Electrodiagnostic studies are an important component of the evaluation of patients with suspected peripheral nerve disorders. The evaluation of the patient with suspected peripheral neuropathy involves consideration of multiple sources of information, including the clinical history, neurologic examination, electrodiagnostic (EDX) studies, and laboratory studies. Key questions such as the pattern of weakness and temporal history of sensory loss should be elicited during the history and physical examination (Table 73–1).^1,3

As an integral part of this comprehensive evaluation, electrodiagnostic studies can confirm and localize peripheral nerve disease, assess the type of nerve fiber involvement (motor and large sensory fibers), determine the distribution of nerve involvement, identify the underlying pathophysiologic process (axon loss, demyelination, or mixed features), determine the severity, and help monitor recovery or treatment effect (Fig. 73–1).

The initial EDX studies in patients with suspected neuropathy should always include motor and sensory nerve conduction studies (NCSs) and needle electromyography (EMG) studies. The initial selection of nerves and muscles to study is guided by the clinical presentation of the individual patient. The NCSs are selected in the region of the patient’s predominant clinical symptoms and pattern of involvement (Table 73–2).²

A leg is usually selected as the site to begin the evaluation because most patients with peripheral neuropathy have symptoms and neurologic abnormalities greatest in the lower extremities. If the leg studies are abnormal, then NCSs are performed in the arm to determine the extent of nerve involvement. If the patient has upper extremity–predominant symptoms, then NCSs are started in the arm. If a patient’s clinical presentation is symmetrical, it is reasonable to perform NCSs on one side only. If the presentation is asymmetrical or a mononeuritis multiplex pattern, comparing studies of involved nerves with uninvolved nerves in the opposite limb helps to demonstrate the multifocal nature of the nerve disease.

NCSs are performed before the needle examination and serve to give an overview of the fiber types involved and the severity of involvement. Next, the needle examination is performed. This should include muscles in the distal and proximal limbs based on the pattern of clinical involvement. For patients with a distal symmetrical pattern of neuropathy, muscles in the distal leg are typically studied first. If the distal leg muscles show normal results, then a distal foot muscle (such as the abductor hallucis or first dorsal interosseous pedis) is studied to evaluate for abnormalities that may be confined to the most distal nerve distribution. If the distal leg muscles show abnormalities, the needle examination is carried up to proximal leg muscles to determine the extent of proximal involvement. In some cases, the lumbosacral and thoracic paraspinal muscles are studied to determine if there is evidence of involvement of the nerves at the root level.

The approach to localization of abnormalities on EDX studies involves combined interpretation of the results of NCSs and the EMG examination. The combination of findings on sensory NCSs, motor NCSs, and needle EMGs can help to confirm a peripheral neuropathy and assess for other potential sites of localization. Commonly performed NCSs and muscles examined on needle EMG for assessment of peripheral neuropathy are listed in Table 73–3.

Sensory nerve studies are particularly helpful for localization to the peripheral nerve because an abnormal sensory nerve action potential (SNAP) indicates disease that is localized at or distal to the dorsal root ganglion. Reduced amplitude of SNAPs in multiple upper and lower extremity nerves is a reliable indicator of peripheral nerve disease. Sensory nerve conduction studies are also abnormal when disease localizes to the brachial or lumbosacral plexus regions but are typically spared when nerve disease is localized to the nerve root level. If a patient with distal lower extremity sensory symptoms has normal sensory nerve conduction studies, the possibility of multiple lumbosacral radiculopathies or a polyradiculopathy should be considered (Table 73–4).

Reduced amplitude of the compound muscle action potential (CMAP) is also seen in peripheral nerve disease but is much less specific for peripheral nerve disease than reduced-amplitude SNAPs. The CMAP may be reduced with other conditions, including anterior horn cell disease, radiculopathy, plexopathy, neuromuscular transmission disorders, and myopathy. However, motor nerve conduction studies may show a number of other abnormalities besides reduced-amplitude CMAPs, which help to localize the disease process to the peripheral nerve. These features are discussed in further detail below (Table 73–5 lists normal values.)