The Near Response 1 and 2 Flashcards
The Near Triad
convergence, accommodation and pupil myosis
Responses can occur independent of the others and hence term ‘synkinesis’ preferred rather than ‘reflex’.
Subjective measurement
- Accommodation can be measured using the RAF rule or done in free space. This is known as the near point of accommodation and accommodative facility.
Objective measurement- Done using the dynamic retinoscopy and autorefractors (Shin Nippon, PlusoptiX)
Near point of accommodation
Maximum increase in optical power that an eye can achieve in adjusting its focus from far to near
Calculation for emmetropes
Accom (D)= 1/ far point (m)- Accom (D)= 1/ near point
Minus lens method
- Tested monocularly
- Patient fixates a small target at near or distance wearing refractive correction
- Minus power lenses are introduced in 0.25D steps
- Patient reports when letters become and remain blurred
Near/distance not comparable
(Momeni-Moghaddam et al 2013)
Minus lens addes to induce…
Blur
Minus lens added examples
-4.00 DS
-4-2.5= -6.5D
Minus lens amplitude= 6.5D
Testing distance 40cm/ 2.5 dioptres
1/0.4(m)
= 2.5 D
NOTE- Since the test is done at 40cm subtract working distance 2.5D from -4DS
RAF rule instructions
Instruct patient to let you know when text starts to blur as this is the end point
Do RAF rule instructions matter?
Goodall & Firth, 2003 findings
Found a clinically significant difference of 6.8cm between the point at which text blurs and the point at which it becomes indistinguishable in early presbyopia subjects.
Recommendations of RAF
1) With target far from eyes and move forwards (Conventional method, clear to blur)
2) With target close to eyes and move back (Modified push-up method, blur to clear)
What is the end point
NOTE- End point is when blur first noticed.
RAF rule literature
Modified push up method
Better for young children as difficult to understand blur.
(Scheiman and Wick, 1994)
Easier to judge end point
(Esmail & Arblaster, 2016)
Repeatability of RAF rule
Conventional method had poor test-retest in children 6 - 10 years. Variation of 3.1D found.
(Adler et al, 2013)
Conventional method gave significantly closer NPA (on average 0.73cm) in RE, LE and BEO but no clinically significant difference. Modified and Conventional push-up methods are not interchangeable.
(Esmail & Arblaster, 2016)
The 2 methods are similar in adults and children (but both different to minus lens method).
(Taub & Shallo-Hoffmann 2012)
Accomodative facility
The ability to rapidly change accommodation to various stimuli.
Tested with flipper lenses.
Accomodative facility method
Can be tested binocular (mostly) and monocular, near and distance.
Pt should wear refractive correction
Present +2.00DS lens
Allow patient time to clear target
Present -2.00DS lens
Allow patient time to clear target
Repeat for 1 minute
Record number of cycles in 1 minute
Recording accommodative facility
Accomm facility +/-2DS 12cpm
Tested with +2 flipper lens and mostly tested binocular
Accommodative facility norms 3 studies
Pandian et al (2006) in 1328 5-8 y/o:
– 7 cycles per min monocularly at near and in distance
– 5.5 cpm monocularly 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 7.3cpm to 10.4 cpm on 2nd test
– Recommendation: Should test binoc only
3 cycles more achieved on 2nd test monocularly in Horwood and Toor (2014)
Dynamic RET methods
Monocular estimation method MEM- lens used
NOTT- RET distance moved
MEM method
Patient fixates on target (at 40cm) attached to retinoscope
Lenses are used to neutralise reflex
Record dioptric value of lens at neutralisation
MEM end result
End result:
+ve lens = lag of accomm (under accommodating)
-ve lens = lead of accomm (over accommodating)
Normal values are between +0.25 D and +1.00 D of accomm lag- accommodative lead is abnormal
With V Against
With motion → plus lens
Against motion → minus lens
Not retinoscopy
Patient fixates on target (often 40cm)
Distance of retinoscope moved to neutralisation point
With motion → move closer to the patient
Against motion → move further away from the patient
Distance from target to retinoscope is calculated to give lead or lag
Neutral point behind target = accomm lag
Neutral point in front of target = accomm lead
If target is 40cm and neutral point is 60cm there is… accommodative lag
Nott retinoscopy further
If target is at 40cm and neutral point is at 60cm, calculate the accommodative lag:
= 1/(0.4(𝑚)) - 1/(0.6(𝑚))
= 2.5D - 1.67D
= 0.83D
Is this normal?- Target is point of neutralisation
Autorefractors
Measurement of refractive power change- accommodation is measured as myopic refractive change
PROS AND CONS OF subjective methods and dynamic RET?
PROS- quick and engaging for children
CONS- poor fixation in nystagmus, high refractive error and can get damage
Accommodation in research
1) Initial non-linear portion
2) Linear but showing a lag of accommodation
3) Response to stimulus reduces
4) No further accommodation as stimulus increased
- Accom response less than demand
Response is less than demand in…
Hypo-accommodation seen in children, accommodative response is less than demand
More so in corrected hypermetropes (Horwood and Riddell 2011
Main components to accommodation and convergence
Disparity
Does accommodation drive convergence?
Accommodation does not drive vergence, neither does vergence drive accommodation, but both are driven by a combination of cues from the outside world
Which factors causing accommodation can be isolated
Closed Loop
Normal conditions
Feedback mechanism
i.e. Know when something is blurred
Open loop mechanism
Feedback loop broken
Allows individual parts of accomm function to be measured
i.e. Unable to tell if target becomes blurred so eliminates accomm due to blur
Opening the loop causes blur
Gabor/ Difference of Gaussian
-No stimulus for accomm, Eliminates blur
Pinhole (0.5mm)
-Dist target, Eliminates blur
Ganzfeld field
-Illuminated empty field with no focusable contours, Eliminates blur
Proximal accommodation
- Rosenfield et al (1991) show that proximally induced accommodation occurs for targets up to 3m away
- 3D accommodation at 20cm
- Graph: Mean open loop
accommodative response drops off after 3m, plotted against target position
Blur driven accommodation
“Blur is the subjective awareness that the edges of a high contrast image are indistinct.” (Horwood, 2022) +ve and –ve retinal image blur is identical
Chromatic effects
Fincham and Walton (1957) suggested chromatic aberrations give a cue as to direction of accommodation adjustment necessary.
* Retinal images have subtle colour fringes, not visible on a conscious level.
* This is due to the spectrum of foci of a retinal image.
* Short wavelengths (e.g. blue 400nm) focus much further forward in the eye than long wavelengths (e.g. red 700nm).
* Red fringe retinal image: focus behind the retina
(under-accommodation).
* Blue fringe retinal image: focus in front of the retina
(over-accommodation).
* Monochromatic light (chromatic aberration neutralised) showed: accommodation was less accurate.
* Reduced sensitivity to blur (Kruger and Pola 1986)
Two pilots both have excellent visual acuity in the examining room. However, at night one can consistently detect and identify targets faster than the other…
WHY?
Darkness reduces other cues
Tonic accommodation TA
When visual stimulus is poor e.g. at night, the eyes adjust involuntarily for a distance determined by the individual amount of ‘tonic accommodation’.
Distance is usually intermediate and varies widely between individuals.
TA aka
* ‘resting focus’
* ‘tonus state’
* ‘dark focus’
Leibowitz and Owens (1978) findings
Reported distribution of dark-focus values for 220 emmetropic/fully corrected college students. All measures were taken with a laser optometer in total darkness. There is a wide range. Average = 1.5 diopters, This corresponds to a focal distance of 67 cm. 1/1.5 = 0.67 m
Convert to cm 67 cm.
Convergence accommodation
The amount of accommodation resulting directly from convergence of the eyes.
CA/C ratio assesses if a known amount of convergence results in excessive or defective convergence accommodation.
Convergence us initiated with…
a base out prism
CA/C ratio in region of
0.1D : 1PD
(Hirani and Firth, 2009 – their mean 0.13±0.04)
For every 10 PD of forced convergence, around 1D of convergence accommodation occurs and must be inhibited to maintain clear vision (relative negative accommodation).
AC/A Ratio value
Its the amount of accommodative convergence
(measured in prism dioptres (PD)) occurring per dioptre of accommodation
4:1 or LESS
So for every 1D of accomm, there is 4PD of accommodative convergence
Response AC/A Ratio
Response AC/A ratio measures the response and uses this in formula.
Response AC/A was higher than stimulus AC/A ratio:
x 1.08 (Alpern et al, 1959) – 4 subjects
x 1.23 (Gratton and Firth, 2010) – 16 orthoptic students
x 1.41*(Miyata et al, 2006) – 63 strabismic patients, 79% exo deviations
– mean was 1.41 higher, but large range (Miyata et al, 2006)
Strabismic patients:
46 patients with intermittent exotropia, 4 with exophoria, 8 with esophoria, and 5 with esotropia
Stimulus AC/A Ratio assumption
assumes the change in accommodation (i.e. the response) which occurs when viewing a target through a minus lens is equivalent to the power of the lens used (i.e the stimulus). Response = stimulus.
e.g. assume 3D accomm for -3D lens
Often a lag of accommodation (response < stimulus)
e.g. 2.5D accomm for -3D lens
When plus lenses used for near often lead of accommodation as incomplete relaxation of accommodation (response > stimulus)
e.g. 4D accomm for -3D lens
Cognitive accommodation; effort increases the level of accommodation
Francis et al (2003):
‘Effort-to-see’ affects vergence and accommodation levels
Instructions to concentrate vs space-out
Horwood and Toor (2014); Horwood et al (2014):
Compared accomm in various exercise groups
‘Effort’ had significant improvement on accomm (and convergence)
Asymmetrical accommodation
Accommodation was thought to be symmetrical between eyes, however this is not always the case. 23% Anisometropic amblyopic children studied had very asymmetrical or even inverse responses in amblyopic eye. Accommodated more in distance and relaxed accommodation for near.
