The evaluation of neurologic or neuromuscular disorders through the application of electrophysiologic techniques, such as electromyography and nerve conduction studies, plays a pivotal role in the diagnosis and management of a wide array of pathologies. As an adjunct to a history and physical examination, electrodiagnostic studies aid in the physician’s diagnostic approach. In this chapter, you will find an introduction to electrodiagnostic studies in the upper extremity, including analysis, interpretation, and common findings to aid in clinical practice.
5 Electrodiagnostic Studies
A. Anatomy of a Peripheral Nerve
See ► Fig. 5.1.
B. Common Nerves Studied/Associated Lesions
Suprascapular nerve (C5-C6)
Compression can occur at suprascapular notch.
Weakness of supraspinatus and infraspinatus.
Upper and middle trunk
Long thoracic nerve (C5-C7)
Weakness/loss of function of serratus anterior (loss of protraction and stabilization of scapula; causes scapular winging).
Musculocutaneous nerve (C5-C6)
Compression/entrapment by coracobrachialis (hypertrophy).
Weakness of coracobrachialis, biceps, and brachialis (flexion at elbow/glenohumeral joint, supination at radioulnar joint).
Ulnar nerve (C8-T1)
Compression most commonly posterior to medial epicondyle of the elbow (cubital tunnel) or at Guyon’s canal in the wrist.
Weakness, sensory loss, pain, and paresthesia in intrinsic muscles of hand and forearm flexors, and in the small and sometimes ring fingers (sensory).
Lateral and medial cord
Median nerve (C5-T1)
Compression/entrapment at elbow (between heads of pronator teres) or more commonly the wrist (carpal tunnel).
Weakness, sensory loss, pain, and paresthesia in nerve distribution (anterior compartment of forearm, thenar muscle group, lumbricals, skin).
Weakness/loss of function of teres minor and deltoid muscle (loss of abduction by 15 to 90 degrees, flexion and extension of shoulder).
Radial nerve (C5-C6)
Compression or trauma (at risk with humeral fractures).
Weakness/loss of function/sensation in posterior compartment ofarm and forearm (extension of elbow and wrist and fingers), sensory loss in dorsal hand.
Conduction velocity (CV)
The speed (meters/second) of an electrical impulse observed between two points.
The amount of time (milliseconds) between stimulation and initiation of compound motor action potential.
Distal peak latency
The amount of time (milliseconds) between stimulation and peak waveform is observed in a sensory nerve action potential.
The difference in voltage observed between two points, proportional to the number and size of nerves depolarized.
The time from onset of stimulus to termination.
Compound muscle action potential (CMAP)
The muscle motor unit response to peripheral nerve stimulation, observed as a waveform.
Sensory nerve action potential (SNAP)
The response of a sensory nerve following stimulation, observed as a waveform.
III. Physiology of Action Potential
The action potential of a nerve is dependent on the integrity of its components and electrical stimulation. The resting membrane potential of a muscle is affected by the concentration of sodium and potassium. Depolarization across the axonal membrane causes a stimulus at the neuromuscular junction, which in turn depolarizes skeletal muscle sarcolemma.
IV. Types of Studies
The study of nerve and muscle integrity by observing electrical activity.
Surface or intramuscular (needle) electrodes are used to stimulate and observe electrical activity that is produced by skeletal muscles. The placement ofelectrodes is dependent on the etiology.
Resting and insertional activity of the muscle is observed for comparison with voluntary muscle contraction.
Frequency, shape, and size of electrical impulses are observed. Various locations within muscle are studied to determine overall function.
Maximal voluntary contraction is used to correlate muscle function with electrical stimulation.
Motor unit action potential (MUAP) is the sum of electrical activity produced by stimulation. This is influenced by the number of motor units recruited. Factors affecting MUAP include fiber number, type, integrity, and metabolism.
B. Nerve Conduction Study
It evaluates the ability for motor and sensory nerves to conduct electrical impulses to help determine the etiology and/or mechanism of symptoms. Determinants include the analysis of factors such as speed (conduction velocity), size (amplitude), and action potential shape. The stimulation and response recording by variously placed electrodes allow the clinician to analyze the integrity of motor and sensory components of the peripheral nervous system.
Motor (► Fig. 5.2).
A peripheral nerve undergoes excitation by a stimulator and the muscle response is recorded by a separate electrode. In the upper extremity, the radial, ulnar, and median nerves are most commonly studied and evaluated.
The amount of time (milliseconds) between stimulation and initiation of compound motor action potential is designated as latency. An increase in latency is seen in loss of integrity or function of peripheral nerves by compression, demyeli-nation, or other etiology.
The amplitude (mV) of a response, or the maximum size response of a generated action potential, can be compared between one or more muscle sites.
A decrease in amplitude can be seen in loss of muscle fiber recruitment, or from structural defects in the nerve or muscle itself.
The conduction velocity is the speed of an electrical impulse observed between two points. The velocity is calculated by stimulating multiple points along the same nerve, taking into account the distance and time it takes for an electrical impulse to propagate.
A decrease in conduction velocity can be observed if the structural integrity of the nerve, fascicles, axon, or myelination is compromised.
Conduction velocity = Distance (cm)/Time (ms).
Sensory (► Fig. 5.3)
A peripheral nerve is stimulated with an electrical impulse and the observed response is recorded and analyzed. The observed response is the summation of all sensory action potentials, known as the SNAP.
Orthodromic—the nerve is stimulated distally and the action potential is recorded proximally (in the natural direction of the sensory response to the dorsal root ganglion and dorsal horn of the spinal cord).
Antidromic—the nerve is stimulated proximally and the action potential is recorded distally.
Parameters studied include latency (peak and onset), amplitude, duration of response, and conduction velocity. The number and size of activated sensory fibers affect the SNAP amplitude. The conduction velocity, duration, and latencies of stimulated sensory response are affected by internal and external components such as compression, transection, demyelination, and axonal injury.
Common nerves studied in the upper extremity include the median and ulnar nerves.
Mixed sensory and motor nerves are routinely evaluated, for example, in evaluating for carpal tunnel syndrome (CTS). The comparison of latencies and conduction velocities along different areas of the nerve provide information about the specific area of injury.
Orthodromic evaluation of sensory and motor true reflex arc
Afferent loop includes type 1a sensory fibers to dorsal root ganglion and dorsal horn.
Efferent loop includes response from ventral horn and motor axons to produce muscle response.
Utility in assessing proximal damage to sensory or motor pathways, including radiculopathies or avulsions (especially cervical injuries).
Evaluation of motor nerve to identify structural integrity issues of proximal nerves such as radiculopathies and demyelinating disorders.
Supramaximal electrical impulse provided distally which travels from the peripheral motor nerve to the ventral horn and back down motor nerve distally.
High sensitivity in identifying demyelinating neuropathies due to proximal conduction slowing.