Thus, activation of each motor neuron generates a motor unit action potential, consisting of the summation of individual muscle fiber action potentials within a single motor unit. The action potential then propagates along the sarcolemma by shifts in sodium and potassium ion-driven currents, generating a depolarizing electrical field along with adjacent membrane segments. When a motor neuron is activated, the action potential travels along the nerve and arrives at the neuromuscular junction, where after a series of electrochemical processes, depolarization of the muscle fiber occurs. Īt rest, the normal muscle membrane has only resting potential therefore, resting muscle EMG is “electrically silent.” Only when the needle electrode is close to the neuromuscular junction can small normal endplate potentials be recorded. The number of motor units, the density of fibers in a single unit, and their special distribution throughout the muscle contribute to needle electrode recordings during electrodiagnostic studies.

Įlectrodiagnostic 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. In such cases, this can lead not only to abnormal EMG but also abnormal motor nerve conductions due to muscle atrophy.

In certain myopathies, distal muscles are preferentially involved (myofibrillar myopathies, distal muscular dystrophies). Testing clinically weak muscles increases the yield of the test. Most myopathies affect proximal muscles therefore, limb-girdle and paraspinal muscles are usually tested. The selection of muscles for electrodiagnostic examination depends on the clinical scenario and technical limitations. 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. 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. NCS is normal in the majority of patients with disorders of muscle and assists in excluding disease mimickers.

NCS usually precedes needle EMG and provides valuable information about the function of sensory and motor nerve fibers. Electrodiagnostic studies are considered an extension of the physical examination and are most useful in the workup of a patient with a suspected myopathy. 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. It consists of nerve conduction studies (NCS) and electromyography (EMG). Electrodiagnostic testing is the core diagnostic modality for patients with a suspected myopathy.

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