CI Atlas · Into the Cochlea: The Medical and Surgical Path of Implantation · Module 05

5Mastoidectomy and the Facial Recess Approach

Every implant system in routine use reaches the cochlea the same way: through a cortical mastoidectomy and a posterior tympanotomy that threads between the facial nerve and the chorda tympani. This is the workhorse approach, but it is also where the procedure's most feared complication lives. This module walks the landmarks, defines the boundaries of the facial recess, weighs the heat and proximity that endanger the facial nerve, and introduces the suprameatal and transcanal routes proposed to bypass that window altogether.

FCortical (canal-wall-up) mastoidectomy

The standard access is a canal-wall-up cortical mastoidectomy that deliberately avoids saucerising the superior and posterior bony margins, which are kept to shelter the connecting leads and stabilise the receiver. The flap is elevated to expose mastoid landmarks: the spine of Henle, the linea temporalis and the mastoid tip, with at least 3 cm of bone exposed above and beyond the mastoid. The descending (mastoid) segment of the facial nerve is traced from superior to inferior using the fossa incudis, the lateral semicircular canal and the digastric ridge as landmarks. Unlike chronic-ear surgery, the sigmoid sinus and mastoid-tip air cells need not be fully exposed; small superior, posterior and inferior overhangs are intentionally left to help control the electrode. The short process of the incus, sitting in the fossa incudis, marks the level of the facial recess and points the drill toward it.[2009][2012]

TThe posterior tympanotomy and its boundaries

The facial recess is a triangular window opened in canal-wall-up surgery, bounded laterally by the chorda tympani, medially by the vertical (mastoid) facial nerve, and superiorly by the fossa incudis. It is opened with progressively smaller diamond burrs, dissecting in the plane between the fallopian canal and chorda tympani, to reveal the incudostapedial joint and the cochlear windows. The chorda tympani can usually be preserved, but if the recess is small or a device needs generous exposure the nerve may be sacrificed, taking care not to tear the tympanic membrane, since the chorda enters the middle ear at the level of the annulus. Bone over the anterior aspect of the vertical facial nerve, including the anteromedial fallopian canal just below the pyramidal process, is sometimes removed to bring the round window into view. The posterior canal wall is thinned to maximum to open the path the electrode will travel from the recess to the cochleostomy, without perforating into the ear canal.[2009][2012]

CRisk to the facial nerve

Facial nerve injury is rare but among the most serious complications; the usual mechanism is heat from the rotating burr shaft passing over the nerve in the floor of the facial recess rather than a direct cut. The protective rules are concrete: copious irrigation while drilling, keeping a thin sheet of bone over the nerve, and angling the drill so the burr shaft stays lateral to and away from the recess floor. Intra-operative facial nerve monitoring is valuable where anatomy is abnormal, since cochlear malformation can carry an anomalous facial nerve and ossification may demand more extensive dissection. Distinct from injury, facial-nerve stimulation on device activation is reported in roughly 6 to 7% of recipients, far higher (up to about 75%) in otosclerosis where current shunts through the abnormal bone. Burr shafts and instruments must never rest on a potentially exposed facial nerve in the recess floor, particularly during cochleostomy drilling.[1997][2009]

The transmastoid posterior tympanotomy is the standard route, giving a direct view of the round window but requiring drilling through the facial recess close to the nerve. The suprameatal tunnel passes above the ear canal instead, averaging 13 mm in adults and 7 mm in children and running 3-7 mm from the canal wall, avoiding the facial recess at the cost of a blind tunnel. The transcanal route is the most direct. Post-implant otitis media occurs in about 20% of children with either route, so middle-ear health matters whichever is chosen. Schematic.

CAlternatives and securing the lead

The suprameatal (Veria/Kronenberg) approach avoids the mastoidectomy and posterior tympanotomy entirely, reaching the middle ear through a tympanomeatal flap and passing the electrode through a suprameatal tunnel drilled superoposterior to the spine of Henle. That suprameatal tunnel is drilled obliquely from posterosuperior to anteroinferior, with a mean length of about 13 mm in adults and 7 mm in children, kept 3 to 7 mm from the bony ear-canal wall and inferior to the middle-fossa dura. When comparing approaches in children, post-implant otitis media occurred in about 20% regardless of whether a posterior-tympanotomy-with-mastoidectomy or a suprameatal-without-mastoidectomy route was used. A short bony groove drilled lateral to the incus body and medial to the chorda creates a fixation tunnel that holds the electrode and keeps it off the canal skin and drum. Within the mastoid, the lead is left without tension at the cochleostomy and can be tucked medial to the incus short process or held with mesh ties to the incus bridge.[2009][2012][2013][2006]

Case 16.5 · Mastoidectomy and the Facial Reces

Three weeks after an uncomplicated cochlear implantation through a standard facial recess approach, a patient develops a House-Brackmann grade II facial weakness that was not present immediately post-op. Drilling notes record the recess was opened with limited irrigation and a thin bony cover over the nerve was preserved.

What is the most likely mechanism of this delayed facial nerve dysfunction?

Self-assessment — Module 52 questions

Question 1

Which three structures bound the facial recess (posterior tympanotomy)?

Question 2

What is the chief rationale for the suprameatal (Kronenberg) approach?

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