The retina: Examination and what it can tell you (Proceedings)

When we examine the posterior segment of the eye we tend to think that we are looking for problems in the retina. I reality we are seeing the vitreous humor, the neural retina and optic nerve, retina pigment epithelium, the vascular coat lining the back of the eye – the choroid, and the outer fibrous coat of the eye – the sclera. The fundus is the area including all of these structures as seen by ophthalmoscopy through the pupil.

Examination

For the most thorough and rapid screening of the posterior segment indirect ophthalmoscopy is the method of choice. Although more difficult initially to master this technique allows you to quickly see and identify abnormalities over a large area of the posterior segment. It does require that the pupil be dilated for the best view. Direct ophthalmoscopy is perceived as an easier technique to learn but it does have limitations in terms of the ease with which lesions can be noticed and identified.

Indirect ophthalmoscopy requires use of a bright light source and ideally a 20D or 28D lens. An assistant restrains the animal and holds the head still. The examiner stands at arm's length from the animal and directs the light source at the eye in a way that the tapetal reflection is easily seen. The 20D lens is then positioned along the light path about 2-3 inches from the eye. The image seen is inverted.

The Normal Fundus

It is important to remember that the structures of the fundus are layered – one on top of the other. The appearance is largely affected by the presence or absence of a tapetum, pigment in the retinal pigment epithelium and choroid and the normal vascular patterns in the eye.

The layers as seen by the examiner are the vitreous, the neural retina (this is to some extent transparent and the only structures identified are the retinal blood vessels), the retinal pigment epithelium, choroid and sclera. The retinal pigment epithelium varies in the amount of pigment present depending on the area of fundus examined (less pigment over the tapetal fundus – more in the non-tapetal fundus) and the overall amount of pigment in the eye (iris in particular predicts what might be seen in the fundus) and the animals coat color.

The choroid is visibly made up of blood vessels and pigment. How much choroid is seen depends on the pigmentation in the overlying RPE in the non-tapetal area of the fundus. The tapetum is a modification of the anterior part of the choroid present in the dorsal half of the fundus– it forms a reflective layer to maximize light absorption by the retinal photoreceptors. In areas of tapetal fundus the rest of the choroid is not seen behind the tapetum.

The sclera is not seen in animals with plenty of ocular pigmentation. In color-dilute eyes the choroid and sclera may be seen to varying degrees reflecting the overall pigmentation of the eye.

What To Look For In The Fundus

When an appreciation for normal is gained by repeated examination, certain disease features of the fundus are readily appreciated.

Lesions involving the vitreous may result in opacities/opacification of this normal transparent substance. Inflammatory and cellular exudates, hemorrhage are seen in cases of uveitis or chorioretinitis or optic neuritis. The overall color of the vitreous may be changed by inflammation. Vitreous liquefaction often accompanies inflammation. Tumors of the posterior segment may protrude into the vitreous cavity. Retinal detachments may be seen anywhere in the vitreous cavity.

Age related asteroid hyalosis if commonly seen in dogs and usually does not affect vision.

In the tapetal fundus the degree of reflection from the tapetum needs to be evaluated. Most lesions in the tapetal fundus result in either an increase or decrease in the reflectance form the tapetum. Such areas can be focal or diffuse. An increase in the tapetal reflectivity implies thinning of the retina overlying the tapetal allowing more light to be reflected to the viewer and less absorbed in the retina. This may occur diffusely (inherited retinal degenerations, glaucoma, sudden acquired retinal degeneration (SARD) or retinal detachment) or focally (retinal scars or inactive and previous chorioretinitis or focal degenerations associated with developmental disease (retinal dysplasia). Often this may also present as color change in the tapetum. In the centre of focal areas of degeneration the retinal pigment epithelium often develops pigmentation giving a darker appearance in the center of the areas of hyper-reflectivity.

A reduction in reflectivity from the tapetal fundus occurs when the retina is thickened by retinal dysplasia or when infiltrates/exudates/hemorrhage is seen in the vitreous or retina overlying the tapetum. This is commonly the case in active chorioretinitis. This appears as dark fuzzy areas in the tapetum and may be accompanied by areas of hemorrhage. With the indirect ophthalmoscope the lesions may appear raised and thickening the retina. In severe cases small bullous detachments may focally separate the retina from the underlying retinal pigment epithelium. Findings of chorioretinitis occur in many infectious diseases in the domestic small animal species and should be an indication to perform a detailed workup for underlying systemic disease.

Primary retinal neoplasia is relatively rare but metastasis or generalized neoplastic infiltrates (lymphosarcoma is most common) may present in the same e way in the fundus. Other lesions which can reduce tapetal reflectivity are areas of retina thickening and folding as occurs in retinal dysplasia.

The non-tapetal fundus has a different appearance with either degeneration or inflammation. Degenerative lesions appear as areas of depigmentation which may expose the underlying choroid. The pigment epithelium reacts by forming denser areas of pigmentation focally within the depigmented areas. In extensive advanced degeneration the choroid may also be lost exposing the sclera. Inflammatory infiltrates and retinal folds appear as lighter colored (white, gray, and beige) focal areas against the darker background of the pigmented retinal epithelium.The retinal vasculature shows changes in various disease states. Retinal vessels are lost in retinal degenerations. In various conditions the vessels may become engorged and dilated (inflammation, hyperviscosity syndrome, cardiac anomalies). Tortuosity of retina vessels is usually a normal variation although it can accompany some developmental diseases and hyperviscosity.

Hemorrhage from retinal vasculature is seen in cases of thrombocytopenia, systemic hypertension and chorioretinitis. Perivascular cuffing with inflammatory cells may be seen around retinal vessels in cases or chorioretinitis. The color of the retinal vessels is due to the blood they contain. In anemic patients the vessels may appear thinner than usual. In hyperlipemic patients (diabetes) the vessels appear pink.

Choroidal vasculature is not seen behind the tapetum or retinal pigment epithelium. When the tapetum is degenerate or hypoplastic or when RPE pigment is lost the choroidal vasculature and pigment is seen. In cases of choroiditis darker areas are seen within the fundus. Choroidal vessels are hypoplastic or dysplastic in certain developmental diseases (collie eye anomaly)

The optic nerve can present with considerable normal variation in color and size and shape in the dog (less so in normal cats or horses). Most of the variation is related to the extent of myelination extending from the nerve into the nerve fiber layer o f the retina (this rarely happens in the cat or horse). The optic nerve may show various changes with ocular disease. Not infrequently abnormalities of size and shape are seen in congenital developmental diseases. Areas of abnormal development may appear as colobomatous defects in the nerve.

Inflammation of the optic nerve appears as a hazy indistinct nerve with an irregular margin, which may appear elevated into the vitreous. The nerve may be more hyperemic and hemorrhages may be present. In severe cases the structure of the nerve may be obscured by infiltrates. Etiologies are same as for chorioretinitis and often indicate systemic disease.

Degeneration of the optic nerve occurs secondary to retinal disease (inflammatory or inherited degeneration), optic neuritis and is a major cause of vision loss in glaucoma. The affected nerve loses myelin, vasculature and becomes a dull gray color.