Retinoscopy OP1201 – Basic Clinical Techniques Part 1 – Spherical refractive error Dr Kirsten Hamilton-Maxwell

Today’s goals By the end of today’s lecture, you should be able to explain Types of refractive error (ametropia) Accommodation on vision in ametropes Distribution of refraction error Basic optical principles of retinoscopy How to perform retinoscopy By the end of the related practical, you should be able to Assess distance refractive error in one meridian using retinoscopy within 10min for both eyes

Refractive error Types of refractive error Accommodation Distribution

Vitreal Chamber (vitreous humour): n = 1.33 Crystalline Lens: n = Anterior Chamber (aqueous humour): n = 1.33 Cornea: n = 1.37 Air n = 1 Visual axis Iris Axial length Retina Optic Nerve Fovea Refractive components of the eyes

Emmetropia: Parallel light from infinite object focussed on the retina Emmetropia: Parallel light from infinite object focussed on the retina Object at infinity 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 length perfectly matched: image clear, no corrective lens required 2 Convergent light rays

5 negative, concave, diverging, corrective lens required to push focus back onto the retina Myopia: Parallel light from infinite object focussed in front of the retina Myopia: Parallel light from infinite object focussed in front of the retina Object at infinity 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, converging corrective lens required to pull focal point forward to retina Hypermetropia: Definition: Parallel light from infinite object focussed behind retina Hypermetropia: Definition: Parallel light from infinite object focussed behind retina Object at infinity 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 at infinity 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 errors

1 Object at infinity 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 Parallel light 2 Near object 2 Divergent light Refractive power unchanged 3 Focal point shifted 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 more convergent 1 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” 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 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.

Development of refractive error In neonates (babies), spherical error shows a relatively normal distribution with slight skewness towards hypermetropia. Average of DS, standard deviation +/-2.00 DS. 57% up to DS hypermetropic 18% between and DS 25% being myopic up to DS. Range reduces rapidly through first two years of life (SD +/ 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 error For the adult eye: Tendency for hypermetropia to reduce until years old (troughs at to DS, depending on the study) Then slight tendency to increase in hypermetropia to the age of 65-75* (peaks at 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.

Retinoscopy – the science What is retinoscopy? The retinoscope Optical principles – how does it work? The ret reflex

Retinoscopy Objective measurement of refractive error Starting point for subjective refraction Used to prescribe where subjective refraction can’t be performed Screening for ocular disease Keratoconus, media opacities Specialist retinoscopy (next year) Accommodation stability Accommodative lag

The retinoscope Eyepiece Light source Spot or streak bulb Collar Moves up and down to change the vergence of the light Rotates to change the angle of the beam On/off/brightness control

A retinoscope head Mirror with central hole Subject Observer Incoming light Outgoing light Variable condensing lens

How does it work? An offshoot of ophthalmoscopy For ophthalmoscopy, purpose is to inspect the fundus For 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 SoSo S1S1 S2S2 Patient Optical principles 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 S2S2 S2S2 No S 2 S1S1 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 direction of movement of light Mirror tilts further forwards Mirror tilts forwards S2S2 S2S2 Far point behind observers pupil Within pupil Outside pupil No S 2 S1S1 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 of observers pupil Reflex disappears Reflex moves up, i.e. against direction of movement of light Mirror tilts forwards Mirror tilts further forwards S2S2 S2S2 Within pupil Outside pupil No S 2 S1S1 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 wider  Greater portion of light falls outside of practitioner’s pupil, so dimmer reflex  Greater excursion before reflex lost, so movement of reflex from seen to not seen becomes slower 1 2

Ret reflex can tell us a lot ReflexObservationMeaning BrightnessDimFar from Rx BrightClose to Rx Streak sizeNarrowFar from Rx WideClose to Rx Movement directionWithNeed more plus AgainstNeed more minus Movement speedSlowFar from Rx FastClose to Rx

The final goal Not 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 infinity Let’s see what this is, and what we can do about it

Clinician S2S2 Patient Working distance 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 prescription we need to compensate Rx = lens power – 1/d So to get neutral, we needed: lens power = Rx + 1/d

Working distance compensation Calculation 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) = – 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 = 50cm WD lens = 1/0.50 = +2.00DS Neutrality for the same patient is still +3.00DS WD lens = +2.00DS Lens 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 head Need to physically change spherical lens in the trial frame before you begin subjective refraction For the working distance lens Acts as a fogging lens to help control accommodation Still need to remove WD-compensation prior to subjective refraction but this is easier because it is a separate lens

Procedure 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 patient May be the only way of determining refractive error for non-communicative or non-cooperative patients Children Non-English speaking Learning difficulties Malingerers Low vision Laboratory animals

Set up Measure your patient’s pupillary distance (PD) Dial your patient’s PD into the trial frame and fit it to your patient’s face Place 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 metres Roughly the length of your outstretched arm Most of you will use a lens between and +2.00DS Illuminate a non-accommodative target Usually the duochrome – ask your patient to look at the green light Turn room lights off

Procedure Hold 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 possible Your head should almost be in the way! Confirm this by asking your patient

Procedure Move the retinoscope collar so it is at the bottom Turn the retinoscope on and rotate the collar so the light is vertical More information about using other positions in the next lecture Shine the light into your patient’s right eye and observe the red reflex Is it bright/dim, wide/narrow? ie. is there a high refractive error?

Procedure Now move the light from side to side 3 or 4 times With/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 movement Then try to find neutrality using smaller steps

The theoretical endpoint for ret where light fills the pupil What you really see is a swirling light Several lenses will give this appearance You will choose the wrong lens if you stop when you first see “neutral” Neutral The real endpoint for ret Keep adding lenses until you see the movement change direction When you see this “reversal”, you know that you are one step too far The previous lens is your endpoint Reversal

What lens to start with? Other than amount and speed of movement seen with ret… Symptoms Myopia or hypermetropia What does the patient know about their Rx? Vision Current spectacles (if available) Ophthalmoscopy result

Procedure Repeat for the left eye Always go back to check the right eye again Especially if hypermetropic in either eye Need to control accommodation Remove working distance lens, check vision Record 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 result When neutrality approaches, reflex movement becomes too fast to judge If 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

Further reading Read Elliott sections Bennett 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!