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"These aren't working for
me." This isn't what we want to hear from our patients.
Before you go remaking or refunding, realize that adaptation
issues are a challenge every optician faces.
Many answers can be found in the
patient's current eyewear, both in what works and what
doesn't. Along with changes in prescription, lens material
also affects visual acuity. Different lens materials have
varying degrees of optical clarity. The Abbe value system
rates lenses based on the amount of aberrations present.
Lenses with lower Abbe values, such as polycarbonate (Abbe
30) are less clear than Trivex® (Abbe 48). There are other
factors to consider aside from Abbe value, such as
refractive index and lens design. Abbe values also vary
between manufacturers for the same lens material.
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Take a look at the lens design. Is
the patient in the best lens for their vision? 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.
If working with a multifocal
patient, is this their first multifocal, or are they
transitioning from a flat top to a progressive? As
technology improves, so do progressive lenses. The patient
may find more comfort and ease of use in one of the modern
free form progressives available.
If there is a notable change in
prescription, it is likely that there will be an adaptation
period. The patient's eyes have grown accustomed to the
current visual situation. The change may bring upon
headaches or fatigue as the patient adapts. A few days of
adaptation is to be expected, however, if the problems
persist or worsen, it is important to follow up with that
patient.
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 the 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.
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.
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.
Most patients value style as well
as functionality. If a patient has chosen a zyl frame to
replace their wire frame or three-piece mount, ensure that
they are aware of the differences. This includes the
decreased vertex distance of a zyl, or the introduction of
adjustable nose pads. When choosing a frame with a shallow
vertical measurement, the layout, and use of progressive
lenses changes. Patients may find the differences in the
progressive as well as the visibility of a smaller eye wire
distracting.
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, however that is beyond the scope
and space of this article. It is a good practice to always
adjust the patient's frame before taking measurements, to
ensure accuracy.
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 to work
closely with the patient's physician, and to monitor any
changes in health or medications.
Adaptation issues will always
exist, but by listening to the needs of our patients,
educating them on what to expect, making them an integral
part of the decision making progress, and doing all that we
can to ensure accuracy and ease of use will certainly make
for less adaptation problems and more satisfied patients.
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