An eye is said to be emmetrope when it works properly. Sometimes it has defects connected with focusing or diseases connected with the lens, the sensibility of receptors or the shape of the cornea. The most common defects are:
– nearsightedness (myopia) is due to the eye’s refractive power being too strong or to the eyeball being too long. In both conditions the light rays are focused in front of the retina giving a confused image. Concave corrective lenses are needed to focus on distant objects;
– Farsightdness (hypermetropia) is caused by the eye’s refractive power being too weak or by the eyeball being too short. In both conditions light rays are focused behind the retina giving a confused image. Convex corrective lenses are needed to get good focus on close objects;
– Astigmatism means that a person cannot focus on the main meridians at the same time because the cornea doesn’t curve correctly.
lenses, affecting the light rays on one of the planes, are required; hard contact lenses are suggested;
– Presbyopia is common in old age. It consists of a reduced ability to focus the eye on near objects because of a gradual loss of elasticity of the cristalline
– Daltonism means colour blindness for some colours.
What are the main eye diseases?
Generally speaking, an eye disease occurs when there is something wrong with the sensibility of the receptors, the lens and the cornea. The most common diseases of the eye are:
– black eyes are bruises of the eyelids and the tissue around the eye; it can be cured by applying a cold compress;
– a stye is an inflammated swelling of the edge of the eyelid and it is caused by bacterial infection;
– conjunctivitis is an inflammation of the conjunctiva (the membrane covering the front of th eyeball and the inside of the eyelids). It can be caused by infection or irritation and is due to bacterial or viral infection. It is not a very serious disease in itself but sometimes it can damage the cornea ( the clear part of the eyeball);
– blepharitis is an inflammation of the eyelid margin. It can be “mild” and consists of redness of the eyelid margin or it can be more severe, affecting the follicles of the eyelashes.
– herpes Zoster (shingles) affects the skin of the eyelids and it can be severe when it affects the cornea and the inner eye.
-glaucoma: is due to an increase in ocular tension or to some atrophy of the pupil involving gradual loss of field of vision.
– keratoconous is a developmental anomaly where the central portion of the cornea becomes progressively thinner as a displaced and increasing protrusion.
Beginning in one eye but later affecting both, it usually manifests itself t puberty.
What are the main eye disorders?
Their symptoms are: blindness, temporary diminished eyesight, impaired vision. We can have blurred vision, that is a loss of visual acuity resulting in unclear visual details; vision loss: that is the inability to perceive visual stimuli; blind spots: that is apparent dark holes in the visual field in which nothing can be seen.
What are the main causes of eye disorders?
They are: aging, the most common cause of decreased vision; cataracts, also common in the elderly; glaucoma, a disease of the eye characterised by increased fluid pressure within the eyeball; complications of diabetes, one of the most common causes of blindness; retinal haemorrhages which can cause more permanent blindness diabetic retinopathy.
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