INTRODUCTION

The retina is the raison d'etre for the eye to exist and its detailed examination obviously must be part of the clinician's ability to investigate sight problems in his patients. Too often the cat's misdemeanors or the dead battery in the ophthalmoscope can destroy the enthusiasm required to perform meaningful ophthalmoscopy, and the complexities of the retina do require exacting comprehension. The investigation of the ocular fundus can present as a real challenge, but help is at hand. In this lecture we will discover the use of the indirect ophthalmoscope and give meaning to the features of disease that it will reveal.

EXAMINATION

The fundus is examined clinically by using ophthalmoscopic techniques, retinal function is assessed electrophysiologically using the electroretinogram (ERG) and ultrasonography is used to evaluate the posterior segment when the ocular media are opaque. However, it is the meaningful use of the ophthalmoscope, particularly the indirect scope, which will identify those lesions on which you will build your diagnosis.

Two types of ophthalmoscope are used--the direct and the indirect types. Although the direct method tends to be used most commonly in small animal practice, indirect ophthalmoscopy offers many advantages. The principles are similar but the important points concerning use are as follows:

1.  Direct

a.  A darkened room is necessary.

b.  A short acting mydriatic should be used (tropicamide 1%)(20 minutes to obtain maximum mydriasis).

c.  The ophthalmoscope is set at 0.

d.  The pupil is located by the tapetal reflection from 25cms ideally using the right eye to examine the patient's right eye and vice versa.

e.  The observer closes up to within2cms of the patient's cornea, gazing through, not at, the hole in the ophthalmoscope.

f.  A focused image is achieved by rotating the disc until a clear sharp image of a blood vessel is obtained.

g.  Approximately 9 degrees of the fundus are viewed and the composite fundus appearance is built in the mind's eye.

h.  There is no stereoscopy and refocusing the ophthalmoscope is necessary to obtain different levels of in-focus view.

2.  Indirect

a.  A darkened room is necessary.

b.  Mydriasis is essential.

c.  The pupil is located from 1 meter (arm's length).

d.  A 20D or 30D condensing lens is placed approximately 5cms from the cornea.

e.  The lens is moved backwards and forwards slightly until a sharp, inverted and reversed image of the fundus is seen within the lens.

f.  25 to 70 degrees of the retina are seen (depending on the lens) and binocular techniques offer good stereoscopic vision.

The combination of a 20D or 30D condensing lens and a focal illuminator will allow an easy introduction to indirect ophthalmoscopy. For long periods of retina screening a spectacle frame instrument will be found preferable to a head band.

THE NORMAL FUNDUS

A working knowledge of the fundus is absolutely essential before the lesions of disease can be identified with accuracy. In the dog there is considerable variation in the appearance of the normal with the bizarre sometimes looking more like disease than normal variation.

Dog

1.  Tapetal fundus--a bright shiny sweep of colour above the optic disc, triangular in shape.

2.  Non-tapetal fundus--the pigmented area which surrounds the optic disc and the tapetal fundus.

3.  Optic disc--white to pink usually circular structure situated usually at the junction of the tapetal and non-tapetal fundi

4.  Retinal vasculature--usually three to four primary veins which meet in a kind of circle on the optic disc. Secondary veins arise from the venous circle. Arterioles (c.25) arise from the edge of the disc.

Variations in the Dog

1.  Colour of tapetal fundus is usually yellow with a green and blue border. Greens, oranges and blues seen.

2.  Size and shape of the tapetal fundus varies and the tapetum is absent in albinotic and sub albinotic dogs.

3.  Non-tapetal fundus is usually darkly pigmented (R.P.E. contains melanin), but in dogs with brown coats and a tan iris, a tigroid appearance is seen. The choroidal blood vessels are seen.

4.  In the merle, the tapetum, retinal pigment and choroidal pigment may all be missing. The picture is one of the superficial retinal and choroidal blood vessels against the white scleral background.

5.  Optic disc is white to pink depending on the dilution of blood vessel glow by myelin and axon material.

6.  Shape/size of optic disc varies with amount of myelin present.

Cat

1.  Very large tapetal fundus

2.  Small round optic disc usually within the tapetal fundus

3.  Three pairs of arteries and veins arising from the edge of the optic disc

4.  The optic nerve is nonmyelinated at the level of the lamina cribrosa and the optic disc thus appears grey

Variations in the cat

Not as many as the dog but colour dilution affects the amount of pigmentation and the form of the tapetum.

THE DISEASED RETINA

There are few ophthalmoscopic manifestations in retinal disease and once these can be identified, diagnosis should be uncomplicated. The common changes are as follows:

1.  Reduced tapetal reflectivity: a feature of active inflammation and bullous retinal detachment.

2.  Increased tapetal reflectivity: a feature of retinal degeneration.

3.  Abnormal pigmentation: a feature of retinal degeneration.

4.  Reduced pigmentation: a feature of retinal degeneration.

5.  Blood vessel congestion: usually associated with inflammation.

6.  Blood vessel attenuation: due to retinal degeneration.

7.  Haemorrhage: usually associated with inflammation but can be seen with other systemic diseases process like hypertension and thrombocytopaenia.

8.  Disc shape: enlarged with active or passive congestion. : distorted or shrunken with retinal and optic nerve degeneration.

In addition there are specific congenital lesions i.e., neuroretinal folds or colobomatous defects, but such lesions are readily recognisable.

CONCLUSION

The ophthalmoscopic feature will dictate the nature of the disease process present and together with the anamnesis, clinical examination and other ancillary tests, diagnosis should be possible.