Otitis Externa Series Part 1: Diagnosis of Otitis Externa
Ashley Bourgeois, DVM, and Wayne Rosenkrantz, DVM, Diplomate ACVD
This article—the first of a 2-part series—discusses diagnosis of otitis externa. Topical therapy for treatment of otitis externa will be discussed in the November/December 2014 issue of Today’s Veterinary Practice (tvpjournal.com).
Otitis externa is a prevalent complaint in patients presented to small animal practices. This inflammatory disease of the external ear canal and/or pinna can have an acute or chronic presentation. Management of otitis externa depends on identifying and treating predisposing and perpetuating factors, as well as primary and secondary causes (Table 1).
|Table 1. Common Predisposing Factors, Primary & Secondary Causes, & Perpetuating Factors of Otitis Externa|
|PREDISPOSING FACTORS||PRIMARY CAUSES||SECONDARY CAUSES||PERPETUATING FACTORS|
FACTORS & CAUSES
Predisposing factors alone do not cause otitis externa, but increase risk for development and persistence of chronic infection. These factors work in conjunction with primary or secondary causes, allowing otitis externa to become a significant problem.
Primary causes of otitis externa are the inciting agent or etiology that directly damages the ear canal’s epithelium, resulting in subsequent inflammation. To prevent recurrent episodes of otitis externa, it is critical that a primary cause be diagnosed and managed.
Secondary causes of otitis externa do not create pathology in a healthy ear; instead, they incite disease in ears affected by a primary cause or predisposing factor. If the inciting cause or factor is inadequately controlled, secondary causes, such as bacterial or yeast overgrowth, typically become chronic issues.
Perpetuating factors are changes in anatomy and physiology of the ear that occur in response to otitis externa. They are most commonly seen in chronic cases and are not disease specific. These factors can accentuate development of secondary infections by providing environments and microscopic niches that favor their persistence.
In severe cases, perpetuating factors can ultimately prevent the resolution of otitis externa by leading to irreversible changes of the ear canal. They are the most common reason that otitis externa fails to respond to medical therapy and, ultimately, requires surgical intervention.
Diagnostic testing begins with a minimum database of:
- Detailed history
- Physical examination
- Ear canal cytology.
Cytologic examination of ear canal discharge provides a brief overview of the aural environment, providing a foundation for therapeutic decisions and advanced diagnostics.1,2It is also the primary tool in identifying bacterial or yeast overgrowth.
Initial cytology should be performed prior to bacterial culture and sensitivity (C/S) testing because bacterial C/S is not recommended if only yeast overgrowth is noted. See In Practice: External Ear Canal Cytology for a stepwise approach to cytologic sample collection.
In Practice: External Ear Canal Cytology
- Carefully insert an applicator tip in the ear canal and, near the junction of the vertical and horizontal canals, collect material for cytologic examination.
- Collect deeper, and generally more representative, samples by passing an ear loop or pediatric feeding tube through an otoscopic cone.
- Transfer samples onto a glass slide, heat fix, and stain with Diff-Quik.
- When examining samples under the microscope, note the:
- Number of bacteria and yeast per oil immersion field (100×)
- Presence or absence of inflammatory cells.
Culture & Sensitivity
Indications for C/S include:
- Suppurative inflammation (including that with bacterial rods, cocci, or no visible organisms) revealed during initial cytology
- Lack of response to appropriate topical and systemic antibiotic therapy
- Systemic therapy required for otitis media or deeper, soft-tissue infections of ear canal
- Resistant strains of bacteria suspected.
Resistant bacteria should be suspected if:
- History of chronic topical therapy
- Rods observed on cytology
- Bacteria persistent on cytologic examination despite appropriate therapy (ie, suspect methicillin-resistant Staphylococcus pseudintermedius).
Ideally, topical or systemic antibiotic therapy should be discontinued 3 to 5 days prior to acquisition of culture samples.
When preparing the sample for submission, include any pertinent information regarding the organisms seen on cytology and a representative cytology slide.
In addition, if rods are observed on cytology—suggesting the presence of Pseudomonasspecies—additional antibiotic sensitivities should be requested with bacterial culture, including:
- Polymyxin B
- Third-generation cephalosporin.
Once the laboratory report is in hand, in addition to susceptibility, it is important to review the reported minimum inhibitory concentration, which helps direct the choice or dose of antibiotic required.
Middle Ear & External Ear Canal Bacterial Spectrums
The spectrum of bacteria and their sensitivity patterns seen in the middle ear (which is lined with ciliated columnar epithelium) and external ear canal (which is lined with epidermis) may differ due to variations in cellular composition. In a study by Cole and colleagues, different strains of Pseudomonas species, based on sensitivity pattern, were cultured from each location.3 Other studies have shown different strains of a bacterial species from a single sampling site.4,5
In addition to detailed history, physical examination, and ear canal cytology, otoscopy is part of a diagnostic minimum database, and can diagnose several conditions that create deeper ear canal disease (Table 2).
|Table 2. Diagnostic Imaging for Ear Disease|
|IMAGING MODALITY||EAR DISEASE/CONDITION EVALUATED|
|Radiography||Detects bony involvement of bullae; has limited value in soft tissue changes, especially in acute cases|
|Computed axial tomography||Aids in differentiation of bony lesions in the bullae from soft tissue reactions|
|Magnetic resonance imaging||Aids in visualizing middle and inner ear and detects presence of fluids, such as endolymph within the cochlea and semicircular canals|
|Ultrasonography||Detects fluid within the tympanic bullae|
Otoscopes must have a strong light and power source, combined with at least 10× magnification that allows focusing within the normal length of the ear canal. Avoid using a battery-operated otoscope that has significantly lost power and light, which results in a diminished view of the deep ear canal.
Either a traditional diagnostic otoscope or surgical otoscope head may be used. The benefits of surgical otoscopes include, in certain cases:
- Enhanced manipulation and angulation within the ear canal
- Passage of tubes or other instruments into the ear canal with concurrent visualization.
Various sizes of otoscopic cones are required to properly examine ear canals based on patient size.
In Practice: Ear Cleaning & Flushing
If you encounter any problems related to use of FVEO, refer to Table 4.
Ceruminolytics help break down larger pieces of waxy debris, such as ceruminoliths, and are gentle and soothing to the epithelium of the ear canal.
- Squalene Urea peroxide
- Carbamide peroxide
- Hexamethyl tetracosane
- Dioctyl sodium/calcium sulfosuccinate
- Triethanolamine polypeptide elite condensate
Fiberoptic Video-Enhanced Otoscopy
Advancement of fiberoptics, improved lighting, and miniaturization of video cameras, combined with rigid endoscopy, has led to development of fiberoptic video-enhanced otoscopy (FVEO). FVEO, despite its expense to purchase and maintain, is extremely beneficial for improved diagnostics, therapy, and client education.
The camera within the fiberoptic tip significantly magnifies and improves visualization of the ear canal. FVEO also facilitates permanent recordings via picture or video of the ear canal—including debris, foreign bodies, and masses—which can be shared with clients and other veterinarians.
Compared with handheld otoscopy, FVEO allows:
- Thorough flushing with water or saline, providing better visualization and magnification
- Observation of fine details, such as small tears of the tympanic membrane, consequently recognized as air bubbles extruding from the middle ear cavity through the tympanic membrane.
In chronic otitis cases, the following imaging techniques may be helpful diagnostic tools. Table 2 lists these additional imaging modalities as well as the types of conditions they can diagnose and evaluate. However, the cost and availability of these diagnostics may make them prohibitive for some clients.
Computed tomography (CT) and magnetic resonance imaging (MRI) have been shown to be more reliable and accurate than radiography.6 CT is most commonly used due to efficiency and expense; however, if soft tissue masses or vestibular disease is suspected, MRI is more accurate. In one study, diagnosis of otitis media by CT was found to have an 86% sensitivity and 89% specificity compared with the gold standard of histopathologic diagnosis.6
Ultrasound has been used for the detection of fluid within the tympanic bulla, with 80% to 100% sensitivity and 74% to 100% specificity compared with the gold standard of CT.7
EAR CLEANING & FLUSHING
Cleaning and flushing the ears is critical for:
- Proper visualization and examination of ear canal
- Determination of disease extent
- Indications for additional diagnostics and case management
- Determination of disease resolution.
See In Practice: Ear Cleaning & Flushing for a stepwise description of appropriate cleaning and flushing. Typically, patients can be maintained with once to twice weekly flushing, but frequency of flushing should be determined on a case-by-case basis.
Otitis externa is a multifactorial inflammatory disease of the ear canals and pinnae that may become chronic. Chronicity is usually due to inadequate control of the primary cause or the presence of a perpetuating factor. Detection of a primary cause and any perpetuating and predisposing factors is essential for complete resolution and prevention of recurrence of otitis externa.
Identification of infectious organisms through cytology is an essential first step for initial treatment. Thorough ear cleaning and flushing coupled with appropriate antimicrobials, based on C/S when necessary, enhance treatment success.
C/S = culture and sensitivity; CT = computed tomography; FVEO = fiberoptic video-enhanced otoscopy; MRI = magnetic resonance imaging
- Ginel PF, Lucena R, Rodriguez J, Ortega J. A semiquantitative cytological examination of normal and pathological samples from the external ear canal of dogs and cats. Vet Dermatol 2002; 13(3):151-156.
- Tater K, Scott D, Miller W, Erb H. The cytology of the external ear canal in the normal dog and cat. J Vet Med A Physiol Pathol Clin Med 2003; 50(7):370-374.
- Cole L, Kwochka K, Kowalski J, Hillier A. Microbial flora and antimicrobial susceptibility patterns of isolated pathogens from the horizontal ear canal and middle ear in dogs with otitis media. JAVMA1998; 212(4):534-538.
- Schick A, Angus JC, Coyner K. Variability of laboratory identification and antibiotic susceptibility reporting of Pseudomonas spp. isolates from dogs with chronic otitis externa. Vet Dermatol 2007; 18(2):120-126.
- Graham-Mize C, Rosser E Jr. Comparison of microbial isolates and susceptibility patterns from the external ear canal of dogs with otitis externa. JAAHA 2004; 40(2):102-108.
- Rohleder J, Jones J, Duncan R, et al. Comparative performance of radiography and computed tomography in the diagnosis of middle ear disease in 31 dogs. Vet Radiol Ultrasound 2006; 47(1):45-52.
- Doust R, King A, Hammond G, et al. Assessment of middle ear disease in the dog: A comparison of diagnostic imaging modalities. J Small Anim Pract 2007; 48(4):188-192.
- Cole LK. Primary secretory otitis media in Cavalier King Charles Spaniels. Vet Clin Small Anim Pract N Am 2012; 42(6):1137-1142.
- Pye C, Yu A, Weese J. Evaluation of biofilm production by Pseudomonas aeruginosa from canine ears and the impact of biofilm on antimicrobial susceptibility in vitro. Vet Dermatol 2013; 24(4):446-449.