It is easy to feel like your head is going to explode when a colleague asks a simple question that makes you stop and think… and think and think. The late Robert A. Kraskin, OD, asked, “Of what value is the +0.75 on the distance refraction to the human organism?” The answer that I (Dr. Harris) had been primed to give was also a simple one. I raised my hand and said, “A buffer.” Then, he asked me to explain what I meant. It was only over time that I developed an understanding of buffers and the value of hyperopia to the visual process.
Finding a Solution
My first exposure to the word “buffer” was in the context of chemistry class and pH values. If you take two unbuffered solutions of equal volume and different pH values and mix them, you end up with a pH halfway between the initial values of each solution. However, things change when you buffer a solution. Simply put, a buffer is a solution that resists pH changes when either an acid or an alkali is added. A misconception is that buffering a solution will fix its pH at 7.0—the border between acid and base—when in actuality, a solution can be buffered to any pH.
Imagine we have a beaker of buffered solution at 8.0pH and a dropper of strong acid at 1.5pH. When the acid drop first attacks the buffered solution, the buffer counteracts the stress of the attacking solution and neutralizes it. The pH of the solution in the beaker remains at 8.0. As we continue to apply drops of acid to the solution, the buffer repeatedly absorbs the attacks, and all is well. However, there is a practical limit to all buffers, and at some point, the buffer runs out of its ability to stabilize the pH in the beaker. At this point, the pH begins to shift toward less base and then less acid as the attack continues.
Stress and Myopia
The analogy here is that the +0.75 is the buffer for accommodative stress, which results from the near-centered visual activities we engage in during our waking hours. That mile marker (+0.75) is the expected distance refraction through which our patients should still be able to read a good 20/20 at distance.
Research into what is now termed near-induced transient myopia (NITM) has discovered variations in refractive findings on a series of regular cycles, such as diurnal or daily cycles, with different lengths. If we assume normal hours for a person’s sleep cycle, we would see no NITM at the beginning of the day, but it would get worse and worse throughout the day, directly proportional to the amount and intensity of sustained close work done and many other associated factors—such as attitude, appraisal, rest and nutrition, to name a few. It is interesting to note that some NITM shows up first in only one meridian with minus cylinder axis 90, which changes as the day progresses and indicates what is happening.
Mapping the +0.75 of hyperopia onto the chemistry example, the +0.75 is the buffering agent. The close work done, with the host of factors modulating the responses to the stressor, is the acid attack. As the near work is carried out, some of the buffer is consumed. Thus, refractions done later in the day on the same patient often show more myopia or less hyperopia than those done earlier in the day. We must take precautions with the refractive data determined later in the day.
|As acid attacks the buffered solution, the buffer fights back and resists an impending pH change.|
Seeing the Signs
Not all patients follow the same cycles. For some patients—those who practice good nutrition, get eight hours of sleep, exercise regularly and use good posture while performing close work—the variation in their distance refractive findings from 8am to 8pm is quite little. But, shift each of those factors to the negative side and you get wide variations over the course of the day, the week (best Monday morning and worst Friday evening) and the school year (best in September and worst in May).
The presence of the +0.75 at distance, through which our patients can still read 20/20, is a measure of their buffer and its availability to help them meet the visual needs of their day. The absence of the +0.75 in the distance refraction means that, at the first sign of NITM creeping in, they will experience distance blur. At first, this blur is short-lived. When patients look up from their computer or phone, things may look a bit blurry before they eventually come into focus. After more time passes, however, that blur becomes worse and takes longer to clear up. Late in the day, the blur may be rather significant and last for an extended period of time. That blur is a sign that the patient lacks proper rest, exercise or nutrition.
In Plane Sight
To confirm this, a colleague from Arizona, Rob Lewis, OD, and I conducted an experiment. We both are garden-variety hyperopes and love to read. When traveling to meetings in each other’s cities, we would meet each other at the airport with retinoscopes in hand. NITM was noted every time, and more often than not, it was asymmetric with some minus cylinder axis 90. When getting off the plane, both of us noted that signs in the distance were blurrier than usual. Those signs would clear up over time, but the longer the flight and the more time spent doing sustained close work, the longer it took for distance vision to return to baseline.
Our patients need their buffers identified and preserved whenever possible. In the meantime, the next time you travel and read more than you usually do, check your vision at distance before and after you do that close work, and see for yourself how well your buffer is or is not protecting your distance visual acuity. Or, on your next family trip, bring along your retinoscope, and be sure everyone reads, reads and reads some more—or plays videogames— and then check their retinoscopy when you arrive.