A discussion about refraction for visually impaired individuals.

I’m often asked how I manage eyeglass prescriptions for my low vision patients, so I thought I’d spend a little time exploring this issue. I’ve presented this discussion in many talks I’ve given, but thought perhaps it was time to put it in writing.

Hope you’ll find this helpful.

Henry Greene

eyeglasses in front of an eye chart

Refraction, of course, is the optometrist’s mainstay. If we can make a sufficient improvement in acuity by refraction, (usually a two-line improvement is required for the patient to experience a functional gain), than obviously that would be our first and most convenient option. A brief retinoscopy through the current eyeglasses (if any) can be valuable to see how close to neutral the reflex is as well as its quality. (Only perform retinoscopy briefly. If you spend too much time you’ll bleach the retina, which will often delay and undermine your exam.) If the reflex is dull due to media issues, consider therapeutic options that might improve it. No amount of lens power will impact acuity if there are significant media opacities. If the patient is post cataract surgery with IOLs, it is unlikely that they will have a significant refractive error. If they do, there is often an astigmatic component.

Generally, by the time a patient gets to a low vision practitioner, if a new refraction would have been of value, it would have already been prescribed. Remember, small changes don’t make big differences! If a patient sees 20/400 a half diopter or 10-degree axis change will be of little value. Changes in refraction will have more potential impact when acuity is 20/80 or better as a modest change might yield 20/60 and that can be helpful. Improving acuity from 20/400 to 20/300 is unlikely to impact the patient’s functional life. So, as a result I will spend more time refining a refraction when acuity is 20/80 or better. I find keratometry can be a very valuable tool. The character of the mires as well as cylinder can often be missed and this data can sometimes make a difference in better-seeing individuals.

I always use a trial frame; I find it more reliable and it allows for a better rapport with the patient. Trial frames are especially important with high cylinders, because it lets the patient maintain their normal posture rather than an unnatural one induced by the phoropter, which can avoid a host of cylinder axis hassles.

Remember to make a sufficient enough power change for the patient to notice. If they can’t notice a half-diopter change, try one diopter changes. There’s no use in making changes smaller than what the patient can reliably respond to- everyone will just get frustrated and you’ll not make any progress.

Remember also that the high-contrast acuity chart is a poor determiner of functional value. After I’ve determined the best refraction, I find it helpful for the patient to look at a low contrast target such someone’s face at the furthest distance that they can normally see it. If the patient can notice a difference, then the Rx change is likely to be of functional value. If they don’t notice a difference than I don’t pursue it further. Keep in mind also that acuity will fluctuate as fixation varies. Don’t let a fleeting acuity improvement fool you into thinking it’s the refraction change that’s helping—it’s more likely a PRL issue. The prescription change has to be enough to make a “real” difference! Remember that if the patient can “almost see something” they still can’t see it!

When refraction is the best you can achieve and acuity remains inadequate for the patient’s goals, than there’s only one option left to further enhance distance vision- make it bigger! And, of course, we have only two ways to do that—walk up close enough to see it, or, make it closer optically (with telescopes!).

The In’s and Out’s of Telescopes for Low Vision: Achieving a Balance between Acuity and Field of View in Prescribing.

As we know, patients often explain that they cannot see far enough away to perform whatever activity they have in mind. They must move closer in order to see it adequately—which they may or may not be able to do, or feel comfortable doing! Patients with 20/40 vision, however, rarely complain of difficulty seeing. Individuals are licensed to drive without restrictions with 20/40 acuity, and children are unencumbered in the classroom with that level of vision.

It is reasonable to assume then, that if we can provide 20/40 acuity through the telescope, most patients should be reasonably satisfied with the functional benefit it provides. So, if the goal is 20/40, a 2x device should be adequate for an individual with 20/80 acuity, 4x for 20/160, and 6x for 20/320. If we prescribe higher magnification to achieve better acuity it will be at the expense of a narrower field of view, which is the major complaint of telescope users. Also, the higher the magnification, the greater the image motion due to head movement that may actually undermine the benefit of the higher power device.

Clinically we find that users have increasing difficulty finding what they’re looking for when fields of view become narrower than about 10 degrees. So we always need to balance adequate acuity with field of view. Personally, I prescribe Galilean telescopes for powers 2.2x and lower, and Keplerian at 3x and higher.

I have found that patients respond most well to telescopic low vision aids when their acuities are 20/200 (6/60) or better. Keplerian telescopes in 4x and 5x can readily provide 20/40 and the telescope fields of view are greater than 10 degrees. I like to tell my ophthalmology colleagues that their goal is to keep their patient’s vision 20/200 or better. Now with the wonderful success of Anti VegF treatments, many patients have acuity much better than that, making their response to low vision aids even more robust.