Electrodiagnosis Brachial Plexopathies

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3.1. Motor Nerve Conduction Studies

Motor nerve conduction studies (MNCS) are useful mostly for demonstrating significant axonal loss in motor fibers. However, routine MNCS will be abnormal in plexopathy only if the nerve under study is derived from the involved trunk or cord of the brachial plexus. Thus, routine MNCS of the median and ulnar nerves provide information relevant only to the lower trunk/medial cord and C8-T1 nerve roots. Unusual conduction studies may be performed for the evaluation of other plexus lesions; for example, information regarding the upper trunk/lateral cord and C5-6 roots may be obtained from MNCS of the musculocutaneous nerve, recording from the biceps brachii. These studies are technically more difficult than routine MNCS and the range of response amplitudes is large; a good rule of thumb when performing these or other technically difficult or unusual studies is to obtain results from the uninvolved contralateral limb and compare the responses for significant asymmetries in amplitude (>50% reduction considered abnormal).

MNCS are often normal in plexopathies but a reduced CMAP amplitude, often associated with mild conduction slowing (due to loss of fast conducting axons) may be observed when axonal loss is significant. Side-to-side comparisons of amplitude are sometimes useful in demonstrating more subtle abnormalities, with a 50% reduction compared to the uninvolved side considered abnormal. MNCS are important for identifying the presence of mononeuropathies that could complicate the diagnosis or clinically mimic brachial plexopathy, such as ulnar neuropathy at the elbow.

3.2. Sensory Nerve Conduction Studies

Sensory nerve conduction studies are essential in the assessment of plexopathies. In contrast to radiculopathies, in which sensory studies are invariably normal, sensory nerve action potentials (SNAP) amplitudes are abnormally reduced in plexopathies as the sensory fibers are injured distal to the dorsal root ganglia.

As with MNCS, amplitude is the most important parameter for demonstrating axonal loss and side to side comparison is important for demonstrating subtle abnormalities. Reduction in SNAP amplitude of 50% or more compared to the contralateral, uninvolved nerve is considered abnormal, even if the absolute value of the SNAP amplitude falls within the normal range. Most important is that the sensory nerve from which the recording is made should reflect the clinically involved dermatome. The choice of which sensory studies to perform in evaluating suspected brachial plexopathy depends on a directed physical examination to determine suspected anatomical level of involvement based on careful mapping of sensory loss, in conjunction with reflex changes and muscle weakness.

The upper extremity nerves most commonly evaluated are the median, ulnar, superficial radial, and lateral antebrachial cutaneous (LAC) nerves; medial antebrachial cutaneous (MAC) nerves are examined less frequently. Relevant sensory nerves for evaluating suspected upper trunk lesions include median (recorded from digits 2 and 1), superficial radial (recorded from snuffbox or digit 1), and LAC. For suspected upper trunk brachial plexo-pathies, LAC and median SNAPs recorded from digit1 are more sensitive for detecting abnormalities than routine median recordings from digit two or superficial radial recordings from the snuffbox. This is due to the contribution from middle trunk fibers to median nerves innervating digit 2 and superficial radial nerves innervating the snuffbox. Radial SNAPs can help differentiate upper trunk from lateral cord lesions, as in the latter the radial SNAP should be preserved.

For evaluation of suspected lower trunk lesions, the MAC and ulnar nerves are the relevant nerves to study. Both the MAC and the ulnar SNAP are derived from the lower trunk and medial cord. In general, the ulnar SNAP is at least as sensitive as the MAC recording in the identification of lower trunk brachial plexus lesions; however, MAC recordings may be helpful in cases where an ulnar neuropathy is present and a superimposed lower trunk or medial cord lesion must be ruled out. The MAC as well as the LAC are best performed on a contralateral asymptomatic limb to allow side to side comparison with the involved limb.

3.3. F-Responses

As in evaluation of radiculopathies, late responses are complementary to the routine conduction studies but do not increase diagnostic sensitivity in most cases as these lesions are predominantly axonal. In addition, the F-responses are non-localizing as F-response latency prolongation may occur from a lesion anywhere from cervical spine to the hand. Like MNCS, F responses are generally only recorded from the motor fibers of the median and ulnar nerves, allowing evaluation of only the lower trunk and C8-T1 nerve roots.

3.4. Needle Electromyography

Needle Electromyography (EMG) is an essential tool for electrodiagnosis of suspected brachial plexus lesions as there is no restriction to the anatomic level and extent of the plexus which can be examined by needle EMG, as opposed to nerve conduction studies. In addition to demonstrating the extent of a given lesion, it is useful in documenting the severity of the lesion, including the presence or absence of axonal continuity to a particular muscle (fundamental in determining prognosis). As in radiculopathy, EMG may demonstrate axonal loss in muscles that appear clinically normal. Conversely, EMG may be used to later demonstrate evidence of reinnervation before it can be identified by physical examination.

Generally, extensive EMG study is required to evaluate suspected brachial plexus lesions as muscles innervated by the relevant trunks, cords, and nerves should be examined. Additionally, the presence of paraspinal muscles abnormalities should be sought in order to rule out associated nerve root injury, particularly in cases of trauma where nerve root avulsion may be a consideration. The aim of the EMG is to demonstrate involvement of multiple muscles innervated by a common brachial plexus structure(s) but different nerves. Muscles outside of the area of suspected brachial plexus involvement that are innervated by these nerves should be examined, in addition to paraspinal muscles, with the expectation that these are uninvolved, thus ruling out radiculopathy or multiple mononeuropathies as the cause for a given clinical syndrome. Table 1 outlines an electrodiagnostic approach to suspected brachial plexus lesions.

Table 1

Electrophysiological Patterns of Brachial and Lumbosacral Plexopathies

Plexopathy SNAP CMAP F-response Needle exam Others

Upper trunk

LAC abnormal Median, radial D1 +/- abnormal

Median and ulnar normal

Normal

Abnormal: (C5 innervated) deltoid, biceps supraspinatus, infraspinatus Partially involved: triceps, brachioradialis pronator teres flexor carpi radialis

Rhomboids, serratus anterior and cervical paraspinal muscles normal

Lower trunk (neurogenic thoracic outlet syndrome)

Ulnar abnormal MAC abnormal

Ulnar and median +/- reduced (related to axon loss)

May be abnormal

Abnormal: (C8-T1 innervated median, ulnar, and radial) abductor pollicis brevis, first dorsal interosseous, extensor indicis propius

Pronator teres, triceps, biceps, and C8-T1 paraspinals normal

Middle trunk

Median abnormal Radial +/- abnormal

Normal

Normal

Abnormal: (C7 innervated) triceps pronator teres flexor carpi radialis

Lateral cord

LAC abnormal Median D1, D2 abnormal

Normal

Normal

Abnormal: biceps pronator teres flexor carpi radialis

Medial cord

Ulnar abnormal MAC abnormal

Median and ulnar +/- reduced

+/- abnormal (related to axon loss)

Abnormal C8-T1 muscles

(median, and ulnar) Abductor pollicis brevis, first dorsal interosseous

Radial innervated C8 muscles (extensor indicis proprius, digitorum communis) normal

(Continued)

(Continued)

Table 1 (Continued)

Plexopathy

SNAP

CMAP

F-response

Needle exam

Others

Posterior cord

Radial abnormal

Lumbosacral Plexopathy

Lumbar plexus (L2-L4)

Sacral plexus (L5-S3)

Saphenous (L4)

abnormal Lateral femoral cutaneous (L2-3) abnormal

Superficial peroneal

(L5) abnormal Sural (S1-2) abnormal

Radial abnormal Median and ulnar normal

Normal

Tibial normal Peroneal normal

Tibial, peroneal +/- reduced (related to axon loss)

Normal

Abnormal:

Proximal and distal radial-innervated muscles triceps, brachioradialis extensor carpi radialis extensor indicis proprius

Abnormal: iliopsoas quadriceps hip adductors

Abnormal:

L4-5: tensor fasciae lata, gluteus medius, tibialis anterior, peroneus longus, and tibialis posterior;

S1: gluteus maximus, biceps femoris, and gastrocnemius.

SNAP, sensory nerve action potential; CMAP, compound muscle action potential; LAC, lateral antebrachial cutaneous; MAC medial antebrachial cutaneous.

As in all axonal injury involving motor fibers, the earliest finding on needle exam is decreased motor unit recruitment (immediate) followed by denervation (i.e., fibrillation potentials, positive sharp waves) within the first 3 wk or so. Reinnervation changes (long-duration, high-amplitude, and polyphasic motor unit potentials) are expected in involved muscles within 3-6 mo after the acute injury. In cases where axonal continuity cannot be demonstrated initially (i.e., lesions characterized by complete denervation in some muscles), serial electrodiagnostic studies may be used to search for any evidence of recovery so that, in cases where no reinnervation is occurring, decisions may be made regarding surgical exploration or nerve grafting.

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