The Normal Fundus
There are so many variations in the normal fundus that it is really important to know what is the range of normal retinas, so as to be able to recognise the abnormal. Looking at many normal eyes is the key to being familiar with the range of appearance occurring in the eye of one species such as the cat or dog.
Differences will be seen in:
The colour of the tapetal reflection.
The position of the optic nerve with regard to the tapetal boundary.
The number and calibre of retinal vessels.
The appearance of the fundus in colour dilute animals where reduction is noted in the presence of the tapetum and ocular pigment.
A Logical Approach to Interpreting Changes in the Fundus
As noted in the first lecture a logical positional approach is needed examining each quadrant of the retina but also a logical structural approach is required.
Are there abnormalities in:
Retinal position: retina only in focus at a plus diopter setting (black numbers on most ophthalmoscopes) indicating retinal detachment or the bottom of the optic disc only in focus at a negative setting (red numbers on most ophthalmoscopes) indicating a colobomatous defect.
Tapetal reflection: hyper-reflectivity indicating retinal thinning or dullness and opacity suggesting cellular infiltration or retinal detachment.
Retinal vessels: engorged or with haemorrhages suggesting hypertension or a hyperviscosity syndrome, with white sheathing around the vessel suggestive of posterior uveitis diminished in size suggestive of retinal atrophy, especially associated with tapetal hyper-reflectivity.
Assessing Retinal Function
Assessing vision is difficult in domestic species but a range of tests is possible. Some test vision behaviourally while others test not vision per se but a part of the visual system. One must always be aware of exactly what one is testing - an intact pupillary light reflex does not define the animal to be visual, to take the most obvious example.
The cotton ball test crudely tests whether an animal can respond to a large moving target.
An obstacle course similarly gives a very gross idea of visual capability.
The pupillary light test only shows that visual pathways are functioning to the level of the lateral geniculate and even then only a few ganglion cell neurons need to be activated to give a response.
The flash electroretinogram gives a photoreceptor mass response that demonstrates that the retina is responding to a flash of light. The erg can be undertaken in photopic (bright light) and scotopic (dim light) conditions to assess cone and rod responses but is more often used to assess whether PRA is present before cataract surgery or in a young puppy after a test mating.
The pattern electroretinogram assesses ganglion cell activity to an alternating checker-board pattern and has been used to document visual acuity in normal dogs and dogs with glaucoma where the ganglion cells rather than the photoreceptors are the cells first damaged by rising intraocular pressure.
Generalised Progressive Retinal Atrophy
Generalized progressive retinal atrophy (gPRA) is a term used for inherited photoreceptor dysplasia or degeneration. Dysplasia implies that the photoreceptor never forms properly while degeneration is used to describe a retina that forms without defect and then atrophies at a later time. Even though different breeds have PRAs which occur at different ages and are caused by different gene defects, they all have the same general ophthalmic characteristics:
Blindness starts with nyctalopia (night blindness) followed by day blindness as well.
Tapetal hyper-reflectivity shows a thinning of the retinal layers.
Retinal vessel attenuation and optic disc pallor accompanies retinal atrophy.
Non-tapetal fundus patchy depigmentation may occur.
Eventually pupillary light reflex is lost though this is not always a reliable test.
Secondary cataract may occur, interestingly in some breeds more so than others.
Diagnosis is arrived at by:
History - breed and age at onset of first signs. In a case of cataract ask whether the blindness preceded the cataract or came at the same time. This can be difficult for owners to answer and you have to be careful not to ask leading questions!
Characteristic posterior segment signs.
Electroretinography for confirmation or in cases of cataract.
The key differential diagnosis is inflammatory retinopathy.
gPRA means it's:
Generalised: if the hyper-reflectivity involves the whole retina then PRA is likely. A post-inflammatory atrophy has sharply defined boundaries.
Progressive: signs of retinal atrophy in PRA worsen with time while a post-inflammatory hyper-reflective retinal lesion stays the same.
There is no treatment for any of these animals, apart from the possibility that anti-oxidants may slow the final progress to complete blindness. But the thing to impress upon an owner distraught to be told their dog is blind, is that this is a progressive condition and so the animal has gradually been having more and more difficulty in seeing for some time. This means that it has happily been learning to use its other senses (much more advanced than ours, of course) to get around without being able to see. Often owners have no idea that there is much of a problem until maybe they move the furniture round or move house. Dogs are amazingly resilient to visual problems as such 'completely blind but nevertheless coping.
Vascular Changes in the Retina
As noted above changes in the retinal blood vessels can occur secondary to retinal atrophy but most occur as a consequence of systemic abnormalities. Anaemia gives pallid tenuous vessels but also haemorrhage in many cases, probably from dysfunction of ischaemic vessel endothelium allowing blood loss across the vessel wall.
Hypertension gives signs of retinal haemorrhage and total retinal detachment in protracted cases but in mild instances signs such as focal flat detachments and 'box-caring' of retinal vessels is seen. In this latter situation the high pressure causes vascular constriction in segments along the vessel. This congestion leads to ischaemia and vessel wall necrosis with so-called insudation of fluid giving a detachment. Hypertension should be confirmed by blood pressure measurement using a sphygmomanometer. Causes of hypertension in the dog and cat may be renal insufficiency or hyperthyroidism, although the idiopathic essential hypertension seen in man occurs rarely in domestic animals. The hypertension should, in cats where it is most commonly seen, be treated with 0.625 mg amlodipine (1/8th of a 5 mg Istin tablet).