Introduction to the retina

Introduction

The retina is a delicate membranous structure that lines the back of the eye, firmly attached at the region of the optic disc (the optic nerve) posteriorly and at the ora serrata anteriorly. This layer of photoreceptors is divided into central and peripheral regions that enable different visual functions:

a. Distinguishing between details
b. Color vision
c. Night vision
d. Peripheral vision
The rule is that retinal disorders are not painful.

The transparent media that fill the eye allow examination of the retina. Frequently used aids are the direct ophthalmoscope, the indirect ophthalmoscope and the slit lamp combined with a biomicroscope. Additional aids such as fluorescein angiography (FA), indocyanine green angiography, echography (US), optical coherence tomography (OCT), scanning laser ophthalmoscope (SLO), X-ray imaging and electrophysiology (ERG) can provide further information in eyes with media opacities. The OCT demonstrates the anatomy of the retina, vitreous and choroid, which can provide further information about the pathological changes in various diseases of the posterior segment. Documentation of the clinical findings (with words or drawings) is an integral part of a complete posterior segment examination.

Pupil dilation can be used in order to perform a full examination of the posterior segment, from the posterior pole up to the anterior boundaries of the ora serrata. Indentation of the sclera, combined with an indirect ophthalmoscope, is a valuable technique that allows viewing of the peripheral retina. Use of a slit lamp combined with a biomicroscope together with contact and non-contact lenses allows viewing of both the posterior pole and part of the periphery of the retina. A slit lamp is used to diagnose thickening of the macula along with other diseases of the retina-vitreous interface and of the choroid. Fluorescein angiography together with indocyanine green angiography are techniques that allow deeper study of the pathophysiology of diseases of the blood vessels of the choroid and retina.

X-ray images and CT allow viewing of foreign bodies that are radiopaque.

MRI can be used to obtain further information about orbital diseases that involve the posterior segment, such as ocular manifestations of thyroid diseases or inflammatory diseases in the posterior segment such as posterior scleritis. Note that the presence of any intraocular, orbital or intracranial foreign body is a contraindication to performing an MRI.

There are retinal diseases that are asymptomatic until they reach an advanced stage, such as diabetic retinopathy.

Possible symptoms of posterior segment diseases:

a.      A temporary or permanent decrease in visual acuity

b.      Changes in color perception

c.       Metamorphopsia – distortion of central vision

d.      Floating bodies – floaters

e.      Photopsia – flashes of light in the visual field

f.       Scotoma – a localized defect in the visual field

g.       Extensive damage to the visual field