Myopia or shortsightedeness is a tyep of refractive error in which parallel of light coming from infinity are focused in front of the retina when accommodation is at rest
MECHANISMS OF PRODUCTION
Aetiologically,myopia may be of the following types:
1. Axial myopia results from increase in anteroposterior length of eyeball. it is the commonest from.
2. Curvatural myopia occurs due to increased curvature of the cornea, lens or both.
3. positional myopia is prodused by anterior placement of crystalline lens in the eye.
4. index myopia results from increase in the refractive index of crystalline lens associated with nuclear sclerosis.
5. Myopia due to excessive accommodation occurs in patients with spasm of accommodation.
MECHANISMS OF PRODUCTION
Aetiologically,myopia may be of the following types:
1. Axial myopia results from increase in anteroposterior length of eyeball. it is the commonest from.
2. Curvatural myopia occurs due to increased curvature of the cornea, lens or both.
3. positional myopia is prodused by anterior placement of crystalline lens in the eye.
4. index myopia results from increase in the refractive index of crystalline lens associated with nuclear sclerosis.
5. Myopia due to excessive accommodation occurs in patients with spasm of accommodation.
OPTICS OF MYOPIA
• Optical system of a myopic eye is too powerful for its axial length.
• Image of distant object on the retina is made up of the circles of diffusion formed by the divergent beam infinity are focused in front of the retina
• For point of the myopic eye is a finite point in front of the eye. Therefore, a near object situated at far point is focused without an effort of accommodation
• Nodal point in a myopic eye is further away from the retina. Therefore, the image formed will be appreciably larger then it would be in the emmetropic eye and in spectacle corrected eye to some extent, this compensates for the poor visual acuity.
• Angle alpha of the myopic eye may be negative, resulting in an apparent convergent squint.
• Accommodation in uncorrected myopic is not developed normally, since they need not accommodate to see the near objects clearly. For this reason, they may suffer from convergence insufficiency, exophoria and early presbyopia as they grow older.
CLINICAL TYPES OF MYOPIA
• Congenital myopia
• Simple or developmental myopia
• Pathological or degenerative myopia
• Acquired myopia
Clinical features
• Present since birth. Congenital myopia is usually diagnosed by the age of 2-3 years.
• High degree of error. Usually the error is of about 8-10 D, which mostly remains constant.
• Anisometropia. Most of the time, the error is unilateral and manifests as anisometropia. Rarely, it may be bilateral.
Diagnosis
• Unilateral congenital myopia is frequently discovered either by routine screening examination or after a strabismus develops because of the associated amblyopia.
• If the myopia is bilateral, the child will generally dispaly some noticeable difficulty in seeing distant object and will tend to hold things very close for viewing.
Diagnosis
• Unilateral congenital myopia is frequently discovered either by routine screening examination or after a strabismus develops because of the associated amblyopia.
• If the myopia is bilateral, the child will generally dispaly some noticeable difficulty in seeing distant object and will tend to hold things very close for viewing.
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