PRESENTING SIGNS

Owners will present the dog or cat because of sudden onset blindness affecting one or, more commonly, both eyes, since many do not notice if their pet is blind in one eye. Adult dogs and cats are normally affected but occasionally young dogs can be presented. The appearance of the eye is sometimes different – cloudy or red for example – and the patient might be depressed or confused. Increased thirst and frequent urination, together with weight loss, might be reported in some animals.

CASE HISTORY

The history is usually brief since the presentation is acute. However, some animals might have a history of recent illness – renal or cardiac disease for example – or might have travelled abroad and thus be susceptible to exotic infections.

CLINICAL EXAMINATION

The animal will be blind in one or both eyes if the detachment is widespread, but if partial in one eye this may be asymptomatic. The affected pupil will be dilated and non- or poorly responsive to constriction in bright light. Thus the patient might have bilateral mydriasis. If unilateral, the normal pupil will constrict on illumination, whereas the abnormal eye will remain dilated on direct illumination but its pupil will constrict if the light is shone in the normal eye. This indirect or consensual light reflex is an indicator of afferent lesions and is termed a positive swinging flashlight test.

Most eyes will be ‘quiet’ with no pain or ocular redness. Some might have cataract formation which could preclude fundus examination – indeed the owner might believe that the patient is blind due to the cataract alone. The eye might also look opaque due to the retinal detachment itself – if a total bullous or rhegmatogenous detachment is present this can be visualized as a grey ‘veil’ floating immediately behind the lens (Figure 49.1). It can be seen on direct illumination with a pen torch without the need for an ophthalmoscope in many cases. If the retina has torn from its dorsal attachments at the periphery behind the lens, it will fall forward over the optic disc, obscuring it, while the tapetum will be more exposed and extremely hyper-reflective above the retinal detachment (Figure 49.2). There will be a complete absence of retinal vessels dorsal to the disc but they may be visualized out of focus in the grey detached ventral retina.

Figure 49.1 Total bullous retinal detachment in a hypertensive cat. The retina can be seen floating behind the dilated pupil like a veil. Exudative retinal detachment.

Figure 49.2 Rhegmatogenous retinal detachment of unknown aetiology in a Manchester terrier. The grey retina can be seen draped over the optic disc, while the tapetum above is highly reflective with no blood vessels over it.

Less severe detachments will require ophthalmoscopy and will manifest as out-of-focus areas of the retina and a reduction in reflectivity due to increased thickness of the retina as it becomes oedematous and fluid accumulates between the retina and tapetum (Figure 49.3). Perivascular cuffing (exudates around the blood vessels) might be noticeable along with retinal, and sometimes vitreal, haemorrhage as the detached vessels are stretched and torn. If the patient is blind in just one eye with a total detachment, the other fundus should be carefully evaluated for early signs of problems – for example, collie eye anomaly, retinal dysplasia, chorioretinitis, vascular anomalies and so on could be present.

Figure 49.3 Dorsal retinal detachment in a pointer with hyperviscosity syndrome – note the retina bulging forwards dorsally, the out-of-focus retina (due to retinal oedema), the perivascular cuffing and engorged, tortuous vessels.

DIFFERENTIAL DIAGNOSES

• Congenital lesions – collie eye anomaly and retinal dysplasia

• Hypertensive retinopathy

• Chorioretinitis with inflammatory exudates (see Table 49.1 for differential list)

• Hyperviscosity syndrome

• Vitreal degeneration and liquefaction

• Hypermature cataract

• Post-surgery – cataract and lens luxation

• Idiopathic or steroid-responsive retinal detachment

• Solid detachment from adjacent pressure (e.g. retrobulbar neoplasia)

• Severe trauma

Table 49.1 Causes of chorioretinitis in dogs and cats

CASE WORK-UP

A fairly extensive work-up can be required to reach a definitive diagnosis in cases of retinal detachment, particularly in adults with serous detachments. Blood samples should be taken for haematology and biochemistry screening. These might show a neutrophilia suggesting infectious disease or hyperviscosity syndromes for example, such that serum electrophoresis is warranted, along with bone marrow biopsy. Renal and hepatic disease might be present. Urinalysis should be considered. Systemic blood pressure should always be taken since hypertension is a common cause of retinal detachment in both dogs and cats. If chorioretinitis is suspected, specific serological testing for the various causes of this can be considered (Table 49.1).

If the fundus cannot be directly visualized, for example because of the presence of a total cataract, then ocular ultrasonography will be invaluable in detecting a retinal detachment. General abdominal and cardiac ultrasonography might also be indicated depending on the preliminary investigation results – looking for cardiac disease or adrenal masses in cases of systemic hypertension for example, or for the location of a primary tumour in hyperviscosity syndromes. Unfortunately, in some instances, the underlying aetiology might not be revealed despite extensive work-up.

Cases of rhegmatogenous retinal detachment do not always require much case work-up since the cause is more frequently ophthalmic rather than systemic. Previous cataract or lens luxation surgery and vitreal degeneration will predispose to large retinal tears and in these situations minimal investigation is required.