Retina - Screen of the Eye, Functions and Conditions

The Screen of the Eye: Retina

Dr.Surya Rao Poodipeddi profile Authored by Dr.Surya Rao Poodipeddi on 31 Jan 2014 - 17:31.

Human eyeball have three layers namely the outer layer called the sclera, which maintains the shape of the eye. Inside the sclera is a layer called the choroid, which supplies nutrients to the eye. the innermost layer is called the Retina.

The Retina structure: The retina is a light sensitive layer at the back of the eye and covers roughly 65 percent of the interior surface of the eye. The retina contains two types of light sensitive cells called Rods & Cones.

The rods respond to dim light and the cones detect color. The rods and cones convert light energy into signals that are carried to the brain by the optic nerve for interpretation of the images. In the middle of the retina lies a small dimple called the Fovea.

The eye’s sharpest vision and location of color perception takes place in the center of the retina.

Roughly 125 million rods are intermingled over the retina. Each rod is about 0.002 mm in diameter and can detect the presence of the dimmest light. The cones though less in number totaling around 6 to 7 millions and measuring about 0.006 mm in diameter can be compared to a low speed color film capable of distinguishing any color pattern in bright light.

The retina is richly supplied by the retinal vessels which leave the retina along with the nerve fibers, through an area called the optic disc. The area of the retina responsible for more detailed vision is called the Macula. The optic nerve is responsible for carrying signals from the retina to the brain through the optic disc. The fovea at the center of the macula packed with millions of light sensitive cells is the most sensitive part of the retina.

Retina conditions: There are far too many conditions, which can affect the retina. Some of the conditions are:

  • Color blindness: This is a sexlinked-inherited disorder, which is transmitted through x-linked chromosomal defect, mainly through a carrier. The inheritance is through a normal mother, an abnormal gene and an affected father, passing it on to all daughters who might not be color blind but may pass on to the next generation. A carrier mother and an unaffected father with a defective gene may pass on the problem to each child with 1 in 2 chance of inheritance. Sons who inherit this gene are invariably color blind where as daughters who inherit this gene, become carriers. Anybody with colorblindness has a reduced ability to distinguish different colors particularly the red and green.
  • Papilledema: This is a condition where a swelling develops over the optic nerve, which carries signals from the light sensitive retina to the brain. High blood pressure, head injuries and tumors in the brain are some of the common causes for edema over the optic disc from where the optic nerve emerges. Treatment depends on the cause. Corticosteroids hold importance in relieving the pressure on the optic nerve and the disc.
  • Retinal Detachment: Normally the retina is attached to the underlying tissue. However, in retinal detachment the retina gets partially separated from the underlying tissue. The problem starts with a small hole through which fluid passes leading to increased separation from the underlying tissue. If the detachment is small it can be sealed through laser surgery under local anesthesia. However in cases where there is considerable detachment, sealing is done under general anesthesia. In either case, early treatment increases the hope of restoring the vision.
  • Retinitis Pigmentosa: This condition is an autonomic recessive heriditary disorder where the light sensitive cells in the retina are progressively degenerated leading to deposits of pigments in affected portion of the retina. There is progressive loss of vision starting from the periphery and finally ending in loss of vision towards the center called a tunnel type vision loss. There is no treatment available for this disorder. However, we can widen the angle of vision through special glasses. It is essential to have genetic counseling to prevent subsequent children from inheriting this disorder.
  • Retinopathy: Whenever small blood vessels of the retina are affected by any underlying cause like diabetes, hypertension, AIDS, pre-term babies and sickle cell anemia we come across a problem called retinopathy where the light sensitive retinal cells are affected. Treatment is based on the underlying cause.
  • Diabetic retinopathy: This condition is usually a complication of uncontrolled diabetes where the retinal blood vessels get damaged. Diabetes can damage blood vessels anywhere in the body. If this happens to the retinal blood vessels, it results in diabetic retinopathy. The main symptom of this condition starts as blurred vision followed by gradual loss of vision including sudden blindness if a blood vessel ruptures.
  • Macular Degeneration: As the name implies, the degenerative process takes place in the light sensitive area of the retina called macula, leading to gradual loss of central and detailed vision. The main symptom is inability to read and recognize faces. There are two varieties of this malady namely dry and wet macular degeneration. The dry variety is not amenable to treatment whereas in the wet variety some treatment can be given through laser surgery.
  • Retinal vein occlusion: As the name implies a block in the retinal vein occurs due to small clots. This causes cessation of venous return from the retina leading to engorgement and bursting of the vein. If the block is in a small vein there may not be any symptoms. However, if a large vein is blocked, whatever vision is lost due to the blockage is permanent. However, by trying to treat the underlying cause we can prevent further loss.
  • Retinoblastoma: This is a rare case of cancer of the retina, affecting children. If detected early the cancer can be treated. However, the child’s vision is severely affected. Genetic counseling is an absolute must and an eye specialist must examine every sibling of the affected child regularly.

Except in disorders of the retina which are not under our control like hereditary conditions, tumors and some rare disorders like color blindness, other disorders can always be prevented if only proper care is taken to prevent complications in cases like hypertension and diabetes. Most importantly periodical checkup of the eyes at least once in a year will go a long way in early detection of most of the retinal problems. Periodical testing of the eyes with an ophthalmoscope gives a lot of information about the retina’s health.

 

*Disclaimer: This is not medical advice. The content is for educational purposes only. Please contact your doctor for any health care issues.