Electromyogram report crucial for patients with suspected myositis
SAN DIEGO — The use of electromyograms and nerve conduction studies are essential to excluding alternative diagnoses and confirming muscle disease when evaluating patients with suspected myopathic disorders, according to a presentation at the 2019 Congress of Clinical Rheumatology West.
“[Electromyograms (EMG)] and [nerve conduction studies (NCS)] are two components that are very helpful in determining whether a patient is experiencing a nerve problem and/or a muscle problem,” Julie J. Paik, MD, MHS, director of clinical trials at the John Hopkins Myositis Center, told attendees. “In myopathy, you will actually find a drop-out of individual muscle fibers. Normally, without myopathy, you will have a healthy motor neuron with an axon, and the motor neurons are not affected.”
However, there are exceptions to the rule, Paik noted, in which the motor neuron could potentially be affected —namely in amyotrophic lateral sclerosis. Although a known nerve disease, rather than a muscle disease, ALS can present similarly to myopathies.
“I have had four patients diagnosed with refractory polymyositis, in which case we performed an EMG and, upon a closer neurological exam, we found that they were misdiagnosed — they actually had ALS,” Paik said.
She noted that paying attention to the EMG report is important to discern whether failures are myopathic or neuropathic in nature.
“In myositis muscle diseases, the motor neurons should not be affected, but if you do have a muscle problem, each individual muscle fiber starts to die out and is inflamed for whatever reason,” Paik said. “If the muscle is affected, then you put the EMG needle in the muscle and ask the patient to contract the muscle; you will not get a normal response, because the muscle fibers are not where they should be.”
Paik noted myopathies are characterized by the presence of polyphasic, short-duration low-amplitude motor unit action potentials, which is “what the neurologist will typically write in the report and determine that this patient has myopathy.”
Another clue in the EMG report that should help distinguish myopathies from neuropathies will be the mention of positive shockwaves for fibrillation, according to Paik.
“That is important because when the neurologist actually inserts the needle into the muscle — I describe it to my patients like ‘a microphone into the muscle’ — they are actually trying to determine if there is membrane instability of the muscle due to the intense inflammation,” Paik said. “[This inflammation] could be from the myositis, but it also could be from adult-onset muscular dystrophy. If that is the case, when they insert the needle, the neurologist will report a lot of ‘pitter-patter’ sound.”
Paik noted that this is referred to as an “irritable myopathy” at the John Hopkins Myositis Center, but on the EMG report, “you are going to look for positive shockwaves and fibrillations. If there is a myopathy, the report will mention that there are polyphasic, shorter-duration, lower amplitude [motor unit action potentials].”– by Robert Stott
Paik JJ. Pathogenesis, evaluation and treatment of the patient with myopathy: Are there triggers? Presented at: Congress of Clinical Rheumatology West; September 26-29, 2019; San Diego.
Disclosure: Paik reports no relevant financial disclosures.