Electrodiagnostic Evaluation of Myopathy

Excerpt

Electrodiagnostic testing is the core diagnostic modality for patients with a suspected myopathy. It consists of nerve conduction studies (NCS) and electromyography (EMG). Despite recent advances in molecular genetics and significant improvement in imaging quality, it is still a pertinent part of the diagnostic process in most patients. Electrodiagnostic studies are considered an extension of the physical examination and are most useful in the workup of a patient with a suspected myopathy.

NCS usually precedes needle EMG and provides valuable information about the function of sensory and motor nerve fibers. NCS is normal in the majority of patients with disorders of muscle and assists in excluding disease mimickers. In some cases, specialized tests, such as repetitive nerve stimulation, can be employed to evaluate disorders of the neuromuscular junction, as another cause of pure motor weakness.

The performance of EMG for evaluation of myopathy involves the placement of a needle recording electrode inside the muscle and analysis of electrical potentials at rest and with muscle activation. The selection of muscles for electrodiagnostic examination depends on the clinical scenario and technical limitations. Testing clinically weak muscles increases the yield of the test. Most myopathies affect proximal muscles; therefore, limb-girdle and paraspinal muscles are usually tested. In certain myopathies, distal muscles are preferentially involved (myofibrillar myopathies, distal muscular dystrophies). In such cases, this can lead not only to abnormal EMG but also abnormal motor nerve conductions due to muscle atrophy.

Electrodiagnostic studies not only allow confirmation of myopathy diagnosis and assist in identifying etiology, but they also can be used for the selection of a suitable site for muscle biopsy or to direct further genetic testing.