Approach to Nerve Conduction Studies, Electromyography, and Neuromuscular Ultrasound

Electrodiagnostic (EDX) studies play a key role in the evaluation of patients with neuromuscular disorders. In selected cases, the additional use of neuromuscular ultrasound (U/S) can add key complementary anatomical and diagnostic information.

EDX studies include nerve conduction studies (NCSs), repetitive nerve stimulation, late responses, blink reflexes, and needle electromyography (EMG), in addition to a variety of other specialized examinations. NCSs and needle EMG form the core of the EDX study . They are performed first and usually yield the greatest diagnostic information. NCSs and needle EMG are complementary and, therefore, are always performed together and in the same setting. Performed and interpreted correctly, EDX studies yield critical information about the underlying neuromuscular disorder and allow other laboratory and diagnostic tests, including neuromuscular U/S, to be used in an appropriate and efficient manner. Sometimes, the information gained from EDX studies leads to specific medical or surgical therapy. For example, a patient with a peripheral neuropathy clinically, who is subsequently found to have an acquired demyelinating neuropathy with conduction blocks on EDX studies, most often has a potentially treatable condition.

Indeed, the value of EDX studies has been validated in several large case studies. In the most recent study by Lindstrom and colleagues, they analyzed data from 1414 consecutive patients and found that EDX studies either changed or confirmed the diagnosis in 52% and 47% of cases, respectively. Even more impressive was that EDX studies resulted in a change in patient management in 63% of patients.

In practice, EDX studies serve as an extension of the clinical examination and should always be considered as such. Accordingly, a directed neurologic examination should always be performed before EDX studies to identify key clinical abnormalities and establish a differential diagnosis. With numerous nerves and literally hundreds of muscles available, it is neither desirable for the patient nor practical for the electromyographer to study them all. In each case, the study must be individualized, based on the neurologic examination and differential diagnosis, and modified in real time as the study progresses and further information is gained.

NCSs and EMG are most often used to diagnose disorders of the peripheral nervous system ( Fig. 1.1 , Box 1.1 ). These include disorders affecting the primary motor neurons (anterior horn cells), primary sensory neurons (dorsal root ganglia), nerve roots, brachial and lumbosacral plexuses, peripheral nerves, neuromuscular junctions (NMJs), and muscles. In addition, these studies may provide useful diagnostic information when the disorder arises in the central nervous system (CNS) (e.g., tremor or upper motor neuron weakness). Occasionally, information from the EDX study is so specific that it suggests a precise etiology. In most cases, however, the exact etiology cannot be defined based on EDX studies alone. It is in some of these cases that neuromuscular U/S is most useful. Neuromuscular U/S, the use of U/S to study the peripheral nerves and muscles, is often able to suggest the precise etiology of the disease, such as chronic inflammatory demyelinating polyneuropathy (CIDP) when a particular pattern of nerve enlargement is seen or inclusion body myositis when a particular pattern of muscle involvement is present. EDX studies provide an initial aid in defining the disorder, giving several key pieces of information, which in some cases can be further refined with neuromuscular U/S.

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Elements of the peripheral nervous system.

Note that the primary motor neuron resides within the spinal cord, whereas the primary sensory neuron, the dorsal root ganglion, lies outside the spinal cord. The dorsal root ganglion is a bipolar cell. Its proximal process forms the sensory nerve root; the distal process becomes the peripheral sensory nerve.