Overview of Otitis Externa

Causes of otitis externa are now defined as primary or secondary, with factors that contribute to or promote disease. It is also standard of care to determine whether the cause is curable or lifelong management is required. Primary causes of otitis externa are those that create disease in a normal ear. They can cause otitis without any other cause or factor and can be subtle; they often go unrecognized by owners and veterinarians until secondary causes develop. Primary factors alter the ear environment, which allows secondary infections to develop. The major primary causes of otitis externa are allergy, autoimmune (eg, pemphigus), endocrine, epithelialization disorders, foreign bodies, glandular disorders, immune-mediated (eg, drug reactions), fungal (eg, aspergillosis), parasites, viral (eg, canine distemper), and miscellaneous (auricular chondritis, eosinophilic diseases, juvenile cellulitis, proliferating necrotitizing otitis of cats). Secondary causes are those that cause disease in an abnormal ear. These causes are relatively easy to eliminate and include bacteria, fungi, medication reactions, overcleaning, and yeast overgrowth.

Factors are elements related to the disease or pet that contribute to or promote the otitis externa by altering the structure, function, or physiology of the ear canal. Factors are subdivided into predisposing factors, which are present before the development of the ear disease, and perpetuating factors, which occur as a result of the inflammation. Predisposing factors include conformation of the ear, excessive moisture, obstruction of the ear canal (eg, polyp, feline apocrine cystadenomatosis), primary otitis media (eg, primary secretory otitis media, otitis media due to neoplasia or respiratory disease), systemic diseases (eg, catabolic states), and treatment effects (eg, alterations of normal microflora, trauma from cleaning). Perpetuating factors include changes in the ear epithelium (eg, failure of migration), ear canal (eg, edema, stenosis, proliferation), tympanic membrane (eg, dilated, ruptured), glandular (eg, sebaceous hyperplasia), pericartilaginous fibrosis (eg, calcification), and middle ear (eg, filled with debris, otitis media). This system is currently referred to as the PSPP classification system. A prognosis should be given for otitis externa. Curable means that the component of the problem is readily resolved with treatment (for weeks) or via surgery. Longterm management indicates that the component of the ear problem may be resolvable, but it can take months of treatment. Lifelong treatment indicates that the owner will need to play an active role in management for the life of the pet.

There is no recognized sex distribution for otitis externa. Young animals may be more commonly affected. There are clear breed predispositions for otitis, which directly reflect the breed predispositions for skin disease (eg, allergies in retrievers and terriers). The most common historical findings are headshaking and aural pruritus.

The first step in physical examination is determination of the severity of pain. This can be done by gentle palpation or petting of the animal. If the ear is painful or the degree of discomfort is high, the animal should be sedated before performing any further diagnostic testing. The second step is gentle palpation and manipulation of the ear canal and pinna to determine the presence of swelling, pruritus, fibrosis, or calcification. The presence or absence of these findings will help determine whether advanced diagnostics are needed, specifically imaging of the ear canal. Next, the outside of the ear should be examined, noting erythema, edema, crusts, scale, ulcers, lichenification, hyperpigmentation, or exudate. The pinnae and periauricular regions should be examined for evidence of self-trauma, erythema, and primary and secondary skin lesions. Pinnal deformities, hyperplastic tissue in the canal, and headshaking suggest chronic otic discomfort. If the otitis is unilateral, the unaffected ear should be examined first to prevent iatrogenic contamination of the unaffected ear with organisms (eg, Pseudomonas aeruginosa or Proteus mirabilis) that may be present in the diseased ear. The unaffected ear may, in fact, be diseased, meaning that the differential diagnosis list should also include causes of bilateral otitis.

Otoscopic examination is often not possible because the ear is painful, swollen, or filled with exudate; sedation is usually required. Swelling of the ear canal often makes it impossible to see the tympanic membrane. A handheld otoscope must have enough light and magnification to clearly visualize the external canal to the level of the tympanic membrane. Disposable otoscopic ear cones are recommended, because studies have demonstrated contamination of cones. Handheld otoscopes are available with a variety of heads, including magnification options and surgical operating heads, that allow for visualization of the ear canal while inserting another instrument. The surgical head is used when biopsies, foreign body removal, or deep flush of the canal is anticipated.

A video otoscope provides magnifcation of the ear canal and tympanic membrane. Most have a working channel through which biopsy instruments, catheters for flushing debris from the canal, and laser tips can be passed. Video otoscopes allow visualization through water and saline to determine the integrity of the tympanic membrane and to facilitate sampling and culture of the middle ear.

During an otoscopic examination, the ear canal should be inspected for changes in diameter, pathologic changes in the skin, quantity and type of exudate, parasites, foreign bodies, neoplasms, and changes in the tympanic membrane. The tympanic membrane should be examined for evidence of disease or rupture. However, in many cases of otitis, the character of the ear canal and tympanic membrane cannot be visualized at all until the exudate is gently flushed from the canal. Samples for cytologic evaluation and culture should be obtained before the ear is flushed. Examination is attempted again after the ear is dried. In chronic cases, the canal may be too stenotic, either from hyperplasia or edema, to be examined. Systemic glucocorticoids given daily for 1 wk may reduce swelling enough to allow examination.

If sedation is not needed, samples for ear diagnostic tests should be collected: skin cytology from the external and inner pinnae, cytology of any exudates present, hair trichograms and skin scrapings for Demodex, and ear swab cytology with mineral oil in young and adult animals (especially cats, because feline demodicosis can present as pruritic otitis). Wood's lamp examinations need to be done with care, keeping in mind that the key color is apple-green fluorescense and that sebum can glow yellow. Dermatophytosis affects the hair of the pinnae and hairs in the concave surface of the ear canal.

Cytologic evaluation of exudate or cerumen taken from the horizontal ear canal may provide immediate diagnostic information. The external ear canals of most dogs and cats harbor small numbers of commensal gram-positive cocci. These organisms may become pathogenic if the microenvironment is changed and encourages their overgrowth. Exudate obtained with a cotton-tipped applicator can be rolled onto a glass slide, stained with a 3-step quick stain or modified Wright’s stain, and examined under a microscope. (A study has shown that heat fixing is not necessary for ear swab cytology.) Smears should be examined microscopically under 4x, 10x, and oil immersion to look for numbers and morphology of keratinocytes, bacteria, yeasts, and WBCs; evidence of phagocytosis of microorganisms; fungal hyphae; and acantholytic or neoplastic cells.

A stained smear can quickly determine whether microbial overgrowth is present. Coccal organisms are usually staphylococci or streptococci. Rod-shaped organisms are usually Pseudomonas aeruginosa, Escherichia coli, or Proteus mirabilis; their appearance in large numbers indicates that a bacterial culture with antibiotic sensitivity should be performed because of their known resistance to many antimicrobial agents. The presence of many neutrophils phagocytizing bacteria confirms the pathogenic nature of the organisms.

The yeast Malassezia pachydermatis is found in low numbers in the ear canals of many healthy dogs and cats. Because yeasts colonize the surface of the ear canal, they are most easily found adhered to clumps of exfoliated squamous epithelial cells. M pachydermatis is identified readily on microscopic examination and its numbers easily assessed. There is no specific number that indicates yeast overgrowth. The key determining factor is whether the ears are pruritic. In addition, if previous treatment did not include antifungal therapy and if otitis externa is recurrent, antifungal therapy is warranted.

A dark exudate in the canal usually signals the presence of either Malassezia spp or a parasite but may also be seen with a bacterial or mixed infection. In addition to stained cytology, otic exudate should be examined for eggs, larvae, or adults of the ear mite Otodectes cynotis and for Demodex mites in dogs and cats, and Psoroptes cuniculi in rabbits and goats. Smears are made by combining cerumen and otic discharge with a small quantity of mineral oil on a glass slide. A coverglass should be used, with the smear examined under low-power magnification. Rarely, refractory ceruminous otitis externa may be associated with localized proliferation of Demodex sp in the external ear canals of dogs and cats and may be the only area on the body affected.

Microbial cultures are taken before otoscopy is completed and before any cleaning is done. Samples for culture should be taken with a sterile culturette from the horizontal canal (the region where most infections arise) or from the middle ear in cases of tympanic rupture. A bacterial culture and antibiotic sensitivity and an antibiotic mean inhibitory concentration should be done.

Histopathologic changes associated with chronic otitis externa are often nonspecific. Histopathologic evidence of a hypersensitivity response may support a recommendation for intradermal allergy testing or for a hypoallergenic diet trial. In addition, biopsies from animals with chronic, obstructive, unilateral otitis externa may reveal whether neoplastic changes are present.

Radiography of the osseous bullae is indicated when proliferative tissues prevent adequate visualization of the tympanic membrane, when otitis media is suspected as a cause of relapsing bacterial otitis externa, and when neurologic signs accompany otitis externa. Fluid densities and proliferative or lytic osseous changes provide evidence of middle ear involvement. Unfortunately, radiographs are normal in many otitis media cases. CT or MRI, if available, should be performed for cases of severe, chronic otitis.

Key to treatment is a discussion with the owner regarding the suspected or known cause of the otitis externa, whether the otitis is curable, and whether treatment must be longterm for resolution or lifelong management will be required. All primary and secondary causes and predisposing factors need to be identified, managed, and treated. Management of pain or pruritus must be included in the initial treatment protocol. Tramadol for the first 5–7 days at 5 mg/kg, PO, tid, may be especially beneficial. In addition, otitis externa is one of the few dermatologic conditions in which glucocorticoids are beneficial in the face of concurrent antimicrobial use or sepsis. Glucocorticoids decrease swelling of the ear canal and may be key to successful treatment. Prednisone or triamcinolone is used most commonly. Duration depends on the severity. Ear hygiene is important; in particular, the hair from the pre- and periauricular area should be clipped, as well as hair from the surface of the inner pinnae and ends of the ears. This facilitates cleaning and treatment of the ears. Plucking of hair from the ear canal is controversial but may be needed to adequately resolve the ear infection. Hair plucking is painful and should be done under anesthesia.

The first ear cleaning should be done in the veterinary clinic, and owners should be instructed not to clean the ears until recheck in 5–7 days. Owners are often unable to clean the ears and/or are too aggressive, causing further damage. Owners should initally focus on administration of topical and/or systemic drugs and can begin to clean the ears after the first recheck and if the otitis is resolving. It is important to remember that topical medications are inactivated by exudates, and excessive cerumen may prevent medications from reaching the epithelium. The ears should be gently cleaned with an ear cleaner that will remove the debris in the canal. Thick, dry, or waxy material requires a ceruminolytic solution such as carbamide peroxide or dioctyl sodium sulfosuccinate (DSS). If rods are seen, the ear cleaner should contain squalene, because one possible cause is Pseudomonas, which can produce a biofilm that protects bacteria from antibiotics. The ears should be thoroughly rinsed with warm water to remove residual ear cleaner. If the tympanic membrane is ruptured, detergents and DSS are contraindicated; milder cleansers (eg, saline, saline plus povidone iodine, Tris EDTA) should be used to flush the ear.

Effective treatment may require both topical and systemic antimicrobial therapy, along with pain medications and glucocorticoids.The duration of treatment may vary from 7–10 days to >30 days, depending on the diagnosis. In treatment of acute bacterial otitis externa, antibacterial agents in combination with corticosteroids reduce exudation, pain, swelling, and glandular secretions. The least potent corticosteroid that will reduce the inflammation should be used (see Corticosteroids).

Most commercial topical products contain a combination of antibiotic/antifungal and glucocorticoids. The volume of the ear canal in most dogs is 1 mL, and adequate treatment requires instillation of at least this volume twice daily. Products with an aqueous base or those that have a thin film should be used; ointments are to be avoided.

Irritating medications (eg, home remedies and vinegar dilutions) should be avoided. They cause swelling of the lining of the ear canal and an increase in glandular secretions, which predispose to opportunistic infections. Substances that are usually not irritating in normal ear canals may cause irritation in an ear that is already inflamed. This is particularly true of propylene glycol. Powders, such as those used after plucking hair from the canal, can form irritating concretions within the ear canal and should not be used.

Systemic therapy should be incorporated into the treatment regimen in most cases of chronic otitis and in any case in which otitis media is suspected. The most common cause of recurrent otitis externa is undiagnosed otitis media. Failure to use systemic antimicrobial therapy is an important cause of chronic ear disease in dogs. Systemic antibiotics should be used when neutrophils or rod-type bacteria are found on cytology, in cases of therapeutic failure with topical antimicrobial agents, in chronic recurring ear infections, and in all cases of otitis media. (Also see Systemic Pharmacotherapeutics of the Integumentary System.) Yeast infections in dogs can be treated with oral ketoconazole 5 mg/kg/day, PO, for 15–30 days. Ketoconazole should not be used in cats; itraconazole 2–3 mg/kg/day for 15–30 days or one week on/one week off is recommended.

Duration of treatment will vary depending on the individual case but should continue until the infection is resolved based on reexamination and repeat cytology and culture. Animals with bacterial and yeast infections should be physically examined, with cytologies evaluated weekly to every other week until there is no evidence of infection. For most acute cases, this takes 2–4 wk. Chronic cases may take months to resolve, and in some instances, a therapeutic regimen must be continued indefinitely.

Methicillin-resistant Staphylococcus intermedius and Pseudomonas otitis (caused by Pseudomonas aeruginosa) have emerged as frustrating and difficult perpetuating causes of otitis because of the development of resistance to most common antibiotics. These infections are often chronic in course (>2 mo) and associated with marked suppurative exudation, severe epithelial ulceration, pain, and edema of the canal. Successful treatment is multifaceted and should include the following steps: 1) identify the primary cause of the otitis and manage it, 2) remove the exudate via irrigation of the ear canal, 3) identify and treat concurrent otitis media, 4) select an appropriate antibiotic from the results of culture and mean inhibitory concentration on the organism and use it at an effective dosage for an appropriate duration, and 5) treat topically and systemically until the infection resolves (weeks to months).

The best treatment of chronic otitis is prevention. In addition to identifying the cause of acute otitis, topical and/or systemic medications should be chosen based on cytology or culture; they should have a narrow spectrum and be specific for the current condition. Aminoglycosides and fluoroquinolone antibiotics should not be used unless absolutely required for successful treatment but are the most common ingredients in topical otic medications. Because many topical products contain a combination of glucocorticoid, antibiotic, and antifungal medications, it is imperative to educate the owner on proper use (frequency and duration). Many owners discontinue treatment when the ear “looks better” before the infection is resolved. Polymyxin B and fluoroquinolone antibiotics have shown the best success in controlling Pseudomonas infections in cases in which resistance has been identified through culture. However, resistance is developing to fluoroquinolones.

Owners should be shown how to properly clean the ears. The frequency of cleaning usually decreases over time from daily to once or twice weekly as a preventive maintenance procedure. The ear canals should be kept dry and well ventilated. Using topical astringents in dogs that swim frequently and preventing water from entering the ear canals during bathing should minimize maceration of the ear canal. Chronic maceration impairs the barrier function of the skin, which predisposes to opportunistic infection. Preventive otic astringents may decrease the frequency of bacterial or fungal infections in moist ear canals. Clipping hair from the inside of the pinna and around the external auditory meatus, and plucking it from hirsute ear canals, improves ventilation and decreases humidity in the ears. However, hair should not routinely be removed from the ear canal if it is not causing a problem, because doing so can induce an acute inflammatory reaction.