Otitis Media

By | 2013-07-19

Otitis media may result from extension of otitis extema through the tympanic membrane, aspiration of pharyngeal contents up the auditory tube (e.g. a sequela to upper respiratory tract (URT] infection in cats), or from hematogenous spread. Extension from otitis externa is the most common cause of otitis media, but otitis media may serve as a perpetuating factor for otitis extema. Developmental abnormalities of the external ear canal and pharynx may also result in otitis media due to the accumulation of secretions in the middle ear. Neoplasia, inflammatory polyps, and middle ear trauma may be associated with secondary otitis media or result in similar clinical signs.

Cholesteatoma is commonly associated with otitis media and chronic otitis extema. A cholesteatoma is a mass of keratinized squamous cells that accumulate within a structure lined with stratified squamous epithelium. The lesion is presumed to develop when a pocket of tympanic membrane becomes adhered to inflamed middle ear mucosa. Significant narrowing of the external ear canal is usually present. Radiographic signs of increased density and bony changes of the tympanic bulla predominate with loss of the air-filled lumen of the external ear canal and concurrent calcification. Treatment is usually limited to total ear canal ablation and lateral bulla osteotomy due to the changes of the external ear canal and mass or accumulation of debris in the tympanic bulla.

The clinical signs associated with middle ear disease often reflect concurrent otitis extema (e.g. head shaking, lethargy, exudate, otic malodor). Significant otic pain, lethargy, inappetence, and pain upon opening the mouth are more suggestive of middle ear involvement. Neurologic signs may be present due to the course of the facial nerve and sympathetic innervation of the eye. Facial nerve paresis or paralysis result in facial asymmetry (i.e. uneven position of the lip commissures, unequal ear carriage, unilateral ptyalism) and abnormal cranial nerve reflexes on neurologic examination (e.g. menace response, palpebral and corneal reflexes, abnormal ear canal, and concave pinnal sensation). Homer’s syndrome, or loss of sympathetic innervation to the eye, can also be complete or partial (i.e. ptosis, miosis, enophthalmia, prolapse of the third eyelid). Otitis interna is usually evidenced by head tilt, abnormal nystagmus, and ataxia and should be differentiated from central vestibular disease based on careful neurologic examination. Otitis interna is not usually associated with ipsilateral hemiparesis or abnormalities in level of consciousness.

Cases of paraaural abscessation usually have concurrent otitis media. The primary cause may be trauma to the external ear canal, severe otitis externa, extension of otic neoplasia, or total ear canal ablation. Signs of middle or external ear disease and soft tissue swelling in the parotid area may be accompanied by draining tracts. A head tilt and pain upon palpation of the area are usually present.

The diagnosis of otitis media is based on a thorough history and physical, neurologic, and otoscopic examinations. A ruptured tympanum strongly suggests otitis media. The pharynx should also be evaluated on physical examination; identification of specific conditions may require general anesthesia due to anatomic location (eg. inflammatory polyps) or pain associated with examination (e.g. otitis media causing temporomandibular joint (TMJ) pain, severe otitis externa). General anesthesia may also be required to perform a complete otoscopic examination in cases of severe otitis externa in which thorough cleaning of the ear is necessary for therapy and diagnosis (i.e. visualization of the tympanum). Significant otitis externa is commonly associated with otitis media; the tympanic membrane is ruptured in up to 50% of dogs with otitis externa, although 70% of dogs with otitis media had an intact tympanic membrane in one study. The tympanic membrane in dogs with otitis extema may be difficult to examine due to secondary changes of the external ear canal, pain associated with otoscopic examination, and accumulation of exudate, cerumen, and debris. Treatment to diminish the severity of otitis externa and general anesthesia may increase the ability to evaluate the tympanum in these cases.

Any case that has significant cerumen, exudate, or debris should undergo careful cleaning of the ear canal to allow evaluation of the integrity and character of the tympanic membrane The presence of a “false middle ear” occurs when large accumulations of debris lodge against the tympanic membrane, causing it to deviate medially into the middle ear. This makes the external ear canal appear elongated and leads to misdiagnosis of a ruptured tympanic membrane.

Gende probing of the tympanic membrane with a red rubber catheter under direct visualization may assist in the diagnosis of small tears in the membrane. If the catheter tip is consistently visible, rupture is unlikely. Alternatively, an aliquot of 1 mL of physiologic saline placed in the horizontal canal should remain stationary; disappearance suggests an opening in the tympanum, allowing the fluid to drain into the middle ear. Movement of the fluid may be blocked by large amounts of debris in the middle ear, even in the presence of a tear in the tympanum.

If the tympanic membrane is visible, its character should be recorded in the medical record for comparison upon re-evaluation. Bulging, increased opacity, and hyperemia may be present with otitis media. If otitis media is suspected, radiographs of the bullae may be made. Lateral oblique and open-mouth views are most helpful for evaluating the tympanic bulla, but positioning for comparison of left and right sides is difficult and requires general anesthesia. Ventrodorsal or dorsoventral views allow evaluation of the air-filled lumen and calcification of the external ear canal. Abnormalities of the bulla include increased opacity, sclerosis, and lysis. Fluid cannot be differentiated from increased soft tissue density (e.g. neoplasia), and absence of radiographic changes does not rule out otitis media. Radiographic changes were absent in 33% of the middle ears in one study of dogs with otitis media confirmed by surgical exploration. Otitis media or neoplasia and otitis interna can cause radiographic evidence of lysis of the petrosal bone.

Other diagnostic tools are available to evaluate patients with otitis media interna. Contrast introduced into the external ear canal followed by radiography, termed canalography, is used to diagnose tympanic membrane perforations. The method is useful for acute tympanic membrane rupture and increases the frequency of diagnosing tympanic membrane rupture with concurrent otitis externa and media beyond that of otoscope alone Advanced imaging with CT and MRI have been studied in normal dogs and dogs with otitis media. CT is considered superior to MRI for bony changes, whereas MRI is better for detection of soft tissue abnormalities in both dogs and cats.

If the tympanic membrane is intact in a dog with otitis media, a myringotomy is performed to obtain samples for culture and susceptibility testing and cytologic examination. Affected dogs are often more comfortable after collection of samples due to decreased pressure in the middle ear after myringotomy. The procedure must be performed with general anesthesia and is usually done after radiography or advanced imaging of the ear. The external ear canal should be thoroughly cleaned and dried prior to myringotomy to avoid contamination with external ear canal debris. Direct otoscopic visualization is used for the procedure. A 20-gauge spinal needle is used to penetrate the tympanic membrane through the caudoventral aspect of the pars tensa. Suction is applied and samples collected — culture and susceptibility takes priority over cytologic examination because cytology is frequently negative, and cultures of the external ear canal do not reflect the middle ear bacteria in the majority of cases. If fluid cannot be aspirated direcdy from the middle ear, 0.5 to 1 mL of warm, sterile saline can be infused through the needle into the middle ear cavity and aspirated. Alternatively, an open-ended tomcat catheter or small, sterile culture swab may be passed into the middle ear cautiously under otoscopic visualization. Pseudomonas species and Staphylococcus intermedium are most commonly isolated, followed by yeast, β-hemolytic Streptococcus, Corynebacterium species, Proteus species, and Enterococcus species. Surgical exploration is rarely required for the diagnosis of otitis media.

Medical therapy of otitis media should be guided by culture and susceptibility results. The external ear canal is flushed and dried as necessary to treat concurrent otitis extema. Flushing is usually performed under the same general anesthetic episode used for diagnostic testing. If the tympanic membrane is ruptured, the middle ear should be gendy lavaged with warm saline. Cytology results, when available, should be used to guide initial therapy. The integrity of the tympanic membrane must be considered when using topical agents to treat concurrent otitis extema: ototoxic medications and vehicles should be avoided if the tympanic membrane is ruptured.

Newly diagnosed cases of otitis media may be started on empiric therapy based on cytology. First-choice antimicrobials include cephalosporins, amoxicillin and clavulonic acid, and fluoroquinolones. Definitive therapy consists of administration of antibiotics based on culture and susceptibility results for a minimum of 4 to 6 weeks. Primary and perpetuating factors of otitis externa should be identified and treated or controlled. Topical medication and flushing of the external ear canal should continue until resolution of clinical signs and normalization of cytology. Gradual improvement of the otitis media is expected within 14 days. The ear canal and tympanic membrane should be evaluated prior to and after discontinuation of therapy. Small tears in the tympanic membrane after myringotomy heal rapidly with appropriate therapy within 2 to 3 weeks.Hs However, re-evaluation of the tympanic membrane in dogs with otitis externa media should precede alteration of the topical agents in the therapeutic plan.

Failure to respond to therapy or chronic or recurrent otitis media warrant re-evaluation for surgical intervention. Total ear canal ablation and lateral bulla osteotomy should be considered in cases with severe secondary changes of the external ear canal and concurrent otitis media. If the external ear canal is not affected, a ventral bulla osteotomy may be performed to remove gross exudate and establish drainage from the middle ear of dogs and cats with chronic or recurrent otitis media. Caution should be taken in considering lateral ear resection and ventral bulla osteotomy in the treatment of concurrent otitis externa and media because lateral ear resection is only an adjunct to medical management of otitis externa.