MYOPIA

 

This is also known as ‘Short Sightedness’. It is defined as a condition of refraction in which parallel rays of light coming from infinity are focused in front of the retina with accommodation at rest. This is because the power of the eyeball is more than +60D.

 Types of Myopia Depending upon the mechanism of production, myopia may be of different types: 

1. Axial myopia: Here the axial length of the eyeball is more than normal (>24 mm). One mm increase in axial length causes three dioptres of myopia. 

2. Curvature myopia: Curvature of cornea or lens is more than normal. One mm increase in curvature causes six dioptres of myopia. Keratoconus and lenticonus are clinical entities that cause high curvature myopia. 

3. Index myopia: Refractive index of the eyeball is more than normal, e.g. in nuclear sclerosis refractive index of the lens is increased causing a myopic shift. 

4. Positional myopia: Anterior displacement of lens in the eyeball causes a myopic shift. 

5. Excessive accommodation as occurs in a spasm of an accommodation causes myopia. 

***Depending upon clinical presentation myopia may be of different types: 

1. Congenital myopia: It is present since birth but parents come to know about the disease at around 2 to 3 years of age. The child usually requires a high concave lens (8-10D) to correct the error. It may be unilateral or bilateral. The child may develop a convergent squint. It may also be associated with other congenital anomalies like microphthalmos, microcornea, congenital cataracts, etc. 

• The child is born with an elongated eye.
• The refraction may be up to -10D.
• Typical fundus changes are seen.
• Progression is rare.

2. Simple myopia: This is the most common variety of myopia encountered clinically. The patient may complain of blurring of vision, eyestrain while reading, writing, or working on the computer and watching television. The school-going child may complain of doing mistakes when he copies matter written on the blackboard. If an error is small, eyestrain symptoms occur more because he tries to compensate for the error and if the error is of a high degree, the patient is not able to see distant objects. On examination, the myopic eyeball is more prominent with large pupils. The fundus may show a temporal crescent. Diagnosis is confirmed by retinoscopy. (PHYSIOLOGICAL OR SCHOOL MYOPIA).*COMMONEST

• The most common is clinical types.
• Does not progress much after adolescence.
• May be up to -5D to -6D.
• No degenerative changes are seen in the fundus, although peripheral retinal degeneration may be seen in later life.
• Associated with good vision with a good prognosis.

3. Pathological myopia: This is also known as degenerative or progressive myopia. There is the rapid progress of myopia in early adult life associated with degenerative changes of the retina and vitreous. It occurs due to a rapid increase in the axial length of the eyeball. Heredity plays the main role because this type of myopia is more common in persons with a family history of high myopia. It is more common in some races like Japanese, Chinese and it is less common in Negroes and Sudanese. Due to rapid growth sclera follows the growth of the retina but the choroid undergoes degeneration followed by degeneration of the retina. General factors like nutritional deficiency, debilitating diseases, wrong posture, poor illumination, and working on a computer for long hours play a minor role in the progress of myopia. 

• Myopia appears in childhood (5-10 years of age) and increasing steadily with age up to 25 years or beyond.
• The final amount of myopia may be -15D to -25D or more.
• There are typical degenerative changes in the fundus.
•  Strongly hereditary and more common in females.
• prognosis is usually poor.

*** If the myopia is more than -6D, it is called high myopia.

Symptoms and Signs of Degenerative Myopia

• A patient with progressive myopia complains of rapid deterioration of vision, vision may not improve to normal status. 

 • Muscae volitantes (a specific type of vitreous floater) and black opacities in front of eyes are complained of by many patients. 

 • Some patients may complain of night blindness due to degeneration of retina. 

 • Eyeball may appear proposed in unilateral cases. Lengthening occurs mainly around the posterior pole. 

• Anterior chamber is deep and the cornea may be bigger than normal. 

• Pupils may be bigger in size and react sluggishly to light. 

 • Fundus may show a big optic disk with a temporal crescent or peripapillary atrophy. A retina becomes very thin and prone to rhegmatogenous retinal detachment. Foster Fuch’s spot (dark red spot due to subretinal neovascularization and choroidal hemorrhage) maybe seen. Cystoid degeneration may occur in the peripheral retina. Posterior Staphyloma may be formed due to ectasia of sclera at the posterior pole lined by choroid and retina. In advanced cases, retinal atrophy may occur and the patient may go blind. 

• Vitreous becomes liquefied with posterior vitreous detachment and vitreous opacities. 

• Visual fields become contracted and ERG reveals subnormal reading due to chorioretinal degeneration.

******* Acquired myopia: This occurs due to exposure to various pharmaceuticals, increase in glucose level, nuclear sclerosis, and increase in curvature of the cornea in conditions such as corneal ectasias.

 The other types of acquired myopia are: 

1. Pseudo myopia: This type of myopia is called false the appearance of myopia that occurs due to excessive accommodation and spasms of accommodation.This type of myopia usually manifest the excessive amount of myopia and decreased amount of hypermetropia. It could cause a severe headache with asthenopic complaints. 

2. Night myopia: In Night myopia the eye has difficulty seeing in low illumination even though the daytime vision is normal. This occurs due to an increase in sensitivity to the shorter wavelengths of light. Younger people are affected more than the elderly. 

3. Space myopia: This type of myopia occurs when the individual has no stimulation for distance fixation.

Symptoms:

• Blurred vision or difficulty in seeing distant objects (Children often cannot read the blackboard, but easily read a book). They tend to go near objects to see clearly.
• Eyestrain 
• Headaches (uncommon) 
• Squinting tendency

Tests:

 A general eye examination or standard ophthalmic exam may include: 
1. Visual acuity, both at a distance (Snellen’s), and close up (Jaeger) 

2. Refraction test to determine the refractive power accurately 

3. Cycloplegic refraction may be required in few conditions to confirm the final prescription 

4. Color vision test to exclude color defect. 

5. Muscle balance test 

6. Slit-lamp examination of the eyes

7. Measurement of the intraocular pressure of the eyes 

8. Retinal examination

 Treatment of Myopia:

Myopia is corrected by appropriate concave lenses either in form of spectacles or contact lens. 

Surgical correction of myopia can be done by photorefractive surgery or exchange of clear lens. Contact lenses are cosmetically better suited with minimal aberrations and maximum visual field, especially in high myopes. However they need motivation and better care on part of the patient. 

LASIK laser surgery is gaining popularity day by day. But it has its own limitations, as very high myopes, are sometimes not fit for surgery due to poor corneal thickness or keratoconus.

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*** OPTICAL CONDITIONS :

In myopia, the parallel rays of light fall in front of the retina and cause a blurred image to fall on the retina. So the distance object appears blurred. In order to see clearly the object is brought closer and the divergent rays come to focus on the retina. This makes the far point in the myopic eye at a finite distance. This distance decreases with the increase in the degree of myopia. Therefore the near object is focused without an effort of accommodation. For these reason, myopes can suffer from convergence insufficiency and exophoria