Basic Disorder Of Human Eye || Most Finding Problem Of Human Eye

       DISORDERS OF THE EYE

Errors of Refraction : These errors can be classified as (1) Myopia. (2) Hypermetropia and Presbyopia, (3) Astigmatism.

 1. Myopia : Myopic persons are shortsighted. They can see the near objects perfectly but find difficulty in seeing the distant objects. The eyeball is longer than normal and the parallel rays are not focussed on the retina but in the front of the retina. The lens may also be more convex. This is corrected by wearing bi-concave lenses, called negative (-) lens, which diverges the parallel rays. Those who use their eyes for near objects, e.g. reading books, watch repairing etc. use concave glasses. Such persons should train their eyes to look at distant objects without glasses.

2. Hypermetropia and Presbyopia : These persons are longsighted.They can see distant objects well but experience difficulty in seeing the near objects. The eyeball is shorter than normal. Divergent rays from near objects are focussed behind the retina. The lens may also remain less convex. This rarely occurs in childhood due to disproportionate growth of the eyeball structures and it may get corrected naturally with age.

 The most common cause of hypermetropia is presbyopia which takes place in old age between 40 and 50 years. It is due to gradual loss of elasticity of the lens and weakening of ciliary muscles. Old lady threading the needle from longer distance is a typical example of presbyopia. This is corrected by wearing convex lenses called positive (+) lenses, which converage the diverging rays. Those who use their eyes for seeing distant objects such as engine drivers use the convex glasses. Old people whose lenses have lost their power of accommodation due to their less elastic lenses suffer from presbyopia and use convex lenses to correct this error of refraction. 

In another form of presbyopia there is a decreased ability to accommodate for near vision. A person suffering from presbyopia holds his book far away in order to see clearly and he needs bifocal lens. The lower part of these glasses is used for the near vision and the top for the distant vision, people who have never worn glasses frequently want "reading glasses" instead of bifocals, as they believe that bifocals will be a difficult adjustment for them. However, reading, glasses will blur their distant vision and they will find that they are constantly taking their reading glasses off and putting them back, once again. Thus, the short time of adjustment to bifocals is well worth their effort.

3. Astigmatism : It is an error or refraction where rays are not sharply focussed on the retina. It results from oblong shape of the cornea and the lens. Thus the curve of one plane differs from that of the other plane. Here the rays of one plane (vertical) and another (horizontal) do not come to a common point of focus, on the retina. This error is corrected by cylindrical lens or by combining cylindrical and spherical lenses. This corrects the error in different planes to focus all rays on the retina to give a correct and sharp image. 

Cataract : Cataract results from an opacity of the lens. It is generally due to degenerative changes in old age. This condition can be treated surgically by removing the lens. Following surgery, the patient may have to wear very thick lenses; however, some people are able to wear contact lenses. Frequently, people have difficulty in judging distances at first and must be particularly cautious in walking up and down the stairs. Now a days, an artificial transparent plastic lens Intra-ocular lense (10L) can be fifted permanently in its place by opthalmic surgeon. Even for removal of cataract lens laser is used to break the lens and then is sucked out. This is called 'pheco' surgery.

Glaucoma : Glaucoma results from a disturbance of the normal balance between the production and drainage of the aqueous humor. The increased intraocular pressure, if it is not relieved, may result in partial or even total blindness. Although it can occur at any age, it is most common in people over 40 years of age. Early diagnosis and treatment are most important in preventing the loss of vision. In acute glaucoma, the patient has attacks of severe pain in and around his eyes, sees halos particularly around lights and has blurred vision. These attacks usually occur suddenly.

Chronic glaucoma occurs more frequently than the acute type. Patients often have no symptoms untill their disease is fairly well advanced. The symptoms are somewhat similar to those of acute glaucoma.

Early diagnosis can be established by examination with an instrument called tonometer. This measures the tension in the eye. All patients with glaucoma should avoid coffee, tea and other caffeine products and must also limit their total fluid intake. It is important that they avoid lifting heavy objects that can increase intraocular pressure and limit activities that cause eye strain and fatigue. Constriction of pupil with the help of parasympathomimetic drugs help in drainage of fluid through the canal of Schlemm and hence decreases intraocular pressure giving relief from glaucoma.

 

Detached Retina : In a detached retina, the nerve layer of the retina becomes separated from the pigmented layer and deprives the nerve layer of its blood supply. Thus, vision is lost in the affected area. The symptoms depend on the size of the affected area but generally the patient reports that he sees flashes of light or a sensation of spots or moving particles before his eyes. The detachment may be treated surgically or if it is a relatively smaller area, it may be corrected by photo-coagulation. 

Photo-coagulation is a procedure in which a beam of light is directed towards the area of detachment. This causes the retina at that point to adhere to the choroid following an inflammatory swelling which re-establishes the contact of the retina with the choroid, restoring vision considerably. 

Conjunctivitis : Conjunctivitis or pink red eye is an inflammation of the conjunctiva due to trauma, allergy or infection. If the etiology is allergy, the patient may be treated with antihistamines, corticosteroids or perhaps can be desensitized to the particular allergen. If it is due to bacteria, it is highly contageous and may be treated with the appropriate antibiotics, applied locally. Resistance may develop in viral infection as the time passes. 

Trachoma : It is a chronic conjunctivitis caused by Chlamydia trachomalis and characterised by progressive exacerbations and remissions with follicular sub-conjunctival hyperplasia, corneal vascularization and cicatrization of the conjunctiva, cornea and lids.

Hordeolum : It is a stye or infection at the edge of the eyelid in an eye lash follicle. Hot, wet compresses help to localize the infection. Within a few days, the lesion will drain and will require no further treatment. Occasionally, it is necessary to incise and to drain the lesion. If a person is subject to frequent recurrences of this type of infection, he should wash his face three or four times a day with an antibacterial soap. 

Chalazion : Small sebaceous glands are located within the upper eyelids. An infection of these glands, called a chalazion can be quite painful and requires surgical treatment of excision. 

Colour-Blindness: This is a defect of retina, in which a person cannot see one or more colours. Some persons are completely colour-blind and see everything in black and white only. Some are partially colour-blind, not seeing particular colours. In males-8.0% and in female about 0.5% are partially or totally colour-blind. The defect lies often in the cones of the retina, sometimes, it may be in the brain also.

Night Blindness : When it is not possible to see in dimlight due to reduced power of dark adaptation, the condition is called "night blindness" .It can be hereditary, retinitis pigmentosa, vitamin A deficiency disease or pathological condition. It has been found that there is relationship between vitamin A and rhodopsin formation. It has photochemical properties and it stimulates rods. The deficiency of vitamin A affects rhodopsin formation and results in night blindness.

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