Chapter 6: Astigmatic Lenses

Question 1

What is astigmatism?

Answer 1

All meridians in the lens do not have the same curvature

Question 2

What are the 2 types of astigmatic lenses?

Answer 2

1. Cylindrical
2. Toric (sphero-cylindrical)

Question 3

What is a cylindrical lens?

Answer 3

• Have one plane srface and other forms part of a cylinder
• in one meridian, lens has no vergence power (the axis).
• in the meridian at right angles to the axis, the cylinder acts as a spherical lens

Question 4

What is the total effect of a cylindrical lens and what is this called?

Answer 4

Formation of a line image of a point object - called the focal line, parallel to the axis of the cylinder

Question 5

What is a Maddox rod formed of?

Answer 5

Series of powerful convex cylindrical lenses mounted **side by side **in a trial lens

Question 6

What is the Maddox rod used for?

Answer 6

Diagnosis of extraocular muscle imbalance

Question 7

How does Maddox rod testing work?

Answer 7

• patient views distant white point source of light through the Maddox rod
• light in meridian parallel to the axis of each cylinder passes through undeviated therefore a row of foci 90 degrees to the axis of the Maddox rod formed - appears as a line
• light incident on MR in meridian 90 degrees to its axis is converged by each cylinder a real line focus between the rod and the eye
• this focus is too close for distinct image to be formed on retina so is scattered over wide area, doesn’t confused composite line image

Question 8

Where is the source of light in relation to the patient when using a Maddox rod?

Answer 8

it must be far enough away for its rays to be parallel on reaching the patient (at least 6m); Maddox rod is close to eye in the trial frame

Question 9

What is important to remember about the image seen by the patient from a Maddox rod?

Answer 9

the line seen lies at 90 degrees to the axis of the rod (not the real line image of the rod)
the Maddox rod glass is tinted red so the composite line image is also red

Question 10

How is the Maddox rod used to test muscle balance?

Answer 10

• rod is palced close in front of R eye 9in trial frame), distant white spotlight viewed by both eyes
• R eye sees a red line at 90 degrees to axis of MR, L eye sees white spotlight
• two eyes see dissimilar images so any muscle imabalance will become manifested

Question 11

How is horizontal muscle imbalance tested for with the MR?

Answer 11

• rod must be horizontal to give a vertical line (and vice versa)
• the position of the red line moves in the opposite direction from the deviation of the eye
• e.g. for the R eye, if the vertical line is to the left of the white dot, indicates exophoria

Question 12

What is a toric lens?

Answer 12

cylindrical surface that is curved in both vertical and horizontal meridians (not to same extent)

meridia of max + min curvature = principal meridia; 90o to each other

Question 13

What is the base curve of a toric lens?

Answer 13

the prinicpal meridian of minmum curvature (and therefore minimum power)

Question 14

What is another name for toric lenses?

Answer 14

sphero-cylindrical lenses

Question 15

Why don’t toric (sphero-cylindrical) lenses produce a single defined image?

Answer 15

principal meridians form separate line foci at right angles to each other - Sturm’s conoid

Question 16

What is the the interval of Sturm?

Answer 16

the distance between two line foci formed by the 2 prinicpal meridians of toric lenses

Question 17

What is the circle of least confusion (akak circle of least diffusion)?

Answer 17

plane where two pencils of light formed by toric lens intersect

Question 18

What type of images are formed at all planes other than the circle of least confusion between Fh & Fv?

Answer 18

blur circle images

Question 19

How is a toric (sphero-cylindrical) lens notated and what does this represent?

Answer 19

• can be thought of as a spherical lens with cylindrical lens superimposed
• writted as a fraction; spherical power is numerator, cylindrical power is denominator
• e.g. power of +2D in one principal meridian and +4D in oher principal meridian can be regarded as a +2D sphere with a +2D cylinder superimposed
• this is written +2.0 DS / +2.0 DC

Question 20

Why might it be useful to calculated the power of the spherical lens of closes overall effect to a given toric lens (spherical equivalent)?

Answer 20

reveals whether the eye is essentially hypermetropic, emmetropic or myopic
this consideration is important in choice of IOL power for individual patient

Question 21

How do you calculate the spherical equivalent power from a toric lens prescription e.g. +2.00 DS/ -2.00 DC?

Answer 21

Algebraic addition of the spherical power and half the cylindrical power:
2+ (-1) = +1.00 DS

Question 22

How does the focal point of a toric lens relate to that of a spherical equivalent lens?

Answer 22

focal point of the spherical equivalent would coincide with the circle of least confusion of the Sturm’s conoid

Question 23

What is a cross cylinder?

Answer 23

• sphero-cylindrical (toric) lens in which the power of the cylinder is twice the power of the sphere and of the opposite sign
• net result is same as superimposing two cylindrical lenses of equal power but opposite sign with axes at right angles

Question 24

What is another name for the cross-cylinder?

Answer 24

Jackson cross-cylinder

Question 25

What are 2 uses of the Jackson cross-cylinder?

Answer 25

1. toric lens used to check orientation of the trial cylinder by superimposing it with its axis lying oblique to the axis of the trial cylinder
2. can also check **power ** of the cylinder by superimposed further cylinders of varying power and sign in the same axis as the trial cylinder

Question 26

How is the Jackson cross-cylinder set up?

Answer 26

• the lens is mounted on a handle which is placed at 45 degrees to the axes of the cylinders (see image)
• the axes marked on the lens are the axes of no power of the individual cylinders
• the power of each cylinder therefore lies at **90 degrees to the marked axis **and coincides with the marked axis (of no power) of the other cylinder (of opposite sign)

Question 27

How do the axes marked on the Jackson cross-cylinder and the axes of refractive power relate to each other?

Answer 27

those marked on the cylinder are in opposite positions from the axes of refractive power

Question 28

How are cross-cylinders named?

Answer 28

by the power of the cylinder (marked on the handle)

e.g. in image would be a 1.00 dioptre cross cylinder (-0.5 DS/+1.0 DC)

Question 29

What are the 2 powers in which cross cylinders are available?

Answer 29

+0.5 and +1.0 D

Question 30

What is the function of the 1.00 D cross-cylinder? 2 things

Answer 30

• used to check the axis of the trial cylinder
• and the power in patients with poor visual acuity

Question 31

What is the function of the 0.5 D corss-cylinder?

Answer 31

used to check the power of the trial cylinder where the patient has good vision

Question 32

What test type must a patient look at during use of the Jackson cross cylinder and why?

Answer 32

test type 2 lines above the smallest they can see
because cross-cylinder blurs the vision and larger letters are used to make discrimination between positions of the cross-cylinder easier for the pt

Question 33

How is the cross-cylinder used to check the axis?

Answer 33

• cross-cylinder held before eye with handle in line with the axis of the trial cylinder; then turned over, and patient asked which position gives a better result
• cross cylinder is held in preferred position and axis of trial cylinder rotated slightly towards axis of same sign on the cross-cylinder
• process repeated until trial cylinder is in correct axis for the eye, at which time rotation of cross cylinder will offer equally unacceptable visual alterations

Question 34

How is the cross-cylinder used to check power of the trial cylinder?

Answer 34

cross-cylinder is held first in one axis then the other overlying the trial cylinder - this increases then decresing the power of the trial cylinder

Question 35

How can a cross cylinder be used to confirm the absence of a cylinder?

Answer 35

• cross-cylinder is offered as an addition to the trial sphere in four different orientations, with its + axis at 90°, 180°, 45°, and 135°.
• If the patient prefers one of these options to the sphere alone, a cylindrical correction is necessary.
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Question 36

What is lenticular astigmatism?

Answer 36

astigmatism caused by the lens (rather than cornea); keratometry readings may not show significant difference but power cross demonstrates difference in different meridia

Question 37

How do you calculate lenticular astigmatism?

Answer 37

difference between overall manifest refraction which shows overral amount of astigmatism and keratometry readings (showing any corneal astigmatism)

Question 38

How are cross-cylinders named?

Answer 38

by the power of the cylinder (marked on the handle)

e.g. in image would be a 1.00 dioptre cross cylinder (-0.5 DS/+1.0 DC)