Pathoanatomy

The anatomy of the rootlets, roots, and the vertebral foramen contributes to the type of injury (avulsion versus rupture) that is observed. At every level, each of the roots is formed by the joining of dorsal (sensory) rootlets and ventral (motor) rootlets off the spinal cord as they pass through the spinal foramen (Fig. 5A). The cell bodies of the sensory nerves lie within ganglia outside the spinal cord (ie, the dorsal root ganglia [DRG]). The rootlets that form the cervical roots are intraspinal and lack connective tissue or a meningeal envelope. This anatomic feature makes them vulnerable to traction and susceptible to avulsion at the level of the spinal cord. The meningeal layers are continuous as roots are formed. For example, the dura changes to epi-neurium within the foramen and is continuous with it. The extraspinal nerve within the foramen has a protective covering formed by the coalescence of the dura.

The spinal nerve is able to move freely within the foramina because it is not attached to it. The nerve roots run down chutes as they emerge from their respective foramina. There is a fibrous attachment of the spinal nerves to the transverse process that is seen in the fourth through seventh cervical root, which firmly attaches the nerves to the transverse process by an epineural sheath, prevertebral fascia, and fibrous slips. C8 and T1 do not have these connective tissue attachments. This anatomic arrangement explains the higher

Axillary Artery Relations

Fig. 4. The relationship of the axillary artery to the cords is an important anatomic relationship. The cords surround the axillary artery and are named for their position with respect to the axillary artery. LC, lateral cord; MC, medial cord; PC, posterior cord. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

Fig. 4. The relationship of the axillary artery to the cords is an important anatomic relationship. The cords surround the axillary artery and are named for their position with respect to the axillary artery. LC, lateral cord; MC, medial cord; PC, posterior cord. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

incidence of root avulsion in the lower two roots compared with the upper three roots.

When an injury causes the tearing of the rootlets from the spinal cord proximal to the DRG, the injury is classified as preganglionic or a root avulsion. Preganglionic injuries may occur centrally, where the nerve is torn directly from the spinal cord, or peripherally (an intradural rupture), where the injury is proximal to the DRG but remnants of the rootlets are still attached to the spinal cord (Fig. 5B). When an injury is distal to the DRG, it is called post-ganglionic. This type of stretch lesion may cause a disruption of the cervical root, or a rupture (Figs. 5C, D, and 6).

The implications that pre- versus postgangli-onic injuries have in surgical reconstruction are enormous. For practical purposes, the nerve connections in preganglionic injuries cannot be restored, and thus alternative nerves (eg, nerve transfers) must be used to reanimate the injured extremity. In postganglionic injuries, the nerve connections can be restored with interpositional nerve grafting to restore function.

Anatomic considerations of preganglionic injuries

Clinical examination and electrodiagnostic or imaging studies can provide evidence to support the anatomic considerations of a preganglionic injury. Physical examination may provide clues that the injury occurred at least close to the level of DRG (or foraminal level). These clues may include weakness of the rhomboids or the serratus anterior or the finding of a Horner's syndrome—the resultant loss of sympathetic outflow to the head and neck results, producing meiosis (small pupil), enophthalmos (sinking of the eyeball), ptosis (lid droop), and anhydrosis (dry eyes) of the ipsilateral face. The sympathetic ganglion for T1 lies in close proximity to the T1 root and provides sympathetic outflow to the head and neck. Because of this close association, avulsion of the T1 root typically causes interruption of the T1 sympathetic ganglion (Fig. 7). Fibrillations in paraspinal muscles (which are innervated by the dorsal primary rami, which arise at the exit of the intervertebral foramen) also would suggest a preganglionic injury. In addition, in preganglionic injury, sensory nerve conduction studies will often be preserved when, clinically, the patient is insensate, because the sensory nerve cell body is intact within the DRG. Chest radiographs may show an elevated hemidiaphragm (from phrenic nerve dysfunction) or cervical films may show transverse process fractures. A CT-myelo-gram may show pseudomeningoceles or an MRI may reveal absent nerve rootlets.

Anatomic considerations of postganglionic injuries

The anatomic configuration of the brachial plexus predisposes it to injury at sites where it is relatively fixed to the surrounding tissues. These points can occur when branches take off from larger nerve structures, or when nerves are tethered by soft tissue (eg, muscles, ligaments, tendons) or osseous structures. One of the more well know points of tethering is that of Erb's point, where the suprascapular nerve comes off the upper trunk. Because the upper trunk is relatively tethered, and the suprascapular nerve and the division of the upper trunk are relatively free, it is a common site of a rupture. The suprascapular nerve is also bound at the suprascapular notch, and with displacement of the scapula from trauma, the suprascapular nerve can sustain a rupture at this location as well. The clavicle can also contribute to brachial plexus injuries at the level of the divisions. The axillary nerve, as it passes posteriorly after its take-off from the posterior cord, is tethered by the soft tissue, which is often a site for its rupture. In addition, the axillary nerve can be injured within the quadrangular space.

Rootlet Plexus

Fig. 5. (A) The root is made from contributions from dorsal (sensory) rootlets and ventral (motor) rootlets that emerge from the spinal cord and coalesce into the cervical root and emerges from the vertebral foramen. It is important to understand that the cell bodies of the dorsal (sensory) portion lie within the DRG. When an injury occurs and the rootlets are torn out of the spinal cord, the injury is classified as preganglionic, because it occurs proximal to the DRG. (B) This type of injury is also known as an avulsion. (C, D) When the cervical root is injured or becomes discontinuous distal to the DRG, the injury is classified as a postganglionic injury. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

Fig. 5. (A) The root is made from contributions from dorsal (sensory) rootlets and ventral (motor) rootlets that emerge from the spinal cord and coalesce into the cervical root and emerges from the vertebral foramen. It is important to understand that the cell bodies of the dorsal (sensory) portion lie within the DRG. When an injury occurs and the rootlets are torn out of the spinal cord, the injury is classified as preganglionic, because it occurs proximal to the DRG. (B) This type of injury is also known as an avulsion. (C, D) When the cervical root is injured or becomes discontinuous distal to the DRG, the injury is classified as a postganglionic injury. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

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Responses

  • Primula
    Where do posterior rootlets exit?
    8 years ago

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