Transcript File - Optometry Peer Tutoring

Retinoscopy
OP1201 – Basic Clinical Techniques
Part 2 - Astigmatism
Dr Kirsten Hamilton-Maxwell
Today’s goals
 By the end of today’s lecture, you should be able
to
 Describe the major types of regular astigmatism
 Explain key issues in retinoscopy
 Describe how to perform retinoscopy in a patient with
astigmatism
 Be aware of procedural adaptations for difficult cases
 By the end of the related practical, you should be
able to
 Assess distance refractive error in both meridians
using retinoscopy, within 10min for both eyes
Astigmatism
 Astigmatism 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 astigmatism
 Two powers and an axis
 Power 1 = most positive (or least negative) meridian
 Power 2 = least positive (or most negative) meridian
 Axis = 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
Note: orientation of line foci will change with cyl. axis,
separation will change with cyl. power.
+ cyl. +2.00 DC,
axis vertical (900)
+ cyl. +1.00 DC,
axis horizontal (1800)
+1.00/+1.00 X 90
(+2.00/-1.00 X 180)
sphero-cylinder
+
=
Note: vertical power gives horizontal line focus,
horizontal power gives vertical line focus
Circle of
least
confusion
Astigmatic cone
Simple myopic astigmatism
Simple hypermetropic astigmatism
Compound myopic astigmatism
Compound hypermetropic
astigmatism
Mixed astigmatism
What does it look like?
Distribution of astigmatism
Power
Axis
 1/3 of all prescriptions are
 With the rule: axis within 15




spherical
1/3 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
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 astigmatism
 As 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
format
 Spectacle prescriptions by optometrists are always
written in sphero minus cyl format
Ret for astigmatism
What does the reflex look like
Finding the axis
Finding the power
Recording your results
Astigmatism
 As 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 orientation
 The axis
 Retinoscopy can measure the powers of both
meridians and determine the axis
Correction of astigmatism
 To correct astigmatism, we need a lens that has a
different power in different meridians
 Cylindrical 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 sphere
 At exactly 90deg to that (always 90deg), add a minuscyl until corrected
In 3D
This example is
against the rule
astigmatism
Retina behind astigmatic cone:
compound myopic astigmatism.
Against in all directions.
-sph., –cyl.x90 (-sph., +cyl.x180)
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 at circle of least confusion: best vision
Retina at front (vertical) line focus:
simple hypermetropic astigmatism.
With vertically, neutral horizontally.
+ sph., –cyl.x90 (or +cyl.x180 only)
Always use –cyl, i.e. not the option in
brackets: move posterior focal line onto
retina with sphere, collapse anterior
backwards with –ve cyl.
Retina in front of astigmatic cone:
compound hypermetropic astigmatism.
With movement in all directions.
+ sph., –cyl.x90 (+ sph., +cyl.x180)
Identifying astigmatism
Oblique movement
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)
 Illuminate a non-accommodative target
 Usually the duochrome
 Turn room lights off
Procedure
 Turn retinoscope to brightest setting, with collar
at the bottom
 Scan along 90 and 180deg to quickly check
adequate fogging in both eyes
 There should be against movement in both eyes
(accommodation control)
 WD lens provides some fog but it will not be enough in
many hypermetropes
 Quick guesstimate of refractive error
 Reflex brightness? With or against movement?
Astigmatism?
Finding the axis
 Return the light to vertical and focus light to
thinnest beam on the face using collar
 Is 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 meridians
 Does the reflex move along the same axis?
 If there is oblique movement, further rotation is
required
Finding the sphere power
 Return the collar to the bottom
 Find 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 week
 As you have found the most hypermetropic meridian,
you’ll be adding plus (or reducing minus)
 Check for reversal
 Refine in smaller steps until neutrality
Finding the cyl power
 Rotate the beam 90deg to the other primary
meridian
 You should see against movement
 Fast = low astigmatism
 Slow = high astigmatism
 Confirm no oblique movement
 Neutralise 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 steps
 Repeat all steps for the LE
 Return to the RE to recheck that you do not need to
add more positive power
 Remove WD lens from both eyes
 Check vision monocularly and record
 Should be within ±0.50D in both meridians and
within 15deg of the axis
 Complete both eyes within 10min
Recording results
 You 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
110deg
 The 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 example
 You 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 horizontal
 Most of your classmates will have this
Another example
 You 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” astigmatism
 Axis within 15deg of vertical
 Some of your classmates will have this
Overcoming problems
 Reflex 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 difficult
 Move closer, try dimmer lighting, or consider use of
tropicamide to dilate pupil
 Asphericity of cornea/lens can result in change in power
with increased distortion in the peripheral pupil
 Concentrate on centre of ret reflex
Overcoming problems
 Lenticular or corneal opacities will make reflex
dimmer
 Slide 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 distortions
 e.g. keratoconus and cataract, which may produce
scissors movement
Scissors movement
Issues in retinoscopy
Controlling accommodation
Optical effects of being off axis
Effect of pupil size
Controlling accommodation
 Intraocular lens can change in shape and thus
change the power of the eye
 Accommodation system is particularly strong/unstable
in young people so needs to be controlled
 The WD lens is part of the solution
 Vision becomes worse if accommodates, so patients
tend to avoid doing this
 Longest working distance possible
 Use non-accommodative target
 Green(? by convention) light on duochrome
Being on axis
 Oblique 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 size
 Small 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 patients
 Large pupils suffer from peripheral aberration
 Look only at the centre
 This will commonly be an issue for your younger patients
Sources of error
 Not being in the right position
 Incorrect working distance (getting too close is most
common)
 Head blocking the patient’s view
 Off axis
 Observation errors
 Failure to obtain reversal
 Failure to locate principal meridian
 Paying too much attention to peripheral movement
with a large pupil
Sources of error
 Not fogging appropriately
 Forgetting to account for the WD lens, or not
removing it when you are finished
 Patient not looking at an appropriate target
Further reading
Read Elliott sections 4.5-4.7
Real examples of ret can be found in Elliott Online