7 Pathology of the retina

The retina is a beautifully complex piece of tissue that functions to convert light to the neuronal impulses interpreted by the brain, manifesting as vision. The retina is microscopically composed of 10 layers, each with an important function. The photoreceptors, composed of rods and cones, are the outermost layer, and are the neurons that convert light to electrical impulse. Impulses are then transmitted through various modulatory layers, including the outer and inner nuclear layers, to reach the ganglion cell layer. The axons of the ganglion cell layer travel along the inner most aspect of the retina and exit the eye through the lamina cribrosa, forming the optic nerve.

Understanding the vasculature of the retina is of some importance, and will help the pathogenesis of some of the lesions described below become more intuitive. The retina is supplied by two sources of blood: the choriocapillaris within the choroid, from which the outer most aspect of the retina obtains its requirements by diffusion; and retinal blood vessels found within the inner most aspect of the retina. Because the inner retina has its own blood supply, it is less susceptible to ischemic injury as compared to the outer retina.

Damage to the retina leads to loss or impairment of vision, and is therefore quite important.

7.1 Retinal detachment

Retinal detachment is a common sequela of a number of different conditions, and the immediate consequence is loss of vision. Because the outer most part of the retina obtains its oxygen and nutrients from the choroid, it quickly becomes hypoxic and atrophic when separated and releases angiogenic factors, which can lead to a PIFM.

7.2 Progressive retinal atrophy

Progressive retinal atrophy (PRA) is a term applied to a collection of diseases with a common outcome: photoreceptor degeneration or disorder. These disorders are congenital and/or inherited, are progressive, lack an inflammatory or toxic cause, and may result in blindness. Histopathology is not particularly helpful for determining the underlying etiology in these cases: they all have the same appearance.

7.3 Collie eye anomaly

Collie eye anomaly is an inherited, congenital, and bilateral condition seen in collies and other breeds. The condition is a failure of embryonic development of the choroid, tapetum lucidum, and sclera. Choroidal hypoplasia is common, as are colobomas near the optic disc.

7.4 Sudden acquired retinal degeneration

This is a rapidly progressing degeneration of photoreceptors that leads to blindness. It is a bilaterally symmetric disease and typically affects adult and/or older dogs of any breed. Loss of photoreceptors leads to blindness, and eventually the condition proceeds to complete retinal atrophy. In its chronic stages, it is indistinguishable histologically from PRA – thus, clinical progression is likely to be of great help to the pathologist.

7.5 Hypertension

Hypertension induced retinopathy is common in dogs and cats. The small vessels of the choroid are particularly affected, which therefore leads to atrophy of the outer retina, and potentially blindness. Leaky hypertensive vessels may lead to retinal detachment through ischemic necrosis of the retinal pigmented epithelium, which normally provides a barrier against fluid loss from the choroid.