Describe the problems of an open mastoid cavity and their prevention.

The Cavity and its Problems

• An open (canal wall down) mastoid cavity is created when the posterior canal wall is removed and the mastoid is exteriorised into the ear canal.
• The disease is exteriorised rather than excised, so the cavity becomes a permanent, skin-lined pocket that the patient lives with for life.
• Why cavities give trouble — the cavity is lined by skin but has lost the normal outward epithelial migration of the ear canal, so keratin and debris accumulate instead of clearing themselves. It is also poorly ventilated, humid, and subject to caloric stimulation by air and water
• The problems, in order of frequency:
• Accumulation of wax and debris, requiring regular cleaning under the microscope, every 6 to 12 months — and lifelong. This is the commonest problem and is the reason follow-up cannot be discharged
• The discharging (wet) cavity — from retained debris, recurrent infection (Pseudomonas), otomycosis, granulation tissue, exposed bone, or residual disease

Prevention and Management

• Prevention AT the original operation IS what determines whether A cavity behaves, and the surgical principles are specific:
• Lower the facial ridge to the level of the facial nerve, so no ledge obstructs the view or the drainage
• Saucerise the cavity — make it a smooth, shallow, round-edged bowl with no overhangs or dependent pockets
• Exenterate all air cells, especially at the tip, sinodural angle and zygomatic root.
• Perform an adequate meatoplasty — a wide meatus is what allows the cavity to ventilate, dry and be cleaned; this single step prevents more problems than any other
• Line the cavity with healthy skin or graft, and remove all diseased mucosa
• Reduce the cavity size where possible — obliteration with muscle, fascia, bone pate or cartilage, or reconstruction of the canal wall.
• Management of an established problem cavity:
• Regular aural toilet under the microscope, which resolves the majority of wet cavities by itself
• Topical antibiotic and antifungal preparations, guided by culture, and acidifying or drying agents; with cautery of granulations with silver nitrate or trichloroacetic acid