What does Refractive error mean

Question 1

EMMETROPIA

Answer 1

Emmetropia’ means the absence of a refractive error, so parallel light from a distant source is perfectly focused on the retina

Question 2

Emmetrope VA

Answer 2

normal distance acuity with no spectacles (uncorrected Snellen acuity of 6/6 or better) ~provided, of course, there is no amblyopia, ocular pathology, or cerebral visual impairment.

Question 3

AMETROPIA

Answer 3

‘Refractive error’ (ametropia) means that an eye does not allow light from a distant source to be focused perfectly on the retina.

Question 4

Which is more common? Myopia or hypermetropia

Answer 4

Myopia is just as common as hypermetropia.

Question 5

What is myopia

Answer 5

‘Myopia (short-sightedness) means that the refractive power of the eye is too great relative to the axial length of the eye; as a result, the image of a distant object lies in front of the retina

Question 6

Causes of myopia

Answer 6

if the refractive power is too high or if the eye is too long

Question 7

How is myopia corrected

Answer 7

By a minus (con- cave) lens, which effectively weakens the refractive power to allow the image to be shifted back on to the retina

Question 8

What is hypermetropia

Answer 8

‘Hypermetropia’ (long-sightedness), also known as hyperopia, means that the refractive power of the eye is too weak relative to the axial length of the eye; as a result, the image of a distant object lies behind the retina

Question 9

How is hypermetropia corrected

Answer 9

by a plus (convex) lens, which effectively strengthens the refractive power to allow the image to be shifted forwards on to the retina

Question 10

What is astigmatism

Answer 10

Astigmatism refers to the refractive power of the eye being different in different meridians. Therefore, light from a point of a distant object cannot forma single point of an image

Question 11

What is Sturm conoid

Answer 11

An eye with astigmatism behaves as a sphero-cylindrical (toric) lens. The principal meridians form separate line foci and between them lies Sturm’s conoid. The dioptric midpoint of Sturm’s conoid is the circle of least confusion, situated at the focal point for the lens spherical equivalent value

Question 12

Where to place the corneal incision in cataract surgery

Answer 12

on the steepest K meridian, the degree of corneal astigmatism is reduced, which can be beneficial to the patient since astigmatism has no refractive advantage

Question 13

What is regular astigmatism

Answer 13

This applies when the meridians of maximum and minimum refractive power are perpendicular to each other

Question 14

Types of regular astigmatism

Answer 14

With the rule
Against the rule
Oblique astigmatism

Question 15

What is WTR astigmatism

Answer 15

• The steepest meridian is vertical, or within 30 degrees of 090 (060- 120), and the flattest is horizontal
• The maximal refractive power of the cornea acts vertically (the steepest K meridian will be near 090)
• The axis of a correcting plus cylinder will be vertical (090), since its power needs to act horizontally to strengthen the relatively weaker horizontal meridian

Question 16

Example of WTR astigmatism on PRESCRIPTION

Answer 16

+1.00/ +1.50 x 090. There is a +1.50 dioptre cyl, which has its axis (denoted by x’) at 090. This cyl is superimposed on a +1.00 dioptre sphere. In the absence of any lenticular astigmatism, one would expect the K value in the vertical meridian to be greater than the horizontal meridian.

Question 17

What is ATR astigmatism

Answer 17

• The steepest meridian is horizontal, or within 30 degrees of 180 (000-030 and 150-180), and the flattest is vertical
• The maximal refractive power of the cornea acts horizontally (the steepest K meridian will be near 180)
• The axis of a correcting plus cylinder will be horizontal (180), since its power needs to act vertically to strengthen the relatively weaker vertical meridian

Question 18

Example of ATR astigmatism

Answer 18

+2.00 / + 1.75 x 180. There is a +1.50 dioptre cyl with its axis (denoted by ‘x’) at 180. This cyl is superimposed on a +2.00 dioptre sphere. In the absence of any lenticular astigmatism, one would expect the K value in the horizontal meridian to be greater than the vertical meridian.

Question 19

What is oblique astigmatism

Answer 19

The maximal and minimal meridians are perpendicular to each other but are not acting at or within 30 degrees of the vertical or horizontal plane for example, a maximal meridian at 050 and a minimal meridian at 140.

Question 20

What is irregular astigmatism

Answer 20

When the meridians of maximum and minimum refractive power are not perpendicular to each other. The most common cause for this is keratoconus (which gives a scissor-like retinoscope reflex that is difficult to neutralise).

Question 21

Causes of irregular astigmatism

Answer 21

*Keratoconus
*Post penetrating keratoplasty

Question 22

What is simple astigmatism

Answer 22

When the eye is plano (emmetropic) in one meridian (ie. the rays in this meridian focus on the retina) and either myopic or hypermetropic in another (ie. the rays in this meridian do not focus on the retina).

Question 23

Example of simple astigmatism

Answer 23

0.00 / +1.50 x 055 implies that no spherical correction is required, but a +1.50 cyl lens is required with an axis at 055 (power acting perpendicularly at 145) to correct the refractive error.

Question 24

What is compound astigmatism

Answer 24

when both meridians are hypermetropic (i.e. the rays in all meridians come to focus behind the retina) or both are myopic (rays in all meridians come to focus in front of the retina).

Question 25

Example of compound astigmatism

Answer 25

+1.00 / +2.00 x 090 implies that a +1.00 spherical lens with a + 2.00 cyl lens with axis at 090 (power acting perpendicularly at 180) is required to correct the refractive error.

Question 26

What is mixed astigmatism

Answer 26

This applies when one meridian is myopic (rays fall in front of the retina) and the other is hypermetropic (rays fall behind the retina).

Question 27

Example of mixed astigmatism

Answer 27

-1.50 / +2.50 x 040 implies that a -1.50 spherical lens with a + 2.50 cyl lens with axis at 040 (power acting perpendicularly at 130) is required to correct the refractive error.

Question 28

How does the cyl axis run on a trial frame

Answer 28

From right to left 0 to 180

Question 29

How to transpose from + to - cyl and vice versa

Answer 29

1) Add cyl to sphere
2) Change cyl sign
3) New axis perpendicular to old axis (+/-90)

Question 30

Transposition example

Answer 30

Transpose —8.00 / +3.00 x 165.
New sphere = (-8.00) + (+3.00) = -5.00 (adding the cyl to the sphere to give
new sphere)
New cyl =-3.00 (changing the sign of the cyl to give the new cyl)
New axis = 165 - 090 = 075 (the new axis is perpendicular to the old axis)
To give ~5.00 / -3.00 x 075.

Question 31

How to calculate spherical equivalent

Answer 31

Spherical equivalent =sphere +(cylinder/2)

Question 32

What is anisometropia

Answer 32

when the difference in refractive prescriptions between the two eyes is sufficiently large to result in troublesome symptoms such as aniseikonia (different image size of single object between eyes) and asthenopia (eye strain —patients are often non-specific, but complain of fatigue, blurred vision, and headache).

Question 33

What difference in dioptres between the eyes causes anisometropia and the troublesome effects

Answer 33

when the difference in spherical equivalent is more than 1.5 dioptres.