unit 6 Flashcards
3 principle measures for near VA
-unaided near va (near vision)
-habitual near va (w own gls)
-optimal near va (w/ best refractive correction)
to focus on a near object the eye needs to do what ?
increase it dioptric power so it will be positive accomodation
when an eye changes focus from near to distance the eye needs to what?
decrease its dioptic power so it is negative accomodation
what is the purpose of accomodation
neutralize negative vergence from a near object
3 main components of the eye involved in the process of accomodation?
-ciliary muscle
-zonules
-crystaline lens
what happens to the eye structures when unaccomodated?
-when looking in distance, the ciliary muscle is relaxed
-the crystaline lens is pulled and stretched into a thinner and flatter form by the zonules
what happens to the structures of the eye when accomodating?
- the ciliary muscles contract causing it to move forward
-this in turn relaxed the tension on the zonules, resulting in an increase in curvature of the crystaline lens
-tonic
-convergence
-proximal
-refelex
-voluntary
tonic accomodation
convergence accomodation
amount of accommodation stimulated by convergence
- in young pxs the accommodative response follows convergence of the eye
what is the reaction time for convergence?
0.2 seconds
-2x faster than reaction time for accomodation
proximal accomodation
amount of accomodation induced by the individuals awareness of the proximity (nearness) of an object
reflex accomodation
the normal involuntary response to blur in order to maintain a clear retinal image
voluntary accomodation
-does not depend on the presence of a stimulus
-ability to relax accommodation from some near focus position is easily learned
amplitude of accomodation
Total amount by which an eye can change in power
-eyes max power minus the eyes min power
Blur technique
-requires the use of the RAG near point rule
-centimetre scale
-diopter scale
-age scale
push up to blur method
-distance correction in place
-RAF rule is placed in a slightly depressed positin to mimic the action of the eyes when reading
-Monocular amps are measured first
-Binocular amplitude is usuaully greater than monocular bc convergence drives accomodation
-USE N5 letter as target
-individual moves the near card along the sliding scale from the end of the rule until the print just blurs
-the amplitude of accomodation is read off at this point
push down to clear method
-individual starts with the print at the end of the scale closest to the face
-move the print away until its just legible
-the amplitude of accomodation is read off at this point
-if both techniques are used, the recorded amplitude of accomodation should be ;
the average of the readings obtained with both methods
What is presbyopia
the amplitude of accommodation decreases gradually with advancing age
when is presbyopia said to be present
When the A of A dropped to around 3.00D
-causes px to require reading gls
what happens to the lens and ciliary muscle with presbyopia?
-the lens capsule gradually loses elasticity
-the lens nucleus begins to stiffen (from about 30 years old)
-thickening of the ciliary muscle and growth of the lens reduce tension in zonules
at what age does presbyopia usually occur/
around 45 years old
clinical evidence suggests that px are able to maintain comfortable near vision if they exert…?
no more than two thirds of their amplitude of accomodation
-symptoms of presbyopia
-near blur
-distance blur after prolonged close work
-eyestrain or headaches
-headache around the temple and forehead
-reading difficulty in poor lighting
presbyopia / ADDformula
ADD= / L / - 2/3 Amp
/L/ = 1/l (working distance in meters)
/L/ = vergence of the incident light arriving at the spec lens
‘normal’ patients may be divided into 3 groups for presbyopia
- pre- presbyopes
- early presbyopes
- late presbyopes
early presbyopes
-around age 40 but some younger px who have abnormal accomodation
-near addition required depends on wokring distance and the accomodation still available
-amplitude of accomodation may still be usefully measured
late presbyopes
-have no remaining accomdation of their own
-reading addition is largrely determined by the working distance
would you measure amplitude of accomodation of pxs over 55?
No it is unproductive so it is rarely attempted
prescribing of a reading addition consists of two phases
- An addition is estimated by considering the age, accomodation and any existing correction that the px may have
- Refine subjectively, taking into account the occupational and lifestyle needs of the px
if the working add is much less than 40cm what would you do to the tentative add?
increase the add
what would you do to the add for computer work at 50-60cm
decrease the tentative add by about 0.50
negative relative accomodation
the max ability to relax accomodation while maintaining clear, single binocular vision
positive relative accomodation
the max ability to stimulate accomodation while maintaing clear, single binocular vision
balancing NRA/PRA
-Adjust phoropter to near PD and attach the near point card
-make sure the optimal distance correction is in place and OU can view the near card
(bring vergence levers in)
-direct pxs attention to letters one or two lines larger than best va
-Ask the px if they are clear
-If they are not, add +0.25 at a time until px reports the letters are clear
-this becomes the initial tentative near add
NRA balancing
-Add plus lenses binoculary, +0.25 at a time, untilt he px reports the first substained blur
-the total amount of plus added is the NRA
-return the lenses in the phoropter to the initial tentative near add found earlier
PRA balanacing
-add minus lenses binocularly, -0.25 at a time until px reports the first sustained blur
-The total amount of minus added is the PRA
Near duochrome / bichrome method
-Start with estimated add in place
-testing monofulcarly, plus lenses are added until the red chart at near is clearer for each eye
-reduce the plus lenses binocularly until the green chart is clearer and the plus lens remaining is the prebyoic add
-NOT COMMONLY USED
The higher the add what happens
-smaller the depth of focus
-the progression characteristics and the near field of view may be affected in PAL
Refining the near add
-aim is to arrive at an add that allows clear and comfortable vision at all distances at which px habitually works
-should be preformed after the distance rx has been determined
what is the range of clear vision
the distance between the artifical far point and the artificcal near point
artifical far point
-the point conjugate with the centre of the macula at the corrected and unaccomodated eye
artifical near point
the point conjugate with the centre of the macula at the corrected but fully accomodated eye
in some cases it is impossible to cover all a pxs requriments with one near add - true or false
TRUE - additional pairs of specs may be required to cover part of the range
-progressives or occuptational progressives may be required
Most common errors in reading adds
-Estimating the tentative add based on pxs age, not WD
-Not determining the pxs near vision needs
-estimating distances instead of measuring w a tape measure
-over plussing
-under plussing - px w a nuclear cataract may require the distance rx to be reduced to reflect the myopic shift
