent symptoms develop approximately twenty years after the initial onset of paralysis.

In 17 patients who fit the clinical criteria of PPS, 102 weakened muscles were encountered. EMG studies were done on all 102 muscles. The EMG patterns that were seen permitted a differential diagnosis between PPS and weakness secondary to other reasons.

In 75 muscles fitting the clinical criteria of PPS, absent insertional activity was seen on EMG and was accompanied by the presence of motor unit action potentials on attempting voluntary movement of the same muscles. In the remaining 27 muscles found to be clinically weakened on examination, normal insertional activity was identified. In some of these, evidence of previous anterior horn cell disease was noted to be present.

By correlating EMG findings with historical and clinical findings, other reasons for weakness could be identified. These reasons include the presence of disuse, incomplete recovery from poliomyelitis, and total absence of recovery following poliomyelitis.

EMG can provide an additional criterion for the diagnosis of PPS. It permits a more exact approach to treatment by the use of non-fatiguing strengthening exercises with beneficial results.

PPS is a clinical entity which has been shown to exist in about 20-30% of individuals who had poliomyelitis during the epidemics preceding the discovery of the poliomyelitis vaccine. In 1986, the clinical criteria for the diagnosis of PPS were established. These include 1) the documentation of previous acute poliomyelitis, 2) a latency period of approximately twenty to twenty-five years during which normal function has been noted to have returned to the previously paralyzed muscles, followed by, 3) a recurrence of weakness in the same muscles that had previously been identified as having been paralyzed or weakened by poliomyelitis, this new weakness associated with severe fatigue and occasional pain in these muscles.

Weichers and Hubbell have identified single fibre EMG changes associated with PPS, indicating the presence of dysfunction in the neuromuscular junctions formed after reinnervation between the terminal axone sprouts and the orphaned denervated muscle fibres. It is postulated that as a result of this dysfunction, these neuromuscular junctions are unable to keep up with the metabolic demands placed upon them. The resultant metabolic fatigue of these neuromuscular junctions results in a gradual decrease, over time, in the number of motor units that are able to function.

Another explanation for the deterioration in function is difficulty for the parent anterior horn cell to keep up with the metabolic demands placed upon it after it has supplied sprouts to the orphaned muscle cells, with a resultant gradual decrease in the number of motor units which can continue to function.

It has been established that clinical weakness occurs only after 50% of the motor units have been destroyed, which explains why there is a latency period of approximately twenty to twenty-five years from the time of the onset of acute poliomyelitis and the perception of recurrent weakness. It also has been noted that the loss of motor units occurring in this manner is greater than what occurs as a result of aging.

The purpose of this present study is to present the results of EMG evaluation of muscle weakness encountered in PPS. These EMG changes have permitted the differentiation between muscles that are weakened as a result of PPS and muscles that are weakened for other reasons in individuals who had suffered from acute poliomyelitis.

As part of a study investigating the effects of non-fatiguing strengthening exercises in PPS, 17 patients admitted into that study were identified as having 102 muscles which demonstrated clinical weakness. By history, 75 of these muscles fit the clinical criteria of PPS. They had initially been weakened or paralyzed during acute poliomyelitis and had recovered and had functioned normally for between twenty to thirty years prior to becoming weakened again. The additional 27 muscles did not fit the criteria of PPS, but continued to demonstrate clinical weakness to the same degree as those muscles that fit the diagnosis of PPS.

After obtaining informed consent, EMG was performed on all weakened muscles using a monopolar needle and a Disa Neuromatic 2000 electromyography apparatus. Data were recorded without full knowledge as to which muscles met the clinical criteria of PPS and which did not meet these criteria. After the EMG study, the results that were obtained were compared with the information which was obtained by history and questionnaire. A total of 102 muscles were studied. A videotape of the procedure was made during one of the latter studies.

All 75 muscles which met the clinical criteria of PPS demonstrated absent insertional activity when the EMG needle was inserted into these muscles. This absence of insertional activity was accompanied by the presence of motor unit action potentials (MUAP) on attempting voluntary movement while the needle remained in place in the muscle. Some of these MUAP had characteristics of previous anterior horn cell disease demonstrating the presence of polyphasic potentials of high amplitude and a decrease in the number of MUAP on attempting voluntary movement. The remaining muscles (27) demonstrated either normal EMG or the presence of previous anterior horn cell disease. All of these latter muscles had normal insertional activity.

As expected, muscles that had been previously paralyzed by acute poliomyelitis but had never recovered, demonstrated atrophy; they had no insertional activity and also had a total absence of voluntary movement and a total absence of EMG activity.

The presence of progressive weakness in PPS is a problem which develops approximately twenty to twenty-five years after the acute onset of poliomyelitis. It is necessary to differentiate this phenomenon from weakness which persists and never changes after acute poliomyelitis in muscles which have only partially recovered or which have never recovered from this disease.

While the reasons for the EMG changes resulting i absent insertional activity are not understood, this finding, together with the persistence of MUAP on attempting voluntary movement in muscles fitting the clinical criteria of PPS, permits a differential diagnosis of weakness in PPS with the use of EMG.

An EMG-based classification therefore can be established defining the types of weakness that are encountered in PPS. This weakness can be secondary to disuse . In this case, there is normal insertional activity and normal EMG. Alternatively, weakness can be secondary to partial recovery from acute poliomyelitis. In the latter case, normal insertional activity will be found, but there may be evidence of previous anterior horn cell disease.

In muscles that never recovered from poliomyelitis, there is no insertional activity and there is also a total absence of EMG activity.

Finally, in muscles that have been affected by the secondary changes associated with PPS, and which meet the clinical criteria of PPS, there is absence of insertional activity. This finding is associated, in the same muscles, with the presence of MUAP on attempting voluntary movement with or without the presence of evidence to indicate previous anterior horn cell disease.

This differentiation permits a more specific physiotherapy prescription for these patients. It is known that muscles previously affected by acute poliomyelitis could not be fatigued during exercise for fear of causing further weakness and/or atrophy. The same problem has been encountered when vigourous exercise has been given to muscles fitting the clinical criteria of PPS. The differentiation by EMG permits the identification of muscles that should be subjected to non-fatiguing strengthening exercises (PPS muscles) and those that can be exercised without regard to fatigue (i.e. all other weakened muscles).

By prescribing non-fatiguing strengthening exercises to muscles identified by EMG as being PPS muscles, the progressive weakness encountered in untreated PPS has been stopped. Approximately 50% of the muscles identified as being weakened from PPS were found to improve in strength, while just under 50% remained at a muscle strength level similar to that found prior to the onset of physiotherapy treatment. A strengthening effect also had been obtained in non-PPS muscles treated with the usual types of strengthening exercises.

In summary, PPS has been identified in 20-30% of individuals who had acute poliomyelitis prior to the discovery of the polio vaccine. It has been noted that clinical weakness is present in these patients in muscles which do not meet the clinical criteria for the diagnosis of this syndrome. By performing routine EMG examination on all weakened encountered in post- polio patients, differentiation as to the various causes of weakness can be established. This differentiation permits a more exact approach to treatment without concern that exercise will cause any further deterioration because of fatigue. It is proposed that the EMG changes seen in PPS, should be included amongst the clinical criteria for the establishment of the diagnosis of this disease.

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