Day 5 (2): Refraction

Question 1

What does refraction mean?

Answer 1

• Determination of the refractive status (and errors) of the eye.
• Selecting a lens of sufficient power with a secondary focal plane coinciding with the far point plane of the eye.

Goals:
1. Comfortable and satisfied patient
2. Best-corrected binocular vision
3. Relaxed accommodation

Question 2

What are the different kinds of refraction?

Answer 2

1. Objective/Manifest Refraction
   - uses a Retinoscope or Autorefractometer
2. Subjective Refraction
   - determine by subjective means the combination of lenses that would provide the patient with the best corrected VA
   - provide patients with two different powered lenses at a time and ask which of the two results to better vision
   - uses trial lens set or lenses mounted to a phoropter
3. Cycloplegic/Wet Refraction
   - using cycloplegic drugs to paralyze the ciliary muscles and relax accommodation (e.g. Tropicamide/Cyclopentolate/Atropine)
   - can be used in both subjective and objective

Question 3

What are the steps in refraction?

Answer 3

1. Get the best UNcorrected visual acuity.
2. Determine the error of refraction (myopia, hyperopia, astigmatism)
3. Get the pupillary distance
4. Do objective refraction
5. Confirm with subjective refraction
6. Measure reading add or bifocals
   - done in pts with presbyopia or > 40 years old
7. Writing the correct glasses prescription.

Question 4

What is the purpose of Pinhole Testing?

Answer 4

To determine if blurring of vision is due to an error of refraction

VA improves if pinhole: (+) Error of Refraction
VA does not improve: Look for other causes

Question 5

What is the pupillary distance?

Answer 5

Distance between the center of one pupil to the center of the other pupil.

Relevance: when creating prescription glasses, the distance between the optical centers of the frame/lens should approximate the pupillary diameter
- if not: (+) prismatic effect –> diplopia, BOV, straining, headache

Distance PD: pt looking straight ahead and fixating on an object at a distance

Near PD: pt looking at a close object

Question 6

What is retinoscopy?

Answer 6

• An OBJECTIVE technique for determining the refractive power of an eye
• Shooting light into the eye and observing the red reflex and how it moves with respect to the movement of the light
• Locates the far point of the eye

Who?
1. Pediatric patients
2. Patients who can’t talk

What are noted:
1. spherical refractive error
2. (+/-) regular or irregular astigmatism
3. (+/-) media opacities or optical defects

Question 7

What are the components of the retinoscope?

Answer 7

Components:
1. Illumination System
- light bulb filament
- mirror
- sleeve: adjusts separation of filament from mirror
2. Observation System
- peephole

Question 8

What are conjugate points?

Answer 8

• Locations or points in space that are OBJECT and IMAGE of one another
• If points A and B are conjugate points: for an IMAGE to form at point B, the OBJECT should be located at point A.

Question 9

What is a Far Point?

Answer 9

• Point in space conjugate to the retina when the eye is not accommodating
• Simply put: for an image to form exactly at the retina, object should be located at the far point
• Refractive status of the eye is a function of the far point location
• A property of an imaging system only (NOT of a lens)

Question 10

What is the Primary Focal Point?

Answer 10

• OBJECT location from which incident light rays would EXIT the lens (and enter the eye) with zero vergence (PARALLEL)
• IMAGE location: INFINITY

PLUS Lens:
- real point outside of the eye

MINUS Lens:
- imaginary point within the eye

Question 11

What is the Secondary Focal Point?

Answer 11

• IMAGE location when light rays with zero vergence (PARALLEL) ENTER a given lens (or the eye)
• IMAGE location: INTERSECTION POINT of the refracted light rays

PLUS Lens:
- real point inside of the eye

MINUS Lens:
- imaginary point outside the eye

Question 12

What is the difference between far point, primary and secondary focal point?

Answer 12

Far Point
- Location of an OBJECT in space so the the IMAGE formed FALLS ON THE RETINA
- Inherent to an optical system (eye) NOT a lens
- Object: Far Point
- Image: Retina

Primary Focal Point
- Location of an OBJECT such that incident rays exit the lens PARALLEL
- Inherent to a lens
- Object: Primary Focal Point
- Image: Infinity

Secondary Focal Point
- Where object from optical INFINITY focuses and forms an IMAGE in the eye
- Inherent to a lens
- Object: Infinity
- Image: Secondary Focal Point (Intersection of refracted rays)

Question 13

Where is the far point and the secondary focal point of an emmetropic eye?

Answer 13

Far Point: a single point located at INFINITY
- so far removed from point of origin that vergence is 0
- parallel rays enter the eye
- can see 20/20 vision or better AT DISTANCE without correction

Secondary Focal Point: RETINA

Question 14

Where is the far point and the secondary focal point of a myopic eye?

Answer 14

Reasons:
1. Cornea or lens too curved (too much converging power)
2. Axial Myopia: globe is too long for converging power of the eye
- more common reason

Far Point: ANTERIOR to CORNEAL PLANE (front of eye)
- to be conjugate with retina, excess convergence should be offset by an equivalent amount of divergence of incident rays
- when light rays enter the myopic eye, they are still divergent enough to counteract the convergence thus focusing on the retina
- reason why myopic eyes see better at near

Note: difference in the amount of myopia is reflected in the DISTANCE between the retina and the far point
- LOW myopia: less convergence/shorter globe –> LESS compensatory divergence needed –> FARTHER far point
- HIGH myopia: more convergence/longer globe –> MORE compensatory divergence needed –> CLOSER far point

Secondary Focal Point: ANTERIOR to RETINA
- parallel incident rays coming from point at infinity meets at the vitreous or in front of the retina
- after intersecting in front of the retina, light rays diverge and forms a BLUR CIRCLE on the retina

Question 15

Where is the far point and the secondary focal point of a hyperopic eye?

Answer 15

Reasons:
1. Cornea or lens too flat (too little converging power)
2. Axial Hyperopia: globe is too short for converging power of eye

Far Point: BEHIND the CORNEAL PLANE (within the eye)
- to be conjugate to the retina, the far point must contribute convergence to the decreased converging power of the eye
- however, converging light does NOT naturally occur in nature
- thus, far point is imaginary

Secondary Focal Point: POSTERIOR to RETINA
- imaginary point behind retina where light rays would have intersected if it had not run into the retina
- rays from infinity NEVER intersect and form a blur circle over the retina

Question 16

Why are the terms nearsightedness and farsightedness confusing?

Answer 16

• Vision is clear if an object is located at its far point.
• The refractive status of an eye is a function of the location of the far point.
• Emmetropic: at distance (far point: INFINITY)
• Myopic: at near (far point: ANTERIOR TO CORNEAL PLANE)
• Hyperopic: none (far point: imaginary point BEHIND THE EYE and not within the light of sight) –> out of focus at every distance

Question 17

What does AGAINST motion in the retinoscope mean?

Answer 17

• Red reflex moves OPPOSITE the light source
• Seen in: High MYOPIC eyes (greater than the working distance)
• Far point: between peephole and the patient
• FP2: anterior to retina
• Instead, blur circle is formed on the retina (light not as bright and intense)

Correction: Add MINUS lens of increasing power
- to diverge incident light rays to compensate for excess converging power of eye and focus exactly on retina
- convergence of light is gradually moved back posteriorly to the retina to achieve neutrality
- if overpowered lens is added: light is focused to an imaginary point behind the retina –> WITH motion

Question 18

What does WITH motion in the retinoscope mean?

Answer 18

• Red reflex moves WITH (in the same direction as) the light source
• Seen in
  1. HYPEROPIC eyes
  2. Emmetropic eyes
  3. Low myopic eyes
• Far point: behind the examiner
• FP2: imaginary point behind the retina
• Instead, blur circle is formed on the retina (light not as bright and intense)

Correction: Add PLUS lenses of increasing power
- to converge incident light rays to compensate for the decreased convergent power of the eye
- the gradually increasing power of the placed lenses slowly focus the light anteriorly into the retina to achieve neutrality
- if overpowered lens is added: light is focused in front of the retina –> AGAINST motion

Question 19

When is NEUTRALITY achieved in retinoscopy?

Answer 19

• Peephole is AT the neutral or far point and is conjugate with the retina
• NO movement of red reflex and an intense and bright reflection filling the entire pupil
• Light converges exactly at a single point in the macula
• Far point: neutral point (where peephole is)
• FP2: at a single point on the retina

Question 20

What parameters are noted in the red reflex seen in retinoscopy?

Answer 20

1. Movement
   - WITH motion: same direction as light source
   - AGAINST motion opposite direction as light source
2. Speed
   - NO movement: target end-point
   - Slow: high grade EoR
   - Fast: low grade EoR
3. Brilliance: reflected light intensity
   - Dull: high grade EoR
   - Bright: low grade EoR
4. Width of red reflex
   - Narrow: high grade EoR
   - Broad: low grade EoR

Low grade: close to neutrality, adjust power of lenses in smaller intervals

High grade: far from neutrality, initially adjust in larger intervals

Question 21

What are the steps in performing retinoscopy?

Answer 21

1. Determine working distance (0.67 m ~ 1.5 D)
2. Determine type of error of refraction
   - WITH motion: hyperope, emmetrope, low myope
   - AGAINST motion: high myope
3. Determine axis of principal meridians
4. Neutralize meridian 1
5. Neutralize meridian 2
6. Determine spherical power
7. Determine cylinder axis
8. Determine cylinder power
9. Subtract working distance power from the gross sphere power

Shortcut:
1. Need to start with a WITH motion. If on initial test you see AGAINST motion, add MINUS lens of increasing power until you see WITH motion.
- to focus the light rays behind the retina
2. Add power in increments of 1 D (MORE PLUS or LESS MINUS) in each meridian until the reflex is neutralized in both meridians.
- to focus light rays gradually forward into the retina
3. Subtract the working distance power (1.5 D) FROM THE SPHERE POWER once gross refractive power is obtained.

Question 22

What is the average working distance and why it is subtracted from the final refraction?

Answer 22

67 cm: average arms length
- equivalent to around 1.5 D power
- a 1.5 D lens will put the far point at 67 cm from the pt’s eye
- may input working distance power on the optical system or manually subtract from the gross neutralizing power

At this distance, if eye is:
emmetropic: neutral point (neutrality achieved)
myopic: (+) against motion
hyperopic: (+) with motion

• In doing retinoscopy, the goal is to know the error of refraction of the eye and make it emmetropic with a far point at INFINITY:

1/Infinity + D = V –> 0 + D = V –> D = V (the vergence of light contributed by lens is the vergence of light exiting to hit the retina)

• This is not possible because testing is done at a WORKING DISTANCE
• This provides some amount of divergence to the incoming light rays (U is NOT 0 –> - 1/n + D = V)
• Thus the NEUTRALIZED POWER is in reality HIGHER than the TRUE POWER of the eye
• To maintain V as a constant and make sure light still falls on the retina, the overpowered lens needed to compensate for the divergence of light rays at the working distance must be subtracted by - 1.5 D to arrive at the true power of the eye

Question 23

How is the axis of the principal meridians determined using a retinoscope?

Answer 23

Once intercept/light source is shone on the eye, note the following parameters of the red reflex that’s reflected back:
1. Break
2. Thickness
3. Intensity
4. Skew: is streak parallel to red reflex?

Goal: BRIGHT, WIDE red reflex close to filling the pupil oriented PARALLEL and moving in the SAME direction as the streak

Meridian 1: tested meridian
Meridian 2: 90 degrees from the tested meridian

Question 24

Reminders when neutralizing the meridians.

Answer 24

1. The light streak tests the power of the meridian it sweeps across
   - i.e. the meridian perpendicular to the streak axis
   - vertical streak (90 degrees axis) moving from L to R: tests the power of the 180 degree meridian
   - horizontal streak (180 degrees axis) moving up or down: tests the power of the 90 degree meridian
2. For practicality: first start with a vertical streak to test the 180 degree meridian
3. Once done with meridian 1, rotate streak by 90 degrees and test meridian 2 with a horizontal streak moving up and down.
4. To avoid confusion, always start with a WITH motion. If on first testing you see AGAINST motion, use a MINUS lens to move light behind the retina and gradually work your way up with less minus or more plus lenses until neutralization is achieved.

Question 25

How is the spherical power computed?

Answer 25

• Once, the two prime meridians have been neutralized, the power of the neutralizing lenses are noted.
• Spherical power = higher power of the two (MORE plus or LESS minus) subtracted by the working distance power
• The higher powered meridian is used because by convention, MINUS cylinders are used.

Question 26

How is the cylinder axis and power determined?

Answer 26

Notes:
1. The power meridian of a cyl is perpendicular to it’s axis
2. The light streak tests the power of the meridian it sweeps across
- vertical streak (axis 90) moving L to R: tests 180 degree meridian
- horizontal streak (axis 180) moving up or down: tests 90 degree meridian
3. Spherical power = higher powered meridian
- because a MINUS cyl is used

Thus,
Cyl AXIS: meridian of the spherical power used OR the axis of the flatter cylinder (if in two cylinder notation)
- if using PLUS cyl: LOWER powered meridian
- if using MINUS cyl: HIGHER powered meridian (more plus, less minus)

Cyl POWER: power that if subtracted from the sph power results to the power of the other meridian

E.g. Retinoscopy –> 2 cylinder notation
180 degree meridian neutralized with 2 D lens
90 degree meridian neutralized with 4 D lens
2 D x 90 = 4 D x 180

Spherical power: 4 D (higher powered of the two)
Cylinder power: - 2 D (to get 2 D in the other meridian)
Axis: 90 degrees (MERIDIAN of the sphere power or AXIS of the flatter cylinder)

Question 27

What two techniques are used to neutralize sphero-cylinders or eyes with astigmatism?

Answer 27

1. Sphere to Sphere Technique
   - use spherical lenses to neutralize both meridians
2. Sphere to Cylinder Technique
   - spherical lens: to neutralize the primary meridian (180 degrees) with a vertical streak moving left to right
   - cylindrical lens: to neutralize the secondary meridian (90 degrees) with a horizontal streak moving up and down

Question 28

What to do if red reflex is oblique?

Answer 28

• Don’t test anymore with a horizontal or vertical streak
• Instead, follow the angle of the reflex and test that meridian and the meridian perpendicular to that

Question 29

What is subjective refraction?

Answer 29

• Involves the pt fixating at a visual acuity chart while the examiner presents a set of lenses of varying powers in front of the patient’s eyes according to the patient’s subjective responses

Goals:
1. Determine refractive error with fully relaxed accommodation
- so that full range of accommodation is available to the patient even at near objects
- avoid ASTHENOPIA (ciliary muscle straining due to overaction) from constant accommodation
2. Binocular balance: accommodation equally relaxed in both eyes
3. Develop and refine a prescription through which the pt can see CLEARLY and COMFORTABLY (clear not always = comfortable)

Tool: Phoropter
- complex lens holder designed to allow the examiner to change lenses efficiently and easily

Question 30

When is accommodation stimulated?

Answer 30

1. Awareness of near object/near vision
2. Attempting to overcome hyperopia
3. Ciliary muscle (accommodative) spasm

Question 31

How is accommodation controlled?

Answer 31

1. Cycloplegics: wet refraction
   - indications: accommodative spasm, latent hyperopia, persistent symptoms after dry refraction
   - disadvantage: may reduce or eliminate normal tonic accommodation (0.50 - 1.50 D) leading to blurred vision when normal accommodation is resumed
   - also causes pupil dilation –> (+) spherical aberration in peripheral pupil
2. Fogging: dry refraction using a plus lens occluder
   - increase plus lenses until best VA decreases to 6/36 or 20/120

Question 32

What are the steps in doing subjective refraction?

Answer 32

Starting point: prescription from retinoscopy/lensometry/ autorefraction

Set-up:
1. patient 6 m/20 ft from VA chart
2. proper centration: ensure eyes are centered in the trial frame with NO pantoscopic tilt and proper vertex distance
3. monocular testing: start with R and occlude L
- opaque occluder: may stimulate accommodation
- plus lenses: decrease in VA by two lines; avoids accommodation

Steps:
1. Refining the sphere/Initial sphere check
2. Refining the cylinder axis
3. Refining the cylinder power
4. Verifying spherical endpoint using duochrome testing
5. Repeat steps 1-5 for the other eye
6. Check near vision and range of accommodation
7. Testing binocular balance for comfort

Question 33

How is the initial sphere refinement performed?

Answer 33

Steps:
1. Test the best UCVA in both eyes by asking pt to read smallest line possible.
2. Place a pinhole in front of the tested eye if VA will improve (= + EoR). The target of subjective refraction is to achieve this level of VA or better.
3. Input sph power, cyl power and axis in the phoropter based on the last prescription or results of objective refraction.
4. Ensure pupillary distance is centered and phoropter is level.
5. Fogging: placing a high PLUS sphere lens so that the smallest line the patient can read is 20/200; done to relax accommodation
6. Defogging: gradually decrease amount of sphere by 0.5 D (2 clicks) decrements by adding MINUS lenses.
- as the chart starts to appear clearer, ask patient to say as soon as the displayed 20/30 (6/9) line is discernible (not necessarily clear).
- refine further by increments (using PLUS lens) or decrements (using MINUS lens) of 0.25 D (1 click)
- start by adding 0.25 D briefly and asking the pt if letters appear clearer, blurrier or same
- if clearer/same: add + 0.25 D to sphere and repeat comparison
- if blurrier: add - 0.25 D and repeat comparison –> if it becomes better: adjust sphere power and repeat comparison
7. Target: 1 -2 lines larger than the UCVA

Goal: highest PLUS lens or the least MINUS lens to achieve best VA

Reminders:
1. If astigmatism is present, defogging places the circle of least confusion on the retina creating a MIXED astigmatism.
2. Don’t deduct too much PLUS power unless you are sure it is necessary to see clearer.
- overly-minused lens can activate accommodation –> if prolonged: eye fatigue and strain
3. If vision is already 20/20: sph power is already refined and accurate
- recheck if overly-minused by adding + 0.5 D
- if better/same: adjust final sph power
- if worse: retain sph power because increment made vision worse

Question 34

How is the cylinder axis and power refined?

Answer 34

3 Techniques:

1. Astigmatic Dial
2. Jackson Cross Cylinder
3. Stenopeic Slit
   - indications: irregular astigmatism, lenticular or corneal opacities, small pupils

Starting point: Cyl in the
- retinoscopy
- lensometry
- autorefractometry
- previous prescription

Question 35

What is an astigmatic dial?

Answer 35

• Subjective method of diagnosing and treating astigmatism
• A dial of sharp lines spaced 10 degrees apart
• Starts with the examiner asking the pt which of the dials appear BLACKER and SHARPER

Example 1: Plano - 1 D x 180 (Simple Myopic Astigmatism)
- One meridian focuses on the retina while the other is in front
- Meridian 1/Vertical: meridian with higher power (1 D)
- Meridian 2/Horizontal: meridian with lower power (Plano)
- Using a light source in the form of a cross:
1. Horizontal points in the cross will appear BLURRED because they are stretched out VERTICALLY
2. Vertical points will appear BLACKER because as they are stretched out VERTICALLY, points superimpose on each other
- Thus: the BLACKER and CLEARER dial = ASTIGMATIC meridian
= POWER meridian of corrective CYL
- To correct: apply a CYL with AXIS 90 degrees from blacker and clearer dial –> corrective power of cyl aligned with astigmatic meridian

Steps:
1. Obtain BCVA using sph only. Correct spherical error only first.
2. Fogging: adding PLUS spheres until can only see 6/18 or 20/60 line
- pushes all focal lines forward in front of the retina
3. Ask patient to identify the BLACKEST and SHARPEST line in the astigmatic dial.
4. Collapse the Conoid of Sturm: add MINUS cyl with the axis perpendicular to the sharpest and blackest line until all lines appear equally sharp and black.
- reduces the Conoid of Sturm and distance between the two focal lines
5. Defogging: reduce PLUS sphere power in decrements of 0.25 D or add MINUS sphere power to get least minus or highest plus and BCVA is attained.

Question 36

What is a Jackson Cross Cylinder or the Flip Cross Technique?

Answer 36

• A single lens that is a combination of a PLUS and a MINUS cylinder of equal strength placed with their axes perpendicular to each other and mounted on a handle.
• Made as a spherocylindrical lens having a spherical component combined with a cylindrical component of twice the power of the sphere of opposite sign
• Affects both meridians (vs simple cylinder lenses where changes are applied to one meridian at a time) –> more noticeable difference to the pt
• Power of the cross cylinder used varies according to the cyl power in the prescription being tested
  1. 0.5 D: + 0.25 and - 0.25
  2. 1.0 D: + 0.50 and - 0.20
  3. 1.5 D: + 0.75 and - 0.75
  4. 2.0 D: + 1.00 and - 1.00
• Red dots: axis of MINUS cyl; power meridian of PLUS cyl
• White dots: axis of PLUS cyl; power meridian of MINUS cyl

Question 37

What are the steps in Jackson Cross Cylinder refinement?

Answer 37

Steps:
AXIS refinement
- straddle axis by placing handle parallel to initial axis
- flip JCC and ask patient which of the two produces sharper vision
- since a MINUS cyl is used, move axis in the trial frame closer to the MINUS cyl of the JCC
- realign the new axis and the handle of the trial frame, reflip the JCC and ask pt which produces clearer vision
- repeat steps but in smaller intervals of axis adjustment
- endpoint: equal or no difference with flipping = final cyl axis

POWER refinement
- align red marker with the new axis, flip and ask pt which produces better vision
- principle: in changing power by X D, difference between powers of the two meridians either gets larger (larger CoS, blurrier vision) or smaller (smaller CoS, clearer vision)
- if clearer with MINUS cyl: add minus JCC power to initial cyl
- if clearer with PLUS cyl: add plus JCC power to initial cyl
- sph adjustment: for every X D change in cyl, adjust sph by X/2 D (half) of OPPOSITE sign because change in power affects both meridians
- target: one line above the best line (to avoid accommodation and an over-minused lens)

Question 38

Test used for the second sphere check

Answer 38

Duochrome Test
- use 2nd line less than BCVA
- test alternately with the green and red filter
- if similar on both red and green: no adjustment
- if clearer on red: add MINUS lens (RAM)
- if clearer on green: add PLUS lens (GAP)

Question 39

Estimated reading add for bifocals based on age.

Answer 39

40 - 45: + 0.75 D - + 1.00 D
46 - 50: + 1.25 D - + 1.75 D
51 - 55: + 2:00 D - + 2.50 D
56 - 70: + 2.50 D - + 2.75 D

Question 40

What are the different types of bifocals?

Answer 40

• Flat top
• Round segment
• Blended segment
• Executive stye
• Progressive (for distance, intermediate and near)

Question 41

Reminders when doing refraction:

Answer 41

1. Accommodation should be relaxed
2. Maximum plus but still with clear VA (avoids accommodation and straining with eye fatigue)
3. Always test frame before prescribing
4. Take into account vertex distance especially for HIGH prescription individuals.

Question 42

What is the reverse Galilean telescope phenomenon?

Answer 42

In pts with myopia, as more MINUS lens is added in front of the eye, the eye generates more PLUS power by accommodating.

Letters appear SMALLER, DARKER and more DISTINCT = misinterpreted as CLEARER.

Reverse of the Galilean telescope: a plus lens placed in front of a minus lens with coinciding focal points causing refracted rays to have larger subtended angles from the eye and images to appear LARGER.

Ask: is this definitely better or are the letters just smaller and darker?

Reminder: avoid additional minus sph power that does not allow more letters to be read –> may cause asthenopia due to constant accommodation.

Question 43

Why and how is binocular balance checked at the end of the subjective refraction?

Answer 43

Final step which makes certain that accommodation has been relaxed equally in both eyes while wearing the trial frame with the prescription lenses.

2 techniques:
1. Alternating Occlusion
2. Prism Dissociation

Question 44

How is alternating occlusion done?

Answer 44

1. While wearing trial frame with the prescription lenses, blur the patient until only upto the 6/12 or 6/15 (20/40 or 20/50) line is seen.
2. Do alternating occlusion of the eyes.
3. Ask the pt if the image is EQUALLY BLURRED on each side.
4. If not, PLUS sph should be added to the eye with better vision.

Target: both sides should be equally blurred = accommodation relaxed into the same degree

Question 45

How is prism dissociation done?

Answer 45

• Most sensitive test of binocular balance
• Can identify as little as 0.25 D sphere differences

Steps:
1. While wearing trial frame with the prescription lenses, blur the patient until only upto the 6/12 or 6/15 (20/40 or 20/50) line is seen.
2. Put a 4-6 PD prism in front of R eye or a 2-3 PD prism in front of each eye. Both eyes should be BLURRED to the SAME degree.
- Base DOWN prism: image is shifted UP
- Base UP prism: image shifted DOWN
3. If not, PLUS sph should be added to the eye with better vision.

Target: both sides should be equally blurred = accommodation relaxed into the same degree