Retina, Vitreous & Uvea
Retina (rɛtini from Latin rēte meaning “net”) is a light-sensitive layer at the back of the eye that covers about 65 percent of its interior surface. It is the third and inner coat of the eye which is a light-sensitive layer of tissue. The optics of the eye create an image of the visual world on the retina (through the cornea and lens), which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These are sent to various visual centres of the brain through the fibers of the optic nerve. The retina is a layered structure with several layers of neurons interconnected by synapses. The only neurons that are directly sensitive to light are the photoreceptor cells. These are mainly of two types: the rods and cones. Rods function mainly in dim light and provide black-and-white vision, while cones support daytime vision and the perception of colour. A third, much rarer type of photoreceptor, the intrinsically photosensitive ganglion cell, is important for reflexive responses to bright daylight. Neural signals from the rods and cones undergo processing by other neurons of the retina. The output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. Several important features of visual perception can be traced to the retinal encoding and processing of light.
The retina has ten distinct layers-
a) Inner Limiting Membrane – basement membrane elaborated by Müller cells (retinal cells)
b) Nerve Fibre Layer – axons of the ganglion cell (neuron located in inner retina) nuclei (note that a thin layer of Müller cell footplates exists between this layer and the inner limiting membrane)
c) Ganglion Cell Layer – contains nuclei of ganglion cells, the axons of which become the optic nerve fibres for messages and some displaced amacrine cells (interneurons in the retina).
d) Inner Plexiform Layer – contains the synapse between the bipolar cell axons and the dendrites of the ganglion and amacrine cells.
e) Inner Nuclear Layer – contains the nuclei and surrounding cell bodies (perikarya) of the amacrine cells, bipolar cells and horizontal cells.
f) Outer Plexiform Layer – projections of rods and cones ending in the rod spherule and cone pedicle, respectively. These make synapses with dendrites of bipolar cells. In the macular region, this is known as the Fiber layer of Henle.
g) Outer Nuclear Layer – cell bodies of rods and cones
h) External Limiting Membrane – layer that separates the inner segment portions of the photoreceptors from their cell nucleus
i) Layer of Rods and Cones – layer of rod cells and cone cells
j) Retinal Pigment Epithelium – single layer of cuboidal cells (with extrusions not shown in diagram). This is closest to the choroid. These can be simplified into 4 main processing stages: photoreception, transmission to bipolar cells (exiss between photoreceptors, act to transmit signals from photoreceptors to ganglion cells), transmission to ganglion cells (neuron located near the inner surface) which also contain photoreceptors, the photosensitive ganglion cells (neuon located in retina), and transmission along the optic nerve. At each synaptic stage there are also laterally connecting horizontal (interconnecting neurons) and amacrine cells (interneurons in retina). The optic nerve is a central tract of many axons of ganglion cells connecting primarily to the lateral geniculate body (relay cetre in the thalamus), a visual relay station in the diencephalon (the rear of the forebrain). It also projects to the superior colliculus (paired structure of the midbrain), the suprachiasmatic nucleus (tiny region in the hypothalamus), and the nucleus of the optic tract. It passes through the other layers creating the optic disc in primates.
Function – An image is produced by the patterned excitation of the cones and rods in the retina. The excitation is processed by the neuronal system and various parts of the brain working in parallel to form a representation of the external environment in the brain. The cones respond to bright light and mediate high-resolution colour vision during daylight illumination (also called photopic vision). The rods are saturated at daylight levels and don’t contribute to pattern vision. However, rods do respond to dim light and mediate lower-resolution, monochromatic vision under very low levels of illumination (called scotopic vision). The response of cones to various wavelengths of light is called their spectral sensitivity. In normal human vision, the spectral sensitivity of a cone falls into one of three subgroups. These are often called blue, green, and red cones but more accurately are short, medium, and long wavelength sensitive cone subgroups. It is a lack of one or more of the cone subtypes that causes individuals to have deficiencies in colour vision or various kinds of colour blindness. These individuals are not blind to objects of a particular colour but experience the inability to distinguish between two groups of colours that can be distinguished by people with normal vision.
Retinal Diseases & Disorders- There are many inherited and acquired diseases or disorders that may affect the retina. Some of them include:-
a) Retinitis Pigmentosa- It is a group of genetic diseases that affect the retina and cause the loss of night vision and peripheral vision.
b) Macular Degeneration describes a group of diseases characterized by loss of central vision because of death or impairment of the cells in the macula.
c) Cone-Rod Dystrophy (CORD) describes a number of diseases where vision loss is caused by deterioration of the cones and/or rods in the retina.
d) Retinal Separation/Retinal Detachment- The retina detaches from the back of the eyeball. The term retinal detachment is used to describe a separation of the neurosensory retina from the retinal pigment epithelium (pigmented cell layer just outside the neurosensory retina).
e) Both Hypertension and Diabetes Mellitus can cause damage to the tiny blood vessels that supply the retina, leading to Hypertensive Retinopathy and Diabetic Retinopathy.
f) Hypertensive Retinopathy- It is damage to the retina & retinal circulation due to high blood pressure (Hypertension). High blood pressure can cause damage to the retina’s blood vessels, limit the retina’s function & put pressure on optic nerve causing vision problems.
g) Diabetic Retinopathy- is a complication of diabetes that causes damage to the blood vessels of the retina. There are generally 2 types of causes of vision loss from diabetic retinopathy-
Diabetic Macular Edema- is the term used for swelling in the central part of the retina. The macula or the central part of the retina is used for sharp, straight-ahead vision. It is nourished by blood vessels that are weakened or begin to leak as a result of diabetes. This causes the central part of the retina to become thickened or swollen & can lead to decreased vision.
Proliferative Diabetic Retinopathy – also called PDR, in this poor circulation of blood in retina may lead to the development of growth factors that can cause new blood vessels & scar tissue to grow on the surface of the retina.
h) Retinoblastoma- is a cancer of the retina. It is a rare form of cancer that rapidly develops from the immature cells of a retina, the light-detecting tissue of the eye. It is the most common malignant cancer of the eye in children, and it is almost exclusively found in young children.
i) Lipemia Retinalis- It is a white appearance of the retina, and can occur by lipid deposition in lipoprotein lipase deficiency.
j) Retinal Prosthesis- In this a microelectronic chip is used to convert light rays to neuronal impulses that can be conveyed to the brain via the preserved connections.
k) AMD (Age-related Macular Degeneration)- In normal aging yellowish deposits called drusen form under the retina. As drusen increases it can affect normal functioning of the retina damaging its cells. This type of AMD is called Dry AMD. Wet AMD occurs when abnormal blood vessels behind the retina start to grow under the macula. Afterwards these blood vessels leak blood & fluid damaging the cells of the retina.
l) Central Retinal Vein Occlusion- also known as CRVO is a condition in which the main vein that drains blood from the retina closes of partially or completely. This can cause blurred vision & other vision problems.
m) Central Serous Chorioretinopathy- commonly referred as CSC is a condition in which fluid accumulates under the retina causing a serous (fluid-filled) detachment & vision loss.
n) Choroidal Detachment- is a spongy layer of blood vessels that lines the back wall of eye between the retina & the sclera. It plays an important role in delivering oxygen & nutrients to the outer half of the retina. The choroid is normally directly next to sclera, but can be displaced by fluid or blood leading to a choroidal detachment (separation).
o) Congenital X-Linked Retinoschisis- also known as CXLRS is an early onset hereditary retinal disease characterized by splitting of retinal layers, particularly in the center of vision (fovea) and/or in the peripheral retina.
p) Epiretinal Membranes- also commonly known as Cellophane Maculopathy or Macular Puckers, are avascular (having few or no blood vessels), semitranslucent fibro cellular membrane that form on the inner surface of the retina. They most commonly cause minimal symptoms & can be simply observed but in some cases they can cause painless loss of vision & metamorphosia (visual distortion).
q) Familial Exudative Vitreoretinopathy- called FEVR, is a hereditary condition where the retinal blood vessels do not develop normally. As a result, scar tissue may develop which as it contracts can detach the retina & result in vision loss.
r) Infectious Retinitis- is an inflammation of the retina resulting from infection by bacteria, viruses, fugi or parasites. These pathogens affect patients differentially depending on characteristics like age, location & immune status.
s) Macular Hole- Macula is situated in the centre of the retina. When a full thickness defect develops in the macula the condition is referred as Macular Hole.
t) Floaters- are those tiny spots, specks, flecks & cobwebs that drift aimlessly around in your field of vision. Floaters occur when the vitreous (gel like substance in eye) slowly shrinks & it becomes stingy & the strands can cast tiny shadows.
u) Persistent Fetal Vasculature- or PFV also known as Persistent Hyperplastic Primary Vitreous (PHPV) is a congenital developmental disorder that occurs when the vascular structures present during the development of the eye fail to wither or regress as they should. The resulting structural abnormalities may lead to impairment of vision.
v) Posterior Vitreous Detachment- as PVD is a natural change that occurs during adulthood when the vitreous gel that fills the eye, separates from the retina.
w) Presumed Ocular Histoplasmosis Syndrome- or POHS causes atropy (wasting) around the optic nerve & multiple scars called histo spots in the choroid. These symptoms are accompanied by new blood vessel growth that starts adjacent to a histo spot.
x) Retinopathy of Prematurity- or ROP is a condition affecting premature infants of low birthweight & young gestational age. It occurs when the development of normal retinal blood vessels which typically require a full term pregnancy for completion is interrupted by premature birth. If the disease progresses, scar tissue can grow. This can lead retinal detachment & vision loss.
y) River Blindness- or Onchocerciasis is a disease caused by infection with the parasitic worm Onchocerca volvulus. Symptoms include severe itching, bumps under the skin, and blindness. It is the second most common cause of blindness due to infection, after trachoma. Infection can cause intense itching, skin discoloration, rashes, and eye disease that often leads to permanent blindness. It is spread by the bites of infected black flies that breed in rapidly flowing rivers.
z) Vitreomacular Traction Syndrome- The vitreous humor is a transparent, gel-like material that fills the space within the eye between the lens and the retina. The vitreous is encapsulated in a thin shell called the Vitreous Cortex and the cortex in young, healthy eyes is usually sealed to the retina. As the eye ages, or in certain pathologic conditions, the vitreous cortex can pull away from the retina, leading to a condition known as Posterior Vitreous Detachment (PVD). This detachment usually occurs as part of the normal aging process. There are instances where a PVD is incomplete, leaving the vitreous partially attached to the retina, and causing tractional (pulling) forces that can cause anatomical damage. The resulting condition is called Vitreomacular Traction (VMT) syndrome.
Torn Retina Surgery- Most retinal tears need to be treated by sealing the retina to the back wall of the eye with laser surgery or cryotherapy (a freezing treatment). Both of these procedures create a scar that helps seal the retina to the back of the eye. This prevents fluid from traveling through the tear and under the retina, which usually prevents the retina from detaching. These treatments cause little or no discomfort.
Laser Surgery (Photocogulation) – With laser surgery, your ophthalmologist uses a laser to make small burns around the retinal tear. The scarring that results seals the retina to the underlying tissue, helping to prevent a retinal detachment.
Freezing Treatment (Cryopexy)- Your eye surgeon uses a special freezing probe to apply intense cold and freeze the retina around the retinal tear. The result is a scar that helps secure the retina to the eye wall.
Detached Retina Surgery – Almost all patients with retinal detachments must have surgery to place the retina back in its proper position. Otherwise, the retina will lose the ability to function, possibly permanently, and blindness can result. The method for fixing retinal detachment depends on the characteristics of the detachment. In each of the following methods, doctor will locate the retinal tears and use laser surgery or cryotherapy to seal the tear.
Scleral Buckle – This treatment involves placing a flexible band (scleral buckle) around the eye to counteract the force pulling the retina out of place. The doctor often drains the fluid under the detached retina, allowing the retina to settle back into its normal position against the back wall of the eye. This procedure is performed in an operating room.
Pneumatic Retinopexy – In this procedure, a gas bubble is injected into the vitreous space inside the eye in combination with laser surgery or cryotherapy. The gas bubble pushes the retinal tear into place against the back wall of the eye. The doctor will ask you to constantly maintain a certain head position for several days. The gas bubble will gradually disappear.
Vitrectomy – This surgery is commonly used to fix a retinal detachment and is performed in an operating room. The vitreous gel, which is pulling on the retina, is removed from the eye and usually replaced with a gas bubble. Sometimes an oil bubble is used (instead of a gas bubble) to keep the retina in place. Your body’s own fluids will gradually replace a gas bubble. An oil bubble will need to be removed from the eye at a later date with another surgical procedure. Sometimes vitrectomy is combined with a scleral buckle.