1 Approach to Nerve Conduction Studies and Electromyography

Electrodiagnostic (EDX) studies play a key role in the evaluation of patients with neuromuscular disorders. Among these studies are included 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 during the same setting. Performed and interpreted correctly, EDX studies yield critical information about the underlying neuromuscular disorder and allow use of other laboratory tests in an appropriate and efficient manner. Likewise, the information gained from EDX studies often 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.

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 in order 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 (Figure 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, and muscles. In addition, these studies may provide useful diagnostic information when the disorder arises in the central nervous system (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.

FIGURE 1–1 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.

Box 1–1

Disorders of the Peripheral Nervous System

Motor neuronopathy

Amyotrophic lateral sclerosis

Spinal muscular atrophy

Infectious (poliomyelitis, West Nile virus)

Mononeuritis multiplex

Neuromuscular junction disorders

Myasthenia gravis

Lambert–Eaton myasthenic syndrome

Localization of the Disorder is the Major Aim of the Electrodiagnostic Study

The principal goal of every EDX study is to localize the disorder. The differential diagnosis is often dramatically narrowed once the disorder has been localized. Broadly speaking, the first order of localization is whether the disorder is neuropathic, myopathic, a disorder of neuromuscular transmission, or a disorder of the central nervous system (CNS). For example, in patients with pure weakness, EDX studies can be used to localize whether the disorder is caused by dysfunction of the motor neurons/axons, neuromuscular junctions, muscles, or has a central etiology. The pattern of nerve conduction and especially EMG abnormalities usually can differentiate among these possibilities and guide subsequent laboratory investigations. For example, a patient with proximal muscle weakness may have spinal muscular atrophy (i.e., a motor neuron disorder), myasthenic syndrome (i.e., a neuromuscular junction disorder), or polymyositis (i.e., a muscle disorder), among other disorders, including those with central etiologies (e.g., a parasagittal frontal lesion). EDX studies can easily differentiate among these conditions, providing key information to guide subsequent evaluation and treatment, which differ markedly among these diseases.

Once the localization is determined to be neuropathic, myopathic, a disorder of the NMJ or of the CNS, EDX studies can usually add other important pieces of information to localize the problem further (Figure 1–2). For instance, the differential diagnosis of a patient with weakness of the hand and numbness of the fourth and fifth fingers includes lesions affecting the ulnar nerve, lower brachial plexus, or C8-T1 nerve roots. If EDX studies demonstrate an ulnar neuropathy at the elbow, the differential diagnosis is limited to a few conditions, and further diagnostic studies can be directed in a more intelligent manner. In this situation, for instance, there is no need to obtain a magnetic resonance imaging scan of the cervical spine to assess a possible cervical radiculopathy because the EDX studies demonstrated an ulnar neuropathy at the elbow as the source of the patient’s symptoms.