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 Refractive components of the eyes Air n =1 Cornea: n = 1.37 Anterior Chamber (aqueous humour): n = 1.33 Crystalline Lens: n = 1.386 -1.41 Vitreal Chamber (vitreous humour): n = 1.33 Fovea Visual axis Axial length Iris Optic Nerve Retina Emmetropia: Parallel light from infinite object focussed on the retina Object at infinity 4 Refractive power and axial length perfectly matched: image clear, no corrective lens required Parallel light rays 1 Light refracted by cornea and lens i.e. refractive power (converging, convex , +ve) 2 Convergent light rays 3 Light focussed on retina Myopia: Parallel light from infinite object focussed in front of the retina Object at infinity 1 Refracted by cornea and lens 4 Either refractive power too strong or axial length too long Parallel light rays 5 negative, concave, diverging, corrective lens required to push focus back onto the retina 2 Focussed in front of retina 3 Blur circle at retina Hypermetropia: Definition: Parallel light from infinite object focussed behind retina Object at infinity 1 Refracted by cornea and lens 4 Either refractive power too weak or axial length too short Parallel light rays 5 Positive, convex, converging corrective lens required to pull focal point forward to retina 2 Light focussed behind retina 3 Blur circle at retina Summary of spherical refractive errors Myopia: focussed in front of retina: requires –ve lens for correction Object at infinity Hypermetropia: focussed behind retina: requires +ve lens for correction Parallel light rays Emmetropia: focussed on retina: no corrective lens required Accommodation: As object comes nearer: • Emmetrope becomes blurred • Hypermetrope becomes more blurred • Myope becomes clearer (until focussed • on retina, then becomes blurred) 1 Object at infinity 2 Near object 1 Parallel light 2 Divergent light Refractive power unchanged 3 Focal point shifted backwards 4 Blur circle at retina 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 4 Distant object now forms blur circle at retina Near object: Divergent light 1 Increase refractive power by increasing lens power (cornea remains unchanged) 2 Becomes more convergent Note: must now alter definitions of emmetropia, myopia and hypermetropia to include the phrase “when accommodation is relaxed” Distribution of refractive errors 70 60 50 % 40 30 20 10 +6 to +7 > +7 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +3 +3 to +4 +4 to +5 +5 to +6 > -8 -8 to -7 -7 to -6 -6 to -5 -5 to -4 -4 to -3 -3 to -2 0 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. Development of refractive error In 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 hypermetropic 18% between +4.00 and +12.00DS 25% 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 error For 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. 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 Observer Subject Outgoing light Incoming light Mirror with central hole 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 Optical principles Clinician Patient S2 S1 The light in the pupil is called the “ret reflex” So Reflex stage: neutral Clinician S1 Neutral position: Far point conjugate with observers nodal point. No movement of reflex, sudden change from red reflex to no reflex. Patient S2 Mirror tilts forwards S2 No effect on reflex Mirror tilts further forwards No S2 Reflex disappears The final goal Non-neutral point: With movement Observer S1 Far point behind observers pupil Far point behind observers pupil. With movement of reflex, gradual change from red reflex to no reflex S2 Within pupil Mirror tilts forwards Outside pupil Subject S2 Reflex moves down, i.e. with direction of movement of light Mirror tilts further forwards Reflex disappears No S2 With movement Non-neutral point: Against movement Observer S1 Within pupil Far point in front of observers pupil. Against movement of reflex, gradual change from red reflex to no reflex S2 Far point in front of observers pupil Mirror tilts forwards Outside pupil Subject S2 Reflex moves up, i.e. against direction of movement of light Mirror tilts further forwards Reflex disappears No S2 Against movement Degree of ametropia and the reflex 1 2 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 Ret reflex can tell us a lot Reflex Observation Meaning Brightness Dim Far from Rx Bright Close to Rx Narrow Far from Rx Wide Close to Rx With Need more plus Against Need more minus Slow Far from Rx Fast Close to Rx Streak size Movement direction Movement speed 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 Working distance Clinician We now have neutral We have added lenses Patient S2 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 So to get neutral, we needed: lens power = Rx + 1/d To get the right prescription we need to compensate Rx = lens power – 1/d Working distance compensation Calculation Rx = lens power - 1/d Working distance lens Before you begin, add a 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 “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 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 +1.50 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 Neutral Reversal • 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” • 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 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 4.5-4.7 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!