Nearsightedness, myopia
Myopia (short sightedness); the optical refractive power of the eye is too high for its axial length so the parallel rays of light are brought to a focus in front of the retina (when the eye is at rest). Accommodation in uncorrected myopes is not developed normally, they may suffer from convergence insufficiency, exophoria and early presbyopia as they grow. Without correction myopic eye cannot have clear distant vision.The far point of clear vision lies nearer than 5 meters depending on the degree of myopia. High myopia is inherited, genes responsible for this is PAX 6 gene. In myopia there is a weakness of connective tissue and endocrine imbalance. Bad hygienic conditions, concomitant diseases, imbalanced calcium change and other factors are also involved. Causes are;
- increase in anterior posterior diameter of the globe= axial myopia
- increase of the curvature of the cornea= curvature myopia
- Increase in refractive index of the crystalline lens associated with nuclear sclerosis=index myopia
- Excessive accommodation= occurs in patients with spasm of accomodation
Clinical forms of myopia:
- Congenital myopia-present at birth. It is mostly unilateral and is diagnosed by the age of 2-3years. It manifests as anisometropia. It is associated with cataract, micropthalmos, aniridia, megalocornea, congenital separation of retina. Usually error is about 8-10Dpt which mostly remains constant
- Ordinary-refractive myopia-less than (-) 4 to (-) 6 Dpt without concomitant degenerative changes, progresses only during school age. Results from normal biological variation in the development of eye. Usually the eyes are large and prominent, slight deep anterior chamber. It is confirmed by retinoscopy
- Excessive, pathologic myopia exceeds (-) 10 Dpt (up to 20 or more Dpt), rapidly progressive, combined with heavy degenerative changes in the posterior eye segment. Usually apparent during the 1st decade of life. Genetic factors might play a role, also increased axial length, increased growth retina, stretching of sclera, degeneration of choroid and retina and vitreous. Symptoms are defective vision, muscae volitantes and night blindness. Signs are prominent eyes appearing elongated, large cornea deep anterior chamber, large pupils. Optic disc appears large and pale and at temporal edge a characteristic myopic crescent is present. Posterior staphyloma is an ectasia or bulging of the posterior sclera due to focal expansion and thinning. Visual field shows contraction and sometimes ring scotoma may be seen.
- Acquired myopia causes are post traumatic, post keratitic, drug induced, etc
Types of myopia according to amount:
- Mild up to (-) 3D