Accommodation and Presbyopia Flashcards
The +1.00Ds Blur Test
- Add +1.00Ds to the distance rx and remeasure the VA
- Record the VA with the +1.00Ds blur lens
- Should blur back 3-4 lines
- If it blurs more than 5 lines, it suggests the rx is over plus
What is accommodation?
- The ability to change the focusing power of the crystalline lens
- lens changes shape
- enables distance and near objects to be in focus on the retina
What is amplitude of accommodation?
Max possible increase in positive crystalline lens power (D)
Theory of accomodation (distance)
- cilliary muscle is relaxed
- anterior zonules fibres are stretched
- lens capsule is under tension
- capsule is stretched, lens flattens to focus at distance
Theory of accommodation (near)
- contraction of ciliary muscles
- less tension of anterior zonules on the lens
- increases curvature (mainly anterior surface, more zonule tension here and also anterior surface is more elastic)
- refractive power increases
- increased curvature converges light rays onto retina
Why is pinhole used and what can it indicate?
Pinhole reduces effective pupil size
- reduces size of retinal blur circles
- If V/VA does not improve with pinhole, then blurring not due to blurred retinal image (ie not due to refractive error)
- you can use the ph to check if any better VA is possible eg after ret or at end of subjective
How do we control accommodation?
In the young px we need to keep accommodation under control while testing distance vision
- px views distant target at 6m
- green background on duochrome - relaxes accommodation
- fogging - should not be more than +2.00Ds required to get reversal or just against movement
How do you control accommodation when changing plus lenses?
- keep original lens in place
- put new lens in front
- remove original lens
How to measure the AA?
We measure the closest distance at which the eye can focus
1) Use of negative lenses - what is the max -ve power the px can accept without a reduction in acuity
2) RAF Rule
What is on the RAF rule
There are 4 diff reading types
- reduced snellen test type
- four lines of times roman text
- reproduction of a section of a telephone directory
- vertical line with central dot for convergence testing
Scale:
- age
- cm
- accommodation
How do you measure AA using the RAF rule?
- bring text towards px eyes until blur detected and note accommodation in D
- take print back until it clears and note accommodation again
- repeat blur and clear points
- take avg
- px encouraged to make maximal effort
- first monocularly
- then binocularly
Limitations/problems using RAF rule
- subjectivity - diff px will respond differently
- px reaction time
- practitioners reaction time
- lighting
- inconsistent calibration
Why do we measure AA?
- need to know how much accom is available for near work
- in older px, ageing process causes reduced accommodation - presbyopia
- in young px, reduced accom suggests uncorrected refractive error, binocular vision anomaly and/ or pathology
Presbyopia
- decrease in accomodative ability of the eye that occurs with ageing
- inability to focus at near
- part of the normal ageing process
- lens hardens with age and less able to change its shape
- the accommodative mechanisms (zonules and ciliary muscle) are fully intact
- the lens itself is less pliable and no longer changes shape when ciliary muscle contracts
- happens gradually until mid age, then can be relatively rapid - can be quite disconcerting for px
- px can’t see near detail clearly with distance rx in place
Management of presbyopia
Prescribe reading glasses
1) Calculate approx addition (add)
- after subjective for dist, measure the AA
- calculate reading add = positive sph lens power
- based on AA or px age
2) Refining addition
- find BVS for near - at the viewing dist the px requires
- keep positive power to a min
- ensure good lighting
- check px requirements and preferences
