Chapter 6: Astigmatic Lenses Flashcards by A D

What is astigmatism?

All meridians in the lens do not have the same curvature

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What are the 2 types of astigmatic lenses?

Cylindrical
Toric (sphero-cylindrical)

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What is a cylindrical lens?

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

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

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

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What is a Maddox rod formed of?

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

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What is the Maddox rod used for?

Diagnosis of extraocular muscle imbalance

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How does Maddox rod testing work?

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

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

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

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

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

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How is the Maddox rod used to test muscle balance?

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

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How is horizontal muscle imbalance tested for with the MR?

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

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What is a toric lens?

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

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What is the base curve of a toric lens?

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

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What is another name for toric lenses?

sphero-cylindrical lenses

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

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

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What is the the interval of Sturm?

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

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

plane where two pencils of light formed by toric lens intersect

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

blur circle images

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

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

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

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

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

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

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

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

What is a cross cylinder?

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

What is another name for the cross-cylinder?

Jackson cross-cylinder

What are 2 uses of the Jackson cross-cylinder?

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

How is the Jackson cross-cylinder set up?

* 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)

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

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

How are cross-cylinders named?

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)

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

+0.5 and +1.0 D

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

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

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

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

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

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

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

* 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**

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

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

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

* 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. *

What is lenticular astigmatism?

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

How do you calculate lenticular astigmatism?

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

How are cross-cylinders named?

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)