Chapter 17 Mastoidectomy—Canal Wall Down Procedure
There are two approaches to mastoidectomy in patients with cholesteatoma and chronic otitis media. The canal wall up (also known as intact canal wall) technique is discussed in detail in Chapter 16. This chapter describes the technique for canal wall down surgery. Related topics discussed in this chapter include the role of atticotomy, mastoid obliteration procedures, reconstruction of canal wall down cavities that have not been previously reconstructed, management of dural venous sinus injury during chronic ear surgery, and facial nerve monitoring in chronic ear surgery.
In the classic modified radical mastoidectomy, the epitympanum and canal wall down mastoidectomy and external auditory canal (EAC) are converted into a common cavity. The tympanic membrane and middle ear are left undisturbed.
The radical mastoidectomy eradicates middle ear and mastoid disease by converting the mastoid antrum, middle ear, and EAC into a common cavity. The tympanic membrane and ossicular chain are sacrificed. No effort is made at reconstructing a middle ear space; however, a tissue plug or graft is usually placed to seal the orifice of the eustachian tube.
In tympanoplasty with canal wall down mastoidectomy, the mastoid air cells are exteriorized and form a common cavity with the EAC. The middle ear is reconstructed by grafting the tympanic membrane and possibly reconstructing the ossicular chain. The terminology can sometimes be confusing. Some authors refer to this procedure as a modified radical mastoidectomy. To be accurate, that term should be applied to the Bondy modified radical mastoidectomy.
In an atticotomy, only a limited portion of the wall of the EAC is sacrificed. A small attic cholesteatoma is exteriorized by drilling the scutum to the limits of the cholesteatoma sac. The defect is reconstructed with a cartilage graft or autologous bone.
Mastoid obliteration procedure refers to a possible modification of the above-discussed mastoid procedures in which soft tissue, bone pâté, or biocompatible materials are used to fill the space of the mastoid cavity in an effort to limit postoperative mastoid cavity problems.
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) 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.
Some authors have argued that canal wall down surgery permits excision of cholesteatoma in the sinus tympani region.1 Anatomically, this is not full visualization because the depths of the sinus tympani are medial to the facial nerve. Nonetheless, Hulka and McElveen2 showed that canal wall down procedures do permit additional visualization in the anterior epitympanum and sinus tympani region. The latter is not fully visualized with any technique. Another relative indication for canal wall down mastoidectomy is 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 recurrent cholesteatoma with 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.
With the exception of the preoperative identification of an attic cholesteatoma in an only hearing ear, usually the decision to perform a canal wall down technique is made intraoperatively. Characteristics such as extensive canal wall destruction by cholesteatoma, a large labyrinthine fistula with an extensive cholesteatoma, and a severely contracted mastoid all are features 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 purposes of exposure. Similarly, as the operation is proceeding 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.
A detailed microscopic examination of the ear is necessary preoperatively. 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 “fish-eye” view of these pockets. This technique permits wider visualization of the pocket. In these cases, computed tomography (CT) scan can be helpful to define whether what seems 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 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 show a large cyst in the antrum, an atticotomy 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 pre-existing canal wall down cavity that is draining or is retaining significant debris, the surgeon must evaluate four specific characteristics of the cavity: (1) 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 dealt with appropriately in the revision procedure.
Audiometric studies are routinely obtained preoperatively. Generally, we repeat audiometric studies done elsewhere before the patient undergoes surgery. Studies particularly should be repeated if the “outside” audiograms do not coincide with tuning fork tests. Special attention must be given to the adequacy of masking with the audiometric studies performed.
As with any otologic procedure, we counsel patients that there are three principal risks: (1) hearing loss that may be total in the operated ear (<1%); (2) dizziness that is usually temporary, but rarely can become permanent; and (3) facial nerve paralysis, which is quite rare, but is a distressing complication for the patient. Generally, patients who undergo canal wall down surgery also have facial nerve monitoring. Our indications and rationale for this are discussed later.
Patients undergoing canal wall down surgery should be specifically advised that the ear canal will be larger postoperatively. Healing after a canal wall down procedure takes longer than in a canal wall up procedure. At the preoperative visit, patients are counseled that they should begin applying otologic antibiotic drops immediately after surgery to begin dissolving the packing. After the initial postoperative visit at 2 weeks, half-strength vinegar irrigations are used to remove residual packing and limit granulation tissue formation. These cavities are usually healed by 2 to 3 months postoperatively.
The patient is placed supine on the operating table with the head turned away. Hair is shaved approximately 2 to 3 fingerbreadths behind the pinna. Adhesive plastic drapes are applied surrounding the edges of the shaved hair. Approximately 3 to 4 mL of lidocaine (Xylocaine) 1% with epinephrine 1:100,000 is injected in the postauricular region and in the posterosuperior aspect of the ear canal. Sterile preparation is accomplished with povidone-iodine solution, allowing solution to enter the ear canal as well.
In cases in which facial nerve monitoring is used, electromyographic needle electrodes are placed in the orbicularis oculi and orbicularis oris muscles and in the forehead and ipsilateral shoulder to act as ground for monitoring and for stimulating. Sterile towels are placed around the prepared area. The area is dried with a sterile towel, and a large sterile adhesive drape is applied that overlies the sterile prepared area and holds the sterile towels in position.
As mentioned previously, the decision to convert a mastoid operation from canal wall up to canal wall down is usually made intraoperatively. The usual steps for tympanoplasty with mastoidectomy surgery would have been accomplished, including canal incisions, as described in the chapters on tympanoplasty (undersurface graft Chapter 12 and lateral graft Chapter 9). The ear has been reflected forward, the middle ear work has been completed, the tympanic membrane remnant has been prepared for grafting, and the mastoidectomy has begun.
The basic principle of canal wall down mastoidectomy surgery is to eliminate all of the disease and exteriorize the mastoid antrum in continuity with the EAC. In addition to removing all diseased air cells, the following four steps are basic to creating a trouble-free mastoid cavity.
Removal of bone from the edges of the mastoid defect saucerizes or bevels the edges of the defect so that there is no overhanging bone obstructing the wider cavity below (Fig. 17-1). This saucerization ensures that there is no disease tissue laterally. Also, saucerization permits soft tissue surrounding the mastoid to slide into the defect. Paradoxically, removing additional bone around the margins of the mastoid cortex in this saucerization step actually makes a smaller cavity rather than a larger cavity.
The boundary between the EAC and the mastoid cavity is defined by the height of the facial nerve. Leaving excessive bone overlying the facial nerve (facial ridge) between the EAC and the mastoid cavity creates a situation in which there is a deep trough on the mastoid side (Fig. 17-2). This high ridge is sometimes referred to as a “beginner’s hump.” Occasionally, novice mastoid surgeons leave a large ridge of bone overlying the facial nerve for fear of injuring it. This ridge creates a difficult situation postoperatively with trapped mastoid spaces that are hard to clean in the office, and it prevents natural cleaning. The surgeon should lower the ridge of bone overlying the facial nerve so that the fallopian canal is barely visible with a thin amount of bone overlying the vertical segment of the facial nerve.