Compound Muscle Action Potential (CMAP) Test

The Compound Muscle Action Potential (CMAP)  test (aka the Exercise EMG) is now being used to diagnose the periodic paralyses. When done correctly this test is reported to positively identify 70%-80% of patients. The CMAP test is based on the observation that patients with Periodic Paralysis have a greater than normal increase in the compound muscle action potential amplitude immediately after two to five minutes of intermittent voluntary contraction. This increase is followed over the next 30 to 40 minutes by a progressive decline in CMAP amplitude to well below the pre-exercise baseline.

In CMAP tests done on a group of people without muscle disease the CMAP amplitude decrement varied from 5.4% to 28.8% (mean 15%). This amplitude decrement was not influenced by age or sex. A decrease of more than 40% in the amplitude of CMAP is considered diagnostic of muscle disease. 

The CMAP is highly specific for periodic paralysis but it must be done properly. Negative results can be expected if the patient has not experienced a paralytic attack in the previous three weeks.  Katz et al. suggest that the CMAP test can be used to determine the effectiveness of therapy by comparing CMAP decrements pre and post medication, so it is obvious the patient must be unmedicated for results to be accurate diagnostically. A 72 hour "wash-out" period of medications taken for periodic paralysis is recommended, although potassium supplement can be taken up to 36 hours (for time release) and 24 hours (for quick release) potassium. We include abstracts (below) which will allow clinicians to determine the nature of this test and locate the articles in the literature.

Summary and Technique

The exercise test was described by McManis et al as a simple electrodiagnostic method to confirm clinical suspicion when the diagnosis of PP was uncertain.

McManis et al studied 21 patients with clinically definite hyperkalemic PP, normokalemic PP, or hypokalemic PP, and compared these patients with a group of healthy individuals. Most patients with PP had a greater than normal increase in the compound muscle action potential (CMAP) amplitude immediately after 2 to 5 minutes of intermittent voluntary contraction.

This was followed over the next 30 to 40 minutes by a progressive decline in CMAP amplitude to well below the pre-exercise baseline. Patients with hyperkalemic PP, on average, had much greater amplitude increments and decrements than patients with hypokalemic PP, but there was overlap between individual patients. In the hands of the experienced examiner this test is reliable 70%-80% of the time.

It is important to note that factors such as the time of day, temperature, serum potassium levels, and recent meals may affect the susceptibility of muscle to exercise in PP. Day-to-day variability may be present in a given individual. Since the CMAP test can be used to determine response to therapy by comparing CMAP decrements pre and post treatments, it is obvious the patient must be unmedicated for the test to be used diagnostically.

The test is based on two previously described observations. CMAP amplitude is low in the muscle weakened by periodic paralysis and the weakness can be induced by exercise. Recording electrodes are placed over hypothenar muscle and CMAP is obtained by giving supramaximal stimuli, which are repeated every 30-60 seconds for a period of 2-3 minutes until stable baseline amplitude is obtained.

Then the patient contracts the hypothenar muscles isometrically for 2-5 minutes, with brief (3-4 seconds) rest every 15 seconds to prevent muscle ischemia. CMAP is recorded every minute when muscle is exercised and every 1-2 minutes after exercise for a period of 30 minutes or until no further decrement is observed in the amplitude of CMAP.

Percentage of decrement is calculated by subtracting the smallest amplitude after exercise from the greatest amplitude after exercise and dividing it by the greatest amplitude after exercise. A decrease of more than 40% in the amplitude of CMAP is considered abnormal.

Kuntzer T, Flocard F, Vial C, et al: Exercise test in muscle channelopathies and other muscle disorders. Muscle Nerve 2000 Jul;23(7):1089-94

We studied the percentage change in compound muscle action potential (CMAP) amplitude and area during and after a 5-min maximal contraction of the muscle. The exercise test (ET) was performed on 64 patients with different muscle disorders and on 46 normal controls. The range of normal ET values was defined as the mean + 2 SD of the control values.

The mean sensitivity of the test was 63% in the whole group with ion channel muscle disorders, the highest sensitivity being seen in primary periodic paralysis (81%) and the lowest in chloride channelopathies (17%). In thyrotoxic periodic paralysis, the ET was abnormal in the three of the four patients studied. In patients with myotonic dystrophy, a smaller than normal increase in CMAP amplitude occurred during and after exercise, whereas in proximal myotonic myopathy a normal initial increase in CMAP amplitude was followed by an abnormal decrement.

We conclude that the ET can be of use in confirming abnormal muscle membrane excitability in patients with calcium and sodium channelopathies and thyrotoxic periodic paralysis. In chloride channelopathy, the test may also be abnormal, but shows no, or only a small, increase in amplitude or area in the immediate post-exercise period. The test may also be abnormal in proximal myotonic myopathy, but is normal in myotonic dystrophy. Copyright 2000 John Wiley & Sons, Inc. PMID: 10883004, UI: 20342063

J. S. Katz, MD; G. I. Wolfe, MD; S. Iannaccone, MD; et al The Exercise Test in Andersen Syndrome: Archives of Neurology / volume:56 (page: 352)

The exercise test was described by McManis et al. as a simple electrodiagnostic method to confirm clinical suspicion when the diagnosis of PP was uncertain. McManis et al. studied 21 patients with clinically definite hyperkalemic PP, normokalemic PP, or hypokalemic PP, and compared these patients with a group of healthy individuals.

Most patients with PP had a greater than normal increase in the compound muscle action potential (CMAP) amplitude immediately after 2 to 5 minutes of intermittent voluntary contraction. This was followed over the next 30 to 40 minutes by a progressive decline in CMAP amplitude to well below the pre-exercise baseline. Patients with hyperkalemic PP, on average, had much greater amplitude increments and decrements than patients with hypokalemic PP, but there was overlap between individual patients. PMID: 10190827, UI: 99205102

McManis PG; Lambert EH; Daube JR: The exercise test in periodic paralysis.; Muscle Nerve, 1986 Oct, 9:8, 704-10

Of 21 patients with clinically definite hypokalemic, hyperkalemic, or normokalemic periodic paralysis, 15 (71%) had a greater than normal increase in compound muscle action potential amplitude during 2-5 minutes of intermittent strong voluntary contraction of the muscle. This increase was followed by a progressive decline in amplitude, which was greater than in a control population and which was most rapid during the first 20 minutes after exercise. The amplitude often decreased to a level below the pre-exercise level. A similar response was seen in six of nine patients with periodic paralysis secondary to disorders such as thyrotoxicosis. This test may have value in the identification of patients with periodic paralysis. Unique Identifier 87064860 More information.