Mechanism and pathoanatomy

Most adult brachial plexus pathology is caused by closed trauma. Nerve injury in these cases is from traction and compression, with traction accounting for 95% of injuries [14]. Following a traction injury, the nerves may rupture, be avulsed at the level of the spinal cord, or be significantly stretched but remain intact (Fig. 1). Following are five possible levels where the nerve can be injured (Fig. 2):

1. The root

2. The anterior branches of the spinal nerves

3. The trunk

4. The cord

5. The peripheral nerve

Root injuries may be further localized with respect to the dorsal root ganglion (DRG). Post-ganglionic (infraganglionic) injuries are located distal to the DRG, whereas preganglionic (supra-ganglionic) lesions are located proximal to the DRG. With both types of lesions, patients present with loss of muscle function. In preganglionic injuries, the nerve has been avulsed from the spinal cord separating the motor nerve fibers from the motor cell bodies in the anterior horn cells. The sensory fibers and cell bodies are still connected at the DRG; however, the efferent fibers entering the dorsal spinal column have been disrupted. Thus, sensory nerve action potentials (SNAPs) are preserved in patients with supraganglionic injuries. In postganglionic injuries, both the motor and sensory nerve cells have been disrupted so there will be abnormalities in both motor action potentials and SNAPs (see Fig. 1) [15]. At present, the repair of preganglionic

Preganglionic Avulsion

Fig. 1. (A) Anatomy of the brachial plexus roots and types of injury. Image A shows the roots are formed by the coalescence of the ventral (motor) and dorsal (sensory) rootlets as they pass through the spinal foramen. The DRG holds the cell bodies of the sensory nerves, whereas the cell bodies for the ventral nerves lie within the spinal cord. There are three types of injury that can occur: avulsion injuries, as shown in image B, pull the rootlets out of the spinal cord; stretch injuries, as shown in image C, attenuate the nerve; and ruptures, as shown in D, result in a complete discontinuity of the nerve. When the injury to the nerve is proximal to the DRG, it is called preganglionic, and when it is distal to the DRG, it is called postganglionic. (B) A clinical example of a preganglionic injury (root avulsion) and a postganglionic injury. In this patient, the C5 root is avulsed with its dorsal and ventral rootlets. The asterisk shows the DRG. The C6 root is inferior and demonstrates a rupture at the root level. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

Fig. 1. (A) Anatomy of the brachial plexus roots and types of injury. Image A shows the roots are formed by the coalescence of the ventral (motor) and dorsal (sensory) rootlets as they pass through the spinal foramen. The DRG holds the cell bodies of the sensory nerves, whereas the cell bodies for the ventral nerves lie within the spinal cord. There are three types of injury that can occur: avulsion injuries, as shown in image B, pull the rootlets out of the spinal cord; stretch injuries, as shown in image C, attenuate the nerve; and ruptures, as shown in D, result in a complete discontinuity of the nerve. When the injury to the nerve is proximal to the DRG, it is called preganglionic, and when it is distal to the DRG, it is called postganglionic. (B) A clinical example of a preganglionic injury (root avulsion) and a postganglionic injury. In this patient, the C5 root is avulsed with its dorsal and ventral rootlets. The asterisk shows the DRG. The C6 root is inferior and demonstrates a rupture at the root level. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

injuries requires a neurotization procedure. Postganglionic injuries may be amenable to surgical repair or grafting.

Root avulsions

Root avulsions are present in 75% of cases of supraclavicular lesions; multiple root avulsions have become more frequent over the past 25 years. There are two mechanisms for avulsion injuries: peripheral and central. Peripheral avulsion injuries are more common, whereas central avulsion injuries are rare and usually the result of direct cervical trauma (Figs. 3 and 4).

The peripheral mechanism occurs when traction forces on the arm overcome the fibrous

Bronchopulmonary Segments

Fig. 2. Anatomy of the brachial plexus. (A) The brachial plexus has five major segments: roots, trunks, divisions, cords, and branches. The clavicle overlies the divisions. The roots and trunks are considered the supraclavicular plexus, whereas the cords and branches are considered the infraclavicular plexus. (B) The relationship between the axillary artery and the cords is shown. The cords are named for their anatomic relationship to the axillary artery: medial, lateral, and posterior. LC, lateral cord; LSS, lower sub scapular nerve; MABC, medial antibrachial cutaneous nerve; MBC, medial brachial cutaneous nerve; MC, medial cord; PC, posterior cord; TD, thoracodorsal; USS, upper sub scapular nerve. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

Fig. 2. Anatomy of the brachial plexus. (A) The brachial plexus has five major segments: roots, trunks, divisions, cords, and branches. The clavicle overlies the divisions. The roots and trunks are considered the supraclavicular plexus, whereas the cords and branches are considered the infraclavicular plexus. (B) The relationship between the axillary artery and the cords is shown. The cords are named for their anatomic relationship to the axillary artery: medial, lateral, and posterior. LC, lateral cord; LSS, lower sub scapular nerve; MABC, medial antibrachial cutaneous nerve; MBC, medial brachial cutaneous nerve; MC, medial cord; PC, posterior cord; TD, thoracodorsal; USS, upper sub scapular nerve. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

Optic Nerve Avulsion
Fig. 3. Peripheral mechanism of avulsion. Peripheral avulsions occur when there is a traction force to the arm and the fibrous supports around the rootlets are avulsed. The epidural sleeve may be pulled out of the spinal canal, creating a pseudomeningocele.

The central mechanism of root avulsion is the result of the spinal cord moving longitudinally or transversely following significant cervical trauma. Spinal bending within the medullary canal induces avulsion of the rootlets [18]. The root remains fixed in the foramen and the epidural sleeve is not ruptured (see Fig. 4).

Injury patterns

Any combination of avulsion, rupture, or stretch may occur following a brachial plexus injury; however, certain patterns are more

Mechanism Brachial Plexus Lesion

Fig. 4. Central mechanism of avulsion. Central avulsions occur from direst cervical trauma. The spinal cord is moved transversely or longitudinally, causing a sheering and spinal bending that results in an avulsion of nerve rootlets.

supports around the rootlets. Anterior roots may be avulsed with or without the posterior rootlets. The epidural sac may tear without complete avulsion of the rootlets. Different injury types account for the different patterns seen on myelog-raphy. The epidural cone moves into the foram-inal canal with peripheral avulsions (Fig. 5). Nagano et al [16,17] have classified avulsion and partial avulsion patterns based on findings at the time of myelogram. The nerve roots of C5 and C6 have strong fascial attachments at the spine and are less commonly avulsed in comparison to the nerve roots of C7 through T1.

Fig. 4. Central mechanism of avulsion. Central avulsions occur from direst cervical trauma. The spinal cord is moved transversely or longitudinally, causing a sheering and spinal bending that results in an avulsion of nerve rootlets.

Nerve Root Injury

Fig. 5. Myelography and CT myelography can be instrumental in determining the level of nerve injury. If a pseudomeningocele (*) is present, there is a greater likelihood of a nerve root avulsion. (A) Multiple root avulsions (*) are clearly seen by CT myelogram. (B). The arrows on the opposite side of the avulsion (*) show the normal dorsal and ventral rootlet outline of the uninjured side. Notice how these outlines are missing on the injured side. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

Fig. 5. Myelography and CT myelography can be instrumental in determining the level of nerve injury. If a pseudomeningocele (*) is present, there is a greater likelihood of a nerve root avulsion. (A) Multiple root avulsions (*) are clearly seen by CT myelogram. (B). The arrows on the opposite side of the avulsion (*) show the normal dorsal and ventral rootlet outline of the uninjured side. Notice how these outlines are missing on the injured side. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

prevalent. Brachial plexus lesions most frequently affect the supraclavicular region rather than the retroclavicular or infraclavicular levels. The roots and trunks are more commonly injured in comparison to the divisions, cords, or terminal branches. Double-level injuries can occur and should be included in the differential. In the supraclavicular region, traction injuries occur when the head and neck are violently moved away from the ipsilateral shoulder, often resulting in an injury to the C5, C6 roots or upper trunk (Fig. 6). Traction to the brachial plexus can also occur with violent arm motion. When the arm is abducted over the head with significant force, traction will occur within the lower elements of the brachial plexus (C8-T1 roots or lower trunk; Fig. 7).

Distal infraclavicular lesions are usually caused by violent injury to the shoulder girdle. These lesions can be associated with axillary arterial rupture. Biomechanically for a cord to rupture, it must be firmly fixed at both ends. The two major mechanisms for rupture are the anterior medial dislocation of the glenohumeral joint and traction of the upper arm with forced abduction [13]. The nerves become injured between two points at which the nerve is either fixed, restrained by surrounding structures, or where it changes direction. Suprascapular, axillary, and musculocu-taneous nerves are susceptible to rupture because they are tethered within the glenohumeral area at the scapular notch and coracobrachialis. Physicians must always consider the possibility of a double crush at the scapular notch and the musculocutaneous nerve at the coracobrachialis. Rupture of the ulnar nerve at the level of the humerus or elbow and median nerve rupture at the level of the elbow are also possible.

Approximately 70% to 75% of injuries are found in the supraclavicular region. Approximately 75% of these injuries involve an injury to the entire plexus (C5-T1); in addition, 20% to 25% of injuries involve damage to the nerve roots of C5 through C7 and 2% to 35% of injuries have isolated supraclavicular injury patterns to C8 and T1. Panplexal injuries usually involve a C5-C6 rupture with a C7-T1 root avulsion. The remaining 25% of plexus injuries are infraclavicular.

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