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Recognizing and Treating Middle Ear Disease
D. White
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Summary
The middle ear is an important and frequently overlooked site of disease in both the dog and the cat. There are substantial differences between the aetiology of the main disease entities in the two species. In the dog, otitis media is most commonly the consequence of a descending bacterial ingress via the intact tympanic membrane as the sequel to chronic otitis externa. In the cat, an ascending aetiology is thought to be responsible for initiating a sterile inflammatory process in the middle ear. Medical and surgical management strategies should be tailored to reflect these differing aetiologies and their pathophysiological processes.
Anatomy
The Dog
The middle ear is phylogenetically derived from the pharynx and separates the external acoustic meatus from the cochlear and vestibular structures of the inner ear. It comprises the tympanic cavity, tympanic membranes, auditory tube and ossicles. The tympanic cavity, formed by the tympanic component of the temporal bone, is a bony shell situated caudal and medial to the zygoma and temporomandibular articulation. The cavity has dorsal, middle and ventral compartments and is lined with respiratory mucosa contiguous via the auditory tube with that of the nasopharynx. The dorsal component, the epitympanum or epitympanic recess, is the smallest of the three chambers and is largely occupied by the incus and part of the malleus. It is lined with cuboidal or squamous epithelium with few cilia. The middle component, the mesotympanum or true tympanic chamber, is a four sided chamber lined with columnar or cuboidal epithelium and variable numbers of ciliated cells.
The Cat
The feline middle ear has a similar anatomic arrangement to that of the dog but its chambers show more distinct separation into a larger ventral cavity, the hypotympanum, and a smaller component rostrolateral to this corresponding to the epitympanum and mesotympanum. The two components are almost completely separated by a bony septum and communicate on their medial aspect through a small slit-like opening which widens into a distinct foramen caudally. This separation gives rise to the characteristic "double shell" profile of the tympanic structures visible on anteroposterior radiographic views.
Pathophysiology
Bacterial Otitis Media: In principle, bacterial access to the middle ear can arise via one of the following three routes:
- the external auditory meatus via the tympanic membrane,
- the nasopharynx via the auditory tube,
- hematogenous distribution.
Extension from the external meatus via the tympanic membrane is considered to be by far the most common route for bacterial invasion in the dog and is usually the sequel to longstanding otitis externa in which there is chronic inflammation and accumulation of bacteria in the external acoustic meatus. The most commonly encountered micro-organisms in canine middle ear infections have been reported to be Staphylococcus spp, Pseudomonas spp, Pathological changes recognised in the middle ear as a consequence of established bacterial infection include thickening of the tympanic membrane and epithelial hyperplasia with thinning of the lamina propria and the development of associated granulation tissue.
Clinical Signs
The clinical signs of otitis media include otorrhea, head shaking and attention to the affected ear(s). Signs of pain include discomfort on aural manipulation, behavioural changes and intermittent alteration of the head carriage with the head carried lower on the affected side with unilateral disease. Sustained head-tilt is not encountered unless there is associated vestibular disease. In patients with inflammatory change extending to involve the nearby temporomandibular joint, signs of pain may be encountered on opening the jaw. These signs are similar to, and often very difficult to differentiate from, those encountered in patients with severe otitis externa. Since most cases of otitis media in the dog are the result of pre-existing otitis externa this is of particular significance since involvement of the middle ear may be easily overlooked. There is some evidence to suggest that signs of pain are more severe or obvious and that stenosis of the external meatus is more severe in patients in which the middle ear has become involved in the disease process. Otitis media may affect any of the nerves exposed within the tympanic chamber and possible neurological changes may include facial nerve palsy and Horner’s Syndrome. Facial nerve dysfunction is characterised by impaired palpebral function and facial and aural drooping. The incidence of facial palsy in dogs with established otitis media is reported to be approximately 10%. Cats affected by middle ear polyps may show a variety of signs depending on the location of the mass.
Diagnosis
A definitive diagnosis for diseases of the middle ear can only be made on the basis of histological, cytological and microbiological examination of samples removed from within the tympanic chamber itself. In the vast majority of cases this information is not readily available and a presumptive diagnosis is reached through a combination of physical and otoscopic examination, diagnostic imaging and other techniques.
Physical Evaluation
Evaluation of the patient should begin with a physical examination to establish if any generalised disease underlying the aetiology of the middle ear disease can be identified. Most cases of bacterial otitis media in the dog are considered to be the consequence of pre-existing otitis externa; therefore particular attention should be paid to the detection of dermatoses which may have initiated this.
Otoscopy
A satisfactory otoscopic examination of the patient with middle ear disease is rarely feasible in the conscious patient and is best performed under general anaesthesia. The identification of a stenotic external meatus or exudate obscuring the tympanic membrane is often a useful starting point for a more detailed examination. Any debris in the external meatus obstructing the view of the tympanic membrane should be removed by irrigation with physiologic saline and gentle aspiration. In many dogs with middle ear disease, the accompanying stenosis of the external meatus resulting from hypertrophic changes in the integument makes further examination difficult or even impossible. The normal tympanic membrane is a shiny, pink translucent structure through which the crescentic shape of the malleus should be clearly visible. The vascular arrangement and the concavity of the membrane should be readily appreciated. Any bulging, loss of translucency, change in colour or perforation of the membrane is suggestive of middle ear disease. A myringotomy should be performed in patients with middle ear disease to permit recovery of samples from the tympanic cavity for cytological, histological and microbiological isolation purposes. In most instances of otitis media, the bacterial isolate and the pattern of antibiotic sensitivity of micro-organisms differs significantly from those found in the horizontal canal. It is essential therefore that microbiological samples should be recovered by myringotomy from the tympanic cavity itself or from the immediate vicinity of the tympanic membrane.
Diagnostic Imaging
Radiographic views of the middle ear may be obtained in the anesthetized patient and include the open mouth anteroposterior, the lateral oblique and ventrodorsal views of the tympani bulla and surrounding structures. Radiological investigation of middle ear disease in the cat, however, is more rewarding in the detection of early disease. Advanced diagnostic imaging techniques such as magnetic resonance imaging (MRI) scanning appear to be much more sensitive in detecting and elaborating disease processes involving the middle ear. MRI has been shown to be capable of detecting early changes in the epithelial lining of the tympanic bulla and accumulations of fluid and cystic growths. Importantly, MRI also provides useful information regarding the surrounding tissues including the petrous temporal bone and extension to involve inner ear structures.
Treatment of Middle Ear Disease
The selection of the most appropriate strategy for the management of middle ear disease will always demand an accurate identification of the disease process. The choice of a medical versus a surgical option for bacterial otitis media in the dog will often require an assessment of the possibility of successful management of any underlying medical disease.
Medical Management of Otitis Media in the Dog
The majority of cases of bacterial otitis media in the dog are considered to be the result of extension of a coexisting otitis externa. Consequently the rationale for medical management should focus not simply on the elimination and control of the infectious process from the tympanic chamber but also on the sustained improvement of the external ear environment to prevent recrudesence of the problem. Many cases of otitis media can be controlled effectively through medical management but only if the changes in the external meatus can be satisfactorily improved. Following otoscopic investigation all debris within the external meatus should be removed to permit visualization of the tympanic membrane and, if the membrane remains intact at this point, myringotomy should be performed. Hence the site for perforation of the membrane is often dictated more by what is available rather than by selection of the optimal. Cases of severe external ear change may necessitate careful, virtually ‘blind’ myringotomy and require considerable digital sensitivity to appreciate the point at which the membrane is breached. Irrigation of the middle ear should be performed using warm physiologic saline instilled gently through the external meatus. Debris should be dislodged by repeated lavage / aspiration cycles with the cannula positioned in the external meatus. Patient repetition of this technique will remove all but the most inspissated debris from the middle ear. Direct irrigation with the cannula tip positioned in the tympanic cavity itself is unnecessary and should be avoided because of the increased risk of barotrauma to inner ear structures. All irrigating fluid should be removed once lavage is complete. Providing that debris has been satisfactorily removed from the middle ear the tympanic membrane has a remarkable capacity for repair and is normally capable of regenerating within 14 - 21 days. Both topical and systemic antibiotic therapies are appropriate for periods of 4 - 6 weeks. Topical antibiotics effective against Pseudomonas, Staphylococcus and Proteus spp. should be selected pending the results of microbiological isolation and antibiograms.
Surgery for Middle Ear Disease
Surgical is indicated where medical management is either no longer effective or deemed unlikely to be effective in the management of middle ear disease. The rationale for surgical management of middle ear disease is twofold, namely:
- elimination of the primary aetiology,
- removal of debris and diseased tissue from within the middle ear.
The selection of the surgical approach to the middle ear is largely dictated by the aetiology and the site of the underlying cause of the problem and the concept of the ventral approach providing superior drainage of the bulla than the lateral approach is controversial. In the dog, chronic otitis externa is the most common aetiology of the middle ear disease and the lateral approach which permits simultaneous access to the diseased tissue of the external meatus predominates. In the cat, the majority of middle ear problems are associated with inflammatory polyp disease and hence the ventral approach, giving superior exposure of the bulla structures but poor access to the external ear is preferred.
Lateral Bulla Osteotomy (total ear canal ablation)
The Dog
Lateral osteotomy of the tympanic bulla is normally combined with total ear canal ablation and is most commonly used in the management of bacterial otitis media in the dog which results from otitis externa with ‘end stage’ changes in the external meatus. The procedure achieves the twin goals of middle ear surgery sequentially, firstly it removes the chronically-diseased tissue of the external meatus which underlies the development of the otitis media through amputation of the tissues forming the external ear canal. Secondly, it permits access to the tympanic bulla itself to allow irrigation / aspiration of pus, the debridement of pathological epithelium and management of any osteomyelitis.
The Cat
Lateral osteotomy of the bulla is performed less frequently in the cat reflecting the lower incidence of middle ear diseases which arise as a consequence of external ear aetiologies. The main indication for lateral osteotomy in the cat is reported to be tumours of the external ear. The procedure is essentially similar to that in the dog although the incidence of neurological complications including Horner’s Syndrome and facial nerve palsy is considerably higher.
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