The Back Of The Normal Eye

Normal retinal anatomyThis is the second of two articles on the normal anatomy of the eye.  This post describes the retina and associated problems.

Pars plana:

This is a flat area extending from the pars plana to the ora serrata.  It does not have a particular function, however, it is an area of the eye through which needles and other instruments can be placed into the eye without causing complications or damaging other parts to the eye.  At times it can be involved in inflammatory problems in the eye.

Ora serrata:

This is the junction between the pars plana and the periphery edge of the retina.  It is significant because straddling this area is the vitreous base, where the vitreous is tightly adherent to the retina.  Because of this tight attachment, significant trauma on the eye can tear the retina away from the wall of the eye causing a dialysis and retinal detachment.

Vitreous cavity:

This is the large space behind the lens, like the air space in a basketball. But instead of air, it is filled with a jelly-like matter call vitreous.  Vitreous is homogenous and clear, normally in early life.  With aging, the vitreous degenerates leading most commonly to symptoms of light flashes and floaters.

However, with the degeneration of the vitreous more serious conditions can occur such as retinal tears, retinal detachments, vitreo-macular traction syndrome, macular pucker, etc.  The vitreous may also be involved in inflammatory conditions, diabetes and cancers.

Retina:

This is the sensory part of the eye, picking up light stimuli and transporting that signal to the brain to be interpreted into sight. It covers the entire inner surface of the eye from the ora serrata to the optic nerve.  It is a very thin layer of tissue which contains the photo receptors and nerve fibers, which carry all the light impulses to the brain.

Any pathology of the retina such as retinitis pigmentosa, myopic degeneration, macular degeneration, trauma, etc. will result in decreased vision.  Any damage to the integrity to the retina such as a hole or tear could lead to a retinal detachment.

Macula:

This is part of the retina, but it is centrally located in the back of the eye.  It is the area of the retina responsible for your best vision.  It gives you your best reading and color vision.  It is a small area measuring about 3 mm in diameter with its center accounting for the most precise vision.

Any image focused outside this area results in decreasing clarity and the further out, the poorer the vision.  The periphery of the retina does not have any photo receptors which can distinguish color, but is adapt for night vision.  Therefore, loss of the peripheral photo receptors results in loss of night vision.

Retinal arteries and veins:

There is only one main artery, the central retinal artery that enters the eye through the optic nerve.  As soon as it enters the eye it divides into an upper (superior) branch and a lower (inferior) branch.  From there, progressively smaller vessel branch off until they become capillaries.

The capillaries eventually combine to become progressively larger veins, and finally the upper (superior) vein joins the lower (inferior) vein, to exit the eye as the central retinal vein. Blood entering the eye through the central retinal artery supply oxygen and nutrients to the inner ⅔ of the retina, which does not include the photoreceptors.  Any obstruction of these vessels would be called a occlusion, which be the result of an embolus or a clot formation.

Choroid:

This is a vascular layer lying just below the retina; it supplies the blood circulation to the outer third of the retina which includes the photoreceptors.  The sources of the vessels which make up the choroid come from branches of the optic artery in back of the eye which penetrates through the sclera.  This layer is like a thin, dry sponge which expands when wet.

Any trauma to the choroidal vessels can cause either seepage of serum into the extravascular space or frank blood.  This would cause expansion inwards into the eye rather than outwards because the sclera prevents any outward expansion.  If the eye is closed, meaning that there is normal pressure within the eye, the expansion would be relative minimal.

However, if there is  and opening like a filtering bleb or valve in glaucoma  patients, or the eye is open during cataract or vitreous surgery, the expansion of the choroid could be massive and result in extensive damage and loss of vision.  Primary malignant melanoma and metastatic can also arise from the choroid.

Optic nerve:

This is the accumulation of all the nerve fibers from the retina which exit the eye.  The exact area at which they exit is the optic disc, which is also where the central retinal artery enters and the central retinal vein exits the eye.  In observing the optic disc and the nerves as they exit the eye, it common to see a small dimple in the center, which is normal.  However, if it is large or asymmetric as compared to the fellow eye, it may suggest the presence of glaucoma.  If the disc appears pale, it indicates optic atrophy; and if it is red and/or swollen, it may indicate local inflammation or pressure in the optic nerve or further back in the brain.

Sclera:

This is the thick, tough outer layer of the eye, which, along with the cornea, maintains the shape of the eye as long as there is pressure within it.  It is whitish in color as observed from the front.  In reality one is seeing it through a transparent layer of tissue called the conjunctiva.

It is within the conjunctiva where all the blood vessels are seen.  Occasionally, the sclera may not keep the eye as round as it normally does due to either weakness or thinning.  This may then result in elongation of the eye, which makes the cornea and lens incapable of focusing an image clearly on the back of the eye.  This results in a condition of myopia (near sightedness).

Thinning on the sides of the eye can be seen as dark areas because the darker choroid layer is   partially exposed.  In contrast to an elongated eye, if the eye is short, sharp focusing may also be impossible, resulting in the condition of hyperopia (far sightedness).  In the very back of the eye the sclera is connected to the optic sheath which is wrapped around the optic nerve as it extends back into the brain.

Dr. Mel Chen
Vitreo-retinal surgeon
Sarasota Retina Institute
Sarasota, Florida