The Issue of Adaptation
“These aren’t working for me.” This isn’t what we want to hear from our patients. Before you fit progressives, please consider the following variables and realize that adaptation issues are a challenge that every Optician faces.
If working with a multifocal patient, is this their first multifocal, or are they transitioning from a flat top to a progressive? (ask questions or review their records) Switching progressive designs on a long time progressive wearer may also cause problems. As technology improves, so do progressive lenses. The first time progressive patient or properly educated consumer may find comfort and ease of use in one of the modern free form progressives available Multifocal wearers who have good visual acuity in both eyes and prismatic imbalance of 1.5 diopters or more at near may be troubled by vertical imbalance. This will present itself as anisometropia and reading material being displaced. Solutions for this problem would be single vision reading lenses, or slab off. Slabbing off would grind a base up prism to cancel the imbalance of the base down component of the Rx. Slab off will leave a visible seam on the lens. While this is less obvious with a flat top design, cosmetically acceptable slab offs are also possible on progressive lenses. Adding an anti-reflective coating will help to decrease the visibility of the seam. Slab off is useful for neutralizing large amounts of prism, however be sure to check with Allentown Optical for availability of the lens design and lens options desired. The measurements taken with each pair of eye glasses are critical. The P.D. (pupilary distance) locates the optical center, which is the point in a lens that a light ray can enter undeviated. This is normally placed in front of the pupil. If this measurement is incorrect, the patient will experience unprescribed prism. This is often described as standing at the bottom of a bowl (base down), or standing on a hill, with vertical objects seeming shorter (base up). Measuring segment heights on both flat top multifocal and progressive lenses involves the optical center and the placement of the segments. These measurements aid in the inset, as well as the placement of the multifocal. If the height is too high or low, it will be difficult or impossible for a patient to use without great discomfort. (observe existing seg measurements, double check your measurements, examine posture of patient, stay square with patient when taking measurements, ask the patient to relax before measuring} Another factor to consider when trouble shooting adaptation issues is the base curve of the lens. Changes to the base curve also affect the patient’s vision. The Corrected Curve Theory is the practice of matching various prescriptions with appropriate curves. Some rules of thumb: the ocular curves (minus side) should fall between -4.00 and -7.00 are best for superior optics. As minus power increases, the base should flatten, while the increase of plus power should call for a steeper base curve. It is recommended to keep the base curve the same when duplicating the Rx or if the change is a diopter or less. These “rules” are merely recommendations, as each situation is different. When using aspheric lenses, it is best to heed the manufacturer’s recommendations. Aspheric lenses will provide a thinner profile, but also provide superior optics. As a patient looks from the optical center, aberrations will increase. Aspheric lenses offer a continuously changing front curvature to avoid or eliminate aberrations. Current progressive designs are aspheric. (Introduced in 1990). All patients have unique visual demands. Multifocals are often a practical solution, but dedicated eyewear may be a better answer. A single vision lens with an intermediate Rx will benefit a musician or one who avidly uses computers. Near vision or reader type lenses provide a wider field of vision for those who enjoy detail oriented hobbies such as needle point, painting and model building. The use of different tints and lens treatments should also be considered for target shooting, skiing, boating, golf, baseball, and other activities where added contrast would be beneficial. The adjustments we make to patient’s eyewear also impact the prescription. Tilt of any nature has the capacity to alter the sphere and to induce cylinder in a patient’s lens. Although patients with stronger prescriptions are more sensitive to these changes, knowing the relationship between these adjustments and the optical center ensure accurate dispensing.As pantoscopic tilt increases, the optical center should be lowered from the pupil. Martin’s Formula of Tilt goes into finer detail explaining how the Rx changes dependent on tilt. It is a good practice to always adjust the patient’s new frame before taking measurements, observing the fit of the patient’s existing glasses. Vertex distance refers to the distance between the patient’s eye and the back of the lens. An example of the effects of vertex distance is moving a lens closer and farther from your eye. A plus lens, such as a hand held magnifying glass, will strengthen as it is moved farther from the eye. Conversely, a minus lens will weaken as the vertex distance is increased. The relationship is further explored with vertex compensation computations, particularly concerning higher prescriptions. Various pathologies can also influence adaptation problems. Patients afflicted with cataracts, retinitis pigmentosa, glaucoma, keratoconus, and age related macular degeneration to name a few will impact the visual acuity of the patient. Visual symptoms can also be part of a larger issue. Diabetes, hypertension, multiple sclerosis, and Alzheimer’s disease are some conditions that effect vision. It is important for the Optometrist/Ophthalmologist to work closely with the Optician as to preexisting conditions of patients being fit for glasses. When dispensing to your patient assure them of RX accuracy, review instruction of use, verify usage with a reading card and explain to them that adaptation will take some time, practice and patience.
Adaptation issues will always exist, but by listening to the needs of our patients, preempting them on what to expect, educating them to aid the decision making progress, and doing all that we can to ensure accuracy will certainly make for less adaptation problems and more satisfied patients