Recognition of fundus lesions can be challenging for many. Firstly the necessary skills to examine the fundus effectively and thoroughly need to be mastered and then having gained a view of the fundus the question of whether what is seen is normal or abnormal needs to be asked. Having decided the fundus has an abnormality then interpreting what the lesion represents is the next challenge.

Examination of the fundus is facilitated by dilating the pupil with a short-acting mydriatic (e.g., tropicamide) unless pupillary dilation is contraindicated (e.g., in animals with glaucoma and, in some instances, those with lens luxation). Proficiency with both direct and indirect ophthalmoscopy is to be encouraged. Indirect ophthalmoscopy readily allows an overview of the fundus, while direct ophthalmoscopy allows for a more magnified view as needed.

There is a wide range of normal variations in fundus appearances, particularly in dogs, but also to a lesser extent in cats. Careful assessment of entire fundus (both tapetal and non-tapetal) should be undertaken. In addition the retinal vasculature and optic nerve head must also be assessed.

The tapetum can vary in both colour and extent. Some animals, particularly colour-dilute individuals, may lack fundus pigmentation and some will also lack a tapetum. In these individuals the underlying choroidal blood vessel pattern can be visualised, the so-called 'tigroid fundus'. In animals with a tapetum it may range in colour in dogs from grey or tan through reds, greens, blues and oranges. The extent of the fundus that the tapetum occupies also varies. In general, miniature breeds of dog will have a smaller tapetal area compared to larger breeds. Cats will more commonly have yellows, greens, blues and less commonly red coloration to the tapetum. The colour of the non-tapetal fundus will be dependent on the degree of ocular pigmentation and may range from lacking pigment through light brown to darkly pigmented.

The retinal vasculature varies between individual dogs. The extent of the vessels can vary as can the tortuosity of vessels. Miniature-breed dogs tend to have less extensive superficial retinal vasculature compared to standard-sized dogs. In the miniature breeds it is not uncommon for this less extensive vasculature to be misdiagnosed as vascular attenuation, which if present would be an indicator of retina degeneration. Cats tend to have less variation in retinal vasculature than dogs. Similarly the normal optic nerve head of dogs shows greater individual variation than that of cats. The myelination of the nerve fibres as they converge on the optic nerve head to penetrate through the sclera at the cribriform plate accounts for the majority of the variation in optic nerve head appearance in dogs. In most dogs myelination of the retinal nerve fibres is gained prior to their penetration through the cribriform plate. The presence of myelin increases the bulk of the nerve fibres meaning that the surface of the optic nerve head is raised from the adjacent retinal surface, and, when viewed ophthalmoscopically, the myelination means that the size of the optic nerve head is greater than that of the cribriform plate, which is obscured from view. The shape and size of the normal canine optic nerve head may vary from circular and relatively flat (less myelination) to a larger triangular-shaped structure that is a little more raised above the retinal surface. An optic nerve head with marked myelination is described as showing pseudopapilloedema because the large size of such an optic nerve head mimics the pathological condition of optic nerve head swelling (papilloedema). In cats the myelination of nerve fibres does not usually take place until the cribriform plate meaning the optic nerve head appears relatively smaller, circular and slightly depressed from the retinal surface.

There is a relatively limited number of altered fundus appearances that occur in diseased eyes. We are very fortunate in that the presence of a tapetum makes it much easier to detect alterations in retinal thickness or transparency than it is in those animals that do not have a tapetum. The tapetum is a reflective structure positioned in the inner choroid and is overlain by the retina.

Any change in retinal thickness or alteration in retinal transparency can affect the ophthalmoscopic view of the tapetum. Retinal thinning allows for increased transmission of light through the retina meaning the tapetum appears more reflective than normal - described as tapetal hyper-reflectivity. If the retina becomes thickened or its transparency is altered, the reflection from the tapetum is reduced, described as tapetal hyporeflectivity. Retinal thinning, seen as tapetal hyper-reflectivity, may occur across the entire tapetal area in conditions resulting in panretinal degeneration, which would include the inherited progressive retinal atrophies as well as non-inherited conditions such as chronic glaucoma. Focal tapetal hyper-reflectivity reflects local retinal death which can occur in conditions such as retinal dysplasias and postinflammatory retinopathies. Tapetal hyporeflectivity can occur with retinal thickening such as that resulting from oedema or inflammatory cell infiltration. Decreased retinal transmission of light can also be an early sign of some forms of progressive retinal atrophy (PRA) and is thought to be the result of loss of the regular arrangement of discs within the outer segments of photoreceptors meaning that there is increased absorption of light as it passes through the retinal layers. This change would precede the development of tapetal hyper-reflectivity, which is a more classic change seen with PRA.

With inflammatory and degenerative conditions pigmentation in the tapetal fundus can occur. This may be the result of pigment proliferation in the retinal pigment epithelial layer that is normally not pigmented where it overlies the tapetum. Such changes are commonly seen in postinflammatory lesions. These lesions should not be confused with the appearance of normal pigmentation where there is a lack of tapetum; in some animals the junction between tapetal and non-tapetal fundus is not a sharp demarcation and at the junction there can be patches of pigmented fundus between islands of tapetum. Pigmentary retinopathies such as retinal pigment epithelial dystrophies can present with brown-pigment spots over the tapetal area. The pigment is a lighter brown colour than that seen with melanin proliferation in postinflammatory type lesions. Tapetal colour change with disease is common. In advanced retinal degeneration the tapetal colour is often altered, which is partly due to the altered reflection from the tapetum.

Changes in the non-tapetal fundus include loss of pigmentation, often accompanied by regions of clumping of pigmentation. This can occur with retinal degenerative conditions and inflammatory conditions. Thickening of the retina with active inflammation can be appreciated as grey-coloured lesions when in the non-tapetal area.

Retinal detachment can vary in appearance depending on the extent of the detachment, the degree of elevation, the colour of the subretinal fluid and the presence or absence of retinal tears or holes. When there is a complete detachment the retina may appear in the vitreous as a grey-coloured membrane with surface vasculature. Focal detachments may appear as raised regions with a less clear view of the underlying structures (such as tapetum or choroid).

Retinal vasculature changes are common findings. These include haemorrhages, which can take on different shapes depending on which retinal layer they involve: whether they are preretinal, intraretinal or subretinal. With inflammatory conditions vascular cuffing may be visualised. With retinal degeneration blood vessel attenuation develops. This can be seen initially as a loss of the finer arterioles but eventually arterioles and venules will both be affected. Alterations in retinal vasculature can be seen with inflammation-increased blood flow caused by systemic disease such as hypertension and hyperviscosity. With hypertension the retinal arterioles are affected and may show focal constriction and dilation and even aneurysms. Hyperviscosity syndrome results in dilation of both arterioles and venules. Lipaemic blood and severe anaemia may also alter the appearance of the retinal vessels.

Changes in the optic nerve head can include developmental abnormalities, for example colobomatous defects and acquired changes including swelling, haemorrhages and atrophy and degeneration. Colobomas involving the optic nerve head appear as pits, the three-dimensional nature of which can be readily appreciated by indirect ophthalmoscopy. Swelling results in an increased prominence of the optic nerve head and often elevation of the adjacent retina. With atrophy there is a loss of myelination resulting in a grey and flattened optic nerve head.

With practice and examination of many eyes the recognition of normality becomes easy and, by extension, the identification of the abnormal becomes possible.