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TECHNOLOGY WATCH
Section Editor: John A. Vukich, MD
Pseudophakic
Accommodation
An update on accommodative physiology, technology, and measurement.
BY ADRIAN GLASSER, P H D
A
ccommodative ability gradually decreases with
increasing age and is lost completely by about
age 55.1 The effects of presbyopia become evident around the age of 45, as the accommodative amplitude recedes beyond a comfortable reading
distance. It is widely accepted that presbyopia results
from stiffening of the lens to a point where the shape of
the lens can no longer be changed by the accommodative mechanism.2,3 Although spectacles can effectively
compensate for this loss, they are a poor substitute for
the active and dynamic accommodation of a young eye.4
There is considerable interest in whether the presbyopic eye’s accommodative ability can be restored. The
idea is not simply to bring back functional near vision,
which can be accomplished with near reading adds, progressive addition lenses, or optical multifocality through
contact lenses, corneal refractive procedures, or multifocal IOLs. Rather, the goal is to restore true active and
dynamic accommodation. Current efforts in this area
include laser or pharmacological modification or softening of the natural phakic presbyopic lens and the development of so-called accommodating IOLs.
ACCOMMODATIVE MECHANISM
Accommodation is a dioptric change in the power of
the eye consequent to contraction of the ciliary muscle.
When the ciliary muscle contracts in the natural, young
phakic eye, the apex of this muscle moves toward the
lens equator.5 This action releases zonular tension
around the lens equator and allows the elastic lens capsule to mold the young, soft lens into a more spherical
and accommodated form. The accommodative increase
in the optical power of the lens comes about by virtue
of an increase in the anterior and posterior surface curvatures of the lens. As a result, the optical power of the
lens increases, and the eye changes focus from distance
to near. A distant object subtends parallel rays at the
cornea, with zero vergence at the eye. A near object has
divergent rays at the cornea; the accommodative
26 ADVANCED OCULAR CARE APRIL 2011
increase in the optical power of the eye is necessary to
overcome this divergence and create a focused image on
the retina.
CALCULATIONS OF ACCOMMODATION
The success of accommodating IOLs will be determined by how well they can produce an accommodative
increase in the optical power of the eye. Simple schematic eye-model calculations demonstrate the accommodative amplitudes that can be achieved based on
movements or changes in an IOL.6,7 An accommodative
increase in the optical power of a pseudophakic eye can
Figure 1. The accommodative response of two schematic
eyes as a function of the decrease in anterior chamber depth.
In the pseudophakic eye, the 20.00 D IOL was moved forward
in the eye. In the phakic eye, the natural accommodative
response was simulated as an increase in lens thickness, a
decrease in anterior chamber depth, and an increase in the
curvature of the anterior and posterior lens surfaces based
on what actually occurs in the young phakic eye. The markedly greater accommodative response in the phakic eye occurs
predominantly due to changes in the curvature of the lens
surfaces but with considerably smaller dimensional changes
than in the pseudophakic eye.
TECHNOLOGY WATCH
Figure 2. An increase in lens thickness, a decrease in anterior
chamber depth, steepening of the curvature of the anterior
and posterior lens surfaces, and the resulting accommodative
response in the natural phakic eye from anatomically based
schematic eye calculations. The changes in lens thickness and
anterior chamber depth are considerably smaller than the
changes in curvature of the lens surfaces, and the former contribute less to the overall accommodative response of the
eye. Negative numbers on the y-axis denote a decrease in
each parameter.
be accomplished in several ways. For example, a 1-mm
forward movement of a 20.00 D single-optic IOL will
produce 1.20 D of accommodation (Figure 1). Accommodative movements of the natural phakic lens, however, are small; for example, lens thickness increases by only
about 300 µm for 5.00 D of accommodation (Figure 2).
A 1-mm forward movement of a single-optic IOL is
therefore unlikely to occur; a 300-µm forward movement of a 20.00 D single-optic IOL would produce only
about 0.40 D of accommodation.
Calculations with an anatomically based schematic
eye that has a natural phakic lens can produce 10.00 D
of accommodation. During accommodation of the natural phakic lens, the anterior surface of the lens moves
forward by about 400 µm.8 The much more efficient
accommodative response in the natural phakic lens is, of
course, not due to forward movement but an increase in
the curvature of the anterior and posterior surfaces. As
the natural phakic lens accommodates, diameter decreases, thickness increases, the anterior surface moves
anteriorly, and the posterior surface moves posteriorly.
More important to the accommodative optical change
in the power of the eye, the curvature of the lens anterior and posterior surfaces becomes steeper. An accommodating IOL that mimics natural accommodation by
changing the curvature of its surfaces will maximize
accommodative capacity.
THE OBJECTIVE MEASUREMENT OF
ACCOMMODATION
In subjective tests of accommodation, the patient is asked
to report on his or her ability to read at near. Although
assessing and understanding patients’ visual performance is
important, it is no substitute for objective accommodation.
Patients’ subjective performance can be misleading. For
example, in a completely presbyopic eye, pupillary constriction alone can increase depth of field, and the patient will
report that text can be moved closer to his or her eye and
still remain in focus. Similarly, a distance-corrected patient
with a multifocal IOL will have some degree of functional
near vision, but this will be due to the multifocal optics and
cannot be considered accommodation. The subjective
measurement of accommodation is affected by factors such
as depth of field and ocular aberrations that have nothing
to do with the active and dynamic increase in the eye’s optical power consequent to the ciliary muscle’s contraction.
Establishing if an accommodating IOL can undergo an
accommodative increase in the optical power of the eye
with an effort to focus at near requires an objective measurement.9-11 Such testing necessitates the use of an objective instrument such as the iTrace wavefront aberrometer
(Tracey Technologies, Inc., Houston, TX) or Grand-Seiko
autorefractor (Grand-Seiko Co, Ltd., Hiroshima, Japan).9-11
Biometry instruments such as optical coherence tomographers (Figure 3) and ultrasound biomicroscopes (Figure 4)
can measure physical accommodative changes in the eye
such as the forward movement of an IOL or a change in
the surface curvature of the lens.
Regardless of which objective method is used, the testing
procedure should be first to measure the distance-corrected
eye while it is viewing a distant target at 6 m to determine
baseline refraction or biometry. A target at 6 m subtends
close to zero vergence at the cornea, so accommodation
Figure 3. Visante OCT image (Carl Zeiss Meditec, Inc., Dublin,
CA) of the anterior segment of the eye, from which the ocular
biometry can be measured to assess accommodative changes.
APRIL 2011 ADVANCED OCULAR CARE 27
TECHNOLOGY WATCH
determined. The accommodative response is calculated by
subtracting the distance measurement from the response
obtained for each near stimulus presented. The result is an
accommodative stimulus-response curve, with error bars
from which the accommodative amplitude can readily be
determined.
Figure 4. Ultrasound biomicroscopy image (Sonomed, Inc.,
Lake Success, NY) of the anterior segment of the eye, from
which ocular biometry can be measured to assess accommodative changes.
Figure 5. A typical stimulus-response function of the accommodative response measured objectively in a phakic patient
with a Grand-Seiko WAM-5500 autorefractor. Three measurements were taken for each stimulus amplitude; the graph
shows the error bars as standard deviations. Using this
approach, it is clear that the accommodative amplitude is
approximately 5.50 D. The solid line represents the 1:1 line. It
is typical that the response lags behind the stimulus and progressively more so with an increasing stimulus demand.
would be relaxed. A compelling accommodative stimulus
should then be presented at several progressively increasing
accommodative demands in predetermined dioptric steps
(Figure 5). Next, the refraction or biometry of the eye
should be measured while the patient is asked to focus on
each progressively closer near stimulus. A compelling stimulus should be used that includes high-contrast letters with
sufficient illumination to make the target clearly visible. The
goal is to provide the most compelling stimulus so as to elicit the strongest accommodative response to each stimulus.
Multiple measures should be taken for each near stimulus
presented so the variance of the measurements can also be
28 ADVANCED OCULAR CARE APRIL 2011
CONCLUSION
If accommodating IOLs are to succeed in restoring
accommodation to the presbyopic eye, their design
should be based on a sound understanding of the mechanism of accommodation and the causes of presbyopia.
Accommodating IOLs with surfaces that change curvature
have considerably greater potential for generating significant accommodation than IOLs that move forward in the
eye without a change in surface curvature. Accommodating IOLs will benefit from technological improvements
in their design and fabrication as well as rigorous and
appropriate clinical assessments of accommodation. That
requires objective—not subjective—measurements. ■
Section Editor John A. Vukich, MD, is the surgical director
of Davis Duehr Dean Center for Refractive Surgery in
Madison, Wisconsin. Dr. Vukich may be reached at (608)
282-2000; [email protected].
Adrian Glasser, PhD, is the Benedict/Pitts professor of optometry and vision sciences and biomedical engineering at the University of Houston
in Texas. Dr. Glasser serves as a paid consultant
to several companies with interests in accommodation and presbyopic and accommodative restoration concepts, including Alcon Laboratories, Inc.; Abbott Medical
Optics Inc.; Encore Vision; LensAR Inc. (including stock
options); PowerVision Inc.; and Tracey Technologies. He has
received an ultrasound biomicroscope from Sonomed USA
and a Grand-Seiko WAM-5500 autorefractor from GrandSeiko Co., Inc., for use in his research. Dr. Glasser may be
reached at (713) 743-1876; [email protected].
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