What anatomy is relevant for facial nerve repair?

Updated: Nov 28, 2018
  • Author: Tang Ho, MD, MSc; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

The facial nerve may be divided into intracranial, intratemporal, and extratemporal components (see the image below). The intracranial portion of the facial nerve may be considered to include the supranuclear component (ie, the voluntary motor cortex, internal capsule, extrapyramidal system, midbrain, and pons) and the facial nerve nucleus and intracranial facial nerve. (See also Facial Nerve Anatomy and Brain Anatomy.)

Surgical anatomy of facial nerve and relevant anat Surgical anatomy of facial nerve and relevant anatomic landmarks.

The intratemporal portion of the facial nerve begins as the nerve enters the internal acoustic meatus and includes the well-described meatal, labyrinthine, tympanic, and vertical segments. The nerve then exits the stylomastoid foramen and soon divides at the pes anserinus. The subsequent branching to the temporal, zygomatic, buccal, marginal mandibular, and cervical branches shows some variability from person to person.

The blood supply to the facial nerve begins with the middle cerebral artery supplying the motor cortex. The facial nucleus in the pons is supplied by the anterior inferior cerebellar artery and the short and long circumferential arteries.

The facial nerve proper is then supplied by the anterior inferior cerebellar artery, the middle meningeal artery, and the stylomastoid branch of the postauricular artery. These tend to overlap; however, the region just proximal to the geniculate ganglion is thought to be somewhat susceptible to vascular compromise secondary to the poorer redundancy present there compared with other areas.

Some discussion of the microanatomy is warranted. Approximately 7000 neuron cell bodies make up the facial nerve, each of which innervates approximately 25 muscle fibers. The axons are surrounded by myelin, produced by the Schwann cells surrounding the axons. The nerve sheath is composed of the following 3 membranes:

  • Epineurium – This is the outer covering, composed of loose areolar tissue, which separates the fascicles and holds them together

  • Perineurium – This is the next inner layer, consisting of cells that are metabolically active and function as a diffusion barrier; the perineurium provides considerable strength to the nerve sheath

  • Endoneurium – This membrane, the innermost layer, surrounds each of the individual nerve fibers

The spatial orientation of the facial nerve has been debated. In the cortex and brainstem, the nerve is spatially oriented. If it continued in an organized spatial orientation through its extra-axial course, this would have implications for the surgeon’s repair technique and ability to help prevent synkinesis; it would also allow some identification of the injured area of the nerve on the basis of clinical findings. Although such clinical observations have been made, the evidence overall suggests that the spatial orientation is not present in the extra-axial facial nerve. [6]

After significant injury, the facial nerve undergoes degeneration of the distal segment, as described by Waller. Sunderland classified such injuries into 5 types or degrees. These 5 types describe the pathophysiologic events associated with each of the disorders that may affect the nerve.

First-degree injury is referred to as neurapraxia, in which a physiologic block is produced by increased intraneural pressure (such as may be produced by external compression). The covering layers of the nerve (ie, endoneurium, perineurium, and epineurium) are not disrupted, and the nerve is capable of stimulation. Full return of function without synkinesis is observed.

Second-degree injury involves a similar mechanism, but the compression is unrelieved and results in degeneration of the nerve axons. This injury is termed axonotmesis, and again, excellent return is expected, though recovery may take several months. Because nerve stimulation is compromised, distinguishing axonotmesis is difficult.

Third- through fifth-degree injuries involve loss of endoneurial, perineurial, and epineurial tubes, respectively. Fourth- and fifth-degree injuries imply partial or complete transection of the nerve. Regeneration is incomplete, and synkinesis is inevitable. Repair of the facial nerve is generally performed in cases of complete paralysis.


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