bifocals Flashcards
(39 cards)
what is presbyopia?
- term given to a normal reduction in accommodation with age
- normal physiological condition
- starts at about 40 years
why does presbyopia happen?
- it happens due to the thickening of lens with age , and as lens continues to lay down layers this means that the lens becomes stiffer and harder to change shape
- when the lens becomes stiffer there is a reduction in the ability to change shape - which means patient’s loose the ability to see closer objects and therefore require some form of correction to help them see it near
what are the glasses options for presbyopia?
- single vision lenses
- single vision lenses allow a patient to see the prescription through the whole lens but only allowed to see one distance at a time
. full lens dedicated to prescription
. two separate pair
. not very useful for people who multitask
. when you have reading prescription on you won’t be able to see anything in the distance
what are other options that have been developed if you don’t want single vision lenses?
1- bifocals
2- trifocals
3-progressive lenses
what are bifocals?
bifocals - focuses at distance and near
- use distance and near prescription
- much more convenient for patient’s allowing them to watch tv and read at same time
what are trifocals?
trifocals - focuses at distance, intermediate and near
- modified form of bifocals which allows to focus at three distances
what are progressive lenses?
- progressive lenses
- these lenses contain prescriptions catering from distance all the way to near vision
- but the size of area that you have to look through for each prescription varies on the type of lens given
- they have some form of peripheral distortion - so the patient has to look through specific areas of lens to get clear vision
e.g.
. varifocals
. enhanced readers/occupational lenses
when are bifocals dispensed?
- bifocals are dispensed when there are problems with accommodation
- usually due to presbyopia
- can also be due to other problems with accommodation such as in children or if there is an issue with the lens
what is the near portion or segment in a bifocal lens?
- near portion ( segment) contain the near vision prescription - which is a combination of the distance prescription and the addition that has been provided to help the patient see up close
- the addition is always a positive value - generally same for left and right eye
what are the advantages of bifocals ?
- bifocals allow clear vision at any 2 distances
- wide reading area- can range in size - so can order a wider or smaller segment
- more convenient than 2 separate pairs
what are the disadvantages of bifocals?
- visible line between distance vision and reading area, so indicative of patient’s age
- smaller seg diameters have limited field of view compared with single vision
- patients over 55yrs will start to require intermediate correction on top of distance and near
e.g. computer screen
may need to recommend trifocal or progressive lens - prismatic effect- induced by segment
- condition called jump -
how are bifocals described ?
- bifocals are described by their segment shape and size
-e.g. D28
D= shape
28 = size - means at its widest point the segment will measure 28mm across
what are the different bifocal shapes that we can order ?
D seg
R seg
C seg
E seg
what is a D seg?
D seg - has a straight top to the segment - so looks like capital letter D on its side
- the value of the number written next to the D refers to the width of the shape at its widest point
what is a R seg ?
R seg- stands for round segment
- the dividing line of this segment is single circle arc
- sometimes the whole of the segment will fit into a lens, in which case it looks like a full circle
- depending on how high up with in the lens we place the segment , you may only see part of circle
what is a C seg?
C segment looks like a D segment , but has a curved top to it
what is a E seg?
- E segment stands for executive segment
- the segment dividing line crosses the whole lens
- for an executive segment you will only write an E seg- you wouldn’t write a size next to the E as the size will go through the whole lens
what are the features of D segs?
- very commonly prescribed lens
- from a cosmetic view point - its reasonable - straight line is noticeable though
- vision - seg exerts base up prism - this is good for minus lenses - this is because as you look down in a minus lens, you naturally experience some base down prism, so the segment exerting some base up can be a good thing as it reduces the amount of prism the patient experiences
- availability - excellent - available in large and small sizes
what are R seg features ?
cosmetic - good - segment not as noticeable
vision - seg exerts base DN prism - good for positive lens
availability - excellent - available in large and small sizes
what are C seg features ?
- cosmetic - better than D segs - curved top not as noticeable
- vision - seg exerts base UP prism - good for minus lenses
- availability - reasonable selection of sized
what are executive lenses features?
- cosmetic - poor - straight line across - most noticeable
- vision - good optics, no jump - one of the best
- availability - harder to get hold of
what are the measurements we need to take to order bifocal lenses?
- need to measure the patient so that the bifocal segment sits in a comfortable place for reading
. need to record
- distance binocular PD
- near binocular PD
- heights
what do we need to take for horizontal measurements?
- we need to measure the patient’s binocular distance vision PD
- we need to measure patient’s near binocular PD
why we do we need to measure the patient’s binocular distance vision PD?
- we do this because when patient is looking through the distance portion of the bifocal, they will be looking through their distance prescription