Chapter 10: Facial Nerve Decompression

Overview

Post-auricular facial nerve decompression is a critical surgical procedure for treating facial nerve pathologies such as Bell’s palsy, traumatic injuries, or tumors. This document provides detailed procedural steps, anatomical landmarks, and clinical considerations for performing this advanced otologic surgery.

Step 1: Cortical Mastoidectomy

Perform a thorough cortical mastoidectomy to expose the mastoid air cells and gain initial access to the facial nerve. This step involves identifying the mastoid segment of the facial nerve and ensuring adequate exposure for subsequent decompression procedures.

Key Anatomical Landmarks:

• Lateral semicircular canal
• Sigmoid sinus
• Tegmen mastoideum
• Sinodural angle

Step 2: Facial Recess Approach

The facial recess is opened by removing bone between the chorda tympani nerve and the incus buttress. This creates a direct path to the tympanic portion of the facial nerve.

Orientation:

• The facial nerve lies medial to the posterior canal wall, below the fossa incudis, and anterior to the horizontal semicircular canal. (Refer to Fig. 17 for anatomical orientation.)

Step 3: Skeletonizing the Descending Portion of the Facial Nerve

Landmarks:

• External genu
• Vasa vasorum of the facial nerve
• Stylomastoid foramen
• Digastric muscle
• Chorda tympani

Using a diamond burr, thin the bone over the descending segment of the facial nerve to an eggshell thickness. Begin at the external genu and proceed inferiorly to the stylomastoid foramen. Avoid over-thinning anteriorly (due to proximity to the external auditory canal) or medially. Ensure profuse irrigation to minimize thermal injury.

Step 4: Exposing the Horizontal Portion of the Facial Nerve

Extend exposure to the horizontal portion of the nerve through the facial recess. Bone thinning is performed along the lateral and inferior borders of the tympanic segment, using the smallest diamond burrs (e.g., 00 or 000) to preserve adjacent structures.

Landmarks:

• Cochleariform process
• Oval window
• Lateral semicircular canal

Ensure adequate irrigation and suction to prevent thermal damage. Bone overlying the horizontal segment is carefully elevated with an elevator for visualization of the geniculate ganglion.

Step 5: “Barber Poling” Technique

Bone removal is performed circumferentially:

1. Unroof the posterior aspect of the mastoid segment.
2. Expose the lateral aspect of the external genu and facial recess.
3. Thin the inferior border of the horizontal segment to the cochleariform process.

Careful rotation of the unroofing process ensures safe decompression without injuring adjacent structures, such as the chorda tympani and ossicular chain. Excessive thinning can lead to fenestration of the horizontal canal or injury to the cochleariform process.

Step 6: Incising the Perineural Sheath

Use a #59 Beaver knife to incise the sheath of the facial nerve, exposing the perineurium. Ensure the blade is sharp to prevent tugging on the nerve. Maintain precision to avoid damage to the underlying nerve fibers.

Clinical Applications

Facial nerve decompression is performed for:

• Bell’s palsy
• Traumatic facial nerve paralysis
• Removal of bone fragments or hematomas
• Accessing medial structures during glomus tumor surgery

Rerouting the facial nerve may be necessary to access medial structures. This involves thinning the bone circumferentially around the nerve before mobilizing it forward.

Indications for Transmastoid Approach

• Temporal bone fractures involving the tympanic or mastoid segments
• Facial nerve tumors localized to the mastoid or tympanic regions

The choice between closed and open tympanoplasty depends on hearing status. Closed tympanoplasty preserves hearing, whereas open tympanoplasty is reserved for cases with non-functional hearing.

Hints and Pitfalls

1. Maintain eggshell-thin bone over the facial nerve to prevent extrusion.
2. Avoid zigzag drilling; use parallel strokes with consistent irrigation.
3. Monitor for landmarks such as the lateral semicircular canal, cochleariform process, and ossicles to avoid complications.
4. Use Brackmann suction tubes to minimize trauma to delicate structures.
5. Bone Thinning: Ensure uniform thinning while preserving a thin layer over the nerve.
6. Irrigation: Use profuse irrigation to prevent thermal damage.
7. Landmark Awareness: Constantly reassess orientation with magnification to prevent errors.

Key Anatomical Landmarks

Intrameatal Segment:

• Lateral: Superior vestibular nerve
• Inferior: Bill’s bar

Labyrinthine Segment:

• Lateral: Ampulla of the superior semicircular canal

First Genu:

• Inferior: Processus cochleariformis
• Posterior: Tensor tympani tunnel

Horizontal Segment:

• Inferior: Stapes
• Lateral: Incus

Second Genu:

• Superior: Lateral semicircular canal
• Inferior: Stapes

Vertical Segment:

• Superior: Lateral semicircular canal
• Inferior: Digastric ridge

Advanced Considerations

1. Mobilization: Circumferential bone removal aids in rerouting the nerve for access to deeper structures.
2. Complications: Monitor for vascular compromise, which can lead to facial paresis or paralysis.
3. Postoperative Care: Ensure meticulous hemostasis and nerve protection during closure.

Conclusion

Mastery of facial nerve decompression requires a systematic approach, thorough knowledge of anatomy, and meticulous surgical technique. This comprehensive guide emphasizes the importance of preserving critical structures while achieving effective decompression for optimal patient outcomes.