In theory, we begin each day with our +0.75 to +1.00 of hyperopia in reserve. We check our digital devices and engage with their “retinal” displays. When we look up, our vision is clear but our refraction is slightly less hyperopic. We don’t notice this, however, because we have a buffer.
As we make our way through the day, this happens more and more, and we give up some hyperopia. If we eat healthy and exercise, though, our chances of ending the day the same way we started it, with no blur at distance, increase. Sleep helps restore our buffer, and, when we wake up the next day, the cycle repeats itself.
The two elements that factor into understanding accommodative stress and hyperopia are the dark focus of accommodation and the hysteresis effect. The rest position of accommodation is at optical infinity or beyond. The dark focus is where our eyes focus in a dark space, 1.00D to 1.50D in front of us.1,2
This 10-year-old male patient’s visual readings were as follows:
The patient did not get plus for distance but did get some plus for near. When some plus is worn for near in this kind of case, it decreases the amount of accommodative effort, causing less of a hysteresis effect and keeping distance measurements from fluctuating. We gave him:
OD plano / +0.75 add, OS plano / +0.75 add
We rarely give the full plus at near, as is obvious by our choice to give less than the near retinoscopy and the full fused cross-cylinder, because we need to leave a small amount of buffer in the accommodative mechanism at near.
Hysteresis is a dynamic lag between an input and an output that disappears if the input varies slower than the output. After accommodating at near for a sustained period and shifting back out to distance, there is a lag in how long it takes for accommodation to fully make its way back to baseline. It may undo 80% to 85% of the shift in a few minutes but can also take a few hours.1 The refraction doesn’t actually change from moment to moment, but the hysteresis effect can make it seem like the hyperopia is disappearing as the day goes on and explains why an emmetropic patient may complain of some intermittent distance blur later in the day.
If we put the entire amount on a +0.75 hyperopic patient in the form of glasses, they will have clear vision in the morning. As they use their digital devices, however, they will look up and notice intermittent distance blur, which typically gets worse as the day progresses. The full correction artificially makes hyperopic patients emmetropic and may consume or “occupy” their buffer so that it is unable to act like a cushion and allow the patient to perform their daily tasks and still come away “unharmed.”
All of this leads to a lens-prescribing directive, which is: unless some specific task requires full correction, leave at least +0.75 of hyperopia at distance uncorrected.
Preserving buffers by not fully compensating all the hyperopia at distance or at near is a hallmark of prescribing. Long-term, we want buffers to be present, but we also don’t want to force a patient into making their buffer bigger. Prescribing just the right amount of plus is key to maintaining good visual function and stabilizing refractive condition over time.
1. Ebenholtz SM. Accommodative hysteresis: a precursor for induced myopia? Invest Ophthalmol Vis Sci. 1983;24:513-5.
2. Neveau C, Stark L. Hysteresis in accommodation. Ophthalmic Physiol Opt. 1995:15(3):207-16.