By Michael Rubin, MD
Professor of Clinical Neurology, Weill Cornell Medical College
Electromyography often is requested in the evaluation of a patient with rhabdomyolysis. It often shows evidence of “myopathy,” but muscle biopsies frequently are non-confirmatory, and genetic testing often is needed for a definitive diagnosis.
Skolka MP, Milone M, Litchy WJ, et al. The utility of electrodiagnostic testing in unprovoked rhabdomyolysis in the era of next-generation sequencing. Muscle Nerve 2024;70:180-186.
Rhabdomyolysis, acquired or hereditary, is a syndrome characterized by acute muscle necrosis, allowing the entry of myoglobin, creatine kinase (CK), and other intracellular proteins and electrolytes into the circulation. Reportedly, 26,000 cases occur annually in the United States, where it may be responsible for up to 7% to 10% of cases of acute renal failure.
Associated with a wide variety of diseases and disorders, several studies report the most common causes to be prolonged exertion during exercise, trauma, or alcohol abuse. Elevation of CK and other muscle enzymes are the hallmark of rhabdomyolysis, with reddish-brown urine of myoglobinuria characteristic, but often not observed. Inherited causes should be explored if the history, physical examination, and initial laboratory studies do not suggest an apparent cause. What role does electromyography (EMG) play in its evaluation, given today’s era of genomic analysis and the oft-performed muscle biopsy?
All patients who underwent EMG at the Mayo Clinic following a resolved episode of rhabdomyolysis between Jan. 1, 2012, and Jan. 1, 2022, were
identified through the clinic’s EMG report and database explorers. Inclusion criteria encompassed age older than 18 years, one or more episodes of unprovoked rhabdomyolysis (defined as a greater than 10-fold elevation of serum CK), and associated with muscle weakness, muscle edema, or myoglobinuria. EMG included nerve conduction studies and needle EMG, assessing motor unit potential parameters by the examining physician, in a quantitative manner at the time of testing.
Patients then were divided into two groups: those with or those without myopathic EMG findings. Myopathy was diagnosed definitively when confirmed by histopathology or genetic testing. Statistical analysis comprised an independent t-test for continuous data and Fischer’s exact test for categorical data, with significance defined as P < 0.05.
Among 66 patients who met inclusion criteria, encompassing 43 men and 23 women, 32 (48%) patients demonstrated a myopathic EMG and 34 patients were non-myopathic. Baseline weakness and elevated CK were more common in the myopathic group, but pathologic evidence of myopathy was found in only 11 of 41 (27%) patients who underwent muscle biopsy, with the remaining biopsies showing no, or only nonspecific, pathologic changes. In the myopathic EMG group, muscle biopsy showed nonspecific fiber type atrophy in three patients without myopathic features but was more likely to have a pathologically specific or genetic diagnosis, including muscular dystrophy.
Among the 15 (23%) patients with a definitive diagnosis, five of 11 who underwent biopsy showed no myopathy on histopathology and their diagnosis required genetic testing. Neither the presence of fibrillation potentials on EMG nor the number of episodes of rhabdomyolysis predicted a higher chance of obtaining a definitive diagnosis. Regardless of EMG findings, a definitive diagnosis of myopathy may be made following rhabdomyolysis with genetic testing or muscle biopsy. EMG did not help to make a definitive diagnosis and may be deferred.
COMMENTARY
Magnetic resonance imaging (MRI) also is useful in the diagnosis of rhabdomyolysis, particularly for evaluating the overall extent of muscle involvement and delineating affected compartments should fasciotomy be needed. Muscles will enhance following intravenous gadolinium (GAD) administration and will demonstrate non-enhancement with a thin peripheral rim of hyperenhancement when there has been progression to myonecrosis, with the caveat that administration of GAD must be judicious in this setting to avoid exacerbating renal dysfunction. Adjacent soft tissues may show T2 hyperintense edema, and long-term follow-up may show muscle atrophy and fatty deposition.
Differential diagnosis includes inflammatory, inherited, or infectious myopathy, with denervation change having similar diffuse intramuscular edema and enhancement in the acute phases. Hence, caution must be exercised in MRI interpretation. In the setting of nonspecific symptoms, imaging can exclude other etiologies of musculoskeletal pain and extremity swelling.1
REFERENCE
- Rixey AB, Glazebrook KN, Powell GM, et al. Rhabdomyolysis: A review of imaging features across modalities. Skeletal Radiol 2024;53:19-27.
