One of the most critical components of your eye is the cornea, the “window” of your eye.  The job of the cornea is to bend, or refract, light rays so that they focus on the retina at the back of your eye.  Refractive errors occur when the cornea fails to focus light rays precisely on the retina.

Normal Eye The structure of the eye owes itself completely to the task of focusing light onto the retina. All of the individual components through which light travels within the eye before reaching the retina are transparent, minimizing dimming of the light. The cornea and lens help to focus light rays onto the retina. This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain.

Myopia Myopia, also known as nearsightedness or short-sightedness, is a refractive defect of the eye, where the person affected usually can see nearby objects clearly but distant objects appear blurred. The opposite of myopia is farsightedness or hyperopia. Myopia is the most common eyesight problem in the world. About one quarter of the adult population in the United States has myopia. In places like Japan, Singapore and Taiwan, as many as one in three or one in two of the adult population is myopic. Myopia is measured in diopters; specifically, the strength of the corrective lens that must be used to enable the eye to focus distant images correctly on the retina. Myopia of 6.00 diopters or greater is considered high, or severe, myopia. People with high myopia are at greater risk of more acute eye problems such as retinal detachment or glaucoma. They are also more likely to experience floaters. Mainstream ophthalmologists and optometrists most commonly correct myopia through the use of corrective lenses, such as spectacles or contact lenses. It may also be corrected by refractive surgery, such as LASIK. The corrective lenses have a negative dioptric value (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

Hyperopia Hyperopia (or more rarely, hypermetropia), also known as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance. As an object moves towards the eye, the eye must increase its power to keep the image on the retina. If the power of the cornea and lens is insufficient, as in hypermetropia, the image will appear blurred. It is corrected by using eyeglasses with lenses of positive curvature (magnifying glasses).

Astigmatism Astigmatism is a refraction error of the eye characterized by an aspherical cornea in which one axis of corneal steepness is greater than the perpendicular axis. Astigmatism causes difficulties in seeing fine detail, and can be often corrected by glasses with a cylindrical lens (i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery. Astigmatism occurs when either the cornea or the lens of the eye is not perfectly spherical. As a result, the eye has different focal points in different planes. For example, the image may be clearly focused on the retina in the horizontal plane, but in front of the retina in the vertical plane. In some cases vertical lines i.e. walls, may appear to the patient to be leaning over like the tower of Pisa.

Presbyopia Presbyopia is the inability of the human eye to focus on objects up close that results with aging. Presbyopia is most likely the result of a decrease in the flexibility of the lens of the eye, changes in its curvature from continual growth, and loss of power of the ciliary muscles (the muscles that bend and straighten the lens). Presbyopia is not a disease as such, but a condition that affects everyone at a certain age. The first symptoms are usually noticed at the age of 40-50. It can start with difficulty reading fine print, particularly if the lighting is poor, or with eyestrain when reading for long periods. More advanced presbyopes may complain that their arms have become "too short" to hold reading material at a comfortable distance. Presbyopia cannot be cured, but the loss of focusing ability can be compensated for by corrective lenses of the converging type. In subjects with other refractory problems, multifocal lenses (such as bifocal or trifocal lenses) or progressive lenses are used. Presbyopia can be corrected with glasses or contact lenses. In some cases, the addition of bifocals to an existing lens prescription is sufficient. As the ability to focus up close worsens, the prescription needs to be changed accordingly. Around the age of 65, the eyes have usually lost most of the elasticity needed to focus up close. However, it will still be possible to read with the help of the appropriate prescription. Even so, you may find it necessary to hold reading materials further away, and you may require larger print and more light to read by. People who do not need glasses for distance vision may only need half glasses or reading glasses. With the use of contact lenses, some people choose to correct one eye for near and one eye for far. This is called "monovision" and eliminates the need for bifocals or reading glasses, but can interfere with depth perception. There are also newer contact lenses that can correct for both near and far vision with the same lens.

Glaucoma Glaucoma is a group of diseases of the optic nerve involving loss of retinal ganglion cells in a characteristic pattern of optic neuropathy. Raised intraocular pressure is a significant risk factor for developing glaucoma. There is no set threshold for intraocular pressure that causes glaucoma — while one person may develop nerve damage at a relatively low pressure, another person may have high eye pressures for years and yet never develop damage. Untreated glaucoma leads to permanent damage of the optic disc of the retina and resultant visual field loss, which can progress to blindness. The most common type, primary open angle glaucoma (POAG), frequently has no symptoms and has been nicknamed "the thief of sight". One factor may be a relative obstruction on the outflow of aqueous humour from the eye. Aqueous humour is produced by the ciliary body of the eye, and then flows through the pupil and into the anterior chamber. The trabecular meshwork then drains the humour to Schlemm's canal, and ultimately to the venous system. All eyes have some intraocular pressure, which is caused by some resistance to the flow of aqueous through the trabeculum and Schlemm's canal. If the intraocular pressure (IOP) is too high, (>21.5 mm Hg), the pressure exerted on the walls of the eye result in compression of the ocular structures. However, other factors such as disturbances of blood flow in the optic nerve head may interact with IOP to affect the optic nerve. In one third of cases of POAG there is statistically normal IOP. This is called normal tension glaucoma (NTG). Because optic nerve examination and perimetry testing are not always done in addition to IOP measurement in those at risk, NTG is underdiagnosed and the condition presents late. Another type, acute angle-closure glaucoma, is characterized by an acute rise in the intraocular pressure. This occurs in susceptible eyes when the pupil dilates and blocks the flow of fluid through it, leading to the peripheral iris blocking the trabecular meshwork. Acute angle-closure glaucoma can cause pain and reduced visual acuity (blurred vision), and may lead to irreversible visual loss within a short time. This is an ocular emergency requiring immediate treatment. Many people with glaucoma experience halos around bright lights as well as the loss of sight characterized by the disease. Primary congenital glaucoma or buphthalmos is a rare genetic disease affecting infants. Newborns present with enlarged globes and clouded corneas. It is thought that reduced trabecular permeability is the cause of increased intraocular pressure. Surgery is the treatment. Secondary glaucoma occurs as a complication of various medical conditions such as eye surgery, advanced cataracts, eye injuries, some eye tumors, uveitis, diabetes or use of corticosteroid drugs.

Cataract A cataract is any opacity which develops in the crystalline lens of the eye or in its envelope. Cataracts form for a variety of reasons, including infrared and microwave exposure, secondary effects of diseases such as diabetes, or simply due to advanced age; they are usually a result of denaturation of lens proteins. Genetic factors are often a cause of congenital cataracts and may also play a role in predisposing someone to cataracts. Some cataract formation is to be expected in any person over the age of 70. Fully half of all people between the ages of 65 and 74 and about 70% of those over 75 have some cataract formation. Cataracts may also be produced by eye injury or physical trauma. Cataracts may be partial or complete, stationary or progressive, hard or soft. An early technique to remove cataracts was couching, which involved using a thin needle or stick to remove the clouding. This technique is known to have existed in Roman times and continued to be used throughout the Middle Ages and continues to be used in underprivilleged "Third world" countries today. In India, however, modern surgery with Intraocular lens insertion in Government and Non Government Organisation (NGO) sponsored Eye Surgical Camps have totally replaced this method. The most effective and common treatment is cataract surgery to remove the cloudy lens. There are two types of surgery that can be used to remove cataracts, extra-capsular and intra-capsular surgery. Extra-capsular surgery consists of removing the lens but leaving the majority of the lens capsule intact. High frequency sound waves (phacoemulsification) are sometimes used to break up the lens before extraction. Intra-capsular surgery involves removing the entire lens of the eye, including the lens capsule, but it is rarely performed in modern practice. The lens is then replaced with a plastic lens (an intraocular lens implant) which remains permanently in the eye.

Retinal Detachment Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. The retina is a thin disc-shaped layer of light-sensitive tissue on the back wall of the eye. Its job is to translate what we see into neural impulses and send them to the brain via the optic nerve. Occasionally, injury or trauma to the eye or head may cause a small tear in the retina, which allows fluid to seep through, and peel it away like a bubble in wallpaper. Detachment is most frequent in the middle-aged or elderly population, and in those with extreme myopia, as their retinas are unusually thin to begin with. Retinal detachment is also an uncommon complication of surgeries such as those used to treat cataracts. There are several ways of treating a detached retina. One way is to inject silicone oil or a gas bubble (pneumatic retinopexy) into the eye to push the retina smoothly back into place. Cryotherapy or laser treatments can then permanently reattach it. If the gas bubble is used, patients may have to keep their heads tilted for several days to more effectively press it against the retina. Another treatment (a scleral buckle procedure) uses very fine silicone bands that are sewed to the outside of the eyeball so they push the wall of the eye against the detached retina area. The bands do not usually have to be removed. After treatment, patients gradually regain their vision, although the visual acuity may not be as good as it was prior to the detachment, particularly if the macula was involved in the area of detachment. However, if left untreated, total blindness can occur in a matter of days.