Diagnostic Approach to Otitis Externa and Media

Introduction

The initial diagnosis of otitis is straightforward, but further examination and tests are needed to identify the organisms and extent of inflammation to select the most appropriate treatment. It is also important to determine the primary, predisposing, perpetuating and secondary factors in cases of chronic and recurrent otitis.

Diagnostic steps include:

• Signalment and history
• Clinical examination
• Otoscopy or video otoscopy
• Cytology
• Culture and antimicrobial susceptibility testing
• Imaging - radiographs, CT and/or MRI

Signalment and History

Clinical Examination

Erythroceruminous Otitis

Erythema and a ceruminous discharge; erosions and ulcers are rare. The inflammation, discharge and chronic pathological changes vary from mild to severe. The ears are more pruritic than painful. Staphylococci and Malassezia are most common.

Suppurative Otitis

Pain, inflammation, ulceration, hemorrhage and purulent discharge. Pseudomonas is most common, but Proteus, other rods, staphylococci or streptococci can also be found. Malassezia is rare.

Clinical Features of Common Primary Causes of Otitis

• Otodectes are associated with dry, dark brown, waxy debris. Mites may be seen in the canal or on microscopic examination of cerumen. They may be difficult to find if there is a hypersensitivity and low mite numbers.
• Atopic dermatitis and adverse food reactions are the most common cause of recurrent otitis in dogs; otitis may be the only clinical sign, but most dogs have skin lesions elsewhere. There is a diffuse erythema of the ventral pinna and vertical ear canal. Eosinophilic granuloma syndrome lesions may be seen in the ear canals of cats with or without lesions elsewhere.
• Contact reactions to topical medications result in ongoing inflammation and a white to purulent discharge with mature non-degenerate neutrophils but few organisms. There is often a history of an initial response followed by relapse and pain on administration.
• Immunosuppressive diseases include hypothyroidism, hyperadrenocorticism and sex hormone alopecia (seborrheic otitis externa).
• Keratinisation disorders result in scaling and seborrhoea of the pinnae and ear canals.
• Immune-mediated diseases can cause ulceration and inflammation of the pinnae and the ear canals. Pustules on the concave aspect of the pinna are common with pemphigus foliaceus. Punched out ulcers and notches can be associated with vasculitis. Juvenile cellulitis causes severe oedematous otitis in young puppies.
• Ceruminous gland tumours (rarely other tumours) can obstruct the ear canal and cause otitis. External tumours can compress and infiltrate the ears.
• Foreign bodies (e.g., grass seeds) are usually easily seen and removed, but might need flushing out under sedation.
• Feline nasopharyngeal polyps may originate from the naso­pharynx, auditory tube or middle ear. Most have otitis media with or without otitis externa.
• Ceruminous/sebaceous hyperplasia is seen in Spaniels, and occasionally other breeds and cats.

Chronic Pathological Changes

The ear canals should be carefully examined to determine the severity of pathology changes. Healthy ears should be pain-free, pliable and freely mobile with pale pink, smooth and thin skin.

Otoscopic Examination

Most dogs accept this conscious. Clean the ears or use anti-inflammatory treatment to open stenosed ears if necessary. Use sedation or anaesthesia in fractious or animals or painful cases.

Otoscopy can identify foreign bodies, Otodectes, Otobius ticks, inflammation, ulceration, stenosis, the condition of the tympanic membrane, exudation and chronic changes. Healthy ear canals have a thin, smooth, pale pink lining with a little waxy pale yellow-brown cerumen. The tympanic membrane should be thin, translucent, sloping ventrally and slightly concave.

Acute inflammation results in moist erythematous swelling. Chronic changes have a firm indurated surface. Ceruminous hyperplasia results in a ‘cobblestone’ appearance, which later develops into single or multiple polyp-like growths. The tympanic membrane may be torn, inflamed, discoloured, thickened and/or convex. Its integrity can be assessed by gently probing with a soft tube. Debris can mimic the membrane, but tubes pass into the middle ear without resistance. If the membrane is not visible, use anatomical cues (e.g., the hairs at the ventral insertion of the tympanic membrane) to check that you are at the right level.

Video Otoscopy

Video otoscopes have many advantages. The magnification, illumination and image quality allow assessment of fine detail. The working channels facilitate ear flushing, foreign body or polyp removal, minor surgery, laser procedures and myringotomy. Image and video capture improves client communication and helps with follow up of cases.

Nature of the Discharge

The dried material at the ear opening can be misleading; always look at material from the ear canals.

Cytology

Cytology samples should be taken from the external ear canals and middle ear as the results may differ. Debris can be collected using cotton buds, catheters, spatulas, curettes or loops. It can be mixed into liquid paraffin to look for mites. Adhesive tape can be used to collect material from the pinna.

Diff-Quik® or Rapi-Diff® type stains are quick and easy, although heat fixing may be necessary with waxy or oily samples. The one-stain method can be very effective for staining microorganisms in thick and greasy smears (put one drop of the basophilic stain on the sample and place a cover-slip).

Large cocci in pairs or clusters suggest Staphylococcus, whereas Streptococcus and Enterococcus are smaller and form chains. Long narrow rods are typical of Pseudomonas, while Proteus and other gram-negatives are shorter or bipolar. Peanut-shaped yeasts are characteristic of Malassezia.

Staphylococci and Malassezia often form overgrowths without neutrophils. Degenerate neutrophils with intracellular bacteria are usually seen with Pseudomonas and occasionally other infections. Neutrophils are also seen in contact reactions and with ulceration. Mucoid slime is evident with biofilm forming organisms.

It is possible to obtain material for indirect impression smears or fine needle aspirates from lesions or the middle ear in the ear canals or middle ear using fine swabs, curettes or long needles passed through an otoscope. This is best done using the working channel of a video otoscope.

Bacterial Culture and Antibiotic Sensitivity

This is not always required. Microorganisms are easily identified on cytology; Malassezia and staphylococci have predictable sensitivity, but gram-negative rods (especially Pseudomonas) are frequently antibiotic resistant. However, antibiotic sensitivity data does not predict the response to topical therapy as local concentrations are ∼1–4,000x those used in sensitivity tests. The response to treatment is best assessed by clinical signs and cytology, but any ear not responding to treatment as expected should be cultured. Material should be taken from the ear canals and middle ear if necessary.

Diagnostic Imaging

Diagnostic imaging is primarily used to investigate otitis media, but it can also be helpful in chronic otitis externa. It’s possible to perform this using sedation but anesthesia facilitates positioning.

In dorso-ventral or ventro-dorsal views the ear canals should be visible and air-filled. Soft-tissue filling indicates occlusion with debris or stenosis. Mineralisation of the ear canals may be seen in severe cases. Diluted soluble contrast material (e.g., 50% Hypaque® diluted 1:10 in saline) can be used to delineate the canals and test the tympanic membrane - if it is ruptured contrast leaks into the middle ear. However, the fluid may not leak through small tears and stenosis may block the flow.

CT is very quick, and highly sensitive and specific for stenosis and occlusion of the ear canals, and bulging or rupture of the tympanic membrane. The ear canals should be thin-walled, open and gently taper to the tympanic membrane. The middle ears should be thin walled and air filled. The density of the material in the ears can differentiate debris from stenosis, and contrast enhancement can be used to identify inflamed soft tissues. Mineralisation of the ear canals is easily seen. MRI is less good for imaging the ear canals but can be useful for imaging soft-tissue structures such as neoplasia, infection and nerves in or around the ears.

Myringotomy

Myringotomy should be considered if the eardrum bulges into the ear canal, is opaque or otherwise abnormal, and/or there is evidence of otitis media. This is best done with a video otoscope, which allows precise puncture in the caudal-ventral pars tensa using a cut catheter, ENT swab or spinal needle. This allows access to the middle ear to see the contents, collect samples for cytology and culture, to flush out debris, and instal medication.