Facial Nerve (VII) - 5 Mark Answer

Facial N., Nervus Intermedius, and Cochlear N. at the internal auditory canal (Shambaugh Surgery of the Ear)

Introduction

• Motor nucleus - in the caudal pons (special visceral efferents)
• Superior salivatory nucleus - dorsal to motor nucleus (general visceral efferents)
• Nucleus of the solitary tract - in the medulla oblongata (taste/sensory)

Note: The superior portion of the motor nucleus (for frontalis and orbicularis oculi) receives bilateral (crossed + uncrossed) cortical input - this is the anatomical basis for sparing of forehead movements in upper motor neuron (UMN) facial palsy. The inferior portion receives only ipsilateral input, so it is affected in both UMN and LMN lesions.

Fiber Components (5 types)

Course and Segments

1. Intracranial (Cisternal) Segment - 24 mm

• Emerges from the brainstem between the pons and the olive
• Crosses the cerebellopontine angle (CPA) alongside CN VIII
• Enters the internal auditory meatus (IAM) - lies anterosuperiorly in the IAC, above the cochlear nerve (superior and inferior vestibular nerves are posterior)
• The nervus intermedius (nerve of Wrisberg) runs as a separate nerve between facial and superior vestibular nerves in the IAC

2. Intracanalicular (Meatal) Segment

• Traverses the IAC; at the fundus, occupies the anterosuperior quadrant (separated from superior vestibular nerve by the vertical crest / Bill's bar)
• Joined here by the nervus intermedius

3. Labyrinthine Segment - 4 mm (shortest, narrowest)

• Runs from the beginning of the fallopian canal to the geniculate ganglion
• Courses superior to the cochlea, between cochlea and vestibule
• Most vulnerable to compression: no epineurium, watershed blood supply (vertebrobasilar + external carotid systems)
• At the geniculate ganglion (1st genu): the nerve makes an acute posterior turn
  • Gives off the greater superficial petrosal nerve (GSPN) - runs in a groove along the middle cranial fossa floor

4. Tympanic (Horizontal) Segment - 13 mm

• Courses in the medial wall of the middle ear (anterior attic), superior to the cochleariform process and oval window
• Ends at the 2nd genu - turns inferiorly just anteroinferior to the lateral semicircular canal
• Most common site of congenital bony dehiscence (especially above the oval window, present in ~25% of ears; bilateral in ~75% of cases)
• Gives off the nerve to stapedius at the pyramidal eminence

5. Mastoid (Vertical) Segment - 20 mm

• Descends from 2nd genu to stylomastoid foramen
• Gives off chorda tympani nerve (~4 mm proximal to stylomastoid foramen)
  • Chorda tympani ascends in a canal lateral and anterior to CN VII, enters tympanic cavity at iter chordae posterius, crosses lateral to long process of incus and medial to malleus, exits at iter chordae anterius (canal of Huguier) through the petrotympanic (Glaserian) fissure
• The facial recess (triangular area used in intact-canal-wall mastoidectomy) is bounded by: incudal fossa (superior), facial nerve (medial), chorda tympani (lateral)

Extracranial Course

1. Posterior auricular nerve - occipital belly of occipitofrontalis, auricular muscles
2. Branch to posterior belly of digastric and stylohyoid

Mnemonic: "To Zanzibar By Motor Car" (Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical)

Intratemporal Branches (3)

1. Greater superficial petrosal nerve (GSPN) - from geniculate ganglion; preganglionic parasympathetic to lacrimal gland and nasal/palatal glands via pterygopalatine ganglion; also taste from palate
2. Nerve to stapedius - from mastoid segment near pyramidal eminence; lesion above this level causes hyperacusis
3. Chorda tympani - from mastoid segment; taste from anterior 2/3 tongue + preganglionic parasympathetic to submandibular and sublingual glands via submandibular ganglion

Clinical Significance

Localization of Lesions (by lost function)

Bell Palsy

• Most common cause of acute unilateral LMN facial paralysis (idiopathic; likely viral - HSV reactivation)
• Complete paralysis of all ipsilateral facial muscles including forehead
• Managed with oral corticosteroids (and antivirals in severe cases)
• Graded using House-Brackmann (HB) grading system (I = normal, VI = complete paralysis)

Ramsay Hunt Syndrome

• Herpes zoster reactivation in the geniculate ganglion
• Triad: LMN facial palsy + vesicles in EAC/pinna + ipsilateral sensorineural hearing loss

Fallopian Canal Dehiscence

• Most common over the oval window (tympanic segment); present in 25% of ears; 75% bilateral
• Renders the nerve vulnerable to suppurative otitis media and surgical injury