practical 3 - measurement of amplitude of accommodation Flashcards
what is the near point Mp?
is the point conjugate with the retina when the eye is in its fully accommodated state
. near point distance is given the symbol b , its dioptric equivalent is B
how is near point measured ?
. the near point is commonly measured using a near point rule (RAF) rule
. it is used to detect problems with accommodation
what is amplitude of accommodation ?
. amplitude of accommodation is the dioptric difference between far point and near point
. Amp o = K (ocular refraction of far point) - B o
- referenced to the eye o for ocular
. if eye is fully distance corrected then the far point will be at infinity and
Amp o = -B o ( neat point )
what is the formula for amplitude of accommodation if we measure to the spectacle plane and eye is corrected ?
. if we measure to the spectacle plane and the eye is corrected
Amp s = -Bs ( near point )
. -B s ( near point ) - we can read this value directly off the RAF rule , if and only if the eye is corrected. if the eye is not fully corrected or we use an additional lens this must be taken into account
what is the normal age for accommodation for a 20 year old ?
9D to 13.5 D
what is the normal range for accommodation ?
normal range is at least +/- 2D up to the age of 45 for emmetropes and corrected ametropes after that range becomes compressed because as we become presbyobic the amount we can accommodate gets less and thus variation also gets less
why do we take multiple reading when measuring accommodation using RAF rule ?
. all measurements have variability
. if this could be clinically significant , we can’t rely on a single value
. averaging gives us a value that is most probably closer to the true value and also a measure of the variability , for example , standard deviation
why don’t we average RET results ?
. ret results vary by +/- 0.95D - 95% of time repeat ret reading will fall within this range
. it takes time and you know the results, which will bias you
. use multiple check tests
. we use subjective refinement instead
what is the difference between push-up and push - down method ?
. push-up - keep target in focus and steadily increase accommodation to limit
. push-down - target can’t be brought into focus- so when we move target away accommodation has to fluctuate and try and find it
. some studies have shown small differences:
. push up gives higher values than push down this is due to depth of field
. binocular higher than monocular notably on push up
what is depth of field ?
. we have range of positions that we can see things in focus even when we have no accommodation .
. part of measuring accommodation
we have range values when we see things clearly even when there is no change in power due to depth of focus
why do we take both push-down and push - up values and average them ?
to remove bias as push up has higher values
why can you get higher results than expected ?
. natural variation
. uncorrected myopia - increase value of amplitude of accommodation
. RAF rule not held at right angles to the face particularly in young
what is the correct position for RAF rule ?
. face at right angle to RAF rule
. natural reading position
why can you have lower than expected results ?
. natural variation
. uncorrected/latent hypermetropia
. sub-normal VA does not lead to reduced amplitude of accommodation
. accommodative insufficiency : some people don’t accommodate as much as other people
.accommodative infacility : some people are less accurate slower with their accommodation
takes them longer to bring things into focus and less good at doing accurately
. accommodative fatigue: repeated muscular effort
what is latent hypermetropia ?
. latent hypermetropia - term we use for hypermetropia where accommodation is being used to compensate it
