Cochlear Implantation

Description

Cochlear implantation involves the placement of an implant system into the cochlea, which then enables transduction of mechanical sound energy into electrical signals that stimulate the cochlear nerve. The implant is thus able to bypass nonfunctioning inner ear hair cells in patients with severe to profound hearing loss. The components of the implant system include a microphone, an external speech processor, and an electrode array that is implanted into the cochlea. During the procedure, the round window of the cochlea is exposed, and a small cochleostomy is made, through which the electrode is threaded. The induction coil of the device is situated in a bony well that is drilled in the mastoid.

Indications for Surgery

Food and Drug Administration (FDA)-approved indications for cochlear implantation vary slightly according to the specific device implanted. The following is a summary of general guidelines.

Adults (>18 years):

• Bilateral severe to profound hearing loss (three frequency pure tone average >70dB unaided in the better hearing ear with word recognition score <20%)
• Minimal benefit from conventional hearing aids (sentence recognition scores <50% in best-aided condition)

Children (< 18 years):

• Age ≥12 months (although FDA guidelines do not include children less than 12 months of age, cochlear implantation has been successfully performed in children as young as 6 months of age)
• Bilateral severe to profound hearing loss (bilateral profound hearing loss if <25 months)
• Minimal benefit from conventional hearing aids (word recognition scores <30% in best-aided condition)
• Lack of auditory skill development
• Enrolled in an educational therapy program emphasizing auditory skills

Contraindications to Surgery

• Medical comorbidities that would preclude surgical or anesthesia risk
• Surgery is not indicated in asymptomatic patients, even if radiographic or endoscopic findings suggesting inflammatory disease are present

Pre-Operative Evaluation

• A complete audiologic assessment should be performed
• Because cochlear implantation increases the risk of pneumococcal meningitis, the Centers for Disease Control and Prevention (CDC) recommend that patients receive adequate immunization against pneumococcus.
• The patient must have tried and failed use of optimized hearing aids
• A computed tomographic (CT) or magnetic resonance imaging (MRI) scan of the temporal bone is performed to identify any anatomic anomalies
• Before being selected as a candidate for surgery, the patient and family typically undergo a rigorous screening process, which may include psychological evaluation, family counseling, and educational programs.

Anesthesia

General endotracheal

Surgical Technique

1. The patient is placed supine on the OR table and general anesthesia is induced via endotracheal intubation.
2. The head is turned and the ear is prepped and draped. Facial nerve monitor leads are placed.
3. The incision site is marked, extending from the temporal region to the postauricular sulcus, and injected with 1% lidocaine with 1:100,000 epinephrine.
4. The binocular otomicroscope is used to inspect the tympanic membrane and rule out acute or chronic ear disease.
5. The marked incision is made and carried down to the temporalis fascia.
6. Temporalis muscle and fascia are harvested and cut into pieces for later use to close the cochleostomy.
7. An incision is made along the linea temporalis , down to the mastoid tip. A musculoperiosteal flap is elevated until landmarks of the bony external canal are seen.
8. A cortical mastoidectomy is performed, leaving the canal wall intact anteriorly.
9. The antrum is entered and the incus and facial nerve are identified.
10. A posterior tympanostomy is performed to open into the facial recess, and the stapes and round window are identified.
11. A pocket is developed deep to periosteum, posterosuperior to the mastoid cavity. A bony well may be drilled to house the stimulator package. The skull may be thin here and care is taken not to breach dura.
12. A trough is drilled extending from the well to the mastoid cavity.
13. A diamond bur is used to drill a 1.5 mm cochleostomy anterior and inferior to the round window. (Figure 19.2) Alternatively, the round window may be exposed by drilling away the round window niche, enabling a right-angled hook to open the round window membrane.
14. The receiver-stimulator is placed into the well, under the previously elevated soft tissue pocket.
15. The stimulator probe is threaded into the inner ear according to manufacturer’s recommendations.
16. The previously harvested muscle and fascia are used to seal the cochleostomy and close the facial recess.
17. If the device has a separate ground electrode, this is positioned deep to the temporalis muscle. The electrode leads are coiled in the mastoid and the postauricular mastoid periosteum sutured in place.
18. The postauricular incision is closed in two layers.
19. The device and electrodes are tested to ensure proper function.
20. A sterile mastoid pressure dressing is applied to the incision and care of the patient is returned to the anesthesiologist for extubation and recovery.

Post-Operative Care

• Cochlear implantation is typically performed on an elective, outpatient basis
• The patient may be discharged home two hours post-procedure if no immediate complications arise
• The patient is discharged with pain medication and a short course of prophylactic antibiotics
• The mastoid dressing should be kept in place for 48 hours after surgery
• A follow-up visit is arranged for two weeks after surgery
• Device stimulation is typically scheduled 3-5 weeks after surgery

Complications

• Damaged or misplaced electrode
• Wound infection/skin flap necrosis
• Injury to local structures, including dura and/or the facial nerve. Delayed facial nerve paralysis is rare but may also occur.
• Cerebrospinal fluid leak/meningitis
• Mastoiditis/postauricular abscess
• Vertigo/dizziness
• Device extrusion
• Device failure