What Does the Retina Do?
The retina is a thin layer of specialized, light-sensitive brain tissue located in the back of the eye. Its main function is to transform light rays into electrical signals so you can see. There are several eye conditions that can impact the retina and, hence, affect your vision. In this blog, we will explain the importance of the retina, common retinal disorders and diseases, and how various procedures and surgeries are utilized to treat these conditions.
Light hits the retina whenever it passes through the eye. From there, cells called photoreceptors in the retina convert light rays into electrical signals that travel through the optic nerve to the brain. The brain translates these signals and interprets them into the images you see.
Because the retina is so crucial to how you interpret images, any disease or damage to that part of the eye can impact how you see images and cause vision changes.
Anatomy of the Retina
The retina measures 32 mm across and the average thickness is about 0.2 mm. The retina contains five types of neurons that each play an important role in its function.
Photoreceptors: Photoreceptors contain photopigments that absorb photons of light to help your brain understand images. There are two types of photoreceptors in the retina: rods and cones. Rods are mainly responsible for peripheral vision and vision at night and in dim light. They help you sense contrast, brightness, and motion. About 95% of the retina’s photoreceptors are made up of rods, which amounts to 120 to 130 million rods. Cones are responsible for central vision in bright light. They specialize in detecting red, green, and blue light. In addition, cones help you sense fine resolution, spatial resolution, and color vision. By comparison, cones account for just 5% of the retina’s photoreceptors – or about 6 to 7 million cones.
Retinal ganglion cells: Ganglion cells serve as the retina’s main output neuron and extend to form the optic nerve, the nerve at the back of the eye that relays messages to your brain to create visual images.
Bipolar cells: Bipolar cells receive information from photoreceptor cells and send them to ganglion cells.
Horizontal cells: Horizontal cells regulate the output of photoreceptors and help adjust your eyes to light – both bright and dim.
Amacrine cells: Amacrine cells are intermediate neurons that make connections with bipolar cells and ganglion cells.
In summary, photoreceptors absorb light once it hits the retina. Rods and cones send signals to bipolar cells, which also communicate with horizontal cells. Bipolar cells send impulses to ganglion cells, which also communicate with amacrine cells. Finally, the messages leave the ganglion cells at the optic nerve and are communicated to the brain.
Further, the retina consists anatomically of the macula (center of the retina) and the peripheral retina (area outside the macula).
- Macula: The macula is home to light-sensitive cones that help you see clearly and focus on fine details. Most of your visual field is produced by the macula. The macula is thicker and more dense than the peripheral retina.
- Peripheral retina: The peripheral retina is home to rods that help with side vision and night vision. The peripheral retina is thinner.
Before light can reach the retina’s photoreceptors, it must first travel through several layers. Here is an overview of the various neurons found in the retina’s layers. The layers are listed in descending order from the vitreous (the clear, gel-like substance that fills up the eyeball) to the choroid (the blood vessels network that is part of the uvea between the sclera and the retina).
- Inner limiting membrane: Separates nervous tissue of the retina from connective tissue of the vitreous humor.
- Nerve fiber layer (NFL): Contains axons of ganglion cells that travel to the optic nerve
- Ganglion cell layer: Contains ganglion cells’nuclei.
- Inner plexiform layer: Contains synapses (a region where nerve impulse are transmitted) between bipolar and ganglion cells.
- Inner nuclear layer: Contains nulcei of bipolar, amarcine, horizontal and Muller cells.
- Outer plexiform layer: Contains synapses between bipolar cells and photoreceptor cells.
- Outer nuclear layer: Contains nuclei of photoreceptor cells.
- External limiting membrane: Separates inner segments from the outer nuclear layer and also keeps the photoreceptors aligned.
- Ellipsoid zone: Separates inner and outer segments of rods and cones.
- COST line: Delineates the outer segment of photoreceptors.
- Retina pigment epithelium: Contains light not captured by photoreceptors.
- Age-related Macular Degeneration: This retinal disease occurs when the macula (the central part of the retina that processes fine-detail vision) deteriorates. AMD can blur your central vision and is most often due to age – it is the leading cause of vision loss in older adults in developed countries.
- Branch retinal vein occlusion (BRVO): BRVO is a condition in which branches of the retinal vein are blocked and cause blood and fluid to leak into the retina. Swelling of the macula may occur and cause central vision loss.
- Central retinal vein occlusion (CRVO): CRVO is a condition in which the central retinal vein is blocked, usually due to a blood clot. The blood clot does not allow blood to flow out of the retina, resulting in blurry vision or sudden vision loss.
- Central serous chorioretinopathy: This condition occurs when fluid leaks from cells between the retina and choroid (tissue under the retina) called the retinal pigment epithelium. When fluid builds up, it can cause visual impairments such as blurry central vision, straight lines that appear bent, or seeing distorted colors (white images can look brownish).
- Cytomegalovirus retinitis: This viral eye infection is caused by cytomegalovirus (CMV), which comes from the family of herpes viruses. CMV is more common in people with weakened immune systems. Symptoms include floaters and a loss of peripheral vision.
- Detached Retina: This condition occurs when fluid passes through a retinal tear and causes detachment.
- Diabetic Retinopathy: Diabetic retinopathy is a progressive eye disease that occurs when blood vessels in the retina become damaged due to either type 1 or type 2 diabetes.
- Flashes: Flashes occur when the vitreous pulls or rubs on the retina and causes flashing lights or streaks in your field of vision.
- Floaters: Floaters occur when fragments of protein fiber in the vitreous cast a shadow on the retina, which becomes noticeable when looking at a plain object.
- Macular Holes: Macular holes involve a defect of the macula, the part of the retina that helps you see clearly. The holes occur due to progressive thinning of the retina.
- Retinoblastoma: Retinoblastoma is a rare eye cancer mostly seen in children under the age of 5. A white pupil – pupils are usually dark – is the most common symptom of retinoblastoma.
- Retinopathy of prematurity: Retinopathy of prematurity (ROP) is an eye disease in premature babies that causes blood vessels to grow abnormally onthe retina and into the vitreous. Some cases of ROP are mild and do not require treatment, but more severe cases can cause vision loss or even blindness.
- Retinal Infection: A retinal infection, also called endophthalmitis, occurs when the intraocular fluids (vitreous and aqueous) become inflamed – usually due to infection.
- Retinal Tears: This condition occurs when the vitreous separates from the retina and pulls to cause a tear.
- Retinitis Pigmentosa: This condition is a group of inherited eye problems that causes rods and cones to degrade over time.
Treatment Options for Retinal Disorders
While the main goal of treating retinal disorders is to improve altered vision or restore vision loss, sometimes the only option is to slow down or stop how the disease or disorder progresses.
- Intraocular injections: Intraocular injections can stop abnormal blood vessels from forming and also prevent them from leaking. Ophthalmologists use anti-VEGF medications that last four to six weeks. Intraocular injections can treat AMD, diabetic retinopathy, and retinal vein occlusion.
- Laser surgery: Laser surgery uses a beam of light to create small burns in the retina. As the opening scars over, it can help seal off retina tears or close off blood vessels. One type of laser surgery called scatter laser photocoagulation uses a laser to shrink abnormal blood vessels associated with diabetic retinopathy.
- Cryopexy: Retinal cryopexy uses extreme cold to freeze parts of the retina and intentionally damage tissue to promote scar formation. Cryopexy can treat peripheral retinal tears or tumors.
- Vitrectomy: A vitrectomy is an intra-ocular surgical procedure that removes most of the vitreous (the clear gel that fills the eye) and replaces it with saline, a gas bubble, or silicone oil. A vitrectomy may be used as part of the process to treat a number of retinal conditions such as a detached retina or diabetic retinopathy.
- Scleral buckle: This procedure is used to correct retinal tears or detachments. The procedure involves externally suturing a flexible silicon band around the sclera (the white part of the eyeball) to push the eye closer to the retina, and counteract any internal traction. Once placed, your eye doctor can use laser surgery or cryopexy to seal any tear(s).
- Pneumatic retinopexy: This procedure involves injecting an expanding gas bubble into the eye to push the retina back into place. Once the retina is back in place, your eye doctor can use laser surgery or cryopexy to correct any tears.