Nerve conduction studies (NCS) and needle electromyography (EMG) are important tools in the diagnostic evaluation of older adults with suspected neurologic disorders. The practitioner must be aware of the spectrum of neurologic disorders known to affect the older adult, as well as various technical issues that arise when performing and interpreting NCS and needle EMG in this patient population. Being aware of these issues will lessen the likelihood of false-positive and false-negative NCS and EMG tests.

NCS and EMG should be considered an extension of the neurologic examination. Primary reasons for referral to the EMG laboratory include pain, sensory loss, weakness, fatigue, and bulbar symptoms (such as diplopia, dysarthria, or dysphagia). These symptoms can be caused by neurologic and nonneurologic disorders. In the elder individual, particularly the medically complicated patient, EMG plays a critical role in establishing a diagnosis.

The roles of NCS and needle EMG are to confirm or refute neurologic disease, localize disease within the peripheral nervous system, characterize the nature of the disease, define disease severity, provide prognostic information, and assess response to treatment (Table 3-6). It is helpful to consider these roles when considering patient referral to the EMG laboratory and when performing electrodiagnostic testing.

NCSs involve the electrical stimulation of motor and sensory nerves in the limbs and face. When performing motor nerve conduction studies, a small recording electrode is placed over the endplate region of a muscle. Electrical stimulation is then performed at consistent sites proximally and distally, specific for the nerve being studied. Electrical stimulation is increased until a maximal electrical potential is generated—the motor compound muscle action potential (CMAP). The amplitude and configuration of the CMAP is noted at both proximal and distal sites of stimulation. The CMAP reflects the electrical summation of all muscle fibers stimulated by that nerve.

By calculating the quotient of distance and time between the proximal and distal CMAPs, the conduction velocity of that nerve can be calculated (recorded in meters per second). In addition, conduction time along the very distal part of the nerve is recorded by calculating the time from distal nerve stimulation to onset of CMAP generation. This is referred to as the distal latency (recorded in milliseconds). F-waves are electrical potentials that can be elicited by antidromic stimulation of motor nerves. Stimulation of F-waves involves propagation of an electrical impulse proximally to the anterior horn cells of that motor nerve, which in turn generate an electrical potential that travels orthodromically down the motor nerve to muscle endplate and is recorded as an F-wave potential. The time to generation of this F-wave potential is considered the F-wave latency. The primary utility of the F-wave latency is to provide an estimate of proximal conduction velocity (at the root or plexus level). The conduction velocity, distal latency, and F-wave latency provide an assessment of the speed of conduction along different segments of motor nerves.

Electrical potentials can also be elicited by stimulation of sensory nerves. These sensory nerve action potentials (SNAPs) are generated by either antidromic
or orthodromic stimulation of sensory nerves and are recorded with small surface or ring electrodes. SNAP amplitude, conduction velocity, and distal latency for individual sensory nerves are collected and analyzed based on the same principles used for motor NCS. The SNAP represents the electrical potential of large, myelinated fibers. Small, unmyelinated or thinly myelinated fibers cannot be identified. This is clinically important in cases of suspected small-fiber neuropathy because NCS can be normal in these patients.

Repetitive stimulation of motor nerves is performed in patients with suspected neuromuscular transmission disorders, such as myasthenia gravis or the Lambert-Eaton myasthenic syndrome. The objective of repetitive nerve stimulation (RNS) is to stress the integrity (safety factor) of the neuromuscular junction. Repetitive stimulation of a motor nerve at a rate of 2 Hz is performed before and after exercise. The elicited electrical potentials are recorded and assessed for an abnormal decrease (decrement) or increase (increment) in amplitude and area.

Needle EMG involves the insertion of a recording needle electrode into a skeletal muscle to assess the electrical activity of the muscle, both at rest and during voluntary activation. The electrical activity is displayed on a monitor and recorded over a loudspeaker to allow simultaneous visual and auditory assessment. During the needle examination, insertional activity and motor unit potentials (MUPs) are evaluated. When evaluating insertional activity, the patient is instructed to keep the muscle relaxed so that there is no volitional activation of MUPs. The needle is then passed through different areas of the muscle so that the resting electrical activity of that muscle can be assessed. Insertional activity is then graded as normal, increased, or decreased.

In normal persons, needle electrode insertion and movement elicits only a brief discharge that ends with or shortly after needle movement. However, a more prolonged discharge can occur when the needle is in the vicinity of the muscle endplate. This is referred to as endplate noise or endplate spikes; endplate noise and endplate spikes sound like a “seashell” and “fat on a frying pan,” respectively. Endplate spikes and endplate noise are found in normal persons and in individuals with neuromuscular disease. Decreased insertional activity typically reflects a longstanding or even old (neurogenic or myopathic) process that has caused muscle atrophy and fibrosis.

Increased insertional activity can be seen with disorders affecting nerve or muscle. These abnormal electrical discharges can be stimulated by needle movement and often persist long after the needle has stopped moving. The nature of the increased insertional activity is characterized based on the firing rate, firing pattern, and configuration of the electrical discharges (Table 3-7). The most commonly seen abnormal discharges in the EMG laboratory are fibrillation potentials and fasciculation potentials. Fibrillations are an electrical potential of a denervated muscle fiber and are best appreciated by their regular rhythmic firing pattern. Fasciculation potentials are electrical potentials generated by an entire motor unit and typically occur sporadically and are nonrhythmic. Fasciculation potentials can be considered “benign” when they are not associated with fibrillation potentials or MUP changes.

The second portion of the needle examination involves collection and analysis of MUPs. MUPs are generated with voluntary activation of the muscle being examined. A single MUP is an electrical potential generated by the discharge of one motor unit. MUP characteristics assessed include duration, height, complexity, firing rate, and stability. All of these features are analyzed and used in determining whether a peripheral nervous system (PNS) disorder is present and, ultimately, whether the underlying process is a disease of nerve or muscle.

A reduction in the size of the MUP is generally observed in muscle disorders but can also be seen in neuromuscular junction disorders and in neurogenic
disorders affecting the nerve terminal. Large MUPs are seen in neurogenic disorders affecting anterior horn cells, nerve roots, or peripheral nerves. MUP recruitment is assessed by determining the number of activated MUPs and the firing rate of the MUPs at a given level of activation. In normal individuals, as the strength of muscle contraction increases, MUP firing rate increases, and additional MUPs are recruited. Reduced recruitment is seen in neurogenic disorders and is noted as a decrease in the number of MUPs and an increase in the firing rate of the activated MUPs. In myopathic disorders, rapid recruitment is seen, whereby increased numbers of MUPs are activated at a given level of contraction.

Single-fiber EMG is a highly specialized technique used primarily in the investigation of suspected neuromuscular transmission disorders (predominantly myasthenia gravis). Some institutions use a highly specialized single-fiber needle, but increasingly, a concentric needle is being used for single-fiber EMG studies (12). The recording and filtering characteristics of the single-fiber study are such that individual potentials from single muscle fibers are collected. Variation in muscle fiber firing times relative to each other is recorded and calculated as “jitter” and “blocking.” Individuals with neuromuscular transmission disorders have increased jitter and may have blocking. Generally, single-fiber EMG is performed when RNS studies are normal. It is also important to understand that disorders other than neuromuscular junction disorders, such as neuropathy, radiculopathy, myopathy, or motor neuron disease, are associated with abnormal single-fiber EMG studies.

Recognition of potential technical issues is critical in the accurate performance and interpretation of NCS and EMG. Limb temperature is one such issue. Limb temperature is typically assessed using a surface thermometer. If the limb temperature is below the accepted range, the limb being studied is typically warmed with a heat pack, heat lamp, or warm water bath. Low limb temperatures can have a significant impact on NCS, resulting in distal latency prolongation and slowing of conduction velocities, and can result in falsely increased motor and sensory nerve amplitudes. Limb edema, which is not uncommon in the elderly patient, can result in falsely low (or even absent) motor and sensory responses. This is due to difficulties with appropriate electrode placement and in achieving supramaximal stimulation. Obesity can have similar effects on nerve conduction studies through similar mechanisms due to increased limb girth.