Otitis media (OM) is by definition disease within the middle ear, and can be acute or chronic.
6.1 AETIOLOGY AND PATHOGENESIS OF ACUTE OTITIS MEDIA
OM can result from descending disease as an extension from problems within the external ear canal; as ascending disease from the nasopharynx via the eustachian tube; by haematogenous spread; or from primary disease within the tympanic bulla. In the dog OM is usually recognized as a progression from chronic infectious otitis externa. This may be due to infection, particularly with Gram-negative bacteria, or from foreign bodies within the canal perforating the tympanum. These may be extraneous material such as grass seeds or ceruminoliths caused by failure of epithelial migration.
Ascending infection, possibly from respiratory infection, occurs in the cat; descending infection is far less common. When disease spreads from the external ear canal it must penetrate the tympanic membrane. This may be because the tympanic membrane has become porous or perforated due to infection, inflammation, or a foreign body. The tympanic membrane can heal in the face of OM trapping abnormal discharge and infection within the tympanic bulla. Careful visualization of the tympanum is therefore essential when assessing an animal for OM. Cholesteatoma occurs when there is invagination of the tympanic membrane into the epitympanic recess. If the tympanic membrane becomes adherent to the cochlear promontory a cholesteatoma can form. When disease starts in the middle ear it may be caused by primary secretory abnormalities or growths within the tympanic bulla. Primary secretory otitis media (PSOM) is most commonly recognized in the Cavalier King Charles Spaniel and can occur without signs of otitis externa. Dogs present with typical signs of OM (see Table 6.2). Nasopharyngeal polyps are rare in the dog; most growths in the middle ear tend to be more aggressive neoplastic processes. Feline PSOM has not been identified. OM is more commonly caused by nasopharyngeal polyps originating within the tympanic bulla. Growths within the feline ear often present with signs of otitis externa, media, and interna. Occlusion of the external ear canal due to an extension of the polyp through the tympanic membrane into the horizontal canal commonly causes infectious otitis externa, often with Gram-negative bacteria. Growth of the polyp within the middle ear frequently leads to damage to the sympathetic nerves that traverse the bulla septum, leading to signs of Horner’s disease, and where pressure is exerted on the round window vestibular disease can also be seen.
The mucoperiosteum which lines the tympanic bulla responds to inflammation by producing mucus: this traps infection within the bulla and perpetuates the inflammatory process. Changes occur within the bulla as the disease becomes more chronic and the simple cuboidal epithelium changes to a pseudostratified columnar epithelium. Where long-standing oedema and an inflammatory infiltrate are present granulation tissue forms within the bulla, making the area less accessible to therapy (Fig. 6.1). As the disease progresses, further fibrosis and calcification of the soft tissue occurs and the bulla progresses to become irreversibly damaged.
6.1.1 Otitis Media as a Result of Descending Disease
Infection
Chronic bacterial otitis externa is the most common descending infectious trigger for OM. Where infection has been present for more than 6 months it is uncommon for OM not to be present. Long-standing disease and chronic change predispose the ear to Gram-negative infection (Fig. 6.2) which tends therefore to be the most frequently identified pathogen. Malassezia rarely causes OM.
Foreign Bodies
Where foreign bodies penetrate the tympanic membrane they can lead to inflammation and secondary infection in the middle ear. Grass awns and other organic material can become embedded in wax in the horizontal canal, causing irritation and leading to head shaking and subsequent perforation of the tympanic membrane (Fig 6.3). Impacted medication, particularly some of the canker powders used by owners and groomers, can lead to damage to the eardrum. This can dry out within the canal causing subsequent damage. Ceruminoliths are accumulations of wax, skin cells, and hair that can form in the horizontal canal as a result of a reduction in epithelial migration (Fig. 6.4), the ear’s natural cleaning mechanism. Where ceruminoliths become adherent to the fine hairs at the base of the horizontal canal they can cause acute discomfort leading to head shaking and tympanic membrane damage (Fig. 6.5).
Cholesteatoma
Aetiology and Pathogenesis
Cholesteatomas may be congenital or acquired and are created when there is a medial retraction of the tympanic membrane into the epitympanic recess due to negative pressure within the tympanic bulla; this may be as a result of chronic OM or abnormalities in eustachian tube function. This invagination of the keratinizing squamous epithelium of the tympanic membrane leads to the formation of a false middle ear within the epitympanic recess as the eardrum can adhere to the cochlear promontory (Fig. 6.6). Congenital lesions are most commonly recognized in young animals and are thought to occur due to abnormalities in the growth of the tympanic membrane. Acquired lesions in humans are thought to be associated with the long-term use of topical products based on propylene glycol. The same link has not been seen in dogs, but acquired lesions can occur as a result of chronic disease especially where there is marked stenosis or occlusion of the canal. Due to the slow progressive growth of the lesion it can causes severe bone changes at the contour of the tympanic bulla, including osteolysis, osteoproliferation, and osteosclerosis; expansion of the tympanic cavity; and sclerosis or osteoproliferation of the ipsilateral temporomandibular joint and paracondylar process. Cholesteatoma can cause lysis of the petrosal part of the temporal bone, leading to intracranial complications.
Clinical Signs
Dogs often have signs of chronic otitis externa; in addition there may be localized pain especially when opening the mouth to eat or bark, and a head tilt.
Key Diagnostic Tests
- Clinical signs may be suggestive.
- Radiography reveals an increased density within the middle ear.
- On CT the cholesteatoma appears as an expansile tympanic cavity mass (Fig. 6.7); this is in comparison to OM where expansion of the tympanic cavity is not seen. Other changes are as described earlier.
Otoliths
Aetiology and Pathogenesis
The origin of otoliths is unknown. They may be caused by the mineralization of necrotic debris from a previous episode of otitis externa where the disease has resolved and the eardrum has healed. They may occur de novo from disease within the middle ear. They have not been recorded in cats.
Clinical Signs
Although they have be associated with active signs of canine OM and in dogs with vestibular disease they are often present as an incidental finding in dogs, asymptomatic for ear disease, undergoing scanning for an unrelated problem. In humans otoliths are known to be a cause of labarynthitis.
Key Diagnostic Tests
- Otoliths are structures that can be identified most easily on CT scans of the middle ear in dogs; they appear as mineralized opacities (Figs 6.8, 6.9).
Figure 6.9 Otoliths can be seen as small mineralized deposits in both tympanic bullae. These are probably incidental findings.
6.1.2 Otitis Media Originating in the Middle Ear
Primary Secretory Otitis Media
Aetiology and Pathogenesis
Secretory abnormalities within the middle ear may be caused through overproduction of mucus within the middle ear or abnormalities in the elimination process, possibly due to failure of the mucociliary escalator and eustachian tube dysfunction. The Cavalier King Charles Spaniel is known to suffer from PSOM. Typically the middle ear contains thick tenacious mucus, ‘glue ear’ (Fig. 6.10), which causes bulging of the intact tympanic membrane.
Clinical Signs
OM due to PSOM is unusual in that there is often no concomitant otitis externa. In contrast to other causes of OM these dogs often demonstrate neurological signs, in addition to the typical signs of OM (see Tables 6.2–6.4). These include ataxia, facial paralysis (Fig. 6.11), nystagmus, head tilt (Fig. 6.12), and deafness which is often acute in onset. Neurological disease is caused by the increase in pressure within the tympanic bulla due to mucus accumulation. Deafness is usually a conductive problem caused by mucus build-up.
Key Diagnostic Tests
- Visualization of the tympanic membrane reveals that is bulges outwards due to the presence of fluid within the middle ear (Fig. 6.13). This can be seen with either a hand-held otoscope or a video otoscope.
- When myringotomy is performed the pressure within the bulla leads to rapid loss of viscous grey–yellow mucus through the myrintogomy hole (Fig. 6.14). Diagnosis can be difficult due to the overlap of the clinical signs with other diseases, notably syringomyelia. Brainstem auditory evoked response (BAER) traces usually show a 50–60% reduction (Fig. 6.15) which usually improves once the mucus has been flushed from the middle ear.
- Radiography or CT of the tympanic bulla reveals changes consistent with a middle ear effusion with or without a concurrent bulla osteitis (Fig. 6.16).
Figure 6.13 Bulging tympanic membrane in a case of PSOM.
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