Errors of Refraction Physiology Notes

Ametropia

The eye with normal refractive power is called emmetropic eye, and the condition is called emmetropia. Any deviation in the refractive power from normal conditions resulting in inadequate focusing on the retina is called ametropia and the eye is called the ametropic eye. The defect is due to the change in the shape of the eyeball.

Ametropia is of two types:

1. Myopia
2. Hypermetropia.

Read And Learn More: Medical Physiology Notes

1. Myopia Or Short Sightedness: Myopia is an eye defect characterized by the inability to focus on distant objects. It is otherwise called short-sightedness because the person can see near objects clearly but not distant objects. In emmetropia, the far point is infinite. In myopia, the near vision is normal but the far point is not infinite, i.e. it is at a definite distance. In extreme conditions, it may be only a few centimeters away from the eye (MYO = half closed; ops = eye).

• Myopia Cause:
  • In myopia, the refractive power of the lens is usually normal. But, the anteroposterior diameter of the eyeball is abnormally long. Therefore, the image is brought to focus a little in front of the retina.
  • In other words, the refractory power of the lens is too strong for the length of the eyeball. The light rays, after coming to a focus, disperse again so, a blurred image is formed upon the retina.
• Myopia Correction: In myopic eye, in order to form a clear image on the retina, the light rays entering the eye must be divergent and not parallel. Thus, the myopic eye is corrected by using the biconcave lens. The light rays are diverged by the concave lens before entering the eye.

2. Hypermetropia Or Long Sightedness: Hypermetropia is an eye defect characterized by the inability to focus on a near object. It is otherwise known as long-sightedness because the person can see distant objects clearly but not near objects. It is also called hyperopia. In this defect, the distant vision is normal but, the near vision is affected (meters = measure).

• Hypermetropia Cause:
  • Hypermetropia is due to the decreased anteroposterior diameter of the eyeball. So, even though the refractive power of the lens is normal, the light rays are not converged enough to form a clear image on the retina, i.e. the light rays are brought to a focus behind the retina.
  • It causes a blurred image of near objects. Hypermetropia occurs in childhood if the eyeballs fail to develop to the correct size. It is common in old age also.
• Hypermetropia Correction: Hypermetropia is corrected by using a biconvex lens. The light rays are converged by a convex lens before entering the eye.

Anisometropia

Anisometropia is the condition in which the two eyes have unequal refractive power. It is corrected by using different appropriate lenses for each eye.

Astigmatism

• Astigmatism is the condition in which the light rays are not brought to a sharp point upon the retina. It is a common optical defect. This defect is present in all eyes. When it is moderate, it is known as physiological astigmatism.
• When it is well-marked, it is considered abnormal. For example, the stars appear as small dots of light to a person with a normal eye. But in astigmatism, the stars appear as radiating short lines of light (A = not; stigma = point).

1. Cause Of Astigmatism:

• The light rays pass through all meridians of a lens. In a normal eye, the lens has approximately the same curvature in all meridians. So, the light rays are refracted almost equally in all meridians and brought to focus.
• If the curvature is different in different meridians – vertical, horizontal, and oblique, the refractive power is also different in different meridians.
• The meridian with greater curvature refracts the light rays more strongly than the other meridians. So, these light rays are brought to focus in front of the light rays, which pass through other meridians. Such irregularity of curvature of tens causes astigmatism.

2. Types Of Astigmatism: Astigmatism is of two types.

1. Regular astigmatism
2. Irregular astigmatism.

 

1. Regular Astigmatism: In this type of astigmatism, the refractive power is unequal in different meridians because of the alteration of curvature in one meridian. But, it is uniform in all points throughout the affected meridian.
2. Irregular Astigmatism: Here, the refractive power is unequal not only in different meridians, but it is also unequal in different points of the same meridian.

3. Correction Of Astigmatism: Astigmatism is corrected by using a cylindrical glass lens having the convexity in the meridians corresponding to that of the lens of an eye having a lesser curvature, i.e. if the horizontal curvature of the lens is less, the person should use a cylindrical glass lens with the convexity in the horizontal meridian.

Presbyopia

• Presbyopia is a condition characterized by the progressively diminished ability of the eyes to focus on near objects with age. It is due to the gradual reduction in the amplitude of accommodation. It progresses as the age advances (presbyos = old; ops = eye). Presbyopia starts developing after middle age.
• In presbyopia, the distant vision is unaffected. Only the near vision is affected. The near point is away from the eye. In presbyopia, the anterior curvature of the lens does not increase during near vision. So, the light rays from near objects are not brought to focus on the retina.

1. Causes Of Presbyopia:

• Decreased elasticity of the lens is because of the physical changes in the lens and its capsule during old age. So, the anterior curvature is not increased during near vision
• Decreased convergence of eyeballs due to the concomitant weakness of ocular muscles in old age.

2. Correction Of Presbyopia: Presbyopia is corrected by using the biconvex lens.