From the Guidelines
The electromyography (EMG) features of myopathy are characterized by polyphasic motor unit action potentials of short duration and low amplitude with increased insertional and spontaneous activity, including fibrillation potentials, sharp waves, or repetitive discharges.
Key EMG Features
- Polyphasic motor unit action potentials
- Short duration and low amplitude
- Increased insertional and spontaneous activity
- Fibrillation potentials
- Sharp waves
- Repetitive discharges As noted in the study published in the Mayo Clinic Proceedings 1, these EMG features are typical of a myopathic process and can help confirm the diagnosis of myopathy. Additionally, a study also published in the Mayo Clinic Proceedings 1 highlights the importance of EMG in diagnosing dermatomyositis, a type of inflammatory myopathy, which also presents with similar EMG abnormalities. The EMG findings are crucial in distinguishing myopathy from other neuromuscular diseases and in targeting a muscle for biopsy, which is the gold standard for confirming the diagnosis of myopathy 1.
From the Research
EMG Features of Myopathy
The electromyography (EMG) features of myopathy include:
- Fibrillation potentials, positive sharp waves, myotonic or complex repetitive discharge, as well as polyphasic potentials, which are non-specific and can occur in both myopathic and neurogenic lesions 2
- Decreased duration of motor unit potentials (MUP), which is the most sensitive and specific parameter for myopathy in conventional EMG, but can also be seen in disorders of the terminal motor fibers or the neuromuscular junction 2
- Abnormal spontaneous activity, such as fibrillation potentials, positive sharp waves, or complex, repetitive discharges, which can indicate an active necrotizing myopathic process and/or unstable muscle membrane potential 3
Diagnostic Yield of EMG in Myopathy
The diagnostic yield of EMG in myopathy can be improved by:
- Combining different techniques, such as manual analysis of individual motor unit potentials and multi-motor unit potential analysis sampled at weak effort, and turns-amplitude analyses such as the cloud analysis and the peak ratio analysis at high effort 4
- Using quantitative analyses and a needle electrode 4
- Combining EMG with other diagnostic methods, such as muscle biopsy and genetic testing 5, 6
Limitations of EMG in Myopathy
The limitations of EMG in myopathy include:
- Low sensitivity and specificity for predicting an inflammatory histology 3
- Limited ability to differentiate between different types of myopathy 4
- Potential for discordance between EMG and muscle biopsy findings, which can be attributed to different muscle sampling and to disorders with both neurogenic and myopathic features 6