Otitis Externa: Inflammation of the Ear Canal

Otitis externa is a common condition that is frequently seen in specialty dermatology clinics and small animal general practices.^1,2 It is defined as inflammation of the ear canal with or without involvement of the pinna.^1–3 Otitis is often considered a final diagnosis, but it is merely a clinical sign. The many factors that can contribute to otitis need to be addressed so that initial infections do not progress to chronic changes, such as irreversible structural damage to the ear canal and cartilage.

Sandra Grable, conducts an examination of the ear canal. Photo courtesy of Bradley Leeb, University of Illinois

Structures of the External Canal

The external ear consists of 3 structures: the pinna, the external ear canal (vertical and horizontal), and the tympanic membrane (TM). The pinna collects and transmits sound waves toward the TM.⁴ It is composed of auricular cartilage covered by skin. The vertical canal begins at the tragus, antitragus, and anthelix cartilages of the pinna; dogs may have few hairs at the entrance. Within the skin of the canals, hair follicles are present and the associated sebaceous and ceruminous glands produce cerumen. These secretions trap debris, protecting the canal and tympanum, and keep the TM flexible and moist.^1,4

FIGURE 1. Normal canine canal showing pars flaccida, pars tensa, manubrium of the malleus, and minimal cerumen.

The TM, which is semitransparent and concave, separates the external and the middle ear. The angle of the TM differs between dogs and cats: 30 to 45 in dogs and 90 in cats.^1,4–6 It has 2 distinct areas: the pars flaccida and the pars tensa (FIGURE 1). The pink, triangular portion that lies dorsally is the pars flaccida, which supplies blood to the pars tensa. The pars tensa is a thin, translucent or opaque region that stretches across the canal and may appear striated. The manubrium of the malleus is embedded in 1 of the 3 layers of the pars tensa and points rostral.^5,6 This knowledge is helpful when viewing video otoscopy images of the TM to identify which ear you are observing.

Classification of Otitis

Otitis externa is classified into 4 categories, which helps in identifying the problem: predisposing, primary, secondary, and perpetuating factors.

Predisposing Factors

The predisposing factors of otitis do not cause ear disease. They only make the animal more susceptible to it or a more severe disease. Predisposing factors may be conformational, obstructive, or iatrogenic; increased moisture in the ear canal is also a predisposing factor.

Conformational factors are attributed to floppy pinnae, such as those seen in bloodhounds, cocker spaniels, and basset hounds. This ear conformation can create a warm, moist environment (increased humidity) due to insufficient ventilation, resulting in the potential for overgrowth of the normal commensal flora. Cats do not appear to be affected by conformational differences of the pinnae, as seen with the Scottish fold.³ Excessive hair in the canal, as seen in poodles and schnauzers, can also decrease ventilation and form hair mats that retain debris and create obstructions.⁷ Shar-peis have stenotic canals that may be predisposed to higher humidity levels and secretions, leading to overgrowth of normal microbial inhabitants.^1,7 An increase in glandular tissue can lead to an increase in cerumen production and debris accumulation, which seems to be more common in cocker spaniels, springer spaniels, and Labrador retrievers.^1,7

Increased moisture in the ear canal can result from swimming, bathing, or high humidity.

FIGURE 2. Ceruminous gland adenoma in the canal of a dog.

Obstructive factors may inhibit epithelial migration (self-cleansing mechanism of the canal), resulting in a secondary Malassezia species or bacterial infection from cerumen accumulation.^1,7 For example, polyps and tumors (FIGURE 2) can prevent drainage of exudate, which predisposes ears to secondary infections. Common ear tumors include ceruminous and sebaceous gland tumors, inflammatory polyps, squamous cell carcinomas, mast cell tumors, and histiocytomas.^1,7

Iatrogenic factors include hair plucking and trauma from overaggressive cleaning with cotton swabs, a common predisposition in cats.³ Hair plucking is standard grooming practice for dogs with excessive hairs in the canal, but it can damage the epithelial lining. Hair plucking is contraindicated in the normal ear because it can cause inflammation and predispose the animal to infection. In some cases, it may be indicated to prevent and manage otitis.^1,7

Primary Factors

Primary factors (BOX 1) are ultimately the reason otitis begins. Adverse food reactions have been reported in up to 40% to 52% of dogs with nonseasonal pruritus.⁹ Numerous clinical signs of adverse food reaction may be present; on the other hand, otitis externa may be the only sign.^2,9,10 It has been reported that 7% of cats had otitis associated with adverse food reaction.¹

Atopy is another common cause of otitis. Patients usually exhibit more clinical signs than otitis; however, as with adverse food reactions, it may be the only sign.^1,2,10,11 Unlike adverse food reaction, it may initially be seasonal.

Parasites, such as Sarcoptes, Notoedres, and Cheyletiella species and harvest mites, can cause pruritus and inflammation on or near the pinnae, which can lead to head shaking and scratching and secondary otitis externa.¹² Otodectes cynotis (ear mites) are seen in up to 50% of otitis cases in cats and in 5% to 10% of cases in dogs.¹ Chronic cases of Otodectes infestation may become secondarily infected with bacteria or yeast. Demodex mites may cause ceruminous otitis externa in dogs and cats.^10,11 Otobius megnini, the spinous ear tick, which is found mainly in the southwestern United States, can attach to the lining of the external ear canal and cause inflammation and otitis externa and can be quite painful. Other hard ticks and harvest mites may affect both dogs and cats; however, they usually infest the pinnae or proximal canal.^10,11

Autoimmune and immune-mediated diseases (BOX 1) are rarely primary causes of otitis. Usually, the animal has other skin lesions on the body in addition to the otitis.

FIGURE 3. (A) Removal of a plant awn from a dog using video otoscopy and forceps. (B) Awn after removal.

Keratinization disorders and endocrinopathies (BOX 1) may alter keratin and cerumen gland production in the external ear, resulting in a ceruminous and seborrheic form of otitis externa.^1,11,14,15 Again, clinical signs other than otitis are usually present.

Foreign bodies, such as plant awns (FIGURE 3), dirt, debris, dried medication, loose hair, and dead insects can all be primary causes of otitis and result in secondary infection.^1,8,10,11,13 Foreign bodies in the ear usually present as unilateral otitis; however, they can be bilateral.^1,11 Hunting and working dogs may be predisposed to foreign bodies.¹⁰

Secondary Factors

Secondary factors of otitis (BOX 2) do not, in themselves, cause otitis. Rather, they are a result of a diseased ear and contribute to the primary factor. Secondary factors must be treated along with the underlying (primary) cause.^1,2

Perpetuating Factors

Perpetuating factors of otitis are changes in the anatomy and physiology caused by the primary and secondary factors. These changes may include a loss of epithelial migration, edema, proliferative changes, stenosis, ruptured TM, otitis media, and even calcification of the canal.^1,10,13

Physical Examination and History

It is important to obtain a complete history while keeping the predisposing and primary factors of otitis externa in mind.¹ Age at onset can help to identify the primary cause. Parasites or food allergies should be considered in patients with onset at age <1 year.^8,16 Atopy is a consideration from approximately age 6 months to 6 years.^2,17 In older animals, although atopy is still possible, neoplasia or endocrinopathies should be considered.¹⁴ Dog breeds such as the cocker spaniel, Brittany spaniel, golden retriever, miniature poodle, and West Highland white terrier (to name a few) and Persian and Himalayan cats may have a predisposition.^1,8,11

Clinical Signs

Signs of otitis externa may include aural pruritus and head shaking (most common), odor, discharge, aural hematoma, pain, and intermittent head tilt.^1,11 The concave aspect of the pinnae and the canal may be erythematous, edematous, and malodorous; have excoriations and discharge; and be painful on palpation.^1,11 The firmness of the canals should be evaluated by gentle palpation of the outside of the canals. Less pliable canals are associated with proliferative changes and calcification. Ear canals that are calcified can rarely be managed with therapy and are generally considered candidates for surgery.¹

Otoscopic Examination

If the ears are painful, an examination will not be easy. Chemical restraint or general anesthesia may be necessary, or treatment to reduce inflammation may be needed before the ears can be effectively evaluated. Treatment may be indicated for as little as 4 days to 1 month or more, depending on the severity.^1,2,11

If the otitis is unilateral, start with the unaffected ear because it will be nonpainful and the animal will be less resistant. Spread of infectious agents is also less likely; however, always change cones between ears or use disposable cones.

When the cone of the otoscope is inserted, the intertragic incisure is a good landmark to determine the insertion.⁵ Be gentle and go slowly down the lateral wall. At the junction of the vertical and horizontal canal is a large ridge of cartilage called the auricular projection. Avoiding this area with the cone is critical; touching this area with the cone will inflict pain, and the patient may quickly lose patience. Before you reach this projection, gently lift, pull out, and slightly lower the pinna. This will help straighten out the canal and enable you to advance the cone just past the projection.^5,18

Diagnostics

Cytology

Cytologic evaluation should be done for every new otitis diagnosis and at every recheck before the decision is made to stop therapy. It is fast, easy, relatively noninvasive, and inexpensive and provides important information for treating otitis effectively. Characteristics of exudates or odor are not acceptable replacements for cytologic evaluation.

To perform cytologic evaluation:

• Collect the exudate with a cotton-tipped applicator. Sample the vertical and horizontal canal by gently swabbing the sides of the canal.
• Roll the swab onto the slide in a thin film. You may use 1 slide for both ears. With the frosted edge of the slide closest to you, roll the left ear down the left side in a single line and the right ear down the right side in a single line.
• It is common practice to lightly heat-fix the slide with a match or lighter because cerumen is lipid in nature, although this may be unnecessary.¹⁹
• Stain with Diff-Quik, rinse, and air dry.
• View the prepared slide with a 100× oil lens with the condenser all the way up, the rheostat turned up, and the diaphragm open.

CYTOLOGIC EVALUATION should be done for every new otitis diagnosis and at every recheck before the decision is made to stop therapy. To perform cytologic evaluation collect the exudate with a cotton-tipped applicator. Sample the vertical and horizontal canal by gently swabbing the sides of the canal. Photo courtesy of Bradley Leeb, University of Illinois

To make a slide to view Otodectes species, collect the exudate, roll the swabs in a few drops of mineral oil on a glass slide, and place a coverslip on top. View under 10× magnification, with the condenser in the down position, the diaphragm closed, and the rheostat dimmed.

Malassezia pachydermatis is a normal resident of the canine ear and can be found in cats as well; Malassezia sympodialis can also be isolated in the feline ear canal.⁸ Because Malassezia species are ubiquitous flora of canine and feline ears, it may be difficult to determine when to treat. Numbers on cytologic examination provide a guideline, along with clinical signs, history, and previous response to treatment.

Common cocci found during cytology include Staphylococcus, Streptococcus, and Enterococcus species (gram positive). Bacilli (rod-shaped bacteria) are typically Pseudomonas species, coliform bacteria, or Proteus species (gram negative). In most cases, performing a Gram stain is unnecessary.¹² Normal commensal bacteria may be found, but abnormal increases and the presence of leukocytes, which are not seen in the normal ear, indicate a secondary bacterial infection.^8,12

While looking at the cytology slide for bacteria, yeast, and leukocytes, you may occasionally discover Demodex mites. They will appear large because you are viewing the slide under 100× oil immersion. Melanin granules may also be observed and should not be mistaken for cocci and/or rod bacteria. These can be differentiated by focusing up and down until you note the color of the structure, which is yellow to brown rather than purple. Quantify your findings and record them in the medical record for comparison on recheck.

Culture

Culture may be performed if cytology results reveal bacteria and white blood cells when severe otitis externa is present and systemic antibiotics are indicated. Culture may not be useful when topical medications alone are used because topical therapies lead to higher concentrations of medication in the ear canal than systemic therapies.¹ Samples for culture are obtained with the same method as used for cytology, using a sterile culturette.

Video Otoscopy

Video otoscopes have become more common in private practices and are found in almost all dermatology specialty practices and universities. A video otoscope is simply an otoscope with a port and a monitor, and it is used to flush, suction, obtain biopsy specimens from any masses, and perform myringotomies. The video otoscope allows the veterinarian to obtain a primary diagnosis, such as foreign objects and parasites; visualize such factors as excessive exudate, erythema, and stenosis; and assess the TM.

Imaging

Imaging examinations, such as computed tomography and magnetic resonance imaging, are often performed before surgery or video otoscopy to look for neoplasia or otitis media. Any masses should be biopsied and the specimens submitted to a dermatopathologist for histopathology.¹²

Other Tools

Other diagnostic tools include skin scrapings, parasite trials, allergy work-ups, and laboratory evaluation. A pinnal–pedal reflex may be elicited by rubbing the edges of the pinna; the reflex is observed if the dog attempts to scratch with its hind leg. Although this reflex suggests the presence of Sarcoptes mites, other dermatologic conditions may elicit a similar response.^9,20

FIGURE 4. Chronic proliferative otitis.

Recurrent Otitis

Patients with recurrent chronic otitis externa (FIGURE 4) should undergo food trials and an atopy work-up to determine whether food and/or environmental allergies may be the primary cause. Laboratory evaluation, such as a thyroid panel and adrenocorticotropic hormone stimulation test, may be considered as well.

Treatment

Always collect samples for cytology and culture (if indicated) and try to visualize the TM before cleaning the ears and instilling topical medications that the veterinarian has prescribed. Demonstrate to clients how to properly clean and medicate the ears before they leave the clinic and review the recommended treatment plan to ensure they understand the instructions. Recheck examinations with cytology are necessary and are usually scheduled in 2 to 4 weeks, before the patient stops any medications. Patient response determines duration of treatment, and client compliance is essential.

Summary

The key to a successful outcome is to use the classification system for otitis, find the underlying (primary) cause, and treat accordingly. A treatment plan that does not target the primary cause will result in recurrent otitis. The inflammation will continue and alter the normal anatomy of the ear.