Everyone knows that new eyewear is something "to get used to," and most people go through the adaptation process with forbearance and few real problems. Unfortunately some patients do not adapt well and may return to your office complaining bitterly. Worse, they may complain elsewhere, leaving others to wonder if you really know how to properly fill an ophthalmic prescription.
To avoid such scenarios, many dispensers follow the "do not suggest problems" credo, fearing that a potential problem may become a self-fulfilling prophecy. A middle-of-the-road approach, such as routinely cautioning patients "Things may look a little different for just a bit" and "Be especially careful driving and judging distances," could help you avoid problems in our litigious society. If you expect significant perceptual distortions, especially with elderly patients who have marginal driving skills, you might restrict driving for a few days and so note their record.
But when the patient comes back complaining, how do you know whether the problem is adaptation or something else? There are some key complaints that can help you sort out true adaptation effects. These include comments such as "These things make me feel a lot taller (or shorter)" or "Everything is tilted."
Taller or Shorter
The taller or shorter problem is the result of the lens' major reference point (MRP) being either higher or lower than it was previously. This phenomenon passes very quickly and you should never have to do a remake for such complaints. If a patient returns to your office with taller/shorter complaints that persist, you can be sure that they have not worn the new lenses for any significant period of time. If the MRPs are too low or too high, especially in progressive addition lenses (PALs), aspherics or high-index lenses, a taller/shorter complaint is a clue that the MRPs need to be moved if you need to eliminate other, usually vague, symptoms of difficult adaptation.
Patients who complain "the tabletop and the soup bowl tilt to the right (or left)" are most often experiencing a cylinder-related problem. This is more common with oblique cylinders because we live in a world of flat planes and vertical and horizontal contours. It is the result of meridional magnification. For a prescription of +3.00D -6.00D x 090, the power in the 090 meridian is +3.00D and the retinal image is relatively magnified in this vertical meridian. In the 180 meridian the power is -3.00 and the retinal image is relatively minified in this, the horizontal meridian. This has the effect of making a square appear to be a "portrait oriented" rectangle. Had the prescription axis been at 180, the magnification/minification effects would have been reversed, yielding a "landscape oriented" rectangle. In a monocular situation, this effect is not hard for the perceptual system to overcome, but a
newly "stretched/compressed" retinal image must be fused with the retinal image from the other eye (which may or may not have changed). As part of this fusional process, the perceptual system "adjusts" the way we perceive our environment leading to some rather bizarre distortions. This is the basis of some of the adaptation effects that our patients find most troubling. If the axis were 045, the meridional magnification would have tended to make the retinal image of the square to become a "leaning diamond" and the final perceptual distortion could be predicted to be even more bizarre and troublesome.
Figure 1. This fellow is myopic and his MRPs are set lower than the MRP in his previous lenses. He is looking downward, but the lower MRPs of his new minus lenses give him an unexpected amount of base up prism causing his line of sight to be deviated over the treetops. The resultøhe feels he has become much taller. If he were hyperopic the thicker part of the lens would have been below his line of sight and he would have the sensation of having become shorter.
Tilting effects are often associated with unequal vertex distances and fronts that have become misaligned. Inappropriate, especially unequal, pantoscopic tilt can be a factor. A twisted bridge that gives both more pantoscopic angle to one eye and causes the front to tilt, making the intended MRP heights unequal, is double-trouble. Factors that commonly contribute to tilting may include a change in cylinder axis and power, a change in the index of the lens material and a change in base curve, particularly if base curve is more steeply curved in one eye than the other. Base curves significantly different from the previous eyewear's base curves are also prone to cause adaptation problems. Some eyecare practitioners insist on keeping base curves the same even though there has been a large power change. This usually doesn't make good, long-term optical sense because of decreased performance in the lens periphery.
Humped-Up and Scooped-Out
Occasionally patients, upon looking downward at the floor or terrain just in front of their feet, will comment that things are "humped-up" or "scooped-out." Golfers can become quite agitated when discussing this effect. These effects are the perceptual result of a change in lateral (base-in or base-out) prism. Frame wrap can be a factor, especially if the dress and work eyewear have little wrap and the sun eyewear has a lot of facial wrap. Fortunately, especially for patients who must wear prescribed lateral prism, this is an effect that is subject to adaptation. Adaptation can be speeded up by telling the patient to keep wearing the new eyewear, stop switching back to the old eyewear, spend time walking on uneven terrain (as in a park) and, if they golf, to practice diligently. These effects pass slowly and may take a full 10 days before perception returns to normal.
Misaligned optic axes are sometimes a factor with scooped-out and humped-up problems as well as with other issues that arise during the adaptation process. The best optical performance occurs when the line of sight passes through the optic axis of the lens, yet too often we neglect to determine just where the optic axis is located.
Correctly Positioning The Optic Axes
A quick, practical way to locate the optic axis of a lens is to sight through a light beam directed toward the lens. A penlight, ophthalmoscope or retinoscope can be used as the light source. At a distance of about 75cm hold the penlight within a few centimeters of your own eye and attempt to sight through the beam toward the patient's pupil. The patient should be directed to look at the light.
If you are actually looking through the beam, the patient's pupil will glow red. In addition, you will see two bright reflections from the spectacle lens. One of these reflections is from the front surface of the lens and the other is from the back surface of the lens. You then need to slowly move, with the patient still looking at your light source, until the two lens reflections are superimposed. At this point you are looking through the optic axis of the lens.
Figure 2. Sighting through a penlight beam is a little tricky. If you aim the penlight at a wall in a darkened room, you will find that the beam is almost always skewed rather than paralleling the barrel of the penlight. This can be very helpful as it allows you to tilt the light to better get your own line-of-sight into the beam. Note the location of the axis relative to the patient's pupil. Also note whether the optic axis is angled down or up and whether it slews to the right or left. This same procedure should be repeated for the other eye and lens. Eyewear used for distance seeing should have the optic axes of both lenses located at about the base of the pupils. These optic axes should be parallel and angled downward slightly so if the patient were standing they would intersect the floor at a point about 17 feet in front of them. This location of the optic axes technique will not work with prism lenses. It is best to practice this on single-vision lenses in the two to three diopter range with little or no cylinder before graduating to more complex lenses. Once you know where the optic axes are located, the appropriate frame bends to bring them into good alignment should be apparent.
I Can't See With These Things
Another problem of adaptation is a failure of the accommodative system to relax when plus power is increased. Patients who need more plus for distance or near and have some accommodative ability have usually been accommodating (increasing the plus power of the eye) to maintain clarity of vision. This habitual accommodation does not always cease immediately when the new lenses are worn. The result is an annoying blur at far distances and complaints of having to hold reading material too close.
The distance blur will pass if the lens power was correctly prescribed, but, since all lenses increase in plus or decrease in minus as the vertex increases, it can be helpful to give the front a little facial wrap and increase the pantoscopic angle to decrease the vertex slightly. Patients experiencing distance blur, especially with increased plus, should clean both the inside and outside of their windshields. A dirty windshield can actually trigger accommodation. For the same reason the new lenses should be kept clean.