Vision and Visual Acuity
Visual acuity comes first. I had a nice lunch a few weeks ago with an ophthalmic instrument manufacturer's marketing representative and one of the company's scientists, and the conversation turned to the mundane topic of visual acuity. We talked about how important accurate refraction is to refractive surgery, and we talked about how you may get variations in visual acuity measurements from one office to the next and from one examination room to the next.
There are so many variables. How's the lighting in the room? Is there glare from the overhead lamp or on the screen? How much do you encourage patients to guess? And especially, in most cases, is the projected image calibrated? Have you asked your ophthalmic laboratory technician to calibrate your visual acuity chart lately? When was the last time you checked the distance from a patient's entrance pupil to the chart? Is the lighting appropriate (480 lux)? If the patient's face is 20 feet from the chart, is the 20/20 letter 8.7mm high? If this distance is other than 20 feet, is the 20/20 letter 0.44 X distance high? Seems like checking all of these things might matter since it's one of the most important tests we do. Our discussion at lunch turned to how new automated, remote control projectors, once calibrated, are probably more likely to provide consistent results.
Why don't we measure low-contrast VA or contrast sensitivity? Too time consuming? What do we do with the result? Almost all of us are at fault here. And yet this is what our patients see. They see things (or don't see them well) in an array of contrasts. Some contact lens patients complain of poor vision, especially late in the day or in low light situations, even when our high-contrast visual acuity measurement is good. This is especially true if their lenses aren't clean or if the surfaces are drying as well as for our patients who have irregular corneas, especially keratoconus patients whose low-contrast VA may be lines worse than that of a normal patient due to aberrations and corneal opacities. In these cases, clean lenses with good surfaces and a better, perhaps aberration-correcting lens may be helpful. So next time you measure good VA and your patient still complains of poor vision, think about the low contrast situation.
And consider checking your acuity calibration as well.