Adapting to the Situation
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One of the first things an ECP should be aware of is what factors may cause adaptation issues to occur.
Most patients will be able to put on their glasses and be fine, but there are those who will require an extra amount of time and care to see as well as possible. It will be helpful to know when the extra adaptation time is more likely to occur and plan for it. The ECP can help the patient through the adjustment process through both understanding and sufficient explanation of the changes that are occurring to the patient.
How prescriptions change images
Plus Lenses
Used to correct hyperopia, or farsightedness and presbyopia, plus lenses are two prisms placed base to base. By placing the prisms base to base, the images viewed by the patient are magnified and therefore appear closer.
Minus Lenses
Myopia, or nearsightedness, is corrected by placing two prisms apex to apex in front of the eye. This causes the images in the eye to appear smaller and farther away. These spherical lenses, along with spherical plus lenses, are the easiest lenses to adapt to.
Sphero-Cylinder Lenses
A sphero-cylinder lens is a more complicated lens and used to correct a prescription with astigmatism. They are usually made by grinding a spherical curve on the front of the spectacle lens and a toric, or cylindrical curve, on the back of the lens. The toric curve creates two principle meridians on the lens 90 degrees apart. Because of this, the power of the lens changes as the eye looks around the lens. As a result, the lens may cause a “swimmy” feeling and images may appear slanted and slightly misshapen. The more cylinder correction or axis changes that occur within the lens, the greater the likelihood that patient will need an adaptation period.
Multifocal Lenses
Multifocal lenses are traditionally progressives, lined bifocals or lined trifocals. Although it can be seen in prescriptions for younger individuals to help with focusing problems, it is more commonly seen in individuals known as presbyopes. Presbyopes are individuals, usually over the age of forty, who have lost some of the ability to accommodate. Accommodation is the process in which the crystalline lens changes its shape to allow the eye to easily change focus for a variety of distances. Multifocal lenses, especially the lined types, can cause an abrupt image jump. Multifocals may also cause a vertical imbalance in a patient that has good visual acuity as well as anisometropia, or a power difference in the 90 degree meridian that causes a prismatic imbalance between each eye. Progressives exhibit distortion along the sides of the corridor of the progressive. The higher the add power the higher the distortion.
Unwanted Prism
As discussed earlier, lenses are prisms placed together. The point at which the two prisms meet is the optical center. This is the point where the light entering the lens is not refracted or deviated. In order for the patient to see properly, the optical center must be placed in the proper position. Horizontally, this placement is placed at the pupillary distance. Vertically, this measurement is called the OC (optical center) height and should be placed along the optical axis. The optical axis is not in front of the pupil. The rule of thumb for OC height is to place the measurement 1 mm below the pupil for every 2 degrees of pantoscopic tilt in the frame. Since most frames are worn by the patient with 10 to 12 degrees of pantoscopic tilt, an average OC height would be 5 to 6 mm below the pupil. When these measurements are inaccurate, the patient experiences unwanted prism along with changes in their prescription powers and/or axis.
Unwanted prism is a major reason why a patient has trouble adapting to his or her new lenses. When the pupillary distance is off the effect is excessive Base In or Base Out prism. The patient will experience a slanting feeling when wearing their glasses. An image will appear high toward the prism base and low toward the prism apex. When the OC height is wrong the result is excessive Base Down or Base Up prism. Unwanted Base Up prism will give the wearer the feeling of walking down hill, of standing at the top of a hill, and vertical objects will seem shorter. Excessive Base Down prism wearers will feel as if he or she is at the bottom of a bowl, or will experience a feeling of walking uphill and vertical objects will appear taller.
Adjust the frame
If the frame is crooked, too tight or too loose, the optical center will not follow the optical axis and the patient’s vision will be off. Proper adjustment is crucial both at the time of fitting and at the time of dispensing. It does not matter how much time was taken in the exam room or in measuring for the lens if the frame does not fit properly. Make sure that the weight is distributed properly along the fitting triangle of the bridge of the nose and the top of the ears. Ensure that the frame front is level and is placed at a good vertex distance from the eye – close to the lashes whenever possible. Vertex is especially important because a minus lens that sits too far away from the eye loses perceived power, and a plus lens that sits too far away from the eye exhibits an increase in perceived power. Adjustment to the temple and along the temple bend will keep the lenses stable in relation to the optical axis. Also, make certain that the pantoscopic tilt is proper for the OC height.
Taking the Measurements
Frame Adjustment
Always adjust the frame prior to taking the lens measurements. Adjustments should be made just like they would be at dispensing. First, have the patient place the glasses on his or her face in the position in which it is normally worn. Now, look at the patient and observe how the frame fits. A proper fitting frame should have the following characteristics, if not make the necessary adjustments.
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The nose pads should lay flat against the bridge of the nose, exhibiting the proper splay, frontal and vertical angles to prevent digging or pinching on the delicate skin of the nose and inner eye.
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The frame front should have a pantoscopic tilt between eight and twelve degrees, assuming the eye is sitting 4 to 6 mm above the datum line. Tilt changes the effective sphere, cylinder and axis of prescriptions, and an excessive amount of pantoscopic tilt can cause the patient to experience blurred vision and eye strain.
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The frame should have the proper face form wrap. This allows the frame to follow the natural curve of the face and enables the patient to have a wider field of view in the distance through increased peripheral vision. For every 1 mm of decentration the frame should have two degrees of face form.
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A close fitting vertex distance is important to prevent vision distortion. A proper vertex distance not only allows the patient to get the full benefit of the width of the lens corridors, but it is of extreme importance to patients who have prescriptions in the higher ranges. Changes in vertex distance change the effective power of the lens. The temples should be angled properly and well fitted at the temple bend and behind the ear – without touching the head forward of the ears.
By pre-adjusting the frame, any fitting problems will be discovered early on and prevent the lens from being placed in an ill fitting frame. It will also minimize any fitting height errors that may result from measuring a pair of glasses that are fitting uneven, too tight or loose or exhibiting an “X” ing problem.
Taking the Fitting Height for a Progressive Lens
Eye care professionals should position themselves in front of the patient at eye level. Have the patient look off into the distance and with a marking pen, dot the center of the patient’s pupil. Remove the glasses and draw a one inch straight line across the dot. Next, have the patient put the glasses back on to verify where the fitting height is. The patient should be looking through the line. Now ask the patient to walk around the dispensary and then resume sitting. Observe the patient’s posture and stance in relation to the fitting height. Adjust the fitting height if necessary. Always verify the fitting height in this manner, even if you have the previous fitting height, because a patient’s posture and stance may have changed since the previous fitting. Another accurate technique is to hold a penlight at your eye level and have the patient look at your open eye. Dot the lens at the corneal reflection caused by the penlight. This technique works great for patients with very dark colored irises.
One should note that a patient will normally have an uneven fitting height. Make sure that the uneven height is ordered. If it is not, the patient may exhibit distorted vision in each eye. This could be the result of:
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one eye looking through the intermediate portion of the lens corridor while viewing the distance
if both fitting heights are placed at the height of the higher eye
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one eye not reaching maximum add power if the fitting heights are set to the height of the
lower eye
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uneven MRP causing a prism imbalance that may cause a problem when converging to near
Pupillary Distance
Monocular PDs should always be taken when fitting a patient with a progressive lens or when a patient is prescribed a power in the higher range. The preferred way to measure the PD is using a corneal reflection pupilometer. This is the preferred method because they help eliminate parallax errors; they work very well on very dark irises, and are known for the accuracy of their measurements. The first step is to adjust the working distance dial to the distance setting which is infinity. Sitting across from the patient, place the pupilometer on the patient’s nose with the forehead bar centered against the forehead. Ask the patient to look into the light and then move the paddle to the center position so both eyes are viewable. Move the hairline to bisect the corneal reflex of each eye. The monocular PD is then displayed on the top of the pupilometer. If the patient displays a phoria, then use the paddle switch to occlude one eye at a time in order to get accurate results. Even with a phoria, if one eye is occluded the brain will force the eye to take up fixation.
To verify that this measurement is accurate, the machine should be calibrated at least weekly. In order to verify accuracy, you should set the PD to 32/32 and place a ruler against the hairlines. The measurement should be 64. If it is, then the calibration is complete. If it is incorrect, note the difference, measure again at 29/29 and 35/35. If the pupilometer is off the same amount in each case, remember to adjust the measurements taken to correct them until the pupilometer can be sent for calibration. If there is no consistency, stop using the pupilometer and send if off for repair.
If the pupilometer is unavailable, the ECP can measure the PD by measuring it using a ruler and a penlight source. This technique takes practice to gain accuracy.
Conclusion
Adapting to glasses is normal. Although every ECP loves the patient that puts on a new pair of glasses and can see everything perfect, this cannot happen in every instance. However, by providing a thorough understanding to all parties, proper measurements, and patience, the adaptation process can run much smoother for both the patient and ECP.
With contributions from: Brian A. Thomas, P.h.D, ABOM
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