OP1201 – Basic Clinical Techniques

Part 2 - AstigmatismDr Kirsten Hamilton-Maxwell

Today’s goalsBy the end of today’s lecture, you should be able

toDescribe the major types of regular astigmatismExplain key issues in retinoscopyDescribe how to perform retinoscopy in a patient with

astigmatismBe aware of procedural adaptations for difficult cases

By the end of the related practical, you should be able toAssess distance refractive error in both meridians

using retinoscopy, within 10min for both eyes

AstigmatismAstigmatism means “not spherical”You will find yourself describing it to

patients as “your eye is shaped like a rugby ball instead of a football”

The difference in curvature (usually of the cornea or crystalline lens) results in the eye having two different powers along two different meridians

In regular astigmatism, the two meridians are exactly 90deg apart

Describing astigmatismTwo powers and an axisPower 1 = most positive (or least negative) meridianPower 2 = least positive (or most negative) meridianAxis = the orientation of the flattest side of the rugby

ball. More specifically, orientation of the least positive (most negative) meridian. Lying on its side = Axis 180 and sitting on its point =

Axis 90

Hint: Look at a trial frame

+ cyl. +2.00 DC,axis vertical (900)

+ cyl. +1.00 DC,axis horizontal (1800)

+

Astigmatic cone

Circle ofleastconfusion=

+1.00/+1.00 X 90(+2.00/-1.00 X 180)sphero-cylinder

Note: orientation of line foci will change with cyl. axis,separation will change with cyl. power.

Note: vertical power gives horizontal line focus,horizontal power gives vertical line focus

Simple myopic astigmatism

Simple hypermetropic astigmatism

Compound myopic astigmatism

Compound hypermetropic astigmatism

Mixed astigmatism

What does it look like?

Distribution of astigmatismPower Axis1/3 of all prescriptions are

spherical1/3 contain an astigmatic

correction of 0.25 to 0.50DC 1/6 contain an astigmatic

correction of 0.75 to 1.00DC remaining 1/6 contain an

astigmatic correction of over 1.00DC

1% contain an astigmatic correction of > 4.00DC

With the rule: axis within 15 either side of horizontal (38%)

Against the rule: Axis within 15 either side of vertical (30%) respectively

All other axes considered as oblique (32%)

Prevalence of oblique astigmatism is unaffected by power, but with the rule becomes more prevalent (and therefore against the rule less prevalent) as astigmatic power increases.

More on astigmatismAs a rule, astigmatism is equal and symmetrical across

the two eyes.Degree of astigmatism is unrelated to spherical errors

between + and -8.00DS. Beyond these values, higher spherical refractive error is associated with higher astigmatic errors.

Can consider +/-8.00DS range as ‘normal’ eyes with ‘normal’ refractive errors.

Errors beyond +/-8.00DS can be considered ‘abnormal’.Higher errors of both spherical and astigmatic type are

increasingly associated with ocular pathology.

“Homework”See what you can find out about how astigmatism

changes with age. In particular:Many babies are born with astigmatism. How much

would be considered “normal” and how does it change in the first 2 years of life?

Why does “against the rule” astigmatism become more common in older patients?

Please revise your Dispensing notes on sphero-cyl formatSpectacle prescriptions by optometrists are always

written in sphero minus cyl format

What does the reflex look likeFinding the axis

Finding the powerRecording your results

AstigmatismAs we have just discussed, the eye can be a

different power along different meridians (in different directions)Astigmatism

The primary meridians are always 90deg apart, but can be in any orientationThe axis

Retinoscopy can measure the powers of both meridians and determine the axis

Correction of astigmatismTo correct astigmatism, we need a lens that has a

different power in different meridiansCylindrical lens, abbreviation DC

When doing ret, we will scan and then correct each of the meridians separately

The (eventual) idea is… Find and then correct the most positive (least

negative) meridian first with a sphereAt exactly 90deg to that (always 90deg), add a minus-

cyl until corrected

Retina at front (vertical) line focus:simple hypermetropic astigmatism.With vertically, neutral horizontally.+ sph., –cyl.x90 (or +cyl.x180 only)

Retina at rear (horizontal) line focus:simple myopic astigmatism.Neutral vertically, against horizontally.-cyl.x90 only (or –sph. then +cyl.x180)

Retina in between line foci:mixed astigmatism.With vertically, against horizontally.+ sph., –cyl.x90 (or –sph., +cyl.x180)

Retina in front of astigmatic cone:compound hypermetropic astigmatism.With movement in all directions.+ sph., –cyl.x90 (+ sph., +cyl.x180)

Retina behind astigmatic cone:compound myopic astigmatism.Against in all directions.-sph., –cyl.x90 (-sph., +cyl.x180)

In 3DIn 3D

Retina at circle of least confusion: best vision

This example is against the rule astigmatism

Always use –cyl, i.e. not the option in brackets: move posterior focal line onto retina with sphere, collapse anterior backwards with –ve cyl.

Identifying astigmatism

Oblique movement

Set upMeasure your patient’s pupillary distance (PD) Dial your patient’s PD into the trial frame and fit it to

your patient’s facePlace a working distance (WD) lens in the back cell

for the trial frame (if using)Illuminate a non-accommodative target

Usually the duochromeTurn room lights off

ProcedureTurn retinoscope to brightest setting, with collar

at the bottomScan along 90 and 180deg to quickly check

adequate fogging in both eyesThere should be against movement in both eyes

(accommodation control)WD lens provides some fog but it will not be enough in

many hypermetropesQuick guesstimate of refractive error

Reflex brightness? With or against movement? Astigmatism?

Finding the axisReturn the light to vertical and focus light to

thinnest beam on the face using collarIs the beam in the pupil aligned with the beam on the

face?Rotate until they are

This will occur in two positions These are the primary meridians

Scan along the primary meridiansDoes the reflex move along the same axis?If there is oblique movement, further rotation is

required

Finding the sphere powerReturn the collar to the bottomFind the most hypermetropic meridian

Slowest “with” or fastest “against”This assumes you are using minus cyls (some

textbooks talk about plus cyl refraction)Neutralise the most hypermetropic meridian first

Use the bracketing technique from last weekAs you have found the most hypermetropic meridian,

you’ll be adding plus (or reducing minus)Check for reversalRefine in smaller steps until neutrality

Finding the cyl powerRotate the beam 90deg to the other primary

meridianYou should see against movement

Fast = low astigmatism Slow = high astigmatism

Confirm no oblique movementNeutralise this meridian using minus spheres

This is an intermediate step!You can, and should, use cyls

Replace the sphere with a minus cyl of the same power, with the axis lined up with your beam

All meridians should now be neutralised

The final stepsRepeat all steps for the LEReturn to the RE to recheck that you do not need to

add more positive powerRemove WD lens from both eyesCheck vision monocularly and recordShould be within ±0.50D in both meridians and

within 15deg of the axisComplete both eyes within 10min

Recording resultsYou will now have used two different sphere powers at

two primary meridians (not including the WD lens)For example: +2.00DS axis 20deg and an additional -1.50DS axis

110degThe highest positive power becomes the sphere power

(+2.00DS)The amount of astigmatism is recorded as cylinder, and is

the difference between the power of the two primary meridians (-1.50DC)

The axis is the position of the beam in the most negative/least positive meridian (110deg)

Result: +2.00DS/-1.50DCx110

Another exampleYou have found

RE -1.00DS axis 90 and an additional -2.00DS axis 180 Sphere power = -1.00DS Cyl power = -2.00DS Axis = 180deg -1.00DS/-2.00DCx180

This is called “with the rule astigmatism”Axis within 15deg of horizontalMost of your classmates will have this

Another exampleYou have found

RE +1.00DS axis 180 and an additional -4.00DS axis 90 Sphere power = +1.00DS Cyl power = -4.00DS Axis = 90deg +1.00DS/-4.00DCx90

This is called “against the rule” astigmatismAxis within 15deg of verticalSome of your classmates will have this

Overcoming problemsReflex is very dim in high prescriptions

Use high powered lenses to see if reflex becomes brighter and movement more obvious

Also look out for differences in brightness in different meridians because this means high astigmatism

Small pupil makes retinoscopy and ophthalmoscopy more difficultMove closer, try dimmer lighting, or consider use of

tropicamide to dilate pupilAsphericity of cornea/lens can result in change in power

with increased distortion in the peripheral pupilConcentrate on centre of ret reflex

Overcoming problemsLenticular or corneal opacities will make reflex

dimmerSlide collar up (but watch how far) and/or move closer

(change WD lens to compensate for change in working distance)

Reflex may become distorted with lenticular or corneal opacities or distortionse.g. keratoconus and cataract, which may produce

scissors movement

Scissors movement

Controlling accommodation

Optical effects of being off axis

Effect of pupil size

Controlling accommodationIntraocular lens can change in shape and thus

change the power of the eyeAccommodation system is particularly strong/unstable

in young people so needs to be controlledThe WD lens is part of the solution

Vision becomes worse if accommodates, so patients tend to avoid doing this

Longest working distance possibleUse non-accommodative target

Green(? by convention) light on duochrome

Being on axisOblique astigmatism is induced if retinoscopy is

performed more than 5deg from the visual axis-0.50DCx90 induced if 10deg from visual axis along

the horizontal

Check that you are almost blocking the fixation target with your head, both horizontally and vertically

Completely blocking the target will induce accommodation

Effect of pupil sizeSmall pupils limit the visibility of the reflex

Use dimmest beam possible (to decrease constriction due to light)

Use shorter working distance Don’t forget to use a different working lens Short WD also helps if your patient has a dim reflex. Eg.

Cataracts

This will commonly be an issue for your older patientsLarge pupils suffer from peripheral aberration

Look only at the centreThis will commonly be an issue for your younger patients

Sources of errorNot being in the right position

Incorrect working distance (getting too close is most common)

Head blocking the patient’s viewOff axis

Observation errorsFailure to obtain reversalFailure to locate principal meridianPaying too much attention to peripheral movement

with a large pupil

Sources of errorNot fogging appropriatelyForgetting to account for the WD lens, or not

removing it when you are finishedPatient not looking at an appropriate target

Read Elliott sections 4.5-4.7

Real examples of ret can be found in Elliott Online