Electromyography and Nerve Conduction Studies

Electrodiagnostic testing encompasses a range of specialized tests, including nerve conduction studies (NCS) and needle electromyography (EMG), that are used to evaluate the conduction of electrical impulses along peripheral nerves. These tests should be considered and performed only after a careful history and physical examination, which are sometimes sufficient to establish a diagnosis of neuromuscular dysfunction without further testing. However, in some cases, the subtlety of sensory or motor deficits necessitates further workup for a conclusive diagnosis. ^[1]

Nerve conduction studies and needle EMG are commonly performed by physical medicine and rehabilitation or neurology specialists to assess the ability of the nervous system to conduct electrical impulses and to evaluate nerve/muscle function to determine if neuromuscular disease is present.

Nerve conduction studies

For this test, a series of surface electrodes are placed at different locations along specific peripheral nerves. The nerve is stimulated at one site and recorded at a different site to determine if the nerve is conducting electrical impulses appropriately.

Each electrical stimulation is recorded as a waveform on a computer and analyzed by the electromyographer performing the test. ^[2]

Standard nerve conduction studies typically include motor nerve conduction, sensory nerve conduction, F waves, and H reflexes.

Sensory and motor nerve conduction studies involve analysis of specific parameters, including latency, conduction velocity, and amplitude. Onset latency is the time it takes for the stimulus to initiate an evoked potential and reflects the conduction along the fastest fibers. Peak latency is the latency along the majority of axons and is measured at the peak amplitude. ^[3] Both are affected by the state of the myelination of the nerve.

The conduction velocity along the nerve also depends on the state of myelination and is often decreased in disorders or trauma that affects nerve myelination, although it may be normal if a few myelinated axons remain intact. Reduction of amplitude of recorded responses generally indicates a loss of axons. ^[4]

These studies, in conjunction with the physical examination and correlation to a set of normative values, assist the electromyographer in diagnosing a multitude of nerve disorders, including entrapment neuropathies, brachial plexopathies, and polyneuropathies.

It is often important to distinguish between sensory and motor nerves, as certain disease processes can affect one or both. Radiculopathy produces motor deficits but does not affect sensory nerves since the anatomic location of the damage is proximal to the dorsal root ganglion. When a lesion is distal to the dorsal root ganglion, such as in brachial plexopathies, both motor and sensory nerves are affected.

Sensory nerve conduction studies

Sensory nerve conduction studies are performed via stimulation of a nerve (ie, sufficient to produce an action potential) at one point and measurement of the action potential at another point along the course of the nerve. ^[5] Peripheral sensory nerves can be used to localize a lesion in relation to the dorsal root ganglion that contains the cell body of the nerve, allowing differentiation of preganglionic disorders (eg, radiculopathies, cauda equina lesions, posterior column disease) from postganglionic disorders (eg, neuropathies, plexopathies). With a preganglionic lesion, the sensory nerve action potential is normal (although clinically abnormal) because axonal transport from the cell body to the peripheral axon remains intact. ^[6]

Motor nerve conduction studies

In motor nerve conduction studies, motor nerves are stimulated and the compound muscle action potential from the muscle is recorded. This corresponds to the integrity of the motor unit. Results of this study can be affected by any process that damages the anterior horn cell body or axon, Schwann cells, the neuromuscular junction, or the muscle cell itself. ^[7] Also analyzed are the size, shape, and morphology of the compound muscle action potential to determine the state of myelination, the number of functioning muscle fibers, and the function of the neuromuscular junction. Since the cell body of motor nerves is located in the anterior horn of the spinal cord, the motor nerve conduction is abnormal in both preganglionic and postganglionic injuries.

Late responses

Distal nerve segments are relatively easy to analyze since they can be studied directly. To study proximal nerve segments, late responses based on conduction along the proximal nerve are used. Late responses include F waves and H reflexes.

F waves

The F wave is a late response involving the motor axons that can be elicited in most upper and lower extremity muscles. A stimulus is applied to a distal motor nerve that travels antidromically from the peripheral nerve, to the anterior horn cell, and a response fires back down the motor neuron and is recorded as a muscle response that occurs after the compound muscle action potential. F waves tend to have lower sensitivity for radiculopathy but can be useful in the assessment of polyneuropathy.

H reflexes

The H reflex is basically an electrophysiologically recorded Achilles muscle stretch reflex. It is performed by stimulating the tibial nerve in the popliteal fossa. From there, the stimulus goes proximally through the reflex arc at that spinal segment, then distally from the anterior horn cell and the motor nerve. It can be recorded over the soleus or gastrocnemius muscles. The H reflex is most commonly used to evaluate for an S1 radiculopathy or to distinguish from an L5 radiculopathy.

Needle EMG is used to assess both nerve and muscle function. A small-diameter monopolar pin or coaxial needle is placed into a muscle to evaluate insertional activity, resting activity, voluntary recruitment, morphology, and size of motor units, as well as motor unit recruitment. The needle electrode examination provides valuable information about the electrical characteristics of individual muscle fibers and motor units, as well as the integrity and innervation of muscle fibers. This test can be uncomfortable for the patient.

Insertional activity

Insertional activity is the electrical activity present as the electrode is passed through muscle cells. These are discharge potentials provoked by the disruption of the cell membrane itself. Careful attention is given to the duration and amount of electrical noise after each movement of the needle. This activity is decreased in atrophied muscle or fatty tissue. Conversely, it is also increased in many abnormal conditions that cause membrane instability, such as neuropathies, radiculopathies, and inflammatory myopathies.

Spontaneous activity at rest

Resting or spontaneous activity is the electrical activity present when the muscle is at rest and the electrode is not being moved. This includes both normal and abnormal spontaneous activity.

Normal muscle should be silent after the needle is inserted; however, if the needle happens to be near the neuromuscular junction, miniature endplate potentials or endplate potentials may be heard or seen. The most common abnormal spontaneous activity is reported as a gradation of either positive sharp waves (PSWs) or fibrillation potentials on a scale of 1+ (transient but reproducible discharges) to 4+ (abundant spontaneous potentials).

Fibrillations result from motor axonal loss that is not balanced by reinnervation. Conditions that cause this include any nerve disorder that affects the motor axon, inflammatory myopathies, and direct muscle injury. Depending on the amplitude of the PSWs and/or fibrillation potentials, the electromyographer can determine how recently the injury to the nerve occurred. Low-amplitude fibrillation potentials suggest that denervation occurred in the remote past, whereas high-amplitude fibrillation potentials suggest an ongoing active denervation process.

Voluntary muscle recruitment

Recording of voluntary recruitment of motor unit action potentials can provide additional information. Reduced recruitment signifies motor axonal loss or functional dropout due to focal demyelination or conduction block. By contrast, increased recruitment with a small voluntary force can be seen with myopathy.

Interpretation

The information gathered from needle EMG is combined with that provided by nerve conduction studies to determine the overall interpretation. The results of the analysis of the collective studies often permits delineation of the type of underlying pathologic process, such as a polyneuropathy, mononeuropathy or entrapment neuropathy, radiculopathy, plexopathy, disordered neuromuscular transmission, or myopathic process. In many cases, one diagnosis explains the abnormalities found on the study, but, occasionally, more than one diagnosis is necessary to complete the interpretation of the electrodiagnostic findings.