Mastoidectomy: Canal Wall-Down Procedure

Definitions

The surgeon may perform a series of mastoidectomy procedures that involve sacrificing a portion or all of the ear canal in continuity with the mastoidectomy.

Indications for Canal Wall-Down Mastoidectomy

Mastoidectomy in chronic ear surgery is designed to eliminate mastoid disease in the face of suppurative otitis media and, more commonly, cholesteatoma of the middle ear or mastoid. Generally, canal wall-up surgery is preferred to maintain the normal anatomic contours of the mastoid. Certain factors are strong indications for canal wall-down surgery, including (1) extensive damage by disease to the posterior canal wall, (2) a severely contracted mastoid with low-lying tegmen and far forward sigmoid sinus preventing adequate visualization through a standard canal wall-up approach, (3) cholesteatoma in an only hearing ear, and (4) labyrinthine fistula in an ear with extensive cholesteatoma ( Box 17.1 ).

Some authors have argued that canal wall-down surgery permits microscopic excision of cholesteatoma in the sinus tympani region. Anatomically, this is not full microscopic visualization because the depths of the sinus tympani are medial to the facial nerve. Nonetheless, Hulka and McElveen showed that canal wall-down procedures do permit additional microscopic visualization in the anterior epitympanum and sinus tympani region. The latter is not fully visualized microscopically with any technique; however, the otoendoscopes permit visualization of this area when used alone or as an adjunct to a microscopic exposure. Another relative indication for canal wall-down mastoidectomy is the failure of previous canal wall-up procedures with recurrent cholesteatoma from epitympanic retraction pockets. The anatomy of a canal wall-down mastoidectomy with full exteriorization of the epitympanum makes retraction pocket recurrences of cholesteatoma unlikely because the whole epitympanum has been exteriorized. Although the use of Silastic in canal wall-up facial recess surgery and staging has been significant in reducing the incidence of residual cholesteatoma with staged canal wall-up surgery, the presence of a scutal edge and a distinct epitympanum in cases with persistent eustachian tube dysfunction can produce recurrent cholesteatoma.

Decision-Making

With the exception of the preoperative identification of an attic cholesteatoma in an only hearing ear, the decision to perform a canal wall-down technique is usually made intraoperatively. Characteristics such as extensive canal wall destruction by cholesteatoma, a large labyrinthine fistula with an extensive cholesteatoma, and a severely contracted mastoid are all identified intraoperatively. As the operation proceeds, the surgeon may discover that certain areas of the mastoid, such as the epitympanum, are poorly visualized and may elect to perform a canal wall-down procedure for the purposes of exposure. Similarly, as the operation proceeds in a patient with multiple previous recurrences in the epitympanum through retraction pockets, the surgeon may elect to convert the mastoid into a canal wall-down cavity, especially if no clear technical reason for failure of the previous procedures has been identified other than chronic eustachian tube dysfunction.

Preoperative Evaluation

A detailed preoperative microscopic examination of the ear is necessary. Attic retraction pockets should be viewed with suspicion if the surgeon is unable to see the depths of the pocket. Using a right-angle pick to feel the depths of such a retraction is often helpful. The use of otoendoscopes has been quite helpful to permit a “fisheye” view of these pockets. This technique permits wider visualization of the pocket. In these cases, a computed tomography (CT) scan can be helpful in defining whether what appears to be a small retraction represents the neck of a cholesteatoma or is merely a small retraction. Although the CT scan may underestimate the extent of disease in this area, a large cyst extending into the antrum would be identified with a CT scan. An attic pocket in which the depths cannot be palpated that is beginning to retain debris is an indication for surgery. If the CT scan does not reveal a large cyst in the antrum or if the mastoid is extremely contracted and sclerotic, an atticotomy approach may be considered.

Similarly, a distinct attic cholesteatoma with a positive fistula test and the subjective symptom of dizziness is also an indication for imaging. The possibility of a labyrinthine fistula must be considered. Especially if there is a large cholesteatoma, a canal wall-down procedure should be considered.

In a preexisting canal wall-down cavity that is draining or retaining significant debris, the surgeon must evaluate four specific characteristics of the cavity: (1) the adequacy of saucerization of the mastoid cortex margins, (2) adequate lowering of the facial ridge, (3) adequate management of the mastoid tip, and (4) adequacy of the meatus. Problems with any of these characteristics can contribute to cavity failures. In chronic drainage situations, areas of persistent mucosalization should be identified so that these are managed appropriately in the revision procedure.

Audiometric studies are routinely obtained preoperatively. Generally, we repeat audiometric studies performed elsewhere before the patient undergoes surgery. In particular, studies should be repeated if the “outside” audiograms do not coincide with the tuning fork tests. Special attention must be given to the adequacy of masking with the audiometric studies performed.

Magnetic resonance imaging (MRI) has become a useful tool in complex ears with previous surgery, obliteration, or evidence of soft tissue in the mastoid on CT that is not clearly cholesteatoma. Non-echoplanar, diffusion-weighted imaging MRI sequences (some proprietary names for these MRI sequences are Haste and Propeller) are useful to identify such tissue as cholesteatoma if the tissue is hyperintense relative to the other soft tissue. Such information can be very useful for surgical planning because cholesteatoma requires excision or exteriorization, whereas scar or uninfected soft tissue can be left undisturbed.