Refractive errors

Question 1

Define visual acuity and compare it to vision?

Answer 1

Visual acuity is the measure of the eye’s ability to resolve fine detail with the optimum correction in place. Vision is the what the eye can resolve unaided.

Question 2

Describe accommodation, the accommodation reflex and its purpose.

Answer 2

Accommodation is the ability of the lens to change shape and therefore the refractory power. (Shorter aka more convex = stronger)

The accommodation reflex occurs when focusing on an object which is nearby.

1. Ciliary muscles contract causing the zonules to relax this causes the lens to become more convex and therefore increase its refractory power.
2. The eyeballs converge. This ability to accommodate in old age is diminished as the lens undergoes sclerosis and therefore it is common for elderly people to lose the ability to focus on near objects (reading glasses) this is known as presbyopia.

Question 3

How is acuity commonly measured and recorded?

Answer 3

Snellen chart 6/6 can see at 6m what the average person can see.

6/12 can see at 6m what the average person can see at 12m etc.

6/12 +2 can read 2 letters on the next line but not complete the line.

Note: does not measure near vision special charts are used for this.

Question 4

What are the refractive components of the eye?

Answer 4

Cornea, Lens, Length of Eye

Question 5

Define the following terms: dioptre, emmetropia, ametropia?

Answer 5

Dioptre: is the unit of measurement for the optic power of the lens and describes the ability to refract light.

Emmetropia: optically perfect lens light naturally focuses perfectly on the fovea. Cornea: 40D Lens: 20D Axial length: 22.2mm

Ametropia: one or more of the optical parameters is not correct resulting in a focussing error.

Question 6

Define the terms myopia and hypermetropia and how they are corrected?

Answer 6

Myopia aka short-sightedness (literally can see near/short things can’t see far things). Light becomes focused infront of the retina because: -the refractive power is too great OR -the axial length is too long >22.2mm Corrected with a diverging lens causes the light to spread out.

Hypermetropia aka long-sightedness (can see far things but not near things easily.) Light does not converge enough and becomes focused behind the retina because: -the refractive power is too weak

Question 7

Define astigmatism and how it is corrected?

Answer 7

Astigmatism is when the eye is not a symmetrical shape instead of being like a football it is like a rugby ball. Therefore the light will be refracted by different amount in different plains. This is corrected by using a lens with power in only one direction. It may also be convergent/divergent if there is an underlying myopia/hypermetropia.

Question 8

What are the main two types of contact lenses compare and contrast them.

Answer 8

Soft lenses.

Advantages:

Flexible and more comfortable.

Better fitting.

Safer for sport and extended wear.

Disadvantages

More expensive.

Dehydrate if left out of solution.

Split easily.

Depositions from tears.

Rigid gas permeable lenses.

Advantages:

Fixed shape and durable.

Good for all day wear.

Small diameter therefore reduced hypoxic risk.

Easy to clean.

Better for astigmatism and irregular corneas.

Disadvantages

Poor initial comfort.

Smaller diameter means it is more likely to fall out of the eye.