the near response Flashcards
what is the near train / near synkinesis
Each of the responses can occur independent of the others and hence term ‘synkinesis’ preferred rather than ‘reflex’
Convergence
Accommodation
Pupil miosis
how is accommodation investigated
Subjective
Near point of accommodation (amplitude of accommodation)
Accommodative facility
Objective
Dynamic retinoscopy
Autorefractors (Shin Nippon, PlusoptiX)
what is the amplitude of accomodation
Max accommodative ability of the eye
Near point of accommodation
(literature – called amplitude)
Range of accomm from far point to near point
how to calculate amplitude of accomodation
Simple to calculate in emmetropes (or if wearing full refractive correction)
1/near point (m) e.g. 6cm = 1/0.06 = 16.7D
(Otherwise… amplitude accomm = 1/far point (m) – 1/near point (m))
how are - lenses used for amplitude of accomodation
Some use minus lenses only
Target at near or distance
Lenses in 0.5D steps
Until notice blur
If done at near, take off working distance e.g. 40cm = 2.5D
Near/distance not comparable (Momeni-Moghaddam et al 2013)
what are the limitations of the Raf rule
Blur or not able to read?
Goodall & Firth, 2003
Difference in end point if different instructions given?
Mean difference 6.8cm in early presbyopic subjects
Recommend end point is when blur first noticed
Modified push-up (or push down) method
Start with target close to eyes and move away until clear
Suggested in young children as difficult to understand blur (Scheiman and Wick 1994)
repeatability and the Raf rule?
The 2 methods are similar in adults and children (but both different to minus lens method) (Taub & Shallo-Hoffmann 2012)
Conventional method poor test-retest in children
Aged 8.1 ±2.1 years
Variation 3.1D
Adler et al, 2013
Modified and Conventional push-up methods are not interchangeable (Esmail & Arblaster, 2016)
Conventional NPA significantly closer RE, LE and BEO
But clinically not significant – on average 0.73cm
what is accommodative facility
Ability to rapidly change accomm to various stimuli
‘Flippers’
+2.00DS/-2.00DS
Alternated ensuring that target is cleared before change
Number of cycles in 1 minute
Recording:
Accomm facility+/-2DS 12cpm
what is the normal range for accommodation facility
Pandian et al (2006) in 1328 5-8 y/o:
7 cycles per min monocularly at near and in distance
5.5 cpm monoculalry in myopes in distance
Greater cpm with increasing age
Adler et al (2018) in 136 4-12 y/o:
Binocular (11.6cpm) lower than monoc (12.7cpm)
Prone to practice effects in younger children
Horwood and Toor (2014) in 156 young adults:
Binoc accomm facility 9cpm
Monoc accomm facility prone to practice effects
Improved from 7.3 to 10.4 cpm on 2nd test
Should test binoc only
what are the 2 methods of dynamic retinoscopy
Monocular Estimate Method (MEM) retinoscopy
(lenses)
Nott retinoscopy
(distance)
what is mem retinoscopy
Lenses used to neutralise reflex when patient fixes on retinoscope (target attached)
Record dioptric value of lens
+ve lens = lag of accomm (under accommodating)
-ve lens = lead of accomm (over accommodating)
what is not retinoscopy
Distance of retinoscope moved
to neutralisation point
Distance from target
calculated to give lead or lag
Neutral point behind target = lag accomm
Neutral point in front of target = lead accomm
E.g. target at 40cm and neutral point at 60cm
Accomm lag = 1/0.4m – 1/0.6m = 2.5D – 1.67D = 0.83D
Is this normal?
what do autorefactors do
Measurement of refractive power change
Accommodation measured as myopic refractive change
Shin Nippon
PlusoptiX
what is accommodative stimulus and response function
When presenting an accommodative stimulus to a participant how much do they actually accommodate?
Initial non linear portion
2 Linear but showing a lag of accommodation
3 Response to stimulus reduces
4 No further accommodation as stimulus increased
what are component of accomodation
Proximal
Tonic
Blur
Disparity
Cognitive
what are the main cues for convergence
Horwood and Riddell (2008, 2009)
Blur, disparity, proximity
Disparity is the main cue
what is the main cue to accomodation
Blur, disparity, proximity
Disparity is the main cue
Not blur
how is accomodation measured in laboratory conditions
Closed Loop
Normal conditions
Feedback mechanism
i.e. Know when something is blurred
Open Loop
Feedback loop broken
Allows individual parts of accomm function to be measured
i.e. Unable to tell if target becomes
blurred
how to induce blur (open the loop)
Closed Loop
Normal conditions
Feedback mechanism
i.e. Know when something is blurred
Open Loop
Feedback loop broken
Allows individual parts of accomm function to be measured
i.e. Unable to tell if target becomes
blurred
what is blur driven accomodation
+ve and –ve blur of retinal image identical – how do we know if too close or too far away?
Stimuli = chromatic aberration – gives cue as to direction of adjustment necessary
Fincham and Walton (1957)
Sense if higher contrast of a blue fringe or red fringe to determine if over or under accomm (not visible on conscious level)
If use monochromatic light (chromatic aberration neutralised) accomm less accurate
Reduced sensitivity to blur (Kruger and Pola 1986)
what is proximal accomodation
Rosenfield et al (1991) show that proximally induced accommodation occurs for targets up to 3m away
3D at 20cm
Graph: Mean open loop
accommodative response
plotted against target
position.
what is tonic accomodation
Dark focus/accomm
Resting point of accomm
Mean 0.9 to 1.2 D
Rosenfield et al (1993)
Tonic accomm in 220 students, measured with laser optometer, found mean 1.52D
what is convergence accomodation
Produced as the eyes converge
Value in region of 0.1D/Δ
(for review see Hirani and Firth, 2009 – their mean 0.13±0.04 D/Δ)
Thus for every 10Δ of forced convergence, around 1D of accommodation occurs and must be inhibited to maintain clear vision (relative negative accommodation).
what is the act ratio
Stimulus AC/A ratio: Assumes response is equal to stimulus (e.g. 3D lens, assume 3D accomm)
Response AC/A ratio: Measures response and uses this in formula
Response AC/A higher
x 1.08 (Alpern et al, 1959) – 4 subjects
x 1.23 (Gratton and Firth, 2010) – 16 orthoptic students
Strabismics – mean x 1.41 higher, but large range (Miyata et al, 2006)
