In the REACT (Rehabilitation Engineering and Clinical Technology) Laboratory at the School of Rehabilitation Sciences, University of British Columbia (UBC), we have embarked on research into the post-polio condition. The research focuses on developing a better method of evaluating the nerve-muscle system in people with a history of polio. Conventionally, these people have been sent to electrodiagnosticians or specialists who use EMG (electromyographic) techniques to look at the firing patterns in muscles, to confirm whether the muscle has been affected by polio in the past and to determine whether there is any recent evidence of motor unit or nerve axon loss. The drawback of these methods is that they are not truly quantifiable. An accurate measure of how much the polio has affected the muscle and nerve cannot be obtained, and therefore a measurement of how much the muscle and nerve has changed over time cannot be reached.
The research method that has been developed in the REACT Laboratory looks at a way of using nerve stimulation to count or estimate the number of motor units in a muscle group. A motor unit is the smallest functional unit of a muscle. It is composed of the cell body in the spinal cord, the nerve axon, all the branches of the nerve axon, and all of the muscle fibres that are attached to that nerve. If there are 20 motor units in a muscle, this means that there are 20 nerve axons coming into that muscle.
With this method, we might study a specific muscle in a population that has no history of polio and find that, on average, people have 100 motor units in this muscle, and then use this technique to measure a polio survivor in an identical test. We may find that the polio survivor has only 50 motor units in the particular muscle.
With this technique, we can follow someone over a long period of time and see whether these motor units decrease over time, i.e. whether the person is actually losing nerve axons. This is important because often when the polio survivor complains of recent weakness, fatigue or muscle cramping, a natural concern is, "Am I losing nerve fibres or motor units?" It is reassuring if the doctor can say, "Your tests now, compared with your tests a year ago, show that your number of motor units is still essentially the same."
Another advantage is that if we embark upon a particular type of therapy, whether it’s a new drug therapy or a type of physiotherapy, the effect of the therapy on the nerve and muscle can be evaluated by measuring the number of motor units before and after the therapy.
The actual research technique involves repeatedly stimulating the nerve and measuring the muscle responses using a computer and surface electrode. The computer uses a sophisticated program to isolate all of these electrical responses and then estimate the number of motor units in the muscle.
At this time, the technique is only able to look at the small muscles of the hand and foot. We are hoping to extend this technique to the bigger muscles of the legs and arms, around the shoulders and hips. One of the problems with electrical stimulation of nerves is that because the electrical stimulator is placed on the skin, the electrical current is lost in the tissues unless the nerve is close to the stimulator.
Recently, we obtained funding from Health and Welfare Canada to undertake a clinical evaluation of this technique. Thirty-six individuals with documented polio were studied. Each person underwent concentric needle EMG examinations in the exterior digitorum brevis (EDB) muscles in the feet and abductor pollicis brevis (APB) muscles in the hands. These same muscles were examined using the new computer-assisted technique, and estimates of motor units in each muscle were made. All of this information was correlated with clinical symptoms. A comparison was made with a control group that had no history of neurological injury.
Preliminary analysis of the results shows that there is a definite correlation between the drop in the motor unit count and the severity of the past denervation noted by concentric needle EMG. Muscles that showed evidence of moderate to severe signs of denervation had, on average, fewer motor units than muscles that were either normal or showing only mild signs of denervation. Foot muscles showed greater reductions in motor units than hand muscles in our study. Further analysis correlating symptoms of fatigue, weakness, and pain with a drop in motor unit count is now underway.
This research is promising in that it offers a non-invasive method of objectively measuring the effects of polio on the nerve-muscle system. These motor unit counts allow an individual to be followed over time without subjecting him or her to repeated concentric needle EMG studies. The correlation with clinical symptoms should help us to develop better prognoses and treatments.
Another direction in our research is in the use of magnetic stimulation instead of electrical stimulation to stimulate the nerve repeatedly. At UBC we have now developed the first magnetic stimulator built in Canada. The advantage of magnetic stimulation is that it is actually used at a distance from the skin and induces a current deep into the tissues. We are hoping to extend this technique to reach nerves that are embedded in deep tissues, as in the ones around the hip and shoulder.
Also being developed is a research technique that examines electrical patterns during muscle contraction. When a muscle is used, a certain number of motor units are called upon to fire, either slowly or quickly. These two pieces of information — how quickly the motor units fire and how many motor units are being used — are very important in understanding how muscles work.
By means of these electrical techniques, we hope to understand better why some polio survivors experience fatigue and complain of weakness rather than compensating with smaller numbers of motor units.
(Dr. Hershler is a specialist in Physical Medicine, Electrodiagnostics and Biomedical Engineering, School of Rehabilitation Medicine, University of British Columbia.)