refraction on human eye 3

Question 1

what is the +1.00DS blur test ?

Answer 1

its a check test you can conduct at the end of subjective refraction to check end point

Question 2

how to conduct +1.00DS blur test ?

Answer 2

. add +1.00DS to the distance and re-measure the VA
. record the VA with the +1.00DS blur lens
. should blur back 3 or 4 lines

Question 3

what does it mean if PX doesn’t blur back 3 to 4 lines ?

Answer 3

. this suggest that final Rx is over-minus

Question 4

what does it mean if it blur back more than 5 lines?

Answer 4

. this suggest that the final RX is over-plus

Question 5

what does an over-minus px prefer on the duochrome?

Answer 5

prefer circle on green

Question 6

what does an over-plus PX prefer on the duochrome?

Answer 6

prefer circle on red

Question 7

what do you want px to see when checking duochrome at the end of BVS but before JCC?

Answer 7

when checking duochrome at the end of BVS but before moving onto refining cyl using JCC
you want px to see equal or prefer green

• this is because you want px to have a little bit of accommodation to accommodate to keep clc on retina

Question 8

what do you want px to see on duochrome at end of subjective refraction ?

Answer 8

at end of subjective refraction px should see equal or prefer red
- this is because you want px to have minimal accommodation and you want maximum plus and minimum minus which is why want PX to see either equal or red ?

Question 9

what is another check test you can use at the end of subjective refraction ?

Answer 9

• pin-hole (PH)
• reduces effective pupil size thereby reducing size of retinal blur circles
• this means that if there is uncorrected refractive error when a patient looks through the pinhole, then VA will improve
• if V/VA doesn’t improve with PH, then blur is not due to blurred retinal image ( i.e. not due to refractive error )
• you can use pin-hole to check if any better VA is possible

Question 10

what is accommodation ?

Answer 10

the ability to change the focusing power of the crystalline lens

• the lens changes shape
• enables distance and near objects to be in focus on retina

Question 11

what is the amplitude of accommodation ?

Answer 11

maximum possible increase in positive crystalline lens power ( D )

Question 12

how does accommodation work for distance vision ?

Answer 12

• ciliary muscle is relaxed
• anterior zonules fibres are stretched
• lens capsule in under tension
• capsule is stretched lens flattens to focus at distance

Question 13

how does accommodation work for near vision ?

Answer 13

• contraction of ciliary muscles
• less tension of anterior zonules on the lens
• increase curvature ( mainly anterior surface; more zonules tension and anterior surface is more elastic )
• refractive power increases
• increased curvature converges light rays onto retina

Question 14

how do we control accommodation while testing distance vision particularly in young px?

Answer 14

• Px views distant target at 6m
• green background on duochrome
• fogging

Question 15

what should you consider when fogging ?

Answer 15

• should not be more than +2.00DS more than required to get reversal or against movement

Question 16

how to control accommodation when changing plus lenses ?

Answer 16

• keep original + lens in place
• keep new lens in front
• ## then remove original lens

Question 17

how to control accommodation when changing minus lens ?

Answer 17

• remove negative lens first then replace with new lens

Question 18

how to measure the amplitude of accommodation ?

Answer 18

• measuring the closest distance at which the eye can focus
  1. use of negative lenses- then establish
  what is the maximum -ve power the patient can accept without a reduction in acuity

2. RAF near point rule

Question 19

how to measure the amplitude of accommodation using RAF near point rule ?

Answer 19

• bring print towards Px eyes until blur detected
• take print back until it clears
• repeat blur and clear points
• take an average
• patient encouraged to make maximal effort
• first monocularly
• then binocularly
• very subjective

Question 20

is the RAF method reliable ?

Answer 20

• no studies testing the repeatability of this method specifically
• other ‘push-up’ or push-down’ methods have been tested and repeatability is lower than with other methods

Question 21

what are the limitation with the RAF rule ?

Answer 21

• subjectivity ( different px will respond differently)
• PX reaction time
• practitioner’s reaction time
• lighting
• inconsistent calibration

Question 22

why do we measure amplitude of accommodation?

Answer 22

• to see how much accommodation is available for near work
  we need to know this because
• in older patient’s , aging process causes reduced accommodation ( presbyopia )
• in younger px reduced accommodation suggests uncorrected refractive error, binocular vision anomaly or pathology

Question 23

what is presbyopia ?

Answer 23

• presbyopia is the decrease in accommodative ability of the eye that occurs with ageing
• happens gradually until middle-age, then can be relatively rapid
• patient cannot see near detail clearly with distance Rx in place or without Rx if emmetropic

Question 24

why does presbyopia occur ?

Answer 24

• lens hardens with age and less able to change its shape
• the accommodative mechanism are fully intact
• lens itself is less pliable and no longer changes shape when the ciliary muscle contracts

Question 25

how can you manage a reduction in the amplitude of accommodation (AA)?

Answer 25

prescribe reading glasses
1. calculate approximate addition ( add)
. after subjective for distance , measure the AA
. calculate reading add
. reading add = positive spherical lens power added to the patient’s distance correction
. either based on AA or patient’s age

2. refining addition
   . find BVS for near - at the viewing distance the px requires
   . keep positive power to a minimum
   . ensure good lighting
   . check the px requirements and preferences

Question 26

how to record reading ADD?

Answer 26

add the reading add to the spherical part of prescription

RE: +1.00/-0.50 x 30
LE : +0.50/-0.75 x 160
ADD +2.25 R+L

reading prescription

RE +3.25/-0.50 x 30
LE +2.75/ -0.75 x 160

Question 27

Example
• Patient reads at 40cm
How much accommodation would be needed?
• Measure the amplitude of accommodation so here for example 2.00 D
- So if we leave ~50% unused, available accommodation is?
• Near addition?

Answer 27

• 1/40cm = +2.50D
• • For example AA = 2.00D
• Aim to leave 1/2 to 1/3 AA unused for sustained comfortable reading (i.e. 1/2 to 2/3 being used for comfortable reading)
• can then use +1.00D of patients Amplitude if 50% is unused
  • Near addition?
  2.50 - 1.00 = +1.50D

 Place binocular add in front of DV Rx and check near VA. - refine by BVS and check reading distance.