1 - Approach to Nerve Conduction Studies and Electromyography

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, SECTION I • Overview of Nerve Conduction Studies and Electromyography