OP1201 – Basic Clinical Techniques

Part 1 – Spherical refractive errorDr Kirsten Hamilton-Maxwell

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

to explainTypes of refractive error (ametropia)Accommodation on vision in ametropesDistribution of refraction errorBasic optical principles of retinoscopyHow to perform retinoscopy

By the end of the related practical, you should be able toAssess distance refractive error in one meridian using

retinoscopy within 10min for both eyes

Types of refractive error

Accommodation

Distribution

VitrealChamber(vitreoushumour):n = 1.33

CrystallineLens:n = 1.386 -1.41

Anterior Chamber(aqueous humour):n = 1.33

Cornea:n = 1.37

Airn = 1

Visual axis

Iris

Axial length

Retina

OpticNerve

Fovea

Refractive components of the eyes

Emmetropia:Parallel light frominfinite object focussed on the retina

Emmetropia:Parallel light frominfinite object focussed on the retina

Object atinfinity

Parallel light rays

1 Light refracted by cornea and lens i.e. refractive power(converging, convex , +ve)

3 Light focussed on retina

4 Refractive power and axial lengthperfectly matched:image clear, no corrective lens required

2 Convergentlight rays

5 negative, concave, diverging,corrective lens required to push focus back onto the retina

Myopia:Parallel light frominfinite object focussed in front ofthe retina

Myopia:Parallel light frominfinite object focussed in front ofthe retina

Object atinfinity

Parallel light rays

4 Either refractive power too strong or axial length too long

2 Focussed in front of retina

3 Blur circle at retina

1 Refracted by cornea and lens

5 Positive, convex, convergingcorrective lens required to pull focal point forward to retina

Hypermetropia:Definition:Parallel light frominfinite object focussed behind retina

Hypermetropia:Definition:Parallel light frominfinite object focussed behind retina

Object atinfinity

Parallel light rays

4 Either refractive power too weak or axial length too short

1 Refracted by cornea and lens

2 Light focussed behind retina

3 Blur circle at retina

Object atinfinity

Parallel light rays

Emmetropia: focussed on retina:no corrective lens required

Myopia:focussed in front of retina:requires –ve lens for correction

Hypermetropia:focussed behind retina:requires +ve lens for correction

Summary of spherical refractive errorsSummary of spherical refractive errors

1 Object atinfinity

4 Blur circle at retina

Accommodation:As object comes nearer:•Emmetrope becomes blurred•Hypermetrope becomes more blurred•Myope becomes clearer (until focussed •on retina, then becomes blurred)

Accommodation:As object comes nearer:•Emmetrope becomes blurred•Hypermetrope becomes more blurred•Myope becomes clearer (until focussed •on retina, then becomes blurred)

1 Parallellight

2 Nearobject

2 Divergent light

Refractive powerunchanged

3 Focal pointshifted backwards

Accommodation allows: •Emmetrope to focus on near object (clear near and distance: but not at the same time!):•Hypermetrope to focus on distant and (to lesser extent) near objects (clear distance and less-so near: as degree of hypermetropia increases, distance also becomes blurred).•Myope: only need to accommodate if object is closer than their far-point (clear near, distance blurred)

Accommodation allows: •Emmetrope to focus on near object (clear near and distance: but not at the same time!):•Hypermetrope to focus on distant and (to lesser extent) near objects (clear distance and less-so near: as degree of hypermetropia increases, distance also becomes blurred).•Myope: only need to accommodate if object is closer than their far-point (clear near, distance blurred)

3 Point of focus shifts forwards, blur circle decreases until point acheived

2 Becomes moreconvergent1 Increase refractive power by

increasing lens power(cornea remains unchanged)

4 Distant object now forms blur circle at retina

Note: must now alter definitions of emmetropia, myopia and hypermetropia to include the phrase “when accommodation is relaxed”Note: must now alter definitions of emmetropia, myopia and hypermetropia to include the phrase “when accommodation is relaxed”

Near object: Divergent light

Distribution of refractive errors

Average of 3 surveys, 8187 eyes. Aged 20-35, (mostly 20-23).

Distribution is skewed : 70% of subjects have 0 to +2.00 hypermetropia.

Distribution is concentrated towards small errors Implies active developmental

mechanism which reduces refractive error

Emmetropization Gender differences in spherical

ametropia: Myopia more common in women

than men (after childhood) e.g. 34.1% and 24.6% respectively.

Ethnic differences in spherical ametropia:

Myopia more common in Jewish, Japanese and Chinese people.

0

10

20

30

40

50

60

70

> -

8

-8 to

-7

-7 to

-6

-6 to

-5

-5 to

-4

-4 to

-3

-3 to

-2

-2 to

-1

-1 to

0

0 to

+1

+1

to +

2

+2

to +

3

+3

to +

4

+4

to +

5

+5

to +

6

+6

to +

7

> +

7

%

Development of refractive errorIn neonates (babies), spherical error shows a relatively normal

distribution with slight skewness towards hypermetropia. Average of +1.50 DS, standard deviation +/-2.00 DS.57% up to +4.00 DS hypermetropic18% between +4.00 and +12.00DS25% being myopic up to -12.00 DS.

Range reduces rapidly through first two years of life (SD +/-1.00 DS at 1 year, +/-0.75 DS at 6 years).

Mean shifts slowly towards emmetropia (emmetropization).Becomes non-normal distribution: percentage of low refractive

errors high, percentage of higher refractive errors become increasingly low.

Development of refractive errorFor the adult eye:

Tendency for hypermetropia to reduce until 25-30 years old (troughs at -0.50 to +0.75 DS, depending on the study)

Then slight tendency to increase in hypermetropia to the age of 65-75* (peaks at +1.50 DS)

Then relatively rapid decrease in hypermetropia/descent into myopia over the age of 75.

Complex pattern due to age related changes in various components of the eye (namely cornea and crystalline lens)

*High myopes follow a different pattern; if they change at all, they tend towards higher myopia.

What is retinoscopy?The retinoscope

Optical principles – how does it work?The ret reflex

RetinoscopyObjective measurement of refractive error

Starting point for subjective refractionUsed to prescribe where subjective refraction can’t be

performedScreening for ocular disease

Keratoconus, media opacitiesSpecialist retinoscopy (next year)

Accommodation stabilityAccommodative lag

The retinoscopeEyepieceLight source

Spot or streak bulb

CollarMoves up and down to change the

vergence of the lightRotates to change the angle of the

beam

On/off/brightness control

A retinoscope head

Mirror with central hole

SubjectObserver

Incoming light

Outgoing light

Variable condensing lens

How does it work?An offshoot of ophthalmoscopy

For ophthalmoscopy, purpose is to inspect the fundusFor retinoscopy, purpose is to inspect light moving

across the fundus

Clinician watches shape and movement of the light within the pupil (the ret reflex)

Trial lenses added until the shape and movement reach a state called “reversal”

This provides an estimate of refractive error

Clinician

So

S1S2

Patient

The light in the pupil is called the “ret reflex”

Reflex stage: neutral

Clinician Patient

No effect on reflex

Reflex disappears

Mirror tilts forwards

Mirror tilts further forwards

S2

S2

No S2

S1

Neutral position:Far point conjugate with

observers nodal point.No movement of reflex,sudden change from red

reflex to no reflex.

The final goal

Observer Subject

Reflex disappears

Reflex moves down,i.e. with directionof movement of light

Mirror tilts further forwards

Mirror tilts forwards

S2

S2

Far point behindobservers pupil

Within pupil

Outside pupil

No S2

S1

Non-neutral point: With movement

Far point behind observers pupil.

With movement of reflex, gradual change from red reflex

to no reflex

With movement

Observer Subject

Far point in front ofobservers pupil

Reflex disappears

Reflex moves up,i.e. against directionof movement of light

Mirror tilts forwards

Mirror tilts further forwards

S2

S2

Within pupil

Outside pupil

No S2

S1

Non-neutral point: Against movement

Far point in front of observers pupil.Against movement of reflex,

gradual change from red reflex to no reflex

Against movement

Degree of ametropia and the reflex

As subject becomes more myopic, the cone of light becomes widerGreater portion of light falls outside of practitioner’s pupil, so dimmer reflexGreater excursion before reflex lost, so movement of reflex from seen to not seen becomes slower

1

2

Ret reflex can tell us a lotReflex Observation Meaning

Brightness Dim Far from Rx

Bright Close to Rx

Streak size Narrow Far from Rx

Wide Close to Rx

Movement direction With Need more plus

Against Need more minus

Movement speed Slow Far from Rx

Fast Close to Rx

The final goalNot quite!The general concept of ret is that we use lenses add

lenses until we see “reversal” and then tweak this until we see “neutral”

However, negative vergence is introduced because we are not located at infinityLet’s see what this is, and what we can do about it

Clinician

S2

Patient

We now have neutral

We have also introduced negative vergence due to our working distance (WD)

= 1/d (m)Where d = distance in m, measured between your ret and patient’s eye

We have added lenses

To get the right prescriptionwe need to compensateRx = lens power – 1/d

So to get neutral, we needed: lens power = Rx + 1/d

Working distance compensationCalculation Working distance lens

For example, if neutrality is achieved with a +3.00DS lens and your working distance is 50cm

Rx = +3.00DS – (1/0.50)= +3.00 – 2.00=+1.00DS

Before you begin, add a “working distance lens”

A +ve lens to cancel out the negative vergence

As in eg., WD = 50cmWD lens = 1/0.50 = +2.00DSNeutrality for the same

patient is still +3.00DSWD lens = +2.00DSLens power = +1.00DS

Rx = lens power - 1/d

WD: Calculation vs lens?For the calculation method

Brighter image because there are less reflecting surfaces (lose 8% of light per lens)

Need to be able to do the calculation in your headNeed to physically change spherical lens in the trial

frame before you begin subjective refractionFor the working distance lens

Acts as a fogging lens to help control accommodationStill need to remove WD-compensation prior to

subjective refraction but this is easier because it is a separate lens

When to do ret

The steps to follow

(A simplified procedure to get you started)

When should I do it?Everyone!It is an objective test - it does not need any input

from the patientMay be the only way of determining refractive

error for non-communicative or non-cooperative patientsChildrenNon-English speakingLearning difficultiesMalingerersLow visionLaboratory animals

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)Lens power = 1/distance between retinoscope and your

patients eye in metresRoughly the length of your outstretched armMost of you will use a lens between +1.50 and +2.00DS

Illuminate a non-accommodative targetUsually the duochrome – ask your patient to look at the green

lightTurn room lights off

ProcedureHold the retinoscope in you right hand and RE

for patient’s RE (swap for LE)Position yourself at your correct working

distance Stretch out your left arm and touch the trial frame

This is your working distance (WD) – always check this

Get as close to your patient’s visual axis as possibleYour head should almost be in the way!Confirm this by asking your patient

ProcedureMove the retinoscope collar so it is at the bottomTurn the retinoscope on and rotate the collar so

the light is verticalMore information about using other positions in the

next lecture

Shine the light into your patient’s right eye and observe the red reflexIs it bright/dim, wide/narrow?ie. is there a high refractive error?

ProcedureNow move the light from side to side 3 or 4

timesWith/against, fast/slow?Is your patient myopic or hypermetropic?Is the refractive error high or low?

Add trial lenses in 1DS steps to the front cell of the trial frame until you see reversal ie. The light moves in the opposite direction)

The refractive error will be somewhere between the first lens that shows with movement, and the first lens that shows against movementThen try to find neutrality using smaller steps

What lens to start with?Other than amount and speed of movement seen

with ret…Symptoms

Myopia or hypermetropiaWhat does the patient know about their Rx?Vision

Current spectacles (if available)Ophthalmoscopy result

ProcedureRepeat for the left eyeAlways go back to check the right eye again

Especially if hypermetropic in either eyeNeed to control accommodation

Remove working distance lens, check visionRecord lens power and vision

If you suspect hypermetropia, you need to check that the LE is fogged BEFORE you begin retting the RE.

Add extra plus until you see against movement in the LE.

Checking your resultWhen neutrality approaches, reflex movement becomes

too fast to judgeIf your have correctly neutralised your patient,

Move towards your patient, effectively moving far point behind observers pupil, you will see “with” movement

Move away from your patient, effectively moving far point in front of observers pupil, you will see “against” movement

If not, the movement will not be opposite in different positions so make small changes to the power (0.50DS) until you can see what I’ve described above

You could achieve the same result by adding ±0.50DS and checking that the movement is opposite

Read Elliott sections 4.5-4.7Bennett and Rabbetts: The eye’s optical system

Real examples of ret can be found in Elliott Online – it’s important to look at these before your next prac!