1. Why Perform a Fundus Examination?
Vision impairment related to the retina
Systemic disease and associated retinal changes
Breed certification--CERF, ACES, European Eye Certificate
2. Methods of Examining the Fundus
Use dim or no room light
Dilate pupil using mydriatic (tropicamide)
Ophthalmoscopy is needed to visualize the posterior segment (vitreous, retina and optic nerve and their associated blood vessels)
Two different types of ophthalmoscopy: direct and indirect
Direct ophthalmoscopy: Requires less practice; cheaper; more magnification (varies inversely with size of eye; e.g., about 20 X in the cat and about 8 X in the horse); can be used with miotic pupils; is of some use in most species.
Indirect ophthalmoscopy: Survey instrument with less magnification (varies inversely with the dioptric power of the lens used and inversely with the size of the eye; e.g., about 4 X using a +20 diopter lens in the cat), but wider field allowing more fundus to be visible at one time (35 degrees instead of 9 degrees)--facilitates examination of peripheral fundus; permits exam at safe distance from fractious patients; provides stereoscopy; brighter illumination (for translucent ocular media); excellent teaching device because a prism can be used to give students a simultaneous view of what the examiner is seeing
3. Equipment Needed
Finoff transilluminator
Indirect ophthalmoscope & 20D or 28D lens
Direct ophthalmoscope (not as useful because it is designed for human optics NOT dogs and cats). Useful in horses.
4. Anatomy of the Retina
Ten layers--outermost layer is the retinal pigment epithelium which faces the choroid
The other nine layers referred to as sensory retina (including the photoreceptors)
5. Normal
Important to recognize normal
Regular examination of normal patients is important
Takes many years to become competent
Variations from normal
Many variations, mainly related to colour
Blue irides = tigroid (no overlying pigment) fundus--Siamese cats, merle Collie, Old English sheepdog, etc.
Dark coated dogs = small tapetal fundus, occasionally no tapetum visible e.g., Chocolate Labradors
Optic disc anatomy can vary especially in dogs (variations in myelination)
Blood vessel appearance varies--note in dogs, vessels cross over optic disc
6. Retinal Diseases Recognised with Funduscopy
Retinal Haemorrhage/Detachment
Especially older cats with hypertension
Also coagulopathies, hyperviscosity and some canine systemic diseases
Best seen using indirect ophthalmoscope but can be often seen with naked eye
Look for areas of detachment (dull, out of focus) and haemorrhage (association with blood vessels and retinal layers determines appearance)
Later, look for areas of hyperreflectivity
Retinal Atrophy
Includes progressive retinal atrophy (PRA), progressive rod-cone degeneration, rod-cone dysplasia and retinal degeneration in Abyssinian cats
Signs very subtle in early stages (attenuation of the peripheral retinal vessels) but will have poor vision in dim light)
Later on, obvious hyperreflectivity and retinal vessel attenuation accompanied by vision impairment including in day light
Taurine deficiency in cats causes retinal degeneration in the area centralis
Collie Eye Anomaly
Defect is choroidal hypoplasia dorsotemporal to optic disc and focal absence of tapetum and RPE revealing abnormal choroidal vasculature
Defects in optic nerve
Occasionally accompanied by retinal detachment and haemorrhage
Seen in collies, Shetland sheepdogs, Australian shepherds and some non-shepherd breeds
Inflammatory Disease
Chorioretinitis (as seen in feline infectious peritonitis--FIP and fungal disease) and retinochoroiditis (as seen in neurotrophic viruses such as canine distemper)
Often see hypertrophy of the retinal pigment epithelium, easily visible in the nontapetal fundus
Perivascular cuffing (vasculitis in FIP)
Congenital Disorders
Retinal dysplasia (also in maternal viral infections such as canine herpes and feline panleukopaenia)
Sudden Acquired Retinal Degeneration (SARD)
Cause unknown but retinitis is suspected as the initiator in some cases
Some cases have endocrinological abnormalities
Fundus changes occur sometime after clinical signs noticed