1. Retinoscopy OP1201 – Basic Clinical Techniques Part 2 - Astigmatism
Dr Kirsten Hamilton-Maxwell

2. 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

3. 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

4. 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

5. Note: orientation of line foci will change with cyl. axis,
separation will change with cyl. power.
+ cyl DC,
axis vertical (900)
+ cyl DC,
axis horizontal (1800) =
+1.00/+1.00 X 90
(+2.00/-1.00 X 180)
sphero-cylinder +
Astigmatic cone
Circle of
least
confusion
Note: vertical power gives horizontal line focus,
horizontal power gives vertical line focus

6. Simple myopic astigmatism

7. Simple hypermetropic astigmatism

8. Compound myopic astigmatism

9. Compound hypermetropic astigmatism

10. Mixed astigmatism

11. What does it look like?

12. Distribution of astigmatism
Power
Axis
1/3 of all prescriptions are 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
With the rule: axis within 15 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.

13. 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.

14. “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

15. Ret for astigmatism What does the reflex look like Finding the axis
Finding the power
Recording your results

16. 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

17. 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 minus-cyl until corrected

18. In 3D Retina behind astigmatic cone: compound myopic astigmatism.
Against in all directions.
-sph., –cyl.x90 (-sph., +cyl.x180)
This example is against the rule astigmatism
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)
Retina in front of astigmatic cone:
compound hypermetropic astigmatism.
With movement in all directions.
+ sph., –cyl.x90 (+ sph., +cyl.x180)
Always use –cyl, i.e. not the option in brackets: move posterior focal line onto retina with sphere, collapse anterior backwards with –ve cyl.

19. Identifying astigmatism
Oblique movement

20. 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

21. 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?

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. Scissors movement

32. Issues in retinoscopy Controlling accommodation
Optical effects of being off axis
Effect of pupil size

33. 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

34. 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

35. 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

36. Sources of error Not being in the right position Observation errors
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

37. 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

38. Further reading Read Elliott sections 4.5-4.7
Real examples of ret can be found in Elliott Online