Facial Nerve Dehiscence

Definition

Anatomy

Etiology & Prevalence

• Congenital/developmental: Most common cause. The bony canal fails to ossify fully; the overlying bone may be as thin as 0.1 mm or altogether absent.
• Prevalence: Cadaveric and CT studies report dehiscence in 6–50% of temporal bones, with significant variation based on methodology. Clinically significant dehiscence is far less common.
• Acquired causes:
  • Chronic otitis media (erosion by cholesteatoma, granulation tissue)
  • Temporal bone trauma/fracture
  • Iatrogenic injury (prior surgery)
  • Otosclerosis surgery
  • Malignant otitis externa
  • Radiation

Clinical Significance

1. Iatrogenic Risk During Surgery

• The dehiscent nerve is vulnerable to injury during tympanoplasty, stapedectomy, cochlear implantation, mastoidectomy, and cholesteatoma surgery.
• An exposed nerve can be mistaken for a soft tissue structure and inadvertently damaged.
• Post-operative facial nerve palsy in the setting of ear surgery should prompt consideration of unrecognized dehiscence.

2. Association with Pathology

• Cholesteatoma: Can cause or worsen FND by eroding the Fallopian canal.
• Conductive hearing loss: A dehiscent nerve herniated over the oval window can impair stapes movement.
• Pulsatile tinnitus: Rare, from transmitted vascular pulsations directly to the exposed nerve.
• Spontaneous facial nerve palsy: In some cases of dehiscence, minor inflammation or middle ear effusion can compress the exposed nerve segment.

Imaging

• Look for: discontinuity or absence of the hyperdense bony covering of the Fallopian canal, best seen on axial and coronal bone-window cuts.
• CT has limited sensitivity for thin bony dehiscences (<0.5 mm); false negatives occur.
• MRI is complementary when soft tissue extension, perineural disease, or nerve enhancement is suspected.

Preoperative HRCT (A) showing absence of Fallopian canal wall at the tympanic segment (arrow), and intraoperative confirmation (B) of exposed facial nerve without bony covering during tympanomastoidectomy.

Management

Preoperative

• HRCT review of all patients before ear surgery to identify dehiscence and plan accordingly.
• Neurotologic referral for complex cases.

Intraoperative

• Careful surgical technique near the oval window and tympanic segment.
• Identify the nerve early and trace its course before drilling or manipulating ossicles.
• If dehiscence is encountered incidentally and the nerve is intact, it can be left alone or the defect covered with fascia/cartilage.

When the Nerve is Symptomatic or Injured

Surgical Repair Options

• Fascial sheath repair: Temporalis fascia or perichondrium placed over the dehiscent segment.
• Nerve decompression: Opening the Fallopian canal proximal and distal to the injury site to relieve pressure.
• Nerve grafting: Greater auricular or sural nerve used for segmental defects > 1 cm.

Key Pearls

• The tympanic segment above the oval window is the most vulnerable and most commonly dehiscent portion.
• Dehiscence is often subclinical but becomes critical in the surgical context — always review HRCT before otologic surgery.
• A dehiscent facial nerve can mimic soft tissue (polyp, granulation) intraoperatively.
• Cholesteatoma is the most important acquired cause of progressive dehiscence.
• Post-operative facial palsy after ear surgery should raise suspicion for pre-existing unrecognized dehiscence.