Predisposive, Primary and Perpetuating Causes of Otitis
World Small Animal Veterinary Association World Congress Proceedings, 2015
P. Forsythe, BVM&S, DVD, MRCVS
Dermatology Referral Service, Glasgow, Scotland, UK

Introduction

Otitis externa is a common presentation in small animal practice.1 Chronic otitis externa cases are complex, frustrating and common reasons for referral. Multiple aetiologies may be involved and good understanding of these is required to achieve a successful outcome. In the late 1980s, August2 raised awareness that ear disease was not just the result of an infective process but that some of the problems arose from changes within the ear canal itself. The most recent classification of ear disease3 describes primary and secondary causes of inflammation and predisposing and perpetuating factors abbreviated to the PSPP system. Nurses have key roles to play in the management of otitis externa and knowledge of the PSPP system is essential.

Ear Canal Physiology in Health and Disease

The ear canal is lined with skin. Hair follicles are present within the external ear canal decreasing in number from distal to proximal. Two types of secretory glands are identified: sebaceous and modified apocrine (ceruminous) glands. Cerumen coats the lining of the ear canal and is comprised of a complex mix of exfoliated cells and glandular secretions that form a protective and antimicrobial layer. Epithelial cell migration is a lateral movement of the stratum corneum that facilitates the transport of cerumen and debris from the tympanic membrane to the external ear canal. This process has recently been demonstrated in the canine ear.4

Pathogenesis of Otitis Externa

The aetiological causes of otitis externa may be divided up into primary and secondary causes of inflammation and the perpetuating and predisposing factors. Factors contribute to ear disease but do not cause inflammation per se; they prevent effective initial treatment of ear disease as well as result in recrudescence if they are not adequately addressed.

Primary and Secondary Causes of Inflammation

These are the agents that produce inflammation within the ear canal.

Primary causes of inflammation are listed in Table 1. The ear canal is lined with skin and many skin diseases can act as primary causes of inflammation within the ear canal. Atopic dermatitis (including adverse food reactions) is the most common primary cause.5 Fifty-five percent of dogs with atopic dermatitis may develop otitis externa and otitis externa may be the only clinical sign of AD. A full spectrum of clinical signs may be attributable to the primary cause of inflammation from subclinical through to very obvious and severe.

Yeast and bacterial infections are the principle secondary causes of inflammation in otitis externa. Alterations in the environment within the ear canal as a result of primary causes of inflammation, provide an environment favourable for the growth of commensal organisms, principally Malassezia and Staphylococcus spp. within the ear canal resulting in infection. Less commonly transient organisms such as Pseudomonas spp. may be involved. Cytological examination is indicated in all cases of otitis externa.

Perpetuating Factors

Perpetuating factors arise as the result of the changing environment within the ear canal and secondary pathological changes. They prevent resolution and result in continued disease. Perpetuating factors are listed in Table 2.

Initial changes can be subtle but can ultimately become the most severe component of chronic disease. Frequently, these factors provide microbial niches that perpetuate infection.

Inflammation within the ear canals results in erythema, oedema, soft tissue swelling, and hyperplasia of epithelia and glandular structures. This results in the formation of clefts within the swollen ear canal lining which harbour micro-organisms and swelling leads to stenosis. Additionally, epithelial migration is impaired in otitis externa and the net effect is a buildup of cerumen within the ear canal. This further encourages bacterial and yeast overgrowth and infection. With prolonged disease, severe glandular hyperplasia may be seen in some spaniel breeds. Continued inflammation can lead to calcification and ossification of the auditory cartilages which may lead to irreversible stenosis.

Otitis media is thought to mainly arise as a result of perforation of the tympanic membrane and the extension of infection into the middle ear cavity. It is associated with chronic disease and rod infections. The middle ear cavity becomes a reservoir of infection resulting in recrudescence of otitis externa following apparently successful therapy. The tympanic membrane may heal trapping infection within the middle ear cavity.6 Clinical signs include depression, pain, head tilt, pain on opening the mouth and deafness but most cases are clinically indistinguishable from otitis externa. The diagnosis can be challenging and may involve imaging, video-otoscopy, tympanic membrane palpation, myringotomy, cytology and culture.

Management of Perpetuating Factors

Prolonged and aggressive therapy may be required to reverse changes. Glucocorticoids are of benefit to reduce swelling and ear canal secretions.

Ear cleaning is probably the most important aspect of treating otitis externa and prevention of further episodes of otitis.

Ear cleaning

 Facilitates examination of the ear canal

 Removes microbes, material that harbours microbes, small foreign bodies

 Exposes the lining of the ear canal to topical therapy

 Prevents inactivation of topical therapy

 Improves barrier function

 Cleaners have many beneficial effects including cerumenolytic, antimicrobial, astringent and acidifying

When dispensing a proprietary ear cleaner the owner should be shown exactly how the preparation should be used. This sort of cleaning is effective in removing material from the distal third or half of the ear canal but is unlikely to remove material impacted within the horizontal canal.

Deep ear cleaning may be indicated in some cases and in the author's practice this procedure is always performed under general anaesthesia using a retrograde flushing technique.7 After soaking in an appropriate cleaner saline is used delivered via a giving set, three-way tap, 20-ml syringe and 4-gage nasogastric feeding tube which passes through the working channel of the video-otoscope. A wider catheter allows aspiration of larger clumps of material. Post procedure analgesia is mandatory and systemic glucocorticoid therapy may be administered for up to five days after the procedure.

Predisposing Factors

Predisposing factors do not cause inflammation by themselves but by altering the microclimate within the ear canal place the patient at increased risk of developing ear disease. Predisposing factors are listed in Table 2.

In one study, conformational abnormalities were the most common predisposing factors for otitis externa.8

Findings vary but purebred dogs with pendulous pinnae and hirsute ear canals are more likely to develop otitis externa whereas dogs with erect pinnae are less likely to be affected.9 It has been suggested that this predisposition may be inherently to do with the breed of dog than any direct effect of the ear type and amount of hair on humidity or temperature within the ear canal.10

The author does not recommend routinely plucking hirsute ear canals unless the dog has had previous episodes of otitis externa.

VIN editor: Tables were not available at time of publication.

References

1.  Hill PB, Lo A, Eden CA, et al. Survey of the prevalence, diagnosis and treatment of dermatological conditions in small animals in general practice. Vet Rec. 2006;158(16):533–539.

  

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Peter Forsythe, BVM&S, DVD, MRCVS
The Dermatology Referral Service
Glasgow, Scotland, UK


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