From exam to eyewear:
Trouble spots and how to avoid them
By Palmer R. Cook, OD
The transformation of refraction data into eyewear isn’t always straightforward. As professionals involved in determining prescription needs and dispensing eyewear, we are all in effect, “eyewear designers.” In a dispensary, the eyewear designer is the person who makes the final decision on the frame and lens parameters, and who authorizes the fabrication of the eyewear. Often, it is the same person who dispenses the eyewear. So, how then do we design eyewear that will optimally address the patient’s examination findings as well as their vision, comfort and appearance expectations? In fact, there are several trouble spots and signs to watch for.
With moderate or high refractive errors, the patient may discover that a slight change in head position creates clearer vision during the exam. If they hold that “good” position they may give erroneous findings during subsequent testing. This leads to good acuity—behind the refractor. Unfortunately changing head position does not adjust the cylinder axis when wearing spectacles. This can also lead the examiner to miss a vertical imbalance until a re-check through the new eyewear reveals the error.
Tilts & Turns
A head tilt can lead to prism and cylinder axis problems, because the eyewear is aligned to the patient’s brow rather than to true level, which is usually the orientation of the phoropter during testing. A head turn may require special testing and may also dictate special placement of the patient’s distance and near MRPs (Major Reference Points) in the eyewear. Opticians and technicians should be alert for obvious head tilt and head turn situations, and should consult with the doctor who wrote the lens prescription before finalizing the eyewear design.
Problems related to patients’ pupilary distance (pd) are fairly common. The examining doctor may measure an overall pd of, say, 65mm. This is quite accurate for the purpose of the examination because the patient’s nose does not support the phoropter. Patients with an unequal half pd can easily center their pupils in the phoropter openings by moving to one side or another. Problems arise however, with the eyewear design when the nose is not centered between the eyes and the distance and near pds need to be placed asymmetrically.
Unequal Refractive Errors
Difficulties can arise in designing comfortable eyewear for patients with unequal refractive errors. This is often because the prism induced when the lines of sight are not passing through the optical centers varies unequally between the right and left lenses. In general, patients can comfortably make small adjustments in convergence or divergence. Therefore the varying amounts of lateral prism experienced on looking to the right or left is tolerated relatively well. A vertical imbalance, on the other hand, is usually poorly tolerated.
Eyewear designers should be particularly careful in selecting progressive designs for patients with unequal powers in the vertical meridians. There may be as much as an 8mm difference in height between the level of the distance reference point and the level of the prism reference point. If you multiply this difference in centimeters by the difference in power between the vertical meridians of the two lenses, you will approximate the amount of vertical prism that your patient will have to fuse for clear, single binocular vision at distance.
A patient who has a hyper eye may have a vertical imbalance induced by his spectacle lenses, even if the powers of the lenses are identical. In practice, most hyper eyes are not compensated for during the examination. If the eyewear designer then orders the appropriate MRP heights (which will be unequal by the amount of the hyper displacement) the patient will have unwanted vertical prism. Some refracting devices have a vertical adjustment to compensate for hyper eye situations. Doctors who make this compensation should indicate this on the ophthalmic prescription. Eyewear designers who are faced with a hyper eye situation should contact the examining doctor before ordering any lenses.
Most examiners test at the same vertex for both near and distance corrections. In spectacles, the vertex is usually longer when the patient is viewing through the add because of the downward viewing angle. If the plus power for reading is quite strong (i.e., in hyperopia), this greater vertex in the reading portion of the spectacles can lead to difficulties for the wearer. Half-eye readers, worn down the nose, have an exaggerated vertex and require a narrower pd than lenses worn in the spectacle plane. Measuring the pd in the plane in which the lenses will be worn will reveal the correct pd for half-eyes.
The Right Questions
Fortunately, most of these problems arise with patients who have unusual anatomical and refractive situations. Many potential problems can be eliminated by taking careful half pds and MRP measurements on all patients, and by being especially watchful when designing eyewear with very strong prescriptions. One of the most helpful of all guidelines is to use a pre-exam questionnaire. If you are an eyewear designer, use a pre-design questionnaire. Include such questions as: Have you ever had a pair of glasses you couldn’t wear? An affirmative answer to this question is a warning flag that should never be ignored.
The eyewear designer must use knowledge, careful observation and sound judgment in applying ophthalmic lens technology. Good communication among doctor, eyewear designer and patient can eliminate a lot of potential problems. It’s important to know a patient’s lifestyle so the doctor can explain the features and benefits of such premium products as anti-reflective lenses, high-index materials, polarized lenses and other special lens designs. Eyewear designers should say “no” when patients are determined to make inappropriate eyewear decisions and they should communicate with the doctor when a potential problem looms on the horizon.
Palmer R. Cook is director of the Practice Support division of Diversified