What nerves are at risk during a tympanoplasty (middle ear surgery)?

Nerves at Risk During Tympanoplasty

The facial nerve (cranial nerve VII) is the most significant nerve at risk during tympanoplasty, with potential for temporary or permanent facial paralysis if injured during middle ear surgery. 1

Major Nerves at Risk

Facial Nerve (CN VII)

• The facial nerve is the most commonly affected nerve during tympanoplasty, with its tympanic segment being particularly vulnerable as it courses through the middle ear 1
• The nerve has complex anatomy with pontine, cisternal, and intratemporal segments that can be affected during middle ear access 1
• Facial nerve weakness is the most common complication in temporal bone surgery, with manifestations ranging from paresis to complete paralysis (7.8% incidence) 1
• Injury mechanisms include direct trauma, stretching, thermal injury from cautery, or compression 1

Chorda Tympani Nerve

• This is a branch of the facial nerve that traverses the middle ear space and is frequently encountered during tympanoplasty 2
• The chorda tympani runs against the medial aspect of the tympanum where it is particularly vulnerable to injury 2
• It provides taste sensation to the anterior two-thirds of the tongue and parasympathetic innervation to submandibular and sublingual glands 1
• Injury can result in taste disturbances, though these are often temporary 3

Anatomical Considerations

Facial Nerve Canal Dehiscence

• High-resolution temporal bone CT is recommended to identify facial nerve canal dehiscence preoperatively 4
• Dehiscence of the facial nerve canal increases risk of injury during surgery as the nerve may be exposed without its normal bony covering 4
• Middle fossa decompression is the preferred surgical approach for labyrinthine segment dehiscence if intervention is needed 4

Chorda Tympani Anatomy

• The chorda tympani nerve is the last collateral branch of the facial nerve in its third intraosseous portion 2
• Average length of the nerve is approximately 31.8 mm with its anastomosis to the lingual nerve located about 24.9 mm below the skull base 2
• The nerve forms an acute angle of approximately 63.2° with the lingual nerve 2

Surgical Precautions

Intraoperative Monitoring

• Intraoperative cranial nerve monitoring is strongly recommended during middle ear surgery to reduce risk of nerve injury 1
• Facial nerve monitoring leads to better facial function outcomes (Level 3 evidence) 1
• Monitoring should include direct electrical stimulation and free-running electromyography 1

Surgical Technique

• Active and adequate exposure of the facial nerve and chorda tympani nerve improves surgical safety 5
• Identification of anatomical landmarks such as the short crus of incus and facial nerve contour helps locate and preserve the chorda tympani nerve 6
• Careful lowering of the posterior wall of the auditory canal helps identify the crista of the chorda tympani nerve 6

Complications and Management

Facial Nerve Injury

• Transient facial weakness is common and typically resolves within six months 1
• Risk factors for facial nerve injury include revision surgery, bilateral procedures, and multiple previous operations 1
• If facial nerve injury is suspected, immediate assessment using House-Brackmann facial nerve grading and electrodiagnostic testing is recommended 4

Chorda Tympani Injury

• When severed, the chorda tympani has potential for regeneration (42.3% of cases show anatomical regeneration) 3
• End-to-end anastomosis of the sectioned nerve produces better regeneration (100%) compared to leaving nerve gap defects (36.2%) 3
• Taste disturbances may recover even without visible nerve regeneration 3

Prevention Strategies

• Routine identification of nerve branches in the operative field is strongly recommended 1
• Careful dissection along fascial planes helps prevent nerve injury 1
• The preauricular approach provides better access with reduced risk of facial nerve injury 1
• In cases of complications, consider whether to stop the operation or proceed with repair 7

Remember that surgeon experience significantly impacts outcomes, suggesting that tympanoplasty should ideally be performed by experienced surgeons or in high-volume centers 1.