Visual acuity is tested by asking the patient to read letters of different sizes. Visual acuity is broken into two pieces a numerator, the top number, and the denominator, the bottom number.
The numerator represents the test distance.
The denominator is the number that represents what the patient can see.
20⁄20 Means that at 20 feet the patient can see what a normal eye can see at 20 feet.
20/60 Means that at 20 feet the patient can see what a normal eye can see at 60 feet.
15⁄20 Means that at 15 feet the patient can see what a normal eye can see at 20 feet.
20/15 Means that at 20 feet the patient can see what a normal eye can see at 15 feet.
You will also need to know how to convert any acuity to “20/x.”
Convert 5/100 to “20/x.”
Is 6/18 better than 20/20?
Answer: 6/18 = 20/60 which is worse than 20/20
If the patient can’t see any of the letters hold up fingers and see is the patient can read them. Recored the distance that the patient was able to correctly count the number of fingers that you were holding. CF5′ means that the patient could count fingers at 5 feet.
If the patient can’t count fingers then wave your had in front of the patients eye. If the patient can tell that you are waving your hand then record that as ‘HM.’
If the patient can’t see your hand waving then shine a light in the patients eye. If the patients sees the light record it as ‘LP’(Light perception). If the patient can’t see the light record it as ‘NLP’(No light perception).
Note: Light perception is different from light projection. While light perception is the ability to see light light projection is the ability to locate where the light is coming from.
Visual acuity rules
There are some rules that a patient need to follow when testing visual acuity.
The patient needs to be corrected with best corrected vision. This means that if the patient has glasses the patient ought to wear those glasses during the exam.
The patient needs to cover on eye while the other is being tested.
The patient should not squint. Squinting acts like the pinhole, which I will talk about later in the post. This will give a falsely high acuity.
Potential Acuity Meter
The potential acuity meter (PAM) is an eye chart that can be attached to the slit lamp and light up. The purpose of the PAM is to see what the patients vision is without medial opacities.
The pinhole will increase VA if the cause of poor VA is the cornea. If the cause of poor VA is an ocular pathology then the pinhole will not improve VA. This works because the hole that the patient looks through blocks all misaligned light rays and allows central rays to focus on the retina.