Before discussing Optics of Myopia we should ratify parishioner phrase in Myopia. Briefly, the uncorrected myopia or nearsighted person. Usually can see up close better than far away. Hence, the term “nearsighted” make sense.
Derived from two Greek root words from myein “to shut” – ops “eye”.
Myopia is the refractive state of the eye in which with normal tonic accommodation, the parallel rays of light are brought to a focus on a point in front of the retinal plane when the eye is at rest.
In myopia the eyeball is usually deformed ; the deformation occurs at the posterior part of the globe only ,the anterior part is normal. The eyeball is usually large and looks prominent. When adducted the equator can be seen.
Refractive Error Definition :
A refractive error is a very common eye disorder . It exists when the rays of light do not focus on the retina means the eye cannot clearly focus the images from the outside world .
The eye’s optical system is to powerful over converging light in front of the retina . ( Myopia/Nearsighted)
The result of refractive errors is blurred vision, double vision , haziness or eye strain . which is sometimes so severe that it causes visual impairment .
Optics of Myopia: In myopes a near object may be focused without any effort of accommodation if it is situated at the punctum remotum . The image on the retina of an object at infinity is made up of circles of diffusion formed by divergent beam. In view of the absence of sharp focus the distant objects appear blurred. The nodal point in myopes is farther away from the retina and the image formed is larger in size which partly compensates for the blur due to absence of sharp focus . Accommodation is of little value to myopes, as any exercise of accommodation would only accentuate his visual problem . It would neither increase his vision nor reduce his myopia . In higher errors of refraction the amplitude of accommodation is small. The patient has no incentive to improve it and convergence does not get an accommodative influence . All these factors result in fatigue of accommodation and eye strain. Due to dissociation of convergence and accommodation exophoria commonly occurs and this may ultimately break into exotropia.
The eye of simple myopia is large and prominent. The Anterior chamber is deeper than normal and the pupil reacts sluggishly to light. The macula is slightly nearer to the disc than in a normal eye . The visual axis gets so altered that in some cases the angle alpha becomes slightly negative which gives an impression of a convergent squint . In myopia more commonly one sees a divergent strabismus.
Optical System :
The optical system is too powerful for it’s axial length , over converging light in front of the retina ( myopia / nearsighted).
Image of Distant Object :
Image of distant object on retina is made up of circle of diffusion formed by divergent beam since the parallel rays of light coming from the infinity are focused in front of the retina.
Fig : Myopic Eye
Far Point of the Myopic Eye : Far point is finite point in front of eye . The far point of myopic eye is 70 cm.
Fig : Nodal Point
Nodal Point : Nodal point is further away from retina . The nodal point of human eye is located 17 mm in front of the retina.
Accommodation : Accommodation in uncorrected myopes is not developed normally , they may suffer from convergence insufficiency , exophoria and early presbyopia as they grow.
Image Formation: In myopia image formed in front of eye which is corrected by placing the negative lenses .
Optical Treatment : Include prescription of appropriate concave lens minimum acceptance providing maximum vision should be prescribed .
Never Overcorrected myopia .
Guidelines For Correcting Low Degree Of Myopia Upto-6D :
- Children younger than 8yrs should be fully corrected and instructed to use their glasses constantly.
- Adult younger than 30 years :
Usually accept their full correction .
- Older than 30 years :
Not able to tolerate a full correction with which the patient feels comfortable .
Prescribe less than full correction with which the patient feels comfortable .