Lapis ISSN 2398-2969

Ear base abscess

Synonym(s): Ear base diverticulum, Aural diverticulosis

Contributor(s): Sarah Brown, Vladimir Jekl

Introduction

Lop-eared rabbits may develop fluctuant ear base swellings. These are often called ‘ear base abscesses’ but infection may or may not be present. Such swellings are thought by some to be a diverticulum or hernia of the ear canal wall, although this may be contentious. Secondary infection and concurrent ear disease can be present.
  • Cause: in lop-eared rabbits the ear canal is deformed and acts like a ‘kinked hose’ at the point where the auditory meatus and tragus meet. As a result, the ear canal becomes occluded and cerumen builds up behind the kink and cannot leave the ear. In lop-eared rabbits the ear canal at the level of the ‘kink’ is soft-walled and therefore likely to become distended to form a pouch or diverticulum. A gradual build-up of bacteria and yeast may lead to otitis externa which is difficult to manage due to the stenosis of the ear canal. In addition to this non-infectious diverticulitis, ascending infection may occur from the middle ear, ie ear base swellings are often associated with otitis media or osteomyelitis of the tympanic bulla. Primary abscessation may occur rarely due to a penetrating wound through the tympanic bulla, eg cat bite.
  • Signs: a palpable swelling at the base of one or both ears. There may be other signs associated with concurrent otitis externa, media or interna such as aural discharge, head shaking, facial paralysis or neurological signs, eg head tilt.
  • Diagnosis: generally pathognomonic when swelling is palpable or visible in this region. Ancillary tests such as diagnostic imaging and culture and sensitivity may be indicated to assess for concurrent ear disease and to assess what treatment is required.
  • Treatment: variable, depending on clinical signs and the severity of any concurrent ear issues. May require treatment ranging from no treatment to simple manual expression of the diverticulum contents, to flushing, to surgery such as lateral wall resection, sometimes coupled with lateral bulla osteotomy, or partial or total ear canal ablation.
  • Prognosis: many lop-eared rabbits appear to cope with long-term ear swellings, especially if there are no other associated clinical signs. It can be difficult to assess their level of pain or discomfort. Secondary infection of the diverticulum is likely to require surgery to attempt a ‘cure’. The prognosis will be more guarded if vestibular and middle ear signs are present. Any facial nerve damage is likely to be permanent.

Pathogenesis

Etiology

  • In lop-eared rabbits, the ear canal is deformed, with a ‘kink’ occurring at the level of the junction of the auditory meatus and tragus, ie the ears droop down at this point.
  • The ear canal may act like a kinked hose, not allowing movement of cerumen out of the ear.
  • The cerumen builds up behind the ‘kink’ and over time can cause an outpouching or diverticulum of the ear canal at this site.
  • Secondary infection (and inflammation, although the inflammatory response in such cases is often minimal) and abscessation may ensue, although it has been argued that this does not represent a true abscess as it does not represent infection within a tissue.
  • Primary abscessation may occur rarely due to a penetrating wound through the tympanic bulla, eg cat bite.
  • Abscesses arising from osteomyelitic tympanic bulla and otitis media/interna may also be associated with ear base swellings.

Predisposing factors

General

  • Anatomical predisposition for cerumen to build up just behind the ‘kink’ in lop-eared rabbits’ ears.
  • One study has shown that lop-eared rabbits show statistically significantly more frequent ear canal stenosis, higher scores of cerumen and erythema, and more frequent potential pain response during ear examination, compared with erect-eared rabbits.

Specific

  • Bite wounds in the area of the tympanic bulla may lead to ear base abscessation (rare).

Pathophysiology

  • Development of a diverticulum of the ear canal wall just behind the ‘kink’ present in lop-eared rabbit ears due to chronic cerumen build-up. This can either continue to be a chronic wax-filled outpouching of the ear canal or may progress to infection with concurrent ear issues such as otitis media, otitis interna Otitis media/interna and otitis externa Otitis externa.
  • A gradual build-up of bacteria and yeast may lead to otitis externa which is difficult to manage due to the stenosis of the ear canal.
  • Ascending infection is also thought to occur from the middle ear, ie ear base swellings are often associated with otitis media.
  • Primary abscessation may occur rarely due to a penetrating wound through the tympanic bulla, eg cat bite.

Timecourse

  • The development of aural diverticuli is likely to be a gradual process over months and potentially years. Secondary infection is likely to have a shorter timeframe.

Epidemiology

  • An individual problem rather than a group issue.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Chitty J (2019) Ear-based swellings in rabbits. Vet Pract Today 7 (1), 62-65.
  • Johnson J C & Burn C C (2019) Lop-eared rabbits have more aural and dental problems than erect-eared rabbits: a rescue population study. Vet Rec 185 (24), 758 PubMed.
  • Hedley J (2018) Dealing with abscesses in rabbits. In Pract 40, 359.
  • Mancinelli E & Lennox A M (2017) Management of otitis in rabbits. J Exotic Pet Med 26 (1), 63-73 VetMedResource.
  • Cosmos R, Bosscher G, Mans C & Hardie R (2016) Surgical management of ear diseases in rabbits. Vet Clin Exotic Anim Pract 19 (1), 189-204 PubMed.

Other sources of information

  • Eatwell K (2013) Diagnosis of Otitis Externa, Media and Interna in Rabbits. Vet Times, April 1. Website: www.vettimes.co.uk (pdf download).
  • Eatwell K (2013) Treatment of Otitis Externa, Media and Interna in Rabbits. Vet Times, May 13. Website: www.vettimes.co.uk (pdf download).
  • Harcourt-Brown F & Chitty J (2013) BSAVA Manual of Rabbit Surgery, Dentistry and Imaging. BSAVA, UK.


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