refraction on human eye 3 Flashcards
what is the +1.00DS blur test ?
its a check test you can conduct at the end of subjective refraction to check end point
how to conduct +1.00DS blur test ?
. 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
what does it mean if PX doesn’t blur back 3 to 4 lines ?
. this suggest that final Rx is over-minus
what does it mean if it blur back more than 5 lines?
. this suggest that the final RX is over-plus
what does an over-minus px prefer on the duochrome?
prefer circle on green
what does an over-plus PX prefer on the duochrome?
prefer circle on red
what do you want px to see when checking duochrome at the end of BVS but before JCC?
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
what do you want px to see on duochrome at end of subjective refraction ?
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 ?
what is another check test you can use at the end of subjective refraction ?
- 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
what is accommodation ?
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
what is the amplitude of accommodation ?
maximum possible increase in positive crystalline lens power ( D )
how does accommodation work for distance vision ?
- ciliary muscle is relaxed
- anterior zonules fibres are stretched
- lens capsule in under tension
- capsule is stretched lens flattens to focus at distance
how does accommodation work for near vision ?
- 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
how do we control accommodation while testing distance vision particularly in young px?
- Px views distant target at 6m
- green background on duochrome
- fogging
what should you consider when fogging ?
- should not be more than +2.00DS more than required to get reversal or against movement
how to control accommodation when changing plus lenses ?
- keep original + lens in place
- keep new lens in front
- ## then remove original lens
how to control accommodation when changing minus lens ?
- remove negative lens first then replace with new lens
how to measure the amplitude of accommodation ?
- 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
- RAF near point rule
how to measure the amplitude of accommodation using RAF near point rule ?
- 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
is the RAF method reliable ?
- 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
what are the limitation with the RAF rule ?
- subjectivity ( different px will respond differently)
- PX reaction time
- practitioner’s reaction time
- lighting
- inconsistent calibration
why do we measure amplitude of accommodation?
- 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
what is presbyopia ?
- 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
why does presbyopia occur ?
- 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
how can you manage a reduction in the amplitude of accommodation (AA)?
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
- 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
how to record reading ADD?
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
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?
- 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.
