lecture 7 - accommodation Flashcards
define convergence
simultaneous rotation of the eyes inwards
how do you measure the near point of convergence
pen to nose
RAF rule
what is the normal value for convergence
10 cm (RAF rule) 5-7 cm (pen torch)
what do you record in px’s clinical records for convergence
- write down the method
- with/without Rx
- distance at which it was carried out
define accommodation
the ability to change the convexity of the crystalline lens in order to obtain a clear image of a near object
3 ways of measuring accommodation
- accommodative amplitude or range
- accommodative facility
- accommodative lag or lead
why is it important to be careful when allowing a myope to wear glasses when measuring near point convergence?
- base in prisms due to -ve lenses will assist convergence
- lens edge and frame contours may block the target
what to record in px’s clinical records for accommodation
AoA RAF
binoc with Rx: 10D x3
monoc with Rx RE 8D x3 LE 8D x 3
how to test convergence objectively
- note the eye which diverges first
- note the distance and when they were able to maintain convergence
- note whether diplopia was noticed
how to test convergence objectively
- note the eye which diverges first
- note the distance and when they were able to maintain convergence
- note whether diplopia was noticed
how to test convergence subjective
- when px reports diplopia
- remember the target will be blurred
why is ti important to measure 3 times?
the px can have convergence fatigue and so it is important to do it to ensure your result is accurate
how do you measure accommodative amplitude or range (2 methods)
near point with RAF rule
minus lenses
how do you use the RAF rule to measure near point
- FULL distance Rx is worn
- find nearest point of Clear vision
- measure in cm and convert to D or look at diopter scale
- target should be N5 or smallest visible print
how do you use RAF rule to measure amplitude of accomm
- push up until px reports blue
- pull back until px reports clear
- repeat
- if difference<1D record mean
- iff difference >1D record range
when using the Raf rule, if the amplitude is less than 2DS (50cm) then what do you do?
use binocular near add eg. +2.00 DS
find near point and subtract add from final result
when using the Raf rule, if the amplitude is less than 2DS (50cm) then what do you do?
use binocular near add eg. +2.00 DS
find near point and subtract add from final result
how to use minus lenses to measure accommodative amplitude or range
- full Rx
- 40 cm
- occlude one eye
- minus lenses placed in front of eye
- increased in -0.25D steps
- encourage px to make print clear
- end point when letters cant be cleared
- repeat eye and binocularly
disadvantage of using minus lenses to measure AoA
very slow
define accommodative facility
change on accommodative effort with change in accommodative demand
- speed of change in accommodation
- rapid refutation from near to distance viewing
how to measure accommodative facility
- must wear full correction
- ask them to look at smallest line then put up flipper lenses and ask them when it becomes clear then keep flipping then count how many cycles the px reports it is clear in a minute
normal value for accommodative facility
9-11 cycles per min
using +/- 2.00D
what to record for accommodative facility
- Rx worn or not
- strength of flippers
- no. of cycles per min
- size of print
what to record for accommodative facility
- Rx worn or not
- strength of flippers
- no. of cycles per min
- size of print
disadvantages of subjective methods of measuring accommodation
unreliable endpoint
- large difference between stimulus and response accommodation due to depth of field eg. small pupils increase depth of field
- changing subjective criteria
what is stimulus accommodation
distance of fixation target in D
what is response accommodation
objective assessment of accommodation
what is an objective method of accommodation
dynamic retinoscopy
how do you carry out dynamic retinoscopy (MEM)
- wear distance Rx
- ask px to look at target same distance as retinoscope
- px fixates and neutralisation occurs when the back focal plane of the eye conjugate with back focal plane of retinoscope
- carry out ret in dim lighting
- neutralise with spheres very briefly
accommodative lag results in what kind of movement on ret reflex
with
accommodative lead results in what kind of movement on ret reflex
against
what distance do you carry out dynamic retinoscopy for the MEM method
40 cm
what distance do you carry out dynamic retinoscopy for the Nott method
33cm
how do you carry out dynamic retinoscopy using the Nott method ?
target at same distance to ret
px fixates on detailed target
neutralise by moving the retinoscope back (lag) and forwards (lead)
define AC/A ratio
amount of accommodative convergence per diopter of accommodation (prism diopter/DS)
AC/A ratio - Heterophoria method
- wear distance Rx
- measure the following: pd (cm) , angle of deviation on distance fixation and angle of deviation on near fixation
- calculate AC/A
- eso = +ve
- exo = -ve
disadvantage of Heterophoria method of AC/A
makes no allowance for proximal convergence
formula for AC/A Heterophoria method
pd + (AngleN - Angle D)/Accomm demand)
work out accomm demand by doing reciprocal of viewing distance
normal range for AC/A
3:1 to 5:1
how to measure AC/A using gradient method
measure change in angle of Heterophoria when viewing through +ve or -ve spheres
- always wear distance Rx
- vary the amount of accommodative demand
- at near, compare PCT with +3.00 and without
- at distance, compare PCT with -3.00 and without
what does the +3.00 lens do in the gradient method?
relaxes accommodation
what does the -3.00 lens do in the gradient method?
induces accommodation
how do you calculate AC/A in the gradient method
ac/a= ( PCT with accom- PCT without Accm ) / Accm demand
what is the near pupil response
when looking at near your eyes miose
how do you measure the near pupil response
- px’s face is well illuminated
- keep light on face constant during eye movement
- observe pupil response
- rapidly shift fixation from distance to near or vice versa
- use a detailed target for both
why is it important to keep the face illuminated constantly when measuring near pupil response
this isolates the near reflex from light response
