• 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.

Presenting signs

• Palpable fluctuant swelling at the base of one or both ears, primarily seen in lop-eared rabbits American Fuzzy Lop Dwarf Lop English Lop French Lop Meissner Lop Miniature Lop.
• Other clinical signs associated with ear disease may be present such as aural discharge (associated with otitis externa Otitis externa), vestibular signs (otitis media/interna Otitis media/interna), etc.
• Facial paralysis may be seen as the facial nerve runs close to the site of ear base swellings .

Acute presentation

• Usually not an acute presentation unless concurrent ear disease gives rise to acute clinical signs such as vestibular signs, eg head tilt Head tilt, circling, etc.

Geographic incidence

• Worldwide.

Age predisposition

• Seen more commonly in mature rabbits due to chronic nature of diverticulum development.

Breed/Species predisposition

• Seen almost exclusively in lop-eared rabbits due to the deformity at the ear base where the auditory meatus joins the tragus. In ‘prick-eared’ rabbits, the ear points upwards due to the interlocking of three auricular cartridges at the base of the ear to form the vertical canal. In lop-eared rabbits, these cartilages are deformed and there is a 3-5 mm gap between them and the ear base, allowing the ear to flop over (and occlude the ear canal).
• Penetrating wounds of the bulla due to predator bites which lead to ear base abscessation may occur in any breed.

Cost considerations

• If simple management of a non-infected diverticulum, then minimal cost implications.
• If secondary infection and abscessation, then surgical intervention is likely to be required, with inherent costs.
• Concurrent ear disease clinical signs, eg vestibular signs may need to be treated medically (with inherent costs) and their implications on prognosis and treatment options offered carefully considered.