All of us would be familiar with the Snellen eye chart. It is the chart used to test the clarity of your vision when you visit an eye specialist. The chart gets its name from Herman Snellen, a Dutch ophthalmologist, who was instrumental in preparing the chart way back in 1862.
The chart has eleven lines in all and the first line has just one alphabet which is very large in size. In the lines below the number of alphabets increases and at the same time the size of the alphabets decreases. The test is taken in a way where one eye is covered and the person is told to read out the alphabets. The visual acuity is determined depending upon the smallest alphabet that the person is able to read. The other eye is tested in the same fashion. The alphabets shown on the Snellen Eye Chart are known as optotypes. The letters are not printed in an ordinary manner but the length, width and height do have some geometrical implications.
The customary Snellen chart has only the following 10 alphabets C, D, E, F, L, N, O, P, T, Z. The chart that is wall mounted is inexpensive and is generally used for a preliminary examination of the eye. The Visual acuity is determined by a formula which is:
Distance from where the test is conducted/ distance where the smallest optotype recognized subtends at an angle of 5 arcminutes. The chart is placed at a standard distance of 20 feet in most countries for the test. The chart also has certain specifications, it should be made on non-reflective material, it should have a matte finish, should be made of durable plastic and also the standard size is 22”*11”.
The Snellen eye chart also has its variations i.e. for illiterate persons and for young children who do not know how to read. For the illiterate, the chart has the alphabet E written in various directions and the ones for young children have symbols on it. The Snellen eye chart has also been a subject for a lot of criticism. Many are of the opinion that patients might memorize the chart and this would end up in a wrong result. Also after testing one eye they may be able to memorize it because of which the other eye may seem perfect. But even with all the criticism the chart has been in use all these years. Even with so much progress made in the field of medicine, the Snellen Eye Chart is the primary test one has to go through which speaks volumes about it.
Vision loss is one of the greatest losses that one can suffer. Retinal damage is one of the leading causes of vision loss. The retina is a tissue that is multi layered and which is present at the rear of the eye. It is about 0.5mm in thickness. The Retina can be affected with various kinds of problems and the treatment varies depending upon the nature of the problem. Retinal surgery comes as a boon to many who are bordering on losing their precious vision. But this does not necessarily mean that every patient’s original vision would be restored. Some would have to bear with the loss of partial vision for life.
Researchers are also contemplating and studying retinal transplant as an option to restore lost vision. Studies have shown encouraging results and this could soon be a reality. Electronic retinas have also been considered and researchers are trying out the benefits of this type of retina to help patients get better vision. Retina is a nerve tissue and damage to the retina can lead to many eye problems. Macular degeneration and Retinitis Pigmentosa are common age related problems. This can lead to blindness if not treated in time. Degenerative diseases are not the only ones that affect the retina and thereby vision. The retina can also be affected due to malnutrition. A lack of Vitamin A in ones diet can at a later stage be the cause for nightblindness.
The retina can be said to be a very crucial part of the eye and any sort of problem with the retina can lead to vision loss if not treated on time. Surgery can help patients with retinal problems and this can help in slowing down the Vision loss process. The retina is quite vulnerable to damage by the free radicals. Diseases that affect the retina are supposed to be the leading cause of loss of vision. The rods and cones which help in sensing the light get affected and this in turn makes the patient unable to see.
Millions of patients the world over are affected by retinal problems. Over the years the damage that is inflicted upon the retina is unfathomable. This leads to many a problem at a later stage and finally ends up in Vision loss. The eyes have to taken care of well for it has to last a lifetime and an eye specialist needs to be consulted regularly so as to arrest any problem at the earliest.
The retina can be said to be a very vital part of the eye and also the most complex. Retinal problems increase the complexities in case of refractive surgery. A refractive error along with problems of the retina is possible in which case vision loss increases. In such cases refractive surgeries help in rectifying the problem even if it is only for a short duration. Lasik surgery results in increasing the intraocular pressure for a while which is not suited for most patients with retinal problems. Given below are a few instances of retinal problems and correction options.
Macular Degeneration: The center portion of the Retina is known as macula and its degeneration is known as macular degeneration. Patients suffering from this disorder lose central vision but retain peripheral vision. Lasik increases the pressure in the eye and is not a favorable option.
Myopia leading to Retinal Detachment: The chances of retinal detachment are high in people suffering from myopia. Pressure in the eye pulls the retina which may lead to its detachment. Higher the intensity of the near sightedness, greater is the chance of developing retinal detachment. Refractive surgery is the recommended mode of treatment for severe cases of myopia. Lasik is not a recommended option for such patients as retinal detachment may occur due to the pressure in the eye during Lasik surgery. Chances of retinal detachment is higher is those patients where the risk already exists. Other refractive surgeries such as PRK would be suitable in such instances.
Diabetic Retinopathy: is a condition which affects diabetic patients where the vessels supplying blood to the retina get damaged. Lasik surgeries which involve the use of suction rings may cause more damage to these blood vessels and is best avoided. Refractive surgeries that do not increase the intraocular pressure are a more suited option but for some patients surgery itself is completely ruled out.
Ocular Histoplasmosis: is a condition that develops due to a kind of fungus. Scarring and also inflammation of the retina are the complications. But in many cases this may not affect the Vision to a large extent. Lasik surgery in such cases is not recommended as it can lead to choroidal neovascularization.
Retinal problems can be of various different types and the treatment options also vary. Wherever refractive surgery is an option Lasik is least recommended. The increase in the eye pressure due to Lasik is further damaging to the retina. Therefore, ophthalmologists rarely recommend Lasik surgeries in retinal complications.
The retina is that portion of the innermost part of the eye which is light sensitive. The retina is like a camera as it helps in creating the image. The photoreceptor cells, cones and the rods are the ones responsible for receiving the light and this is sent to the brain through the optic nerves by transforming it into image forming signals. Actually the optic nerve can be said to be a central tract which is the connecting factor of the retina and the brain. Another important feature of the retina is that it is the only part which belongs to the central nervous system (CNS) which can be directly imaged.
The retina is made up of many layers, 10 to be precise. The different layers from the innermost to the outer one are listed below:
- Inner Limiting Membrane
- Nerve Fiber
- Ganglion Cell: the one responsible for producing the optic nerve fibers.
- Inner Plexiform Layer
- Inner Nuclear Layer
- Outer Plexiform Layer
- Outer Nuclear Layer
- External Limiting Membrane
- Photoreceptor Layer
- Retinal Pigment Epithelium
The retina is quite a complex portion of the eye. An adult’s retina is said to have a diameter of about 22 mm. the retina constitutes 72% of the entire area of a sphere. The retina is made up of 75-150 million rods and 7 million cones, figures which are even difficult to fathom. An image is created with the functioning of both the cones and the rods. The cones are the ones that react when bright light falls and decides upon high resolution and color vision whereas the rods react to dim light and arbitrate regarding lower resolution vision, night vision and also black and white vision. Cones that are sensitive to light that are green, red and blue in color are absent in certain individuals which causes color blindness problems and various other color vision deficiencies. Since humans have 3 cones as compared to other mammals which have two (they lack the red sensitive cone), the color vision of humans becomes superior. The retina functions in a truly amazing pattern by compressing the image size to fit the scope of the optic nerve.
The retina is also prone to various diseases such as macular degeneration, retinal separation, retinoblastoma and many others. There are various instruments and treatment methods available today so as to diagnose and treat these problems at the earliest. The retina is a crucial part of the eye and for good vision one would need a healthy retina lifelong.