Transcript File - Optometry Peer Tutoring

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
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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!