Middle ear and mastoid surgery

land has been one of the greatest triumphs of the evolutionary process (perhaps the greatest has been the rewriting of complex neural patterns to allow mankind to walk on two legs). This process has overcome the problems of delivering airborne sound to the auditory sensory cells that lie in the fluid filled spaces of the inner ear. The sensory ‘hair cells’ convert sound energy to neural impulses in the acoustic nerve in much the same way as microphones convert sound to an electrical signal. The answer in reptiles, birds and mammals has been the development of a flexible tympanic membrane with air both sides of it, connected to the fluids of the inner ear by a rigid bony mechanism called the ossicular chain. Sound is absorbed by the membrane then transmitted by the ossicles [malleus (hammer), incus (anvil) and stapes (stirrup) in mammals] through an opening in the bony shell surrounding the inner ear. This opening is called the oval window and in it sits the footplate of the stapes. In this way hair cells can be stimulated by airborne sound. The middle ear, which contains the ossicles, originates from the first ‘gill cleft’ (branchial arch) of our aquatic ancestors. This cleft becomes the Eustachian tube (ET), the middle ear spaces and the air-filled mastoid cavity and is lined with mucosal tissues that resemble the lining of the nose and sinuses in that they produce mucus and absorb oxygen. The ET connects the middle ear spaces with the back of the nose and the whole system lies within what is called the petrous (from the Greek petra meaning rock) temporal bone. This is extremely hard and gives protection to the sensitive structures within. Running through the petrous temporal bone is the facial nerve, with its origin in the brainstem supplying INTRODUCTION the muscles of facial expression. With the development of the middle ear mechanism the facial nerve is forced to take a complex course through the middle ear where it lies protected by only a very thin bony shell (Fallopian canal) before it turns to leave through the base of the skull deep to the tip of the mastoid. In mammals, unlike birds and reptiles, there is the additional development of a skin-lined external ear canal from the first branchial groove on the surface of the developing embryo. Thus, the ear canal and the external surface of the eardrum are covered with skin (ectoderm) whilst the middle ear structures are covered with mucosa (endoderm). The middle ear is in effect an air-filled space within the body lined with epithelium similar to that lining the nose. As far as ear disease is concerned this causes two problems. The middle ear mucosa absorbs oxygen at a greater rate than carbon dioxide is released into the middle ear spaces and this rate is increased if the mucosa is inflamed. Continued absorption results in a fall in middle ear pressure so that the eardrum becomes indrawn and less efficient at absorbing sound. Mucus and cellular debris are also produced by the lining of the middle ear and have to be removed. The ET undertakes the dual need of replenishing the middle ear gases to prevent the pressure falling and of removing debris. Unfortunately, in man, and particularly in children, ET function in many individuals is not up to this task. The reasons are many but the outcomes are very similar.