Ординатура / Офтальмология / Английские материалы / Notes on Veterinary Ophthalmology_Crispin_2005

RABBIT OPHTHALMOLOGY

Box 5.1 Common and important rabbit ocular and adnexal conditions

•Traumatic damage (especially cornea)

•Congenital defects such as colobomas of the retina, choroid and optic nerve head (and, less commonly, other ocular tissues such as the lens)

•Orbital infection

•Globe prolapse

•Ocular lipid infiltration and deposition linked with high fat (usually high cholesterol) diets

•Blepharoconjunctivitis and conjunctivitis (e.g. myxomatosis, rabbit syphilis, bacterial infections)

•Dacryocystitis

•Ulcerative keratitis

•Lens-induced uveitis and cataracts associated with E. cuniculi

•Neoplasia, most commonly squamous cell carcinoma affecting the eyelids

•Glaucoma

ORBIT

Traumatic globe prolapse

The prominent nature of the globe means that it is relatively easy to displace it from the orbit. Prognosis for retention of vision after the globe has been restored to the orbit is guarded.

Orbital abscess and orbital cellulitis

Orbital infection is common in rabbits, and may be associated with septicaemia or dental disease (Figure 5.2). Diagnosis is as for other species, and the common presentation is exophthalmos and third eyelid prominence. Most cases are unilateral.

Differential diagnosis

•Whilst bilateral exophthalmos may be a consequence of disseminated infection (e.g. pyaemia and septicaemia) it is more likely to reflect the jugular engorgement that may accompany cardiothoracic disease, including neoplasia (e.g. lymphoma and thymoma).

•Prolapse of the nictitans gland or the Harderian gland of the third eyelid should be distinguished from third eyelid prominence due to a space occupying lesion of the orbit.

Treatment

•Attend to any dental problems

•Drainage is difficult, even if orbital exenteration is performed, because of poor access and the tenacious nature of the pus

•Systemic antibiotics effective against Staphylococcus spp and Pasteurella multocida are indicated – instillation of gentamicin after evacuation may help

EYELIDS AND CONJUNCTIVA

Conjunctival overgrowth

The cause of this condition is unknown and it may be either congenital or acquired (Figure 5.3). As with symblepharon surgery, there is a tendency for the condition to recur following simple resection, so referral for more complex procedures, such as the Arlt technique, should be considered.

RABBIT OPHTHALMOLOGY

RABBIT OPHTHALMOLOGY

Neoplasia

Squamous cell carcinoma is the most common eyelid tumour to be encountered, and is usually treated surgically ± cryotherapy.

LACRIMAL SYSTEM

Normal tear production shows marked breed variation, and zero Schirmer I tear test readings can be recorded in apparently normal eyes. Average Schirmer I tear test values are about 5mm/minute. When unilateral ocular surface disease is thought to be associated with tear film abnormalities, careful comparison of both eyes, including Schirmer tear testing, is essential.

Dacr yocystitis

Aetiology

•Dacryocystitis is common and is usually a consequence of dental disease causing obstruction of the nasolacrimal duct and secondary bacterial infection

•Staphylococcus spp and Pasteurella spp are important isolates from rabbits with epiphora or frank dacryocystitis, but other bacteria (e.g. Bordetella spp, Moraxella spp, Neisseria spp and Streptococcus spp) may also contribute

•Predisposing factors include poor husbandry (e.g. dietary imbalance, poor ventilation, ammonia build up)

Clinical signs

•The rabbit is unusual in that drainage of tears takes place through a single large punctum located in the ventromedial fornix, and a profuse white ocular discharge can be seen to originate from the lacrimal punctum (Figure 5.4(a,b))

•Conjunctivitis and, less commonly, corneal ulceration or a corneal abscess, may also be present

Management

•Address any underlying husbandry problems

•Take samples for culture and sensitivity

•For mildly-affected cases, topical chloramphenicol drops are applied directly into the lacrimal punctum after careful cleaning

•For more serious cases, the lacrimal punctum is cannulated with a 22or 23-gauge catheter or similar, and gentle irrigation with sterile water used to restore patency, followed by application of aqueous antibiotic solution (e.g. chloramphenicol or fusidic acid, or a combination of fusidic acid and gentamicin drops)

•The catheter can be reinserted for each treatment or left in situ, but the latter is not always well tolerated and the catheter needs to be fixed in place

•In the later stages of dental disease, parenteral treatment with antibiotics, e.g. cephalexin given by subcutaneous injection at a dose rate of 20mg/kg for 5–10 days, may be required

•Treatment may need to be repeated at regular intervals, especially if the dental disease progresses

Prognosis

The prognosis is guarded to poor, depending on the extent of any underlying dental disease and the adequacy of the husbandry.

(a)

(b)

Figure 5.4 (a) Dacryocystitis, with extensive periocular changes, including a copious ocular discharge. (b) A closer view of the ventral conjunctival sac shows pus emerging from the single lacrimal punctum and secondary corneal involvement.

CORNEA

Keratitis

Ulcerative keratitis is not uncommon and is often the result of trauma from fibrous bedding or feed. There also appears to be a form of recurrent epithelial erosion.

Lipid deposition

Corneal lipid deposition will occur readily if fat-rich foods are fed by well meaning, but misguided, owners. The rabbit, as an obligate herbivore, is particularly susceptible to the adverse effects of high circulating cholesterol. Normal circulating levels of total cholesterol are usually less than 3mmol/l; on a high cholesterol diet the levels often exceed 25mmol/l. Accelerated atherosclerosis develops if these abnormal levels are maintained, and there is widespread infiltration and deposition of lipid in the tissues, including those of the eye (Figure 5.5(a,b)).

RABBIT OPHTHALMOLOGY

RABBIT OPHTHALMOLOGY

(b)

(a)

Figure 5.5 (a) Bilateral ocular lipid deposition as a result of a diet that included butter and cheese. In the right eye lipid can be visualised in the cornea, iris, ciliary body and lens. (b) The left eye is not so heavily infiltrated (with acknowledgement to S.D. Carrington).

UVEAL TRACT

Uveitis

Uveitis may be a consequence of disseminated infection or the result of lens infection by the ubiquitous microsporidium parasite Encephalitozoon cuniculi (Figures 5.6(a,b)). Intralenticular Encephalitozoon cuniculi (probably acquired in utero from an infected dam) causes eventual lens rupture (usually the anterior lens capsule) and release of lens material with consequent phacoclastic uveitis.

Encephalitozoonosis can be treated by phacoemulsification to remove the lens contents, and agents such as oral albendazole (15–30mg/kg daily) or fenbendazole (10–20mg/kg daily) over 2–4 weeks, may eliminate infection.

LENS

Cataract

Aetiology

•Intralenticular E. cuniculi

•Trauma

•Inherited cataract

•Posterior capsular lens opacities may be observed in conjunction with persistent hyperplastic primary vitreous

GLAUCOMAS

Glaucoma

Congenital glaucoma (buphthalmos) is known to be inherited (recessive) in the New Zealand White rabbit, but is also seen in other breeds. It develops in early life (1–3 months of age) and is associated with pectinate ligament dysplasia.

Clinical signs

•Most typically corneal oedema and globe enlargement (Figure 5.7)

•Severe cupping of the optic disc is an easily-recognisable feature, but early cupping is not easy to diagnose because of the depth and extent of the normal physiological pit

Treatment

Treatment may not be necessary as the condition is not painful, and the intraocular pressure often returns to normal when degenerative changes in the ciliary body cause a reduction in aqueous production.

RABBIT OPHTHALMOLOGY