VA's
Terms
undefined, object
copy deck
- When do you do VA's in the exam?
- VA's are the first preliminary test except with a chemical burn.
- Visual Acuity Definition
- The resolving ability of the eye to see two things as separate
- What is the "normal" resolving power of the eye?
- able to detect a gap with in 1'minute of arc (with or without correction)
- 3 ways to detect VA's
-
1. Minimal detectable resolution (can obj be distinguished from background)
2. Minimum separable angle (group of points or lines are separable and distinct)
*3. Recognition resolution (most used by clinicians) - what are optotypes?
- test targets based on grid work
- What is the universal method of measuring VA?
- Snellen Acuity Chart
- What are the dimensions of most snellen letters?
- 5 units high and 4 units wide
- What is the best snellen letter?
- E
- How many minutes of arc does a 20 foot optotype subtend at 20 feet?
- 5' minutes of arc (its details subtend 1 minute)
- what would a 30 foot letter subtend at 20 feet?
-
20/20='5
so...
5/20=?/30 answer=7.5 - What would a 20/200ft letter subtend at 20 feet?
-
20/20='5 min of arc
so...
5/20=?/200 answer=50'min of arc - How is the snellen fraction related to MAR?
- Snellen fraction=1/MAR
- What are some limitations of the snellen acuity chart?
-
-letter sizes between lines do not change by a constant ratio.
-between row and between letter spacing is not proportional to letter size.
-legibility for optotypes varies between different size levels. - What is the snellen fraction?
- test distance/distance at which the smallest letter read subtends 5' minutes of arc
- record VA as the smallest line of letters that not __ letters were missed
- two
- What is the general correlating VA's with refractive error?
-
for every .25D of refractive error accounts for approximatly one line of visual acuity. if the patient has cylinder take the spherical equivilant.
-count from 20/20
-works up to 2D
extra line if cyl is oblique - where does 1M letter subtend 5' minutes of arc?
- 1 meter
- when would you use teller acuity cards?
- infants and non-responsive patients
- when would you use the LEA chart?
- on children
- When would you use the HOTV chart?
- on amblyopes
- when would you use the feinbloom chart?
- on low vision patients
- how would you measure near vision for a low vision patient?
- Bailey-Lovie Chart
- are there any ever any reasons not to take VA's?
- NO! Always attempt it!
- what are some clues that you may be dealing with a malingerer?
-
-rapid drop off
-exagerated difficulty
-near and distance do not correlate - when should you do a pin hole acuity?
-
when "entering acuities" are 20/40 or worse.
(entering acuities means with correction if that makes it better) - if an eye is not correctable to 20/20 what should you do?
- Try and figure out why working from the front of the eye to the back
- When would you use the BAT brightness acuity test?
-
when you suspect that acuity would be worse in a glare situation. (Cataract looks bad but VA is okay)
Can justify cataract surgery - How are you considered legally blind?
- 20/200 or worse in the best eye OR less than 30 degrees of visual field in widest meridean of the better eye.