Electrodiagnostic Evaluation of Brachial Plexopathies

Anatomy and Physiology

The brachial plexus is a complex network of peripheral nerves composed of axons originating from spinal cord segments C5 through T1. Sensory neurons arise from dorsal root ganglia, while motor neurons originate from the ventral horn of the spinal cord. These fibers exit the spinal canal as mixed spinal nerves containing both sensory and motor components, then contribute to the brachial plexus.[1][3]

After leaving the spinal canal, the roots pass laterally between the anterior and middle scalene muscles. The 5 roots (C5–T1) combine to form 3 trunks: superior, middle, and inferior. These trunks travel laterally across the first rib and beneath the clavicle before entering the axillary region.[3] Each trunk divides into anterior and posterior divisions while passing beneath the clavicle. These divisions reorganize to form 3 cords that are named according to their relationship to the axillary artery: the lateral, medial, and posterior cords.[3] The cords give rise to the terminal peripheral nerves supplying the upper extremity, the axillary, musculocutaneous, median, radial, and ulnar nerves. Damage at any level of the plexus can lead to predictable patterns of motor weakness and sensory deficits corresponding to the muscles and cutaneous territories supplied by the affected structures.[1][3]

Multiple mechanisms can produce injury to the brachial plexus. Traction injuries are the most frequently reported mechanism and may occur in high-energy trauma, falls, or obstetric complications. Other causes include penetrating trauma, compression injuries, ischemia, inflammatory neuropathies, and tumor infiltration.[1][3] A careful motor examination of muscles supplied by individual plexus elements can help narrow the localization of the lesion before electrodiagnostic testing is performed.[4]

The supraclavicular region of the plexus is commonly affected because of its relatively exposed anatomical position. Injuries in this area often involve the upper roots or upper trunk and may occur when the head is forcibly displaced away from the shoulder, producing traction on the plexus. This mechanism is classically associated with Erb palsy.[3][5]

In contrast, injuries involving the lower portion of the plexus are less common and typically occur when the arm is subjected to excessive upward traction, for example, during a fall while grasping an elevated object. Damage to the lower roots may result in the clinical picture known as Klumpke palsy, characterized by intrinsic hand muscle weakness and sensory loss along the medial forearm and hand.[5] Penetrating trauma or compression may affect any portion of the plexus. Examples include injuries to terminal nerve branches within the arm, compression from crutch use in the infraclavicular region, or external compression from heavy shoulder straps in the supraclavicular area.[6] Clinical manifestations depend on the specific nerves involved. Therefore, a detailed understanding of the motor and sensory functions of each terminal branch of the brachial plexus is essential for localizing pathology during clinical assessment and electrodiagnostic evaluation.[7]

Axillary nerve: Derived from the posterior cord with contributions from C5 to C6 nerve roots. The axillary nerve supplies the deltoid and teres minor muscles and provides sensory innervation to the lateral shoulder through the superior lateral cutaneous nerve of the arm. Injury can lead to weakness in shoulder abduction and sensory loss over the lateral aspect of the shoulder.

Musculocutaneous nerve: Originates from the lateral cord (C5–C7) and innervates the coracobrachialis, biceps brachii, and brachialis muscles. The musculocutaneous nerve also provides sensation to the lateral forearm through the lateral antebrachial cutaneous nerve. Injury may cause weakness of elbow flexion and forearm supination.

Median nerve: Formed from contributions of both the lateral and medial cords (C5–T1). The median nerve innervates most forearm flexor muscles and several intrinsic hand muscles and supplies sensation to the palmar surface of the first 3 digits and half of the fourth digit. Proximal injury may result in impaired forearm pronation, weakness of wrist and digit flexion, and loss of thumb opposition.

Ulnar nerve: Arises from the medial cord (C8–T1) and supplies the flexor carpi ulnaris, the medial portion of the flexor digitorum profundus, and most intrinsic hand muscles. The ulnar nerve sensory distribution includes the medial one and a half digits. Injury may produce weakness of finger flexion and clawing of the hand.

Radial nerve: Derived from the posterior cord (C5–T1). The radial nerve innervates the triceps brachii, brachioradialis, and extensor muscles of the forearm and provides sensory innervation to portions of the posterior arm, forearm, and dorsal hand. Injury may lead to weakness of wrist and digit extension with associated sensory deficits.

Understanding these functional distributions allows clinicians to correlate patients' symptoms with electrodiagnostic findings to localize pathology within the brachial plexus.

Indications

Electrodiagnostic testing is commonly used to evaluate disorders of peripheral nerves, neuromuscular junctions, and muscle. In patients with suspected brachial plexus pathology, these studies help confirm the presence of nerve injury and assist in determining the anatomical location of the lesion.[1][7] Postoperative brachial plexopathy has also been reported following shoulder arthroplasty procedures, highlighting the importance of early recognition and electrodiagnostic evaluation.[8] Electrodiagnostic testing remains a cornerstone for evaluating plexus disorders because it helps localize lesions and assess the degree of axonal involvement.[9]

Electromyography and nerve conduction studies can also help characterize the type of nerve injury present. These tests help distinguish axonal injury from demyelinating processes and can identify whether the pathology primarily involves motor fibers, sensory fibers, or both.[10] Electrodiagnostic evaluation may additionally help determine whether the underlying process is focal, multifocal, or diffuse. That distinction can be useful when differentiating brachial plexopathies from other neuromuscular disorders, including radiculopathies, peripheral neuropathies, neuromuscular junction disorders, or primary myopathies.[1] Electrodiagnostic studies may also provide information regarding the severity and chronicity of nerve injury, which can assist clinicians in estimating prognosis and guiding treatment decisions.[1]

Small-fiber lesions, such as those involving A-delta and C fibers, often produce normal findings on routine electrodiagnostic studies because these fibers are not well evaluated by standard nerve conduction techniques.[10] In selected cases of suspected plexus pathology in which electrodiagnostic findings remain inconclusive, proximal cutaneous nerve biopsy may provide additional diagnostic information.[11] Advanced imaging techniques, including contrast-enhanced 3-dimensional MR neurography, may help identify structural abnormalities of the plexus when used in conjunction with electrodiagnostic studies.[12]

Contraindications

Electrodiagnostic studies have no absolute contraindications. These procedures are generally considered safe and have been performed in a wide range of patient populations, including pregnant patients. Most individuals tolerate the procedure well. However, some patients may experience increased sensitivity to electrical stimulation or needle insertion during testing.[13] Several relative contraindications should be considered when planning an electrodiagnostic evaluation. Active infection at the site of needle insertion represents a relative contraindication to needle electromyography. In these situations, insertion of the recording electrode through infected tissue should be avoided to reduce the risk of spreading infection.

Electrical stimulation used during nerve conduction studies is typically safe. However, caution is advised when testing near implanted cardiac devices or indwelling central lines, because electrical currents theoretically could be transmitted to the heart and provoke arrhythmias. For this reason, stimulation near such devices should generally be avoided.[14] Areas affected by significant edema or lymphedema are often avoided during needle insertion due to theoretical concerns about infection risk, although reported complications specific to lymphedema are rare. The most frequently reported complication of electromyography is localized discomfort or pain at the needle insertion site. Some patients may have difficulty tolerating the discomfort associated with needle electrode placement.[13] Patients receiving chronic anticoagulation therapy or those with coagulation disorders have an increased risk of bleeding following needle insertion. In these individuals, needle examination should be limited to superficial muscles to reduce the risk of hematoma formation.[15]

Equipment

Commercial electromyography and nerve conduction study systems are widely available and typically include several essential components for generating, recording, and analyzing electrical signals from peripheral nerves and muscles.

Computer: Used to process, store, and display recorded electrical signals obtained during electrodiagnostic testing.

Speakers: Provide audible feedback corresponding to levels of electrical activity detected during the study.

Printer: Produces graphical recordings and reports of electrodiagnostic data for interpretation and documentation.

Amplifier: Increases the magnitude of very small electrical signals so the signals can be accurately recorded and analyzed.

Stimulator: Delivers controlled electrical impulses to peripheral nerves during nerve conduction testing.

Surface electrodes: Placed on the skin to stimulate nerves or record electrical responses during nerve conduction studies.

Disposable needle electrodes: Inserted into muscle tissue to measure electrical activity during electromyography.

Technique or Treatment

Electrodiagnostic evaluation of suspected brachial plexopathies generally includes 3 primary components: nerve conduction studies (NCS), electromyography (EMG), and somatosensory evoked potentials. Nerve conduction studies are performed using surface electrodes placed on the skin. One electrode delivers an electrical stimulus to the nerve, while another records the response at a different location along the same nerve pathway. These studies are commonly divided into motor and sensory components. In motor nerve conduction testing, stimulation of a motor nerve produces a compound muscle action potential that is recorded from the corresponding muscle. Sensory nerve conduction testing involves stimulating sensory fibers and recording sensory nerve action potentials. Parameters measured during these studies include amplitude and latency of the evoked responses. Amplitude reflects the magnitude of the response, whereas latency represents the time interval between stimulation and detection of the response. Conduction velocity can then be calculated using the measured distance between electrodes and the response latency. Abnormal findings such as reduced amplitude, prolonged latency, or slowed conduction velocity may indicate nerve injury within the tested segment.[7][16]

Measurements obtained from the affected extremity are often compared with those from the contralateral extremity. By repositioning stimulating and recording electrodes along the nerve, clinicians can help localize the site of neurological injury. Electromyography is typically performed as the second component of electrodiagnostic evaluation. During electromyography, a small needle electrode is inserted into the muscle to record electrical activity generated by motor units. Electrical activity is assessed during needle insertion, at rest, and during voluntary muscle contraction. Interpretation of electromyographic findings requires clinical experience because the assessment involves both quantitative measurements and qualitative analysis of motor unit action potentials. Reduced motor unit recruitment often suggests a neuropathic process, whereas abnormal recruitment patterns may indicate myopathic or neuromuscular junction disorders.[7][16]

Abnormal spontaneous activity, such as fibrillation potentials, may be detected in muscles affected by axonal injury. These potentials typically appear several weeks after nerve damage, resulting from the denervation of muscle fibers. If voluntary effort produces no detectable motor unit activity, the finding may indicate severe injury or complete disruption of the nerve. The presence of voluntary activity suggests that some axons remain intact. During recovery, electromyography may demonstrate polyphasic motor units and increased motor unit amplitude, reflecting ongoing nerve regeneration and reinnervation.[7]

Somatosensory evoked potentials represent an additional electrodiagnostic technique used in selected cases. While nerve conduction studies and electromyography primarily evaluate postganglionic peripheral nerve injury, somatosensory evoked potentials assess the sensory pathway from peripheral nerves to the central nervous system. Electrical stimulation applied to distal nerves produces signals that are recorded at multiple levels along the neural pathway, including the spinal cord and cerebral cortex. This testing can help distinguish preganglionic from postganglionic injury and may assist in identifying central nervous system involvement when peripheral localization is uncertain.[7][16]

During electrodiagnostic evaluation, lesions of the brachial plexus are most commonly localized to the level of terminal branches, cords, the superior trunk, or the nerve roots. Lesions are less commonly identified at the level of the divisions or middle and inferior trunks because these regions do not give rise to easily testable terminal branches. Localization begins by identifying the affected nerve distribution based on clinical findings and electrodiagnostic abnormalities. Characteristic patterns of weakness and sensory loss correspond to specific components of the plexus.[17]

Examples of anatomical localization include:

C5 root: Dorsal scapular nerve involvement.C5 to C6 ± C7: Long thoracic nerve involvement.Upper trunk: Suprascapular nerve involvement.Lateral cord: Musculocutaneous, lateral pectoral, and lateral antebrachial cutaneous nerves.Medial cord: Ulnar, medial pectoral, medial brachial cutaneous, and medial antebrachial cutaneous nerves.Combined lateral and medial cords: Median nerve involvement.Posterior cord: Axillary, radial, upper subscapular, lower subscapular, and thoracodorsal nerves.

After clinical localization is suspected, nerve conduction studies are performed in the relevant nerve distributions. Abnormal sensory nerve action potential or compound muscle action potential findings, such as reduced amplitude or prolonged latency, indicate pathology between the stimulation and recording sites. By adjusting electrode placement along the nerve, the lesion's approximate location can be further defined. Electromyography is then performed in muscles innervated by the suspected nerve distribution. Findings such as fibrillation potentials or reduced motor unit recruitment support denervation in the corresponding muscle group. If electrodiagnostic testing fails to identify a peripheral lesion, a central lesion should be considered. In such cases, somatosensory evoked potentials or imaging studies, such as magnetic resonance imaging, may help determine the anatomical location of the pathology.[18]

Complications

The most frequently reported adverse effect associated with electrodiagnostic testing is mild discomfort at the site of needle electrode insertion. Some patients may experience temporary soreness, tenderness, or minor bruising after the procedure.[13] Infection at the site of needle insertion has been reported, but it occurs very rarely when appropriate sterile technique is used during electromyography. Bleeding complications such as hematoma formation are uncommon but may occur, particularly in individuals receiving anticoagulant or antiplatelet therapy. In such patients, coagulation status should be considered when planning needle examination.[15][19] Electrodiagnostic studies have not been shown to cause complications during pregnancy, and the procedure is generally considered safe when clinically indicated.

Direct nerve injury resulting from needle electromyography is extremely uncommon and has rarely been reported in the literature. Pneumothorax is the most serious potential complication of electromyography of muscles near the thoracic cavity, particularly when evaluating truncal or paraspinal muscles. Although this complication is rare, clinicians should be aware of the possibility when performing needle examination in these regions.[20] Electrical activity generated by implanted devices, such as deep-brain or vagal nerve stimulators, may interfere with electrodiagnostic recordings. Coordination with the patient’s treating clinicians may be necessary to temporarily deactivate these devices before testing when accurate recordings are required.[1]

Clinical Significance

Electrodiagnostic studies have an important clinical role in the evaluation of neuromuscular disorders. These tests assist clinicians in confirming suspected neurologic injury, determining the location of pathology within the peripheral nervous system, and estimating the severity of nerve involvement.[1][7] Radiation-induced plexopathy represents an important delayed complication of cancer therapy and may mimic tumor recurrence or other causes of brachial plexus dysfunction.[21] Electrodiagnostic findings are frequently used to guide surgical planning for nerve transfers in patients with focal neuropathies and plexus injuries.[22]

Results from recent reviews support the use of clinical assessment, electrodiagnostic testing, and imaging when evaluating suspected neuralgic amyotrophy and other plexus disorders.[23] In many cases, disorders affecting peripheral nerves or the brachial plexus present with overlapping or nonspecific symptoms such as weakness, sensory disturbances, or pain. Electrodiagnostic testing can help differentiate among conditions that may produce similar clinical findings, including radiculopathies, plexopathies, mononeuropathies, and generalized neuropathic disorders.[1][7]

In brachial plexopathies specifically, nerve conduction studies and electromyography help identify the level of injury and characterize whether the lesion primarily affects motor fibers, sensory fibers, or both. These findings provide valuable information when determining whether the pathology involves the nerve roots, trunks, cords, or terminal branches of the plexus.[7] Electrodiagnostic studies may also be performed serially to monitor nerve recovery following injury. Changes in motor unit recruitment, the appearance of polyphasic motor units, and improvements in conduction parameters may indicate reinnervation or recovery of nerve function over time. These findings can help guide clinical decision-making regarding conservative management, rehabilitation, or surgical intervention.[1][16] Electrodiagnostic findings are often interpreted in conjunction with imaging studies such as magnetic resonance imaging or ultrasonography, which provide structural information about the brachial plexus. Combining electrodiagnostic and imaging findings can improve diagnostic accuracy and assist clinicians in planning treatment strategies for patients with plexus injury.[2][18]

Enhancing Healthcare Team Outcomes

Electrodiagnostic evaluation contributes to patient-centered care by providing objective information that helps clinicians diagnose and treat neuromuscular disorders. Patients with suspected brachial plexus injuries are often initially evaluated by primary care clinicians, emergency clinicians, neurologists, orthopedic surgeons, neurosurgeons, or pain medicine specialists. These clinicians may refer patients for electrodiagnostic testing to better define the location and severity of neural injury.[1][7]

Electrodiagnostic studies are typically performed by clinicians trained in neurology, physical medicine and rehabilitation, or related specialties with expertise in neuromuscular disorders. During testing, collaboration with nursing staff or trained technicians can help ensure patient safety, appropriate monitoring, and patient comfort throughout the procedure. Communication among members of the health care team is essential for accurate interpretation of electrodiagnostic findings. Clinical history, physical examination findings, imaging studies, and electrodiagnostic results must be integrated to establish the correct diagnosis and guide treatment decisions. Interdisciplinary collaboration improves the likelihood of identifying the precise site of injury and determining the most appropriate treatment strategy for each patient.[2][18]

Electrodiagnostic testing can also assist in determining prognosis and monitoring recovery over time. Repeated studies may demonstrate evidence of nerve regeneration or reinnervation, which can help guide decisions regarding rehabilitation, surgical intervention, or continued conservative management.[16] Although high-level randomized controlled trials evaluating electrodiagnostic testing specifically for brachial plexopathies remain limited, the available clinical evidence supports its usefulness as a diagnostic adjunct in the evaluation of neuromuscular disorders.[24] Through coordinated care involving referring clinicians, electrodiagnosticians, rehabilitation specialists, and other health care professionals, electrodiagnostic studies can contribute to improved diagnostic accuracy, more targeted treatment planning, and better patient outcomes.