Dispensing astigmatic lenses

Question 1

What is the most common cause of astigmatism?

Answer 1

A variation of the curve of the cornea

Question 2

What is the second most common cause of astigmatism?

Answer 2

The lens of the eye may not lie absolutely perpendicular to the visual axis

Question 3

What is the cause of lenticular astigmatism?

Answer 3

The crystalline lens may be toroidal in shape.
(the axis of the lens may coincide with the visual axis of the eye, but it will still produce more dioptric power in one meridian than the other)

Question 4

What are the 5 types of astigmatism?

Answer 4

1. Simple hyperopic astigmatism
2. Simple myopic astigmatism
3. Compound hyperopic astigmatism
4. Compound myopic astigmatism
5. Mixed astigmatism

Question 5

What is irregular astigmatism?

Answer 5

As astigmatism in which the principal powers lie in meridians that are not 90° apart

Question 6

What is keratoconus?

Answer 6

A rare cause of irregular astigmatism, where the cornea begins to bulge
forward and becomes more pointed at its apex, rather like the tip of a bullet.

Question 7

What can be done to help those with keratoconus?

Answer 7

Rigid contact lenses can help
correct keratoconus in its early stages, providing a spherical refracting surface, but corneal
transplant is often ultimately required.

Question 8

How can astigmatism be corrected?

Answer 8

Spectacles, contact lenses or refractive surgery

Question 9

What frames should be avoided for those with high levels of astigmatism?

Answer 9

rimless, supra or side supra frames, frames with double rim is also not advised as the rims can separate if the lens edge is thicker

Question 10

In toric contact lenses, what can happen if astigmatism is left uncorrected?

Answer 10

Leaving the aberration uncorrected results in less precise visual acuity which will be most problematic in dark conditions due to pupil enlargement

Question 11

What is added to toric lenses to keep them in place?

Answer 11

toric contact lenses have a prism weight at the base of the lens to ensure that the cylinder axis of the contact lens remains in close orientation to the visual axis.

Question 12

What are the three curves on a toric lens?

Answer 12

*The base curve: the lowest numerical power on the toroidal surface
* The cross curve: the highest numerical power on the toroidal surface
* The sphere curve: the curve on the opposite side to the toroidal surface

Question 13

What calculation allows us to find the front surface power of a toric lens?

Answer 13

(base curve/cross curve)/sphere curve

Question 14

What calculation allows us to find the back surface power of a toric lens?

Answer 14

sphere curve/(base curve/cross curve)