Facial Nerve

Anatomy and Course

• The facial nerve (Cranial Nerve VII) is a mixed nerve that controls the muscles of facial expression and conveys taste sensations from the anterior two-thirds of the tongue.
• It originates in the pons and exits the brainstem at the cerebellopontine angle.
• The nerve then enters the internal auditory canal, runs through the facial canal in the temporal bone, and exits the skull via the stylomastoid foramen.
• Within the temporal bone, the facial nerve gives off the greater petrosal nerve, nerve to stapedius, and chorda tympani.

Nuclei of the Facial Nerve

Motor Nucleus:

• Located in the pons.
• Controls muscles of facial expression.

Superior Salivatory Nucleus:

• Provides parasympathetic innervation to the lacrimal, nasal, and palatine glands.

Nucleus of the Solitary Tract:

• Receives taste sensations from the anterior two-thirds of the tongue.

Spinal Trigeminal Nucleus:

• Processes pain and temperature sensations from the ear.

Segments of the Facial Nerve

Segment	Anatomy	Symptoms	Clinical Significance
Intracranial (Cisternal) Segment	Extends from the brainstem to the internal auditory meatus.	Complete facial paralysis on the affected side. Loss of taste sensation from the anterior two-thirds of the tongue. Decreased lacrimation and salivation.	These symptoms might be accompanied by other cranial nerve deficits if the cause is a cerebellopontine angle tumor or similar pathology.
Meatal (Labyrinthine) Segment	Runs within the internal auditory canal.	Facial paralysis, including the forehead (frontalis muscle). Hearing loss or tinnitus if the vestibulocochlear nerve is also affected.	Commonly seen in acoustic neuroma or during vestibular schwannoma surgery.
Tympanic (Horizontal) Segment	Lies within the temporal bone.	Facial paralysis. Hyperacusis (increased sensitivity to certain frequencies and volume ranges of sound) due to stapedius muscle paralysis.	Middle ear pathologies like cholesteatoma or chronic infection can affect this segment.
Mastoid (Vertical) Segment	Descends in the mastoid bone.	Facial paralysis. Possible alteration in taste sensation.	Mastoid surgeries or ear infections can damage this segment.
Extratemporal Segment	Emerges from the stylomastoid foramen and branches in the face.	Paralysis of the muscles of facial expression on the affected side, often sparing the forehead in cases of partial damage due to dual innervation. If the main trunk is affected, all facial expressions are impaired.	Commonly injured in facial trauma or parotid gland surgery.

Terminal Branches and Innervation

• After exiting the stylomastoid foramen, it branches into five main branches: Temporal, Zygomatic, Buccal, Marginal Mandibular, and Cervical.
• These branches innervate the muscles of facial expression, including the muscles of the forehead, eye, cheek, and neck.

Functional Aspects

• The nerve is responsible for facial expressions, eyelid closing (orbicularis oculi muscle), and lip movement.
• It also innervates the stapedius muscle in the middle ear (impacting hearing) and conveys taste sensation from the anterior tongue.

Clinical Significance

• Pathologies affecting the facial nerve can lead to facial paralysis or paresis, most commonly Bell’s palsy.
• The nerve is at risk during surgical procedures in the parotid region, temporal bone surgeries, and cerebellopontine angle surgeries.
• Preservation of the facial nerve during surgical interventions is paramount to prevent facial asymmetry and functional impairments.

Clinical grading of facial nerve function (House and Brackmann)

Grade	Function Description	Clinical Sx
1	normal facial function in all areas
2	mild dysfunction	gross: slight weakness noticeable on close inspection; may have very slight synkinesis at rest: normal symmetry and tone motion: forehead: slight to moderate movement eye: complete closure with effort mouth: slight asymmetry
3	moderate dysfunction	gross: obvious but not disfiguring asymmetry; noticeable but not severe synkinesis motion: forehead: slight to moderate movement eye: complete closure with effort mouth: slightly weak with maximal effort
4	moderate to severe dysfunction	gross: obvious weakness and/or disfiguring asymmetry motion: forehead: none eye: incomplete closure mouth: asymmetry with maximum effort
5	severe dysfunction	gross: only barely perceptible motion at rest: asymmetry motion: forehead: none eye: incomplete closure
6	total paralysis no movement

Surgical Considerations

• Techniques like interfascial dissection are employed to protect the nerve, especially in procedures involving the scalp and temporal region.

Preserving frontal branch of the facial nerve through a subfascial dissection