Chapter 10

The External Ear Canal

Koranda A. Wallace, Heather L. DeHeer and Reema T. Patel

Anatomy of the External Ear

The external ear consists of cartilage and the overlying skin, which create the pinna and external acoustic meatus (between the base of the pinna to the tympanic membrane). The auricular cartilage determines the shape and appearance of the pinnae and supports the vertical ear canal. The annular cartilage, found at the base of the auricular cartilage, supports the horizontal and external ear canal. The skin covering the cartilage within the canal contains sebaceous glands, tubular ceruminous glands, and small hair follicles (Figure 10-1).¹

Figure 10-1 Hematoxylin and eosin (H&E) section of the normal feline vertical ear canal with hair follicle (arrowhead), sebaceous glands (arrow), and ceruminous glands (asterisk). (Magnification 200×, Bar = 100 um.)

Etiology and Pathogenesis of Otitis Externa

Otitis externa, inflammation of the skin and adnexal structures of the ear canal, is commonly encountered in veterinary patients. It is estimated to affect 10% to 20% of canines and 2% to 6% of felines presented for veterinary care.2,3

Causes of otitis externa are multifactorial and are commonly divided into primary, predisposing, and perpetuating factors, which are discussed briefly below.

Diagnosis of Otitis Externa

Most cases of acute otitis externa can be readily managed by using the information gained from a thorough history, physical examination, otoscopic examination, and cytologic evaluation of the ear canal secretions. More advanced or chronic cases may require culture and susceptibility testing, biopsy, diagnostic imaging, endocrine testing, and assessment of allergic skin disease.

Cytologic Evaluation of Ear Canal Secretions

Cytologic examination of otic secretions is a simple, inexpensive, and rapid test to assist in the diagnosis and treatment of otitis externa. Physical characteristics, if not guided by cytology, may be misleading and unreliable. The primary goal of cytology of the external ear is to identify overgrowth or infection that may contribute to otitis externa. Cytology should be performed at recheck examinations as a means to monitoring and adjusting therapy.

Collection and Staining of Samples

Samples of the ear canal secretions for cytologic evaluation are collected using separate cotton-tipped swabs for each ear canal. Samples should be collected after performing otoscopic examination, to avoid obscuring the tympanic membrane with compressed debris, and prior to introduction of any cleaning agents or medication. The most clinically relevant samples are obtained from the deeper horizontal canal rather than the superficial vertical canal.³ This can be accomplished in larger patients with insertion of a cotton-tipped swab through an otoscopic cone (Figure 10-2). However, surrounding circumstances such as painful ears, stenosis, and inflammation may make acquisition in this manner difficult without sedation. Another method to obtain samples is to carefully pass a swab into the ear canal, without the aid of an otoscope, aiming for the junction of the vertical and horizontal canal. Avoid straightening of the ear canal to avoid damage to the tympanic membrane.³ If the patient requires anesthesia or sedation, ostoscopy and ear flushing, among other techniques, can be used to acquire samples. Samples should always be collected from both ears as animals that appear to have unilateral otitis may also have mild, less apparent disease in the other ear.^7–9

Figure 10-2 Smears of horizontal ear canal secretions may be collected by passing a cotton-tipped swab through the cone of an otoscope after otoscopic examination.

After secretion from each canal has been collected, separate slides should be prepared for parasite identification and for routine staining. The slides must be labeled to indicate which ear was sampled. Slides for parasite identification should remain unstained, and the otic exudate should be mixed with a small amount of mineral oil, cover-slipped, and microscopically viewed on low power with the condenser down.

To prepare slides for routine staining, the swab is gently rolled onto a clean, dry slide in a thin layer, as thick smears are difficult to evaluate. Heat fixing neither systematically increases nor decreases numbers of yeast on specimens; although it is recommended by many to prevent loss of high lipid content, it is not necessary.3,10,11 After the material on the slide is allowed to air-dry, it is stained with any of the usual hematologic stains (e.g., Diff-Quik or Wright stain). It is recommended to have two sets of staining jars, one reserved for ear cytology and one reserved for other samples (e.g., blood smears, mass aspirates) as yeast and bacteria from ear cytologies may overgrow in the stain solution and contaminate other slides.

Gram staining can be used for additional information on bacterial type; however, it is more time consuming and may be unnecessary, given that most bacterial cocci are gram positive and most bacterial rods are gram negative.

Cytologic Examination

Cerumen

Cerumen, with its high lipid content, does not take up stain and provides the background for many normal ear swab cytologies (Figure 10-3).

Figure 10-3 Ear swab from a normal dog shows some staining and nonstaining epithelial cells and ceruminous debris. (Wright-Giemsa stain. Magnification 100×, Bar = 200 µm.)

Keratinocytes

Keratinocytes, including occasional nucleated forms, are noted in normal ears of both dogs and cats. Normal dogs were noted to have 3.9 keratinocytes per 40× high-power field (hpf) and normal cats were noted to have 8 per 40× hpf.¹³ The finding of nucleated forms should not be mistaken for a pathologic process (parakaratotic hyperkeratosis).

The ear canals of clinically normal dogs often contain small numbers of bacteria. The bacterial concentration typically is low enough that one sees only occasional or no bacteria on cytologic preparations (Figure 10-4). Many of these bacteria are potentially pathogenic and may colonize the ear canal when normal conditions are altered.^3,^6,^8,¹⁴ In animals with bacterial otitis, cytologic evaluation of ear canal secretions often reveals large numbers of bacteria free in the smear (Figure 10-5). Unfortunately, no definitive rule exists for deciding if the bacteria are clinically relevant and warrant treatment. The decision should be based on severity of clinical signs and cytologic findings. Semiquantitative criteria to assess relevance of bacterial populations have been proposed on the basis of their numbers per 40× hpf as follows (Table 10-1): Bacterial counts expected in normal dogs vary among studies and have been reported as a median of 0 cocci (to averaging 5 cocci or fewer).^14,12,13 Abnormal numbers have been reported to be an average of 25 or more organisms, with 6 to 24 organisms being in the “gray zone.” Bacterial counts expected in normal cats vary among studies and have been reported as a median of 0.3 cocci per 40× hpf in one study¹³ and an average of 4 or fewer cocci in a second study.¹² Abnormal numbers have been reported to be 15 or more organisms, with 5 to 14 organisms being in the “gray zone.” Neither study identified bacterial rods as part of the normal ear cytology of dogs or cats.