What is Myopia

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.

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.

Diseases and Disorders

Age-related macular degeneration, or AMD, is a physical disturbance that affects the centre of the retina, called the macula. The macula is the part of the eye responsible for our most acute vision, which we use when reading, driving, and performing other activities that require fine, sharp, or straight-ahead vision

What is HYPERMETROPIA

Hypermetropia (hyperopia) or long sightedness is the refractive state of the eye wherein parallel rays of light coming from infinity are focused behind the retina with accommodation being at rest thus the posterior focal point is behind the retina, which, therefore,receives a blurred image. 

Types of Hypermetropia 

There are three clinical types of hypermetropia:

1. Simple hypermetropia is the commonest from. It result from normal biological variation in the development of the eyeball. includes: 

• Axial hypermetropia due to congenital/developmental short eyeball,and 

• Curvatural hypermetropia due to congenital/developmental flatter cornea.

Simple hypermetropia may be hereditary. Inheritance is usually dominant, which may be irregular.

2. Pathological hypermetropia result due to either congenital or acquired condition of the eyeball which are outside the normal biological variations of the development. 

3. Congenital pathological hypermetropia is seen in following conditions:

• Microphthalmos,

• Microcornea, 

• Congenital posterior subluxation of the lens,and 

• Congenital aphakia.

What The Cornea

                                         The cornea 

The cornea is a highly transparent structure of meniscus from approximately 12 mm in diameter and slightly smaller vertically than horizontally. the center thickness is usually between 0.5 and 0.6 mm.

A thin layer of lacrimal  fluid normally covers the anterior surface, but it is too thin to affect the power appreciably and may be ignored in this context.

To first approximation, both surfaces may be regarded as spherical, the radii of curvature having values approximately ➕7.7 mm (anterior) and ➕6.8 mm (posterior) 

The refractive index of the corneal  substance may be taken as 1.376 and that of the aqueous humour, in contact with the back surface of the cornea, as 1.336 By applying equation   F=(n'-n) R, ( where F= power of spherical refracting surface, R = reciprocal of radius of curvature in metres, n= refractive index of first medium and n' = refractive index of second medium), the tow surface power of the cornea may be found as follows: 


     • Anterior surface                  
                                                  1000(1.376-1)
                      Power       F 1 = ➖➖➖➖➖➖➖➖
                                                    +7.7 
                                             =    +48.83 D 
   
     • Posterior surface 
                                                 1000(1.336-1.376)
                      Power        F 1 = ➖➖➖➖➖➖➖➖
                                                         + 6.8 
                                               = -5.88 D

The power of the cornea as a whole is, therefore, about + 43 D, over tow-thirds of the total power of the eye. 
The light entering the eye is refracted markedly at anterior corneal surface for tow reasons: first, because of curvature and secondly, because of the big difference in refractive indices of air (1) and cornea (1.37). When the eyes are unprotected under water, the anterior surface of the cornea has its power greatly reduced, the retinal image then  becoming inordinately blurred. 

The aspherical  shape of cornea's anterior surface is responsible for a baseline astigmatism of 0.25 -0.5 D in almost every human eye. Further, a changing corneal surface's radius of curvature with distance from the centre of the pupil to the pupillary margin is responsible for a spherical aberration of 0.21-162 D ( depending on the specific corneal from ) in a normal human eye for a pupil of 4 mm in diameter.r

Optics of the eye

EYE AS OPTICAL INSTRUMENT

As an optical instrument the eye be compared with a camera as below
 1.Eye lids act shutter of the camera.
 2.Cornea and crystalline lens act as focusing system of the camera.
 3. Iris   acts as diaphragm which regulates size of the entering the eye.
 4. Choroid helps in forming the darkened interior of the camera.
 5. Retina acts as light-sensitive plate or film on which image is formed.

To be more precise,the functioning of the eye can be considered to be analogous to a closed circuit colour TV system as depicted in (Fig).
The optic nerve and its connection convey the details of the image to the occipital region of the cerebral cortex,where they are processed before reaching consciousness.