Retinoscopy and Subjective Refraction Notes

TODAY'S GOALS

• By the end of this lecture, you should be able to:
  • Describe the major types of regular astigmatism.
  • Explain key issues in retinoscopy.
  • Be aware of procedural adaptations for difficult cases.

ASTIGMATISM

• Astigmatism means “not spherical.”
• It can be described 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 90 degrees apart.

DESCRIBING ASTIGMATISM

• Two powers and an axis define astigmatism.
  • 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.

Sphero-cylinder examples

• Examples of recording astigmatism:
  • +1.00/+1.00 \times 90 which is the same as (+2.00/-1.00 \times 180)
  • + cyl. +2.00 DC, axis vertical (90deg)
  • + cyl. +1.00 DC, axis horizontal (180deg)
• Astigmatic cone: Represents the range of focal points in an astigmatic eye.
• Circle of least confusion: The point where the image is least blurred.

Types of Astigmatism:

• Simple Myopic Astigmatism
• Simple Hyperopic Astigmatism
• Compound Myopic Astigmatism
• Compound Hyperopic Astigmatism
• Mixed Astigmatism

What does it look like?

• Astigmatism causes blurred vision at certain orientations.
  • Representation includes:
  • Original
  • Horizontal Focus
  • Vertical Focus
  • Compromise

ASTIGMATISM and Retinoscopy

• The eye can have different powers along different meridians (in different directions).
• Astigmatism the primary meridians are always 90 degrees apart but can be in any orientation.
• The axis is key to defining astigmatism.
• Retinoscopy can measure the powers of both meridians and determine the axis.

CONTROLLING ACCOMMODATION in Retinoscopy

• 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 light (by convention) on duochrome.

BEING ON AXIS during Retinoscopy

• Oblique astigmatism is induced if retinoscopy is performed more than 5 degrees from the visual axis.
  • -0.50DCx90 induced if 10 degrees 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.

OVERCOMING PROBLEMS in Retinoscopy

• 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 the center of the retinal reflex.

OVERCOMING PROBLEMS with Media Opacities

• 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

• Scissors movement is a tricky retinoscopy finding that can be seen in certain conditions.

SOURCES OF ERROR in Retinoscopy

• 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.
• 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.

ASTIGMATISM CORRECTION

• 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 first is corrected with a spherical lens and the second is corrected with a cylindrical lens.
• Find and then correct the most positive (least negative) meridian first with a sphere.
• At exactly 90 degrees to that (always 90 degrees), add a minus-cyl until corrected.

IDENTIFYING ASTIGMATISM

• Look for Oblique movement.

Retinoscopy PROCEDURE

• Turn retinoscope to the brightest setting, with collar at the bottom.
• Scan along 90 and 180 degrees 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 hyperopes.
• Quick guesstimate of refractive error reflex brightness? With or against movement? Astigmatism?

FINDING THE AXIS

• Return the light to vertical and focus light to the thinnest beam on the face using the collar.
• Is the beam in the pupil aligned with the beam on the face?
• Rotate beam until they are (This will occur in two positions and these are the primary meridians)
• Return the collar to the bottom and 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

• Find the most hyperopic meridian, which is the slowest “with” or fastest “against” (SWFA) movement.
  • This assumes you are using minus cyls (some textbooks talk about plus cyl refraction).
• Neutralize the most hyperopic meridian first.
  • Use the bracketing technique from last week.
  • As you have found the most hyperopic meridian, you will be adding plus (or reducing minus).
• Check for reversal and refine in smaller steps until neutrality.

FINDING THE CYL POWER

• Rotate the beam 90 degrees to the other primary meridian.
  • You should see against movement.
  • Fast = low astigmatism.
  • Slow = high astigmatism.
• Confirm no oblique movement.
• Neutralize this meridian using minus cyl.
  • The markings on the lens should line up with the ret streak.
  • Focus the ret beam temporarily if this helps with alignment.
• All meridians should now be neutralized.

FINAL STEPS in Retinoscopy

• 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 15 degrees of the axis.
• Complete both eyes within 10 min.

RECORDING RESULTS from Retinoscopy

• You will now have used two different lenses for each of the primary meridians (not including the WD lens).
  • For example: +2.00DS axis 20deg and -1.50DC axis 110deg.
• The highest positive power becomes the sphere power (+2.00DS).
• The amount of astigmatism is the amount of cyl you have added (-1.50DC).
• The axis is the position of the beam in the most negative/least positive meridian (110deg).
• Result: +2.00DS/-1.50DCx110

SPHERICAL REFRACTION: Best Vision Sphere

• First step is to check that the spherical correction is correct following retinoscopy.
• Best vision sphere (note difference in definition).
• There are a number of different procedures:
  • Plus/minus test
  • +1.00DS test
  • Duochrome test
• All are monocular.
• Rule of thumb – “give the most positive lens that provides maximum vision.”
  • Minimize accommodation = increase comfort
• There is a twist to this rule when preparing for cross-cyl.

SPHERICAL REFRACTION: Set-up

• Is the same for all of these procedures.
• Assume that retinoscopy has been completed, the working distance lens has been removed, and the vision measured.
  • (For now, assume that the cylindrical correction is correct).
• Turn the room lighting back on.
• Occlude the eye that is not being tested.

PLUS/MINUS TECHNIQUE

• Using positive and negative lenses the goal is to find the right balance to achieve the "in the right place equally blurred" position on the circle of least confusion.

HOW THE CHANGES LOOK TO THE PATIENT

• When using trial lenses of -0.50DS, 0.00DS, and +0.50DS patients see:
  • When the correction is -0.50DS the view:
  • Vision results - 0.25DS
  • Overall the view of the image: Smaller and darker/blurred than +0.25DS
  • When the correction is 0.00DS the view:
  • Vision results - 0.50DS
  • Overall the view of the image: Not as sharp as +0.25DS and only appears fair/blurred
  • When the correction is +0.50DS the view:
  • Vision results - 0.25DS
  • Overall the view of the image: Same/smaller and darker than -0.50DS
• YOU ARE EXPECTING A VISION CHANGE OF ONE LINE PER 0.25DS! Hint: Only if the lenses are clean. Use the cloth in your kits to clean each lens as you use it.

EXAMPLE Testing Plus power

• Starting point: +1.00
• When testing using -1.00 the patient relays the view and comparison appears better.
• When testing using +0.25 the patient relays the view and comparison appears the same.
• The correct answer is is +1.00

EXAMPLE Testing Minus power

• Starting point: +1.00
• When testing using -1.00 the patient relays no difference in the view and comparison.
• When testing using +0.25 the patient relays they see a paler view.
• The correct answer is +1.00
• NEXT - If you are testing a hyperope with accommodation, do not remove the trial frame lens until the new one is in place

HOW TO ASK THE QUESTION!

• What you say is important as you can lead the patient towards a particular answer without realizing it. You will get the WRONG answer if you are not careful.
• When adding plus…
  • Is it better, worse, or still the same?
  • Compare that to “is it better?” only
  • You could reject a lens that was “the same” when it is actually telling you that your patient had been accommodating to make their vision clear.
• When adding minus…
  • Is it clearer, or smaller and darker?

“THEY LOOK THE SAME” interpretations

• If patient answers “Same” or “No difference” at first presentation
  • You already have the right lens (Confirm by changing Rx power to demonstrate a definite response).
  • Vision is too poor due to inaccurate retinoscopy result, or pathology is present (Increase lens power and repeat).
  • Small pupils (Increase power and repeat).
  • Be wary of inducing accommodation!

+1.00DS BLUR TEST

• Over-plussing should induce a blur circle on the retina, hence reducing vision.
• Should blur by 1 Snellen line per 0.25DS.
• If distant light currently focussed on retina, +1.00DS should cause 4 lines of blur.
  • Should blur back from 6/6 to about 6/18.
• We can use this to check if our sphere is roughly correct+1.00 lens.

INTERPRETATION AND ACTIONS when using a +1.00DS lens

• If vision is less than 6/18 blur circle too small, the original focal point is behind the retina, add plus.
• If vision is greater than 6/18, blur circle too large, the original focal point is in front of the retina, add minus.

LIMITATIONS WHEN USING +1.00DS BLUR TEST

• It isn’t always 4 lines or 6/18.
  • Not everyone starts from 6/6.
  • The 0.25DS/line rule was determined using LogMAR.
• Pupil size makes a difference.
  • Small pupils = reduced blur (pinhole effect).
  • Large pupils = increased blur.
• This test does not work under conditions where the 0.25D/line relationship breaks down.
  • Blurred vision, due to significant refractive error. (e.g., your ret didn't work out so well).
  • Ocular pathology causing a decrease in best corrected acuity.
• If change in power is significant (>0.50DS), use another test to confirm.
• BEING EQUAL IS MORE IMPORTANT THAN 6/18

DUOCHROME TEST

• Uses longitudinal chromatic aberration to determine the refractive error.
• Whichever color is focussed nearest to the retina will be seen as clearest.
  • Emmetrope = equal vision with red/green.
  • Myope = red appears clearer.
  • Hyperope = green appears clearer.

PERFORMING THE DUOCHROME TEST

• Switch on duochrome test and establish that the patient can see the ring targets.
• Ask “Are the circles sharpest and clearest on the red or on the green background?”
• Alter power by 0.25DS according to patient's response.
  • Minus if red clearest, plus if green clearest.
• Repeat until no difference seen.
• Be wary of accommodation and red-preference.

LIMITATIONS OF DUOCHROME

• The ring targets are usually constructed of ring thicknesses equivalent to 6/9 (inner) and 6/12 (outer) Snellen equivalent targets.
• Will not work if vision is less than 6/12.
• The difference in focal position due to chromatic aberration is 0.50DS.
• Will not work if prescription is significantly incorrect.
• Small pupil will reduce size of blur circles (Difference between the clarity of red and green is reduced).
• Reduce room lighting for older patients.
• Be prepared to use a different test.