Amplitude of Accommodation Flashcards
What is amplitude of accommodation
the dioptric distance between far point and near point of accommodation
What is far point
point conjugate with the retina of the unaccommodated eye
what is the near point
point conj. with retina of max accomm eye (for objective amplitude) OR closest distance that can be seen cearly (for subjective amplitude)
The equation for amplitude says
AMPL = FAR PT (D) - NEAR PT(D)
If an uncorrected 3D myope has a near point of 25cm, what is the amplitude
AMP = -3 - (-4)
AMP = 1D
What are the 5 methods to measure amplitude of accomm
- push up
- push down
- push away
- minus lens (ML)
- objective techniques (dynamic ret and autorefractor)
What is push up
a target is pushed towards the pt until they report first sigh of sustained blur (FSSB)
- this represent pt near point
- when pt is corrected near pt = amplitude
how can the push up technique be measured
OD, OS and binocular OU
How is target size relevant
change in target size = change in ability to notice blur
- lowering target size = increased blur detection = decreased amplitude .. occurs in PUSH UP
- increasing target size = decrease blur detection = increased amplitude .. occurs in MINUS LENS
according to snellens fraction m the patients near point can also be
the viewing distance, numerator of fraction
What is the push down technique
similar to push up, the target it brought towards the pt until full blur > push away until fully clear
typically the responses of push up are
over estimate of the actual amplitude
typical responses of push down are
under estimate of the actual amplitude
What is push away amplitude
placing target close to pt eye, ensure blur > push target away from eye until they can identify the target (not fully clear)
Whats the 2 major problems with push away amplitude
- end point is when the subject can notice the target, which is not first point of blur > most likely target is farther than needed
- children have high amplitudes, hard to make large jumps
What is minus lens amplitude
- target is at 40cm (2.5D)
- minus lenses added over distance Rx until response for FSSB
What does the amplitude equation say
amplitude = 2.50 + added ML before blur
where 2.5D is the AS
each diopter of minus lens power increases the accommodative stimulus by
1.00D
How can the minus lens amplitude be measured
monocularly only
what is proximal accommodation
assists to find max accommodation, proximal accommodation should be increased
what happens to proximal accommodation during minus lens amplitude
stays constant
what happens to proximal accommodation during push up
increases
What is positive relative accommodation (PRA)
An increase in accommodation while the vergence remains constant
How is PRA measured?
adding minus lenses binocularly
which measurement of amplitude is more favorable, objective or subjective
Objective - measuring the point conjugate with the retina where the pt exerts maximum accommodation
what are considered objective techniques
dynamic ret
autorefraction
what are considered subjective techniques
push up
push away
push down
what is always true about the relationship between objective and subjective amplitdue
objective is always < subjective due to blur
how do you determine when blur will be reported
when AS - AR > half (DOF in Diopters)
Based on Duane’s table, what amplitude does a 40 year old have
5.5
Hoffstetter Eq: for minimum expected amplitude =
15 - 0.25 * (age in yrs)
Hoffstetter Eq: for mean expected amplitude
18.5 - 0.3 * (age in yrs)
Hoffstetter Eq: for maximum expected amplitude
25 - 0.4 * (age in yrs)
the findings of Duanes are based on what kind of accommodation ?
accommodative stimulus (AS), a subjective amplitude
what are the clinical measurements of accommodative response
Dynamic Ret (cross -Nott)
dynamic cross cylinder
near duochrome
What is dynamic cross cylinder
a subjective test to quantify the lead or lag of accommodation
- corrects the pt for distance
introducing a +/- 0.50 JCC, negative axis vertical over their distance rx
if a patient has a LAG of accommodation, which FL is closer to the retina
HFL are closer, thus they appear clearer to them
AR < AS
In dynamic cross cylinder , plus spheres are added until
the two sets of lines appear equal (COLC on retina)
the amount of plus added to achieve equality equals
lag of accommodation
if a pt has a LEAD of accomm. which FL will be closer to the retina
VFL are closer to retina, thus appears clearer to pt
AR > AS
the amount of minuses added to achieve equality equals
the lead of accommodation
what happens when a pt accomm. accurately on the target
AS = AR
VFL and HFL are equidant from target, both lines appear clear
how is dynamic cross cylinder measured
binocular and monocularly
what age qualifies for accurate dynamic cross cylinder measurement
35yr old +
for near ducochrome test what does pt response of green and red mean respectively ?
green = lag of accom
red = lead of accom
what is duochrome testing measuring vs. not ?
helps identify the presence of lead and lag but not the measurement of how significant it is
