COMPLAINTS OR CLUES
Off-axis errors tend to cause floors and tabletops to
appear tilted. Floors that appear to be humped up or
scooped out when the patient looks downward at a
few feet in front of them is usually caused by lateral
prism, either basein or base-out. Power errors can
cause blurring at distance or near, and inappropriatebase
curves can lead to comments such as "thingsare-
clear-but-something-isn't-right." These kinds of
complaints can be important clues that can help you
resolve your patient's difficulties more quickly.
Symptoms, signs, diagnosis and treatment is a classic
chain for resolution of problems with eyewear.
Patients tend to focus on their symptoms so you will
certainly hear about them. Signs, the things you observe
such as the fit of the eyewear or pressure marks
on the nose or behind the ear, often give important
clues. For example, right-handed patients who take
their eyewear off with one hand may spread the left
temple. The result can yield unequal vertex distances
(longer on the right) as well as segs that appear to be
too high on the right and too low on the left.
Your diagnosis of a spread left temple will guide you
to adjust the left temple inward rather than making
the mistake of adjusting the right temple upward
to lower the right seg or adjusting the left temple
downward to raise the left seg. A soccer ball hit at the
juncture of the left temple and the front could cause
a similar problem. In this case your realignment plan
would be the same, but should be followed by suggesting
a sturdy pair of sports glasses. Clinical Tip: Always
correct unequal vertex distance before realigning
the frame if the optical centers (or MRPs) are at
unequal heights.
Patients sometimes return having never worn the
new eyewear long enough for adaptation to occur. If
you hear complaints of the eyewear causing the patient
to be taller or shorter, you can be assured that
the patient has not worn the lenses for any significant
period of time. The taller/shorter phenomenon is an
adaptation effect that passes very quickly. The effect
is caused by lenses of intermediate or higher power
being fitted with the optical centers at different
heights relative to the pupils in the new Rx compared
to their location in the old Rx. Your job is to decide
which height will serve the patient best in the long
run. Usually placing the optical centers at about the
six o'clock position relative to the pupils works well
for most patients. Clinical Tip: Specify MRP heights
(or center of pupil heights for PALs) for every lens you
order. Otherwise your lab will place the MRPs at the
mounting line (i.e. halfway from the top to the bottom
of the lens) and this is frequently a root cause for
patient difficulties.
Whether your preliminary evaluation reveals a possible
cause for your patient's problem or not, you
must evaluate the lenses for a number of potential
errors. This should be done systematically and carefully.
Never assume that a "quick check" is sufficient
because the lenses passed the quality control of your
laboratory and the check-out procedure of your own
office.
If the lenses are PALs, use the OLA's lens identifier
manual to be sure the manufacturer's markings are
as they should be. Look carefully for areas of distortion,
waves and warpage by reflected light as well
as by transmitted light. Check lens thicknesses and
base curves. The power, thickness and curvature
of a lens determine its magnification and patients
tend to be sensitive to magnification differences,
especially those that interfere with comfortable binocular
vision. Your lensometer's lens clamp and lens
table should be used to assure accuracy in power and
prism readings.
Blurring at near is common with an increase in plus
(either overall or via an add). Generally this is due to
over-accommodation and it will pass as the patient
"learns" to relax all that accommodative effort. Clinical
Tip: Ask patients with increased plus to use extra
light for near work for the first few days in the new
prescription. This increases the depth-of-field and a
normal reading distance can be easily maintained.
The increased depth-of-field also optically widens
the corridor and the reading area for PAL wearers and
it makes adaptation easier, especially if they are firsttime
PAL wearers.
Although the fit and fabrication of your patient's eyewear
may be right on target, patient difficulties can
arise from problems with the design. If you suspect
a design problem, it is particularly important to compare
the previous eyewear with the new.
CAREFUL OBSERVATION IS IMPORTANT
Careful observation is the key to unraveling many visual
difficulties caused by problems with the fit of the
eyewear. Although the patient may be annoyed by
the frame sliding down his nose, he will probably not
realize that the increase in vertex also increases the
plus power of his add resulting in a too short reading
distance. The same increase in vertex narrows the
usable reading areas of his lenses so that more head
movement is required to read a column of print and
the lens corridors may no longer be available for binocular
viewing making them partially or wholly unusable.
Looking at the patient from above makes it easy to
detect unequal vertex distances. By looking at the
patient from the side it is easy to evaluate inappropriate
pantoscopic tilt. As you observe from the side,
you can hold a fixation target (e.g. your thumb) so
that the patient looks straight ahead. If you then lower
your thumb slowly so the patient's lines-ofsight are depressed you can
easily tell if the lashes have sufficient clearance and you can also
estimate whether the lines-of-sight intersect the lenses at an
improper angle .
Excessive adaptation problems and difficulties with
vision are often caused by errors in fabrication or by
the fit of the eyewear. If the problems are just an issue
of adaptation, symptoms will lessen with about
three days of constant wear for most patients and
should resolve within another seven days or so. Patients
sometimes can even adapt to minor errors in
fabrication, but simply telling the patient the issue
is one of adaptation is inappropriate. If your patient
feels strongly enough about the issue to return to
your office for resolution, you should always give the
matter a full measure of your attention.
A SIMPLE TEST IDENTIFIES POTENTIAL CAUSES
The optical center of the lens represents the point at
which light rays pass through undeviated. The optic
axis passes through this optical center at right angles
to the lens surfaces. Unless prism has been prescribed,
the very best lens performance occurs when
the patient's line-of-sight coincides with the optical
axis of the lens.
If you want to assure the best lens performance, locate
the optic axis rather than simply marking the optical
center. To locate the optic axis use a direct ophthalmoscope
or penlight at a distance of 20 inches or
so from the patient. The patient should be directed
to look at the light and you should observe the two
reflections of the light (from the front and back lens
surfaces). By moving as you keep the light directed
toward the patient's pupil you will find one location
at which the two reflections are superimposed to appear
as a single reflected point of light. Once located,
you need to note the relationship of that reflection
to the patient's pupil and the orientation of your own
line-of-sight to the patient's facial plane.
Most patients do best if the optic axes are straight
ahead, parallel and depressed a few degrees downward.
If the optic axes are splayed outward the frame
may have too much wrap. Note: If the frame is intended
to wrap significantly, your lab should be directed
to make the appropriate changes in power and MRP
location to compensate for the optical changes induced
by the wrap.
Optic axes that point more upward in one eye than
the other indicate a twist in the bridge that might
otherwise be difficult to detect. Unwanted vertical
prism also becomes apparent when the optic axis position
in one eye is higher or lower than in the other
(relative to the patient's pupils).
CHALLENGES & OPPORTUNITIES
Patients who return with eyewear problems represent
an opportunity to do more than simply resolve
problems and complaints. They allow you to develop
your skills in eyewear design and they will help you
anticipate and avoid future eyewear problems. If you
are persistent and meticulous in addressing these
challenges, your patients will respect your dedication
to their needs and to your profession.
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