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.
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.