Accuracy of Snellen chart with poor vision

[MeganW]

My vision has been pretty poor since I was about 8 (I'm 22 now) and has been progressively worsening over the years. I have an astigmatism and I am near-sighted. I wear either contacts or glasses, with a prescription of 5 and 4.5 (I can't remember if it's + or -).

At my recent eye exam, the optometrist estimated my vision at around 20/1000. She couldn't give me a definitive number, as apparently the Snellen chart does not accurately diagnose visual function at such low acuity. My question is, is there any way to be accurately tested and given a visual acuity score?

[Forum-OD-MP]

yes, there is.  most eye doctors dont do it b/c it doesnt mean all that much.  we usually consider anything over about 20/400 to be not-very-functional vision, so we usually dont bother measuring much worse than that.

but any (and every) acuity can be accurately measured.

the fastest way to do it is by using finger counting:

20/800       CF10' (Count Fingers at ten feet)
20/1000      CF 8'
20/1143      CF 7'
20/1333      CF 6'
20/1600      CF 5'
20/2000      CF 4'
20/2666      CF 3'
20/4000      CF 2'
20/8000      CF 1'

http://www.mdsupport.org/library/acuity.html    

there are other ways to do it, but they involve some serious math:

http://www.ascrs.org/publications/jcrs/guestfeb04.html

the question is, why do you need to know?  if its a curiosity thing, you can certainly satisfy it using the above methods, but most things that would would need good acuity for (military, etc) require much better vision than 20/400.  

[aimee37]

Well, my daughter (11 months) has optic nerve hypoplasia. Her vision is assumed to be quite low. Some of the parents in my ONH group have had their kids' acuity tested at as good as 20/400, so I wanted to attempt to understand what my daughter may or may not be seeing. If I can accurately score my acuity, perhaps it would give me insight into her abilities.

[aimee37]

ah, a legitimate question.

in children really the only way i know of to fairly accurately test an infant or toddler's visual acuity is with a rotating optokinetic nystagmus drum.  pretty much only a pediatric optometrist or ophthalmologist will have a set of them, and eye/vision research institutions.  i know my optometry school had them...SUNY Optometry in Manhattan, NY

you may already know this but a rotating optokinetic nystagmus drum is a cylinder that turns.  the cylinder has vertical black & white "stripes" on it (a "grating") of varying sizes.  larger/fatter "stripes" are for lower acuity like 20/400, 20/2000, etc.  smaller/skinnier "stripes" are for better acuity like 20/40, etc.

the drum is placed in front of the child and rotated.  if the child has good enough acuity to see the size of the grating/"stripes" then the child's eyes will dart back & forth, following the grates.  this is called "optokinetic nystagmus" or OKN.  the idea is keep getting smaller in size until you find the smallest grating size that produces an OKN response and thats the infant's acuity.

you said you are nearsighted (thats negative or minus) -5.00 in your worst eye.  thats an acuity of approximately 20/500 or so

[aimee37]

The pediatric ophthalmologists have used the drum before, with limited results. How can they distinguish between optokinetic and regular nystagmus? My daughter's strabismus and nystagmus are both pretty profound. We're going to schedule a visual evoked potential test, to help determine how many impulses are reaching her visual cortex. She does not track objects, and has a very limited response to visual stimuli. We're hoping for the best, and preparing for the worst. She receives vision therapy once a week as well.

Thank you for your time.