Vision evaluation involves a lot more than making patients read the eye chart. Here are some basic steps in visual acuity testing, as well as some tips on getting the best results during the process.
‘Ask the patient to cover their right eye and read the smallest line they can see, and then do the same with the left eye’ — this is how one is generally taught to evaluate the visual acuity of a patient. And if this is what you have been taught, then are not the only one.
If you are among the lucky ones, you may have received a few additional instructions, such as, ‘make sure the eye not being evaluated is fully covered’ and ‘ensure that the patient is not looking out of the bifocal segment of the glasses when checking distance acuity’. Is this all you really need to know to properly assess the visual acuity of a patient? Absolutely not!
This article discusses the basic steps used in visual acuity testing and offers tips on how to get the most accurate results.
TESTING DISTANCE VISION
Step 1 : Occlude the left eye
Watch the patient while the chart is being read to make sure there is no intentional or non-intentional peeking. Amblyopic children are especially prone to peek when the eye with the better vision is covered. If the patient wears spectacles, consider using a clip-on occluder. Simply hang the occluder from the frame to cover the eye, switching sides as needed.
When using a hand-held occluder, instruct the patient to keep the occluder touching the side of the nose. An alternate method is to hold the occluder yourself.
Step 2 : Ensure the patient is looking at the chart through the distance portion of the glasses when checking vision with correction.
Raise the exam chair for short patients to keep them from bending their neck back to look at the chart. This is particularly important when the patient wears a progressive lens as bending the neck to look upward will put the line of sight through the intermediate portion of the lens, adding plus power that is not to be used for distance viewing.
Look at the position of the frame on the face. It is easy to push the frame up on the face when covering the eye with a hand-held occluder. Or, the patient may be looking over the top of the frame.
Step 3 : Read the smallest line of letters possible.
Don’t stop at 20/20. If you don’t ask the patient to read the smallest line they can, you might not be able to get a true acuity assessment. A patient who reports decreased vision at their yearly eye exam but sees 20/20 may be told that their vision is the same as last year, when in reality they could have seen 20/15 last year and there truly is a decrease in vision.
A patient with a hemianopia may only read the first few letters on the line. If you don’t push them to read the next line, you may miss that they are only seeing half of the chart.
Step 4 : Occlude the right eye and repeat steps 1-3.
The right eye is generally tested first, however, in the case of a patient whose vision is normal in the right eye and very bad in the left eye, the eye with the worse vision may be tested first to avoid chart memorization.
TESTING NEAR VISION
Basically, the same steps and tips used in distance testing are used in near testing. However, there are a few additional tips that will help you achieve the best results:
Make certain that the patient is looking at the near card through the bifocal portion of the glasses when testing with correction.
Near vision should be tested at a distance of 14 inches to 16 inches.
Hand the patient the near card before instructing them where to hold it. You can learn a lot by observing the natural place that the patient puts the card. For example, if you hand a patient the near card and he either pulls his head back or pushes his arm out further to see the card, he most likely needs more plus power to see at near.
Observe the pupil reaction when the patient looks from the distance to the near card. The normal reaction is an equal constriction of both pupils.
There are times when the basic steps have to be altered a bit due to specific circumstances such as those listed below.
PATIENTS WITH AMBLYOPIA – THE CROWDING PHENOMENON
Amblyopia is a relatively common vision disorder that occurs when the nerve pathway from one eye to the cortical area of the brain does not develop during childhood. Strabismus is the most common cause of amblyopia. However, a significant difference in refractive error in the eye, uncorrected high refractive error, or vision deprivation as seen with ptosis and congenital cataracts can also interfere with normal cortical visual development. One characteristic of amblyopia is called ‘the crowding phenomenon’. The patient demonstrates an inability to read an entire row of optotypes, but can read a single optotype of the same size.
Let’s look at a real-life clinical scenario:
During a routine vision screening at school, the eye care practitioner recommended that Andrew, a 5-year old, be sent for a comprehensive eye examination, after the child performed poorly on a vision test. The optometrist checked Andrew by displaying single Snellen letters and recorded his vision as 20/20 OD and 20/40 OS. He prescribed a patching regimen and scheduled a return visit.
Brad is the optometrist who performed the pre-testing when Andrew returned. He tested the vision by showing Andrew full lines of Snellen letters and recorded his vision as 20/20 OD and 20/60 OS.
Has the vision actually decreased? We have no way of knowing since the original test was performed with single letters and the follow-up test was performed with a full line of letters.
Using single letter testing on a patient with amblyopia can mislead the doctor into believing that the vision is better than it really is. For this reason, visual acuity should always be measured with an entire line of letters. If you wish to note what the vision is with single letter testing also, record the vision as: OD: 20/60, single letter 20/40. Testing both the single letter and the full line vision will evaluate the potential macular vision (single letter) and the functioning vision (full line).
PATIENTS WITH NYSTAGMUS
Nystagmus is an involuntary, oscillating, rapid movement of the eyeball. Occluding one eye increases the eye movement, which will result in worse acuity than the patient’s normal, functional acuity. For this reason, visual acuity should be tested using a high power convex lens such as a +10.00 diopter, instead of an occluder, over the non-tested eye. The high plus power lens will fog the non-tested eye enough so that the patient will not be able to see the chart and allow light to enter the eye keeping the eye movement from increasing.
Visual acuity testing is an extremely important facet of the eye examination and involves more than asking the patient to read the chart. Much of the eye care practitioner’s decision-making process is based on the results of visual acuity testing.
A patient may lose their independence by having their driver’s license revoked or be considered legally blind based on the results of a visual acuity test. A young person may have to give up dreams of becoming an airplane pilot because he failed the acuity test. Make sure you document the most accurate acuity assessment possible.
The term ‘malingering’ is used when a patient is dishonest concerning visual ability. It is most commonly seen in school age children who want to wear glasses. Children like to imitate their peers. Therefore if a friend gets glasses, the child may suddenly develop problems seeing.
If it is suspected that the child wants glasses, a pair of ‘glasses’ can be made by using a trial frame with plano lenses. If the child reports the vision is improved with the trail frame glasses, malingering should be suspected. Malingering can also occur when a patient may receive additional compensation for decreased vision, or in a patient with an emotional disturbance. You should never accuse the patient of being dishonest about their vision — just note the factual findings on the chart. For e.g., the report could read ‘Visual acuity improved from 20/40 to 20/20 with plano trial lens’.