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

 

 

The Normal Eye

Over the last year we have talked a lot about problems with the eye. What we would like to do now is take a step back and talk about the normal eye. We hope that this will help add understanding to our previous articles. The picture was created by our own retina specialist, Dr. Chen.

Cornea:eye

This is the clear front portion of the eye.  It excludes the white portion of the eye which is the sclera (to be discussed later). Simplistically, it has 3 basic layers (more in reality).  The first is the epithelium, the second is the stroma, and the third is the endothelium. The epithelium covers the front surface of the cornea and is kept smooth by the tears produced elsewhere.  Without the wet film over the epithelium, the surface is roughened and the vision decreased, and if it is severely roughened and/or abraded, nerve endings are exposed which results in pain.  Common disorders of the cornea are dry eye syndrome, foreign bodies, ulcers, trichiasis, edema, etc. The stroma makes up the major portion of the cornea, creating its curvature and participating in the focusing of the image to the back of the eye.  An irregular loss of or scarring of the stroma will result in distortion or hazing of the image projected to the back of the eye and thus blurred vision. The endothelium is a single cell layer on the back side of the stroma.  It is responsible for keeping the stroma and epithelium dry.  Loss of the endothelium cells permits any fluid entering the epithelium or stroma to result in swelling and then, loss of vision.  Loss of the endothelium cells are most commonly caused by trauma, multiple eye surgeries or hereditary.

Anterior chamber:

This is the space behind the cornea and in front of the iris, it is filled with a liquid called aqueous, which is produced by the ciliary body (to be discussed later).  The fluid remains clear, but in cases of inflammation,  cells and flare are observed.

Iris:

This is the colored part of the eye, which is seen through the cornea.  It acts like the shutter of a camera, adjusting the amount of light which enters the eye.  Closure of the pupil, or constriction of the pupil, occurs when in bright light and when focusing up close. Common disorders include trauma which damages the muscles around the pupil (the round opening in the center of the Iris) causing it to become irregular.  With inflammation the edge of the pupil may adhere to the surface of the lens preventing it from either opening up or closing down, depending where the adhesion occurs.  Severe trauma may also cause the iris to tear away from its peripheral location near the cornea.

Ciliary body:

This is the structure just behind and connected to the iris.  It has two functions.  The first is to produce aqueous and, the second is to adjust the lens, thus focusing the image on to the macula (to be discussed later). Aqueous is important to the eye because it keeps the pressure in the eye.  Without the aqueous the eye would collapse, just as a basketball would without air.  The continuous production of aqueous is necessary because there is a drainage system in the eye which allows for its outflow.  If there were not an outflow the pressure in the eye would elevate leading to glaucoma. Damage to the ciliary body would result in low pressure in the eye and effect vision.

Zonules:

These are fine fibers that originate from the ciliary body and connect to the periphery of the lens.  As the muscles in the ciliary body contract, the zonules pull on the lens resulting in its flattening thus decreasing its refractive power, and if the muscles relax, there is not tension of the lens through the zonules, and the lens thickens, thus increasing the refractive power of the lens.

Lens:

This is the primary means by which an image is focused on the back of the eye, and more specifically on the macula.  As mention above its ability change its focusing power is due to the ciliary body through the zonules.  The lens is also the object of discussion when dealing with cataracts. Nuclear cataracts are the most common type.  They are due to aging the center of the lens initially becomes less pliable, resulting in a shift toward myopia (near sightedness) and presbyopia (inability to focus up near).  With further aging there is greater hardening of the central portion of the lens, which also becomes discolored, progressing from light yellow to dark yellow, then turning to brown, and in unusually advanced cataracts, black. Cortical cataracts involve the part of the lens peripheral to the nucleus of the lens.  It can develop clefts, vacuoles and/or opacities, causing disturbances in the uniformity of the lens and thus decreased vision.  Rarely the cortex can swell to such an extent that it pushes so tightly on the back of the iris that it prevents the natural flow of aqueous out of the eye resulting in a form of glaucoma. Posterior subcapsular cataract involves an opacification on the back surface of the lens.  It can vary in density, but even if small it can significantly affect vision when the pupil constricts which occurs in bright lighting and reading.   Mel Chen, MD Vitreo-Retinal Surgeon Sarasota Retinal Institute  Sarasota, Florida