A member of our PPN Support, Education and Advocacy Group asked for information about EMG’s, also known as CMAP’s. These tests are often used to diagnose or rule out Periodic Paralysis. Unfrotunately, the results of these may or may not diagnose Periodic Paralysis depending on several factors: if it is done correctly, the type of Periodic Paralysis an individual has, whether they have had an episode recently, whether they are presently in an paralytic episode and more.
The procedure can be painful and uncomfortable for some individuals, but not unbearable. If it is done properly, it can diagnose only a few forms of Periodic Paralysis. A NEGATIVE TEST RESULT WILL NOT MEAN THAT YOU DO NOT HAVE PERIODIC PARALYSIS. Research indictes that some individuals who have been genetically diagnosed have negative results on the test or varying results on sequential testing.
A decrease of more than 40% (of the amplitude) is considered enough to diagnose PP. Any more than that is complicating to explain. If someone has not had an episode of paralysis in the three weeks before the test, it would be expected to be normal. The forms which may be detected using EMG (CMAP) testing are calcium and sodium channelopathies and thyrotoxic periodic paralysis.
So, for a diagnosis to be made from the results of an EMG/CMAP many things need to be considered. The following is more specific information and links compiled, which explain all of the difficult to understand specifics. Some forms of PP can easily be detected and others cannot and that depends on many factors and that also includes if the testing is done correctly. So, a negative result does not mean that some one does not have PP, just that it was not able to be detected.
For Hypokalemic Periodic Paralysis:
"Diagnosis can be achieved through a specialized form of electromyographic (EMG) testing called the long exercise test. This test measures the amplitude of a nerve response (called the Compound Muscle Action Potential or CMAP) for 40 to 50 minutes following a few minutes of exercise. In affected patients, there is a progressive fall in the amplitude of the potential.....Standard EMG testing cannot diagnose a patient unless they are in a full blown attack at the time of testing."
Info on all forms:
The presence of a progressive and marked decrease in the amplitude of compound motor action potentials (CMAP) during a long exercise test [McManis et al 1986].
During an attack, EMG findings are not specific; EMG demonstrates a reduced number of motor units and possibly myopathic abnormalities.
Five patterns (I-V) of abnormal responses to SET and/or LET in periodic paralyses and nondystrophic myotonias have been described [Fournier et al 2004]. Genetically defined periodic paralyses specifically result in: Pattern IV (no or rare myotonic discharges, increase of CMAP on SET, immediate increase and late marked decrease in LET), more commonly seen in the hyperkalemic type OR Pattern V (no myotonic discharges, normal response to SET, no immediate increase but late marked decrease in LET), more commonly seen in the hypokalemic type.
A false negative normal pattern may be noted in some individuals who have a disease-causing mutation, especially in asymptomatic individuals or those who have not recently had a paralytic attack [Tengan et al 2004].
"EMG: In between attacks, there may be fibrillation
and complex repetitive discharges, increased by
cold and decreased by exercise (in hypokalaemic
periodic paralysis). During attacks, EMG will show
electrical silence, in both hyper- and hypokalaemic
One more about all forms.....
short exercise protocol.14 However, the short exercise test does not have significant diagnostic value in PP as most patients show no abnormalities.14 The LET is more useful in these patients. Some patients with hyperkalemic PP exhibit electrical myotonia; this finding is not described in hypokalemic PP.11,14
this decrement may or may not be preceded by an initial increment in CMAP amplitude/area.14,16,20 However, there is some cross over between pattern IV and V in hyperkalemic and hypokalemic PP, which may be explained by the proportion of hypokalemic PP patients who have an SCN4A mutation.14 Pattern V is most typical