Motor Nerve Conduction Studies

The basis of motor nerve conduction studies is that a nerve is electrically stimulated while multiple parameters are concurrently measured to determine how well the nerve transmits an impulse. One electrode is placed over a muscle and a second electrode is placed over the tendon insertion of that muscle. The examiner stimulates the nerve at a certain distance from the muscle, and the evoked response is recorded. Latency is defined as the time from stimulation of the nerve until the evoked response, which represents depolarization of the muscle, occurs. The examiner will then stimulate the nerve at a more proximal site, and the latency at this site is also measured. The velocity of conduction between these two points is calculated by dividing the distance between them by the difference between the latency of the distal stimulation site and the latency of the proximal stimulation site. However, the conduction velocity from the distal stimulation site to the muscle cannot be determined because of intrinsic delays at the neuromuscular junction [1].

The F wave is also measured in motor nerve conduction studies. When a nerve is stimulated, the axon depolarizes not only distally but also proximally. The distal depolarization causes the evoked response discussed above, while the proximal depolarization travels to the spinal cord via antidromic conduction of alpha motor neurons. Anterior horn cells are then activated, and the impulse travels orthodromically to depolarize the muscle, which is the F response. The F response can be variable and is a test of motor function only. They are helpful in determining if there is a disturbance in the function in the proximal nerve. For example, if distal nerve conduction studies are normal but the F wave latency is prolonged, there may be a lesion of the proximal nerve [2].

Sensory Nerve Conduction Studies

An active electrode and a reference electrode are placed over the nerve being studied, and the nerve is stimulated either proximally or distally to the electrodes. If stimulated proximally, antidromic conduction occurs. If the nerve is stimulated distally, orthodromic conduction occurs. The time from the onset of the stimulus to the onset of the action potential is measured and is divided by the distance between the two electrodes. This value represents the sensory conduction velocity [3].

When the nerve conduction velocity is decreased, it is usually due to a disorder of the myelin, but this may also be seen in disorders affecting larger axons. Dysfunction of the axons more commonly causes a decrease in the amplitude of evoked motor and sensory responses because fewer fibers are able to conduct the response [3].

Reflex Studies

In reflex studies, a sensory nerve is stimulated and a motor response is recorded and measured. The H reflex is described as the electrical counterpart to the muscle stretch reflex of the ankle jerk. The posterior tibial nerve is submaximally stimulated at the popliteal fossa to selectively depolarize the large Ia afferent fibers. The impulse is carried to the dorsal horn of the spinal cord, where it then depolarizes alpha motor neurons in the anterior horn. The impulse travels back to the soleus muscle and causes it to contract. Consequently, the H reflex helps to determine the integrity of both the S1 motor and sensory nerves. While it is most common to evaluate S1 radiculopathies, H reflex testing has also been used to assess C6–C7 lesions by stimulating the median nerve and measuring the response at the flexor carpi radialis muscle. L3–L4 lesions can also be assessed by stimulating the femoral nerve and recording from the vastus medialis muscle [3]. The amplitude of the H reflex is increased after spinal cord injuries. H reflexes are still present during spinal shock even though deep tendon reflexes are lost.

Electromyography

Electromyography evaluates and records electrical changes within a skeletal muscle. In the resting state, muscle fibers have a transmembrane potential of 70–90 mV. The inside of the cell has a negative charge relative to the extracellular space. When a nerve impulse reaches the neuromuscular junction, acetylcholine is released and initiates an action potential that spreads across the muscle fiber and causes contraction. A monopolar needle electrode is commonly used to perform electromyography. It is a wire electrode coated with insulating Teflon, sparing the tip because that is where the recording occurs. The small-diameter needle is inserted into the muscle, and there is a surface electrode on the patient’s skin. The needle is attached to an oscilloscope with an amplifier, so electrical activity of the muscle can be observed as waveforms. The examiner can also hear the characteristic sounds of various potential changes encountered.

The examiner will evaluate spontaneous activity of the muscle by instructing the patient to relax the limb as much as possible. Insertional activity is the response that occurs when the needle is inserted into the muscle. A burst of spike potential occurs up to 100–300 ms after the conclusion of needle motion. A prolonged time of insertional activity can be a sign of neuromuscular disease, while a decreased insertional activity time may indicate loss of muscle tissue. There is normally no electrical activity in resting muscle. However, there can be normal spontaneous activity if the needle electrode is near a motor end plate. Abnormal spontaneous activity includes fibrillation potentials and positive sharp waves, which are seen if the muscle fiber membrane is electrically unstable. Fibrillations occur when the muscle fiber loses continuity with its motor nerve, thus allowing for spontaneous depolarization. They are associated with lower motor neuron diseases such as radiculopathies, neuropathies, and anterior horn cell pathology, as well as myopathies, hypokalemia, and hyperkalemia. Fibrillations have been reported in cases of spinal cord injury as well, though this is controversial [3]. Positive sharp waves are associated with the same disorders as fibrillations.

Somatosensory Evoked Potentials

Somatosensory evoked potentials (SSEPs) are a technique to evaluate the function of the ascending spinal tract. They are obtained by stimulating a peripheral nerve, usually the median or ulnar at the wrist or the tibial or peroneal at the ankle, and recording the response from the patient’s scalp. They are often used intraoperatively to monitor spinal cord function. SSEPs can evaluate the function of the distal and proximal peripheral nervous system, as well as the spinal cord and brain, since the potentials are carried by sensory nerves peripherally and by the dorsal column-lemniscal system centrally. They have been reported to be of some use in determining ambulation outcomes [4]. Iseli et al. discovered that patients with ischemic spinal cord injury had similar motor and sensory deficits as patients with traumatic spinal cord injury. Both groups also had pathological SSEP recordings [5]. It is important to remember that inhalational anesthetic agents decrease response to SSEP. When general anesthesia is needed for testing, total intravenous anesthesia(TIVA) is preferred to limit confounding factors and increase accuracy. A combination of propofol and remifentanil has been shown to cause less decrease in cortical SSEP than desflurane and remifentanil [6]. SSEPs can also be used intraoperatively for neurovascular cases to predict postprocedural neurological deficits and guide interventions during procedures [7, 8]. When SSEP, motor evoked potentials, and nerve conduction studies are added to International Standards for Neurological Classification of Spinal Cord Injury, they provide invaluable information that helps clinicians guide treatment and predict long-term prognosis.

This chapter reviews the most common and relevant modalities available to ascertain potential spinal cord pathology by electrophysiological assessment. When coupled with radiographic findings, electrophysiologic testing increases accuracy and allows clinicians to formulate better treatment plans for patients. Electrophysiologic testing also has the ability to detect neuronal pathology even when gold-standard radiographic studies like MRI do not show obvious lesions. As technology continues to evolve at a rapid pace, novel techniques will continue to improve medical decision-making and enhance patient care.