(Investigative Ophthalmology and Visual Science. 2002;43:700-708.)
© 2002
by The Association for Research in Vision and Ophthalmology, Inc.
Computational Evaluation of the Role of Accommodation in Pigmentary Glaucoma
Jeffrey J. Heys1 and
Victor H. Barocas2
1 From the Department of Chemical Engineering, University of Colorado, Boulder, Colorado; and the
2 Department of Biomedical Engineering, University of Minnesota, Minneapolis, Minnesota.
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Abstract
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PURPOSE. Accommodation has been proposed as the cause of the bowing of
the posterior iris that occurs in eyes with pigmentary dispersion
syndrome. A mathematical model of the anterior eye is needed to explore
the elastohydrodynamic effects of accommodation on both the aqueous
humor dynamics and the contour of the iris.
METHODS. A mathematical model of the coupled aqueous humor–iris system was used
to predict the effects of accommodation on the iris position and
pressure distribution in the aqueous humor.
RESULTS. The mathematical model predicts that accommodation produces a pressure
reversal—the anterior chamber pressure being higher than the posterior
chamber—and iris movement into a more concave configuration. Total
time for accommodation, iris modulus, iris attachment point, and
trabecular meshwork permeability all had little or no effect on the
iris contour and pressure change. The amount of accommodation, however,
had a dramatic effect on both the amount of iris curvature and
especially the pressure reversal. For accommodation resulting
in a 0.6-mm change in anterior chamber depth, the pressure in the
anterior chamber was more than 1.0 mm Hg higher than that in the
posterior chamber, compared with a pressure difference of less than 0.1
mm Hg for accommodation resulting in a 0.2-mm change in anterior
chamber depth.
CONCLUSIONS. The results confirm that accommodation produces bowing of the posterior
iris and the magnitude of the bowing is a strong function of the amount
of accommodation.
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Introduction
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Insight into pigmentary glaucoma (PG) has increased
dramatically during the past 20 years, but the condition is still not
completely understood.1
Campbell2
first
proposed that rubbing of the concave iris against the zonules causes
the release of pigment in eyes with pigmentary dispersion syndrome
(PDS) and PG. The liberated pigment floats with the aqueous currents
and is primarily deposited on the posterior cornea surface in the form
of the Krukenberg spindle and on the trabecular meshwork (TM).
The cause of iris concavity in PG and PDS is not known with absolute
certainty, but there is substantial evidence3
for the
reverse-pupillary-block theory proposed by Karickhoff.4
Reverse pupillary block implies that aqueous humor can flow forward
through the pupil under normal conditions, but if the pressure in the
anterior chamber is higher than in the posterior chamber, the iris is
pushed against the lens and prevents backward flow from the anterior
chamber into the posterior chamber. The higher pressure in the anterior
chamber applies a net force to the iris, causing it to be displaced
posteriorly and assume a concave shape. The cause of the supposed
pressure reversal is a fundamental question that must be answered
before PG is fully understood.
There are a number of theories about how the pressure becomes elevated
in the anterior chamber. When Karickhoff4
originally
proposed reverse pupillary block, he also suggested that "walking,
certain head positions, or eye movement could create enough of a
difference in anterior- and posterior-chamber pressures to cause the
‘iris valve’ to open and close." It is unlikely that this is the
case, because of the rigidity of the cornea (modulus = 10.3
MPa5
) and the near incompressibility of water.
Pavlin et al.6
7
first proposed accommodation as the cause
of the pressure reversal between the anterior and posterior chambers
resulting in bowing of the posterior iris. They theorized that as the
lens moves forward during accommodation, the pressure in the anterior
chamber is increased. The aqueous could exit through the TM, but,
according to the theory of Pavlin et al., this would happen relatively
slowly. The aqueous would be prevented from flowing back into the
anterior chamber by reverse pupillary block, and the higher anterior
chamber pressure would cause the iris to bow posteriorly. Ultrasound
scans in patients with PDS show a dramatic change in iris contour
before and after accommodation.7
Ultrasounds in patients
with PDS after peripheral iridotomy are equally dramatic in the absence
of iris contour change during accommodation.7
Similar
concavity can occur after accommodation in healthy
individuals,8
an observation that led Sokol et
al.9
to hypothesize that the position of the iris’s
insertion in combination with accommodation may determine whether PDS
develops. They found that individuals with PDS had an iris insertion
0.12 mm more posterior relative to the TM than did normal individuals.
We have developed a mathematical model of passive deformation of the
iris in response to aqueous humor flow.10
In this study,
we used our model to analyze whether the hydrodynamic effects of
accommodation contribute to the development of posterior iris curvature
in PDS.
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Methods
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Model Specification: Base Case
A detailed description of the two-dimensional axisymmetric model
is presented in Heys et al.10
We present herein a brief
review of the model’s equations and a summary of the changes for the
modeling of accommodation.
Figure 1
shows schematically the region of the eye that is modeled, including
the structures that correspond to various boundaries. The
aqueous humor has physical properties close to those of water
(see Table 1
) and is modeled using the Navier-Stokes equations
 | (1) |
 | (2) |
where v is velocity and P is pressure. The
Reynolds number in the eye is normally very small (
0.01), but the
acceleration of fluid during accommodation results in Reynolds numbers
on the order of 1, requiring the full Navier-Stokes equation.
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Figure 1. Model domain showing the boundaries and dimensions of the model and the
boundary condition applied along those boundaries.
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Our experiments on bovine iris13
showed that the tissue is
incompressible and linearly elastic under small deformations, so the
incompressible linear elastic equations are used to model the iris
 | (3) |
 | (4) |
where u is the displacement from the rest position,
which is assumed to be planar, and G is the shear modulus (a
measure of stiffness in response to shape change). The modulus of the
human iris has not been measured to our knowledge, and we therefore
assumed for our base case that the iris has a modulus of 9 kPa, based
on our findings in bovine iris.
The boundaries of the region that is modeled represent different
structures within the eye and are modeled with a variety of boundary
conditions. The cornea is modeled as a 0.5-mm-thick elastic shell with
a modulus of 10.3 MPa.5
The TM and vitreous are modeled as
porous solids, but the hydraulic conductivity of the
vitreous–retina–sclera pathway is set to zero (i.e., no posterior
outflow). The specific conductivity of the TM is set to 2.1 x
10-9 m/s · Pascal, which corresponds to a
steady state IOP of 16 mm Hg. Setting the posterior outflow pathway
conductivity to a nonzero value affects the position of the iris, but
this effect is negligibly small for all reasonable values.
Pressure-independent outflow through the uveoscleral route is included
along the same boundary as the TM outflow. The lens is modeled as a
rigid solid with the position of the lens surface being determined by
the current accommodation level. The final boundary represents the
ciliary bodies and is modeled as a fluid source with a volumetric flow
rate of 2.5 µL/min.16
Accommodation is modeled by moving the boundary that defines the
anterior surface of the lens. Three quantities are needed to model
accommodation: (1) the initial and final positions of the lens, (2) the
total time required for the lens to move between the initial and final
positions, and (3) the velocity of the lens as a function of time
during accommodation. Parameters (1) and (2) are a strong function of
age,17
which may be important, because PG primarily
affects younger individuals and can decline with age.
Koretz et al.18
demonstrated that the anterior lens
surface position y can be described accurately as a function
of distance from the center line r and time t by
parabolas of the form
 | (5) |
where a(t) and c(t)
are functions of time determined experimentally from ultrasound or slit
lamp measurements. The change in anterior chamber depth, which is
normally measured along the symmetry axis passing through the pupil, is
highly age dependent and critical for calculating the function
a(t). Our base case assumes that the anterior
chamber depth changes by 0.2 mm during full
accommodation.19
20
The second function,
c(t), is calculated by assuming that a point near
the periphery of the lens remains fixed. This assumption is justified
by the fact that the human eye is focused primarily by changing the
curvature but not the position of the lens. A point approximately 4 mm
from the pupillary axis is used in the model, based on the measurements
of Cook and Koretz.21
The time required for full accommodation varies between the dominant
and nondominant eyes, focusing direction (far-to-near versus
near-to-far) and age. Ibi22
reported accommodation times
ranging from 0.52 to 0.94 seconds. Because patients with PG tend to be
younger (i.e., accommodating over a shorter time17
) our
base case assumed that accommodation requires 0.5 seconds. The velocity
of the lens is approximated as being constant and purely axial in the
model. Croft et al.17
and Ibi22
have both
measured the velocity of the anterior lens surface throughout
accommodation, and, with the exception of a short initial and final
transition, the velocity is nearly constant in the measurements.
Because the model includes only a limited region of the eye, aqueous
humor outflow equal to the change in lens volume within the region
modeled is necessary to assure conservation of mass. In other words, as
the lens in the model domain moves forward, an equal amount of fluid is
forced posteriorly to reflect a constant lens volume.
In summary, the base case for accommodation has the following
properties: Giris = 9 kPa, change in
anterior chamber depth = 0.2 mm, accommodation time = 0.5
seconds, and kTM = 2.1 x
10-9 m/s · Pascal. The effect of variation of
each parameter will be compared with the standard case in the Results
section.
Model Specification: Case Studies
Case studies serve two important purposes. First, they allow us
to account for potential inaccuracy in parameter values caused by lack
of reliable data or individual variation. Second, they provide a
mechanism for evaluating the significance of an effect. We therefore
performed a series of case studies to evaluate how different parameters
affect the iris contour.
As stated earlier, the modulus of the iris in our base case (9 kPa) was
based on our experimental results in bovine iris. In light of the
importance of this parameter and the potential for interspecies and
individual variation, we performed a second study with a less stiff
iris, setting Giris at 3 kPa.
Our base case model of accommodation assumed that the lens moves 0.2 mm
during full accommodation. Cook and Koretz,21
however,
reported a change in anterior chamber depth of approximately 0.6 mm in
an 18-year-old individual. Due to the uncertainty in the change of the
anterior chamber depth, we studied the case of 0.6-mm accommodation in
addition to the base case. Similarly, we consider a long accommodation
time of 1.0 seconds in contrast to our base case of 0.5 seconds.
Finally, we considered the fact that PG patients have higher IOP and
presumably lower TM conductivity, which could affect the model results.
We therefore performed studies on a model with reduced TM
conductivity—specifically, a reduction to 0.7 x
10-9 m/s · Pascal, corresponding to a very
high steady state IOP of 42 mm Hg.
Numerical Solution of Model Equations
The model equations were solved using the standard Galerkin
finite element method, as described previously.10
The
solution method was modified by allowing finite element nodes to slide
along the surface of the lens to minimize element distortion.
During accommodation, the lens deforms anteriorly toward the iris and
nearly comes into contact with the iris. In theory, actual contact
between two smooth surfaces is impossible because of the infinite
stress developed in the lubricating fluid layer on incipient contact.
Inaccuracies in numerical solution, however, can cause the computed
position of the iris to overlap slightly with the lens, leading to
breakdown of the simulation. Various methods have been developed to
combat this sort of problem, such as the use of a Lagrange multiplier
to push back the nodes that have penetrated the lubrication
layer.23
Our approach was to enforce a no-contact zone by
introducing a stress on the iris normal to the lens. The additional
stress is related to the distance between the iris and lens by the
equation
 | (6) |
where nlens is the vector normal to
the lens surface at the nearest point on the lens surface, A
and 
are adjustable coefficients, and d is the minimum
distance from the iris to the lens. The coefficient corresponds to
how far the no-contact zone extends into the aqueous humor, and the
coefficient A corresponds to how strongly the no-contact
zone is enforced. The contact prevention force can affect the results
if it is applied at greater distances (or equivalently greater
strength), as shown in Figure 2
. Based on the data in Figure 2
, the maximum value of used in all
studies was 3 µm. In addition to having little effect on the
hydrodynamics of interest, the use of a small separation layer is
consistent with the fact that the posterior iris surface is
rough,24
and therefore, although some regions may contact
the lens, the entire surface is unlikely to make contact.
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Figure 2. The effect of the contact-prevention force on apparent contact and flow
between the chambers. If the contact-prevention force is applied over a
longer distance, it has a significant impact on the passive iris
displacement. Apparent contact and flow between chambers was measured
at the final time step of accommodation. Results are based on an iris
modulus of 9 kPa and A = 5 x
104.
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Quantification of Model Results
We used two quantities introduced by the experimental literature
to characterize our iris contour simulation results. Current
high-frequency ultrasound biomicroscopy (UBM) devices are
unable to resolve the gap between the iris and lens (maximum resolution
of UBM is 40–50 µm25
). As a result of this limitation,
many UBM researchers report an apparent iris–lens contact distance,
which is the distance over which there is no resolvable space between
the iris and lens. The apparent contact distance was calculated from
the model solution by determining the distance over which the iris and
lens are separated by less than 50 µm.
The contour of the iris is significant in many forms of glaucoma, and
as a result, many researchers report the curvature of the iris from UBM
measurements. Iris curvature is defined as the maximum distance between
the posterior surface of the iris and the line connecting the posterior
iris root to the posterior surface of the iris at the pupil
periphery.26
Thus, if the posterior iris surface were
completely linear in a micrograph, the curvature would be zero. If the
iris were convex, the curvature would be positive, and if the iris were
concave, as in PG, the curvature would be negative. The iris curvature
was calculated from the model results.
We also estimate the IOP from the model results. IOP was determined by
calculating the pressure in the anterior chamber adjacent to the TM.
Because there is no significant resistance to flow in the anterior
chamber, the pressure is essentially uniform at the IOP throughout the
anterior chamber.
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Results
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Base Case
The results for the base case (G = 9 kPa, decrease
in anterior chamber depth of 0.2 mm, and an accommodation time of 0.5
seconds) are shown in Figure 3
. Before accommodation (Fig. 3a)
, the iris is nearly linear, but as
accommodation begins, aqueous humor is displaced anteriorly by motion
of the lens. At the same time, aqueous must flow posteriorly around the
lens to because of constant lens volume (Fig. 3b)
. After accommodation
(Fig. 3c) , the iris concavity is significantly increased because of
reverse pupillary block. Finally, if the eye is allowed to stay
accommodated for 300 seconds (Fig. 3d)
, pupillary block and steady
secretion of aqueous into the posterior chamber drive the iris away
from the lens.
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Figure 3. The aqueous humor region (gray) and iris
(black) (a) before accommodation,
(b) 0.15 seconds after the start of accommodation,
(c) at the end of accommodation, and (d) long
after accommodation. (b) The flow of aqueous humor and the
contour of the iris shows how aqueous must leave the posterior chamber
to make up for the volume of lens entering the domain. The iris is
deformed to a concave shape from a linear configuration, and the iris
is in near contact with the lens over a much larger region.
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The data shown in Figure 3
are presented in quantitative form in Figure 4
. Accommodation was initiated at time = 0 seconds, but the model
was first run for 2 seconds to ensure that the solution was at the
preaccommodation steady state. Figure 4a
shows a dramatic increase in
apparent contact during accommodation, but the iris drifts away from
the lens during the next 300 seconds, nearly returning to the
preaccommodation level of contact. The iris curvature is initially near
zero, indicating an approximately linear iris (Fig. 4b)
. Accommodation
causes the iris to be deformed into a concave configuration indicated
by negative iris curvatures in Figure 4b
. After accommodation, the iris
returns to an approximately linear contour.
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Figure 4. The effect of accommodation on the iris and aqueous humor.
(a) Apparent iris–lens contact, (b) the
curvature of the iris, (c) the pressure difference between
the anterior and posterior chambers, and (d) the IOP. In all
figures, accommodation was initiated at time = 0, complete
accommodation occurred in 0.5 seconds, and the base case parameters
were used (Giris = 9 kPa, change in
anterior chamber depth, 0.2 mm).
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Under steady state conditions, the posterior chamber must be at a
higher pressure than the anterior chamber, because the normal flow of
aqueous humor is from the posterior chamber into the anterior chamber.
During accommodation, however, the movement of the lens causes the
pressure in the anterior chamber to be temporarily higher than the
pressure in the posterior chamber (Fig. 4c)
. The peak pressure in the
anterior chamber is approximately 0.1 mm Hg higher than that in the
posterior chamber. Two minutes after accommodation, the pressure in the
posterior chamber again becomes higher than that in the anterior
chamber, and a steady state condition is achieved in approximately 250
seconds. The IOP increases by approximately 1 mm Hg during
accommodation, because of the forcing of aqueous humor into the
anterior chamber by the movement of the lens (Fig. 4d)
.
Case Studies
The effects of varying specific parameters from the base case were
summarized by comparing the apparent contact, iris curvature, and
pressure difference between chambers at three different times: (1)
before accommodation (time < 0 seconds), (2) immediately after
accommodation (time = 0.5 seconds or 1 second), and (3) the final
steady state in the accommodated eye (time = 300 seconds).
Figure 5
summarizes the effects of reducing the iris modulus from 9 to 3 kPa.
The lower modulus resulted in very little change in both apparent
contact and iris curvature. However, the pressure difference between
chambers was significantly less immediately after accommodation. This
was caused by a larger gap between the iris and lens in the low modulus
case, which allowed more flow to partially equalize the pressures.
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Figure 5. Case study of iris modulus changing from a base value of 9 kPa (cf.
Fig. 4
) to 3 kPa. Reducing the iris modulus to 3 kPa had little effect,
other than reducing the pressure difference between the anterior and
posterior chambers immediately after accommodation.
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The effects of increasing accommodation time to 1.0 second from the
previous value of 0.5 seconds are shown in Figure 6
. The pressure difference between chambers at the end of accommodation
is reduced slightly in the slower accommodation case, but there is no
change in apparent iris–lens contact length or iris curvature. The
reduced pressure difference is the result of the aqueous’s having more
time to drain through the TM during accommodation.
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Figure 6. Case study of time required for complete accommodation changing from
base value of 0.5 seconds to 1.0 second. The change had almost no
effect on the iris or aqueous humor. The pressure in the anterior
chamber was slightly lower immediately after the 1.0-second
accommodation, because of more extensive drainage from the anterior
chamber during accommodation.
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Figure 7
displays the effects of two different amounts of anterior depth change
on apparent contact, iris curvature, and the pressure difference
between chambers. For the base case, we assumed that the anterior
chamber depth decreases by 0.2 mm during accommodation, but when the
depth change was increased to 0.6 mm (based on measurements in an
18-year-old man21
) the differences in the model results
were dramatic. The magnitude of iris curvature at the end of
accommodation was increased by approximately 200%, and the apparent
contact distance was increased by more than 70%. The most significant
change occurred in the pressure difference between chambers immediately
after accommodation, which was more than 10 times greater than the
standard case. Figure 8
shows the aqueous humor and iris regions for a decrease in anterior
chamber depth of 0.6 mm at the same time points as in Figure 3
. The
iris curvature immediately after accommodation was significantly more
concave in the case of greater anterior chamber depth change (Fig. 8c)
.
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Figure 7. Case study of extent of accommodation, which is shown as a change in
anterior chamber depth, changing from the base value of 0.2 mm to a
value of 0.6 mm. Enlarging the accommodative range had a significant
effect on the curvature of the iris and the pressure difference between
the anterior and posterior chambers.
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Figure 8. The aqueous humor– (gray) and iris-
(black) based change in anterior chamber depth of 0.6
mm, using the same time points in as Figure 3
. (a) Before
accommodation, (b) 0.15 seconds after start of accommodation
(including the flow of aqueous humor), (c) end of
accommodation, and (d) steady state. The greater extent of
accommodation resulted in significantly more iris curvature.
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When the iris insertion point was moved 0.12 mm posterior to the
previous position, the magnitude of the iris curvature increased before
and long after accommodation, but it was nearly equal to the standard
case at the maximum immediately after accommodation (Fig. 9)
. The posterior iris attachment causes the pressure difference between
the chambers to be decreased due to pupillary block by an approximately
fixed amount (0.02 mm Hg) throughout the accommodation event. This
reduction in pressure difference causes the apparent contact maximum to
be decreased slightly from the standard case.
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Figure 9. Case study of iris attachment, placed 0.12 mm posterior to the base
case attachment point. The change caused little change in iris
curvature and a decrease in pressure difference between the anterior
and posterior chambers for the posterior iris attachment, compared with
the standard case.
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Figure 10
summarizes the apparent iris–lens contact, iris curvature, and
pressure difference at two different levels of TM permeability. The
iris position was unaffected by the trabecular permeability, and the
only significant effect of a lower permeability was an increase in the
IOP before, during, and after accommodation.
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Figure 10. Case study of TM permeability decreasing from base value of 2.1 x
10-9 m/s · Pascal to 0.7 x 10-9 m/s
· Pascal. The mathematical model predicts no change in iris curvature
or pressure difference. The only effect of reduced permeability was an
increase in IOP to 42 mm Hg.
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Discussion
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We applied a mathematical model of the eye to the study of the
physical effects of accommodation on aqueous humor dynamics and iris
deformation. The hypothesized relationship between accommodation and
iris–lens contact observed in PDS and PG has received special
attention. The most significant result is the confirmation of UBM
measurements suggesting that hydrodynamic effects driven by
accommodation can cause the concave iris contour observed in the PG
eye. The changes in apparent contact and iris curvature were
qualitatively the same as experimental measurements in eyes undergoing
accommodation. The model predicts that the curvature of the iris
becomes more concave as a result of accommodation, which agrees with
the UBM measurements of McWhae et al.8
The IOP during
accommodation increased by approximately 1 mm Hg, but we are not aware
of any experimental evidence to support this prediction. A number of
researchers27
28
have observed a decrease in IOP after
accommodation, but these results are typically based on pressure
measurements after 10 or 15 minutes of a near focus activity (i.e.,
reading). The decrease in IOP is generally attributed to an increase in
outflow facility after accommodation,28
a phenomenon not
included in the current model.
It is known from the clinical observation4
of aqueous
humor flowing posteriorly through a hole in the iris after laser
iridotomy that the pressure in the anterior chamber is transitorily
higher in eyes with PG. The higher pressure in the anterior chamber
must be temporary because the pigment from the iris is largely
deposited throughout the anterior chamber, including the TM, indicating
normal flow from the posterior chamber into the anterior chamber. The
model predicts that in cases of normal accommodation (i.e., an anterior
depth change of 0.2 mm) the pressure in the anterior chamber is only
slightly higher than that in the posterior chamber after accommodation
(0.1 mm Hg, Fig. 4
) and remains higher for approximately 120
seconds. If, however, the anterior chamber depth changes by 0.6 mm
during accommodation, the anterior chamber pressure becomes
significantly higher than the posterior chamber pressure (1.0 mm Hg,
Fig. 7 ) and remains higher for approximately 220 seconds.
The model provides quantitative predictions for the iris curvature,
pressure changes, and time scales involved under a variety of
conditions. Greater accommodative change, increased iris modulus, and a
posterior iris attachment all increase the effects of accommodation on
iris curvature and apparent contact length, indicating an increased
risk for mechanical rubbing between the iris and lens and potential
pigment liberation. The extent of accommodative change, measured by the
change in anterior chamber depth, had the greatest impact on the change
in apparent contact and iris curvature after accommodation. The
enhancement of accommodation effects in persons that have a larger
anterior chamber depth change (i.e., younger
individuals29
) could help explain why PDS and PG normally
occur in younger persons. There are many other factors that could
explain the development of PG and PDS in younger persons, including
pupillary block, that prevent development in older persons.
The model predicted, as might be expected, a lower anterior–posterior
pressure difference peak in eyes that accommodate more slowly (Fig. 6)
.
Because a lower pressure difference would reduce the force pushing the
iris against the lens, the possibility of iris–lens and iris–zonule
contact would also be reduced. Thus, the model predicts a decrease in
the potential of PDS-PG as the eye ages and accommodation time
increases. The effect is very small, however, and it is unlikely that
slower accommodation plays any significant role in lessening the
severity of the disease, so the pharmacologic slowing of accommodation
cannot be considered a likely treatment option, based on the modeling
results.
Posterior iris insertion led to increased pupillary block and thus
decreased iris–lens contact, which in turn implies less likelihood of
the loss of pigment from the iris pigment epithelium. Based on the
results of the mathematical model, we conclude that posterior iris
attachment increases the potential for the development of PDS and PG by
a nonhydrodynamic mechanism. One possibility is that the posterior iris
attachment increases the potential for contact between the concave iris
and the zonules.
There are two questions that we hope to pursue in the future: What
features of young male myopes lead to an increased risk of development
of PDS in that group,30
and why do few eyes have PDS, even
though all eyes accommodate? We have demonstrated that the large
accommodative range of the young could contribute to the
reverse-pupillary-block effect, but we saw reduced reverse pupillary
block in cases with posterior iris insertion. Better understanding of
structural and anatomic variations between patients with PDS and those
without, as it becomes available, will be incorporated into the model
and used to pursue these two key questions.
The focus of this study was fairly narrow. We did not consider
blinking, which some have argued is responsible for the pressure
reversal in PDS and PG,31
and although we are convinced by
our results that accommodation must play an important role in the
determination of iris contour, we cannot eliminate other effects, such
as blinking, that may also be significant. We were restricted to radial
symmetry in this model, forcing us to consider only symmetric effects.
The relationship between pigment erosion and decreased outflow facility
through the TM remains unclear.32
33
34
Although that
relationship is beyond the scope of the present study, its
understanding is clearly critical to assessing the importance of iris
contour in PDS and PG.
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Footnotes
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Supported by a Bioengineering Research Grant from the Whitaker
Foundation, Rosslyn, Virginia (VHB); National Institutes of Health
Grant R01-EY12291-01; and from National Science Foundation Vertical
Integration of Research and Education Grant DMS-9810751 (JJH). The
computer simulations were made possible through the support of
University of Minnesota Supercomputing Institute for Digital Simulation
and Advanced Computation.
Submitted for publication July 30, 2001; accepted October 18, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C. 
1734
solely to indicate this fact.
Corresponding author: Victor H. Barocas, Department of Biomedical
Engineering, 7-106 SBE, 312 Church Street, University of Minnesota,
Minneapolis, MN 55455; baroc001{at}umn.edu
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References
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|---|
-
Epstein, DL (1997) Pigment dispersion and pigmentary glaucoma Epstein, DL Allingham, RR Schuman, JS eds. Chandler and Grant’s Glaucoma ,220-231 Williams & Wilkins Baltimore.
-
Campbell, DG (1979) Pigmentary dispersion and glaucoma Arch Ophthalmol 97,1667-1672[Abstract]
-
Pavlin, CJ, Burns, PN, Foster, FS (1998) Use of microbubble ultrasound contrast agent to demonstrate the iris valve effect in human eye bank eyes Am J Ophthalmol 126,719-720[Medline][Order article via Infotrieve]
-
Karickhoff, JR (1992) Pigmentary dispersion syndrome and pigmentary glaucoma: a new mechanism concept, a new treatment, and a new technique Ophthalmic Surg 23,269-277[Medline][Order article via Infotrieve]
-
Smolek, MK (1994) Holographic interferometry of intact and radially incised human eye-bank corneas J Cataract Refract Surg 20,277-286[Medline][Order article via Infotrieve]
-
Pavlin, CJ, Harasiewicz, K, Foster, FS (1994) Posterior iris bowing in pigmentary dispersion syndrome caused by accomodation Am J Ophthalmol ,114-116
-
Pavlin, CJ, Macken, P, Trope, GE, Harasiewicz, K, Foster, FS (1996) Accommodation and iridotomy in the pigment dispersion syndrome Ophthalmic Surg Lasers 27,113-120[Medline][Order article via Infotrieve]
-
McWhae, JA, Piemontesi, RL, Crichton, ACS (1996) Blinking and iris configuration in PDS (letter) Ophthalmology 103,197-199[Medline][Order article via Infotrieve]
-
Sokol, J, Stegman, Z, Liebmann, JM, Ritch, R. (1996) Location of the iris insertion in pigment dispersion syndrome Ophthalmology 103,289-293[Medline][Order article via Infotrieve]
-
Heys, JJ, Barocas, VH, Taravella, MJ (2001) Modeling passive mechanical interaction between aqueous humor and iris J Biomech Eng 123,540-547[Medline][Order article via Infotrieve]
-
Scott, JA (1988) A finite element model of heat transport in the human eye Phys Med Biol 33,227-241[Medline][Order article via Infotrieve]
-
Beswick, JA, McCulloch, C. (1956) Effect of hyaluronidase on the viscosity of the aqueous humour Br J Ophthalmol 40,545-548
-
Heys, J, Barocas, VH (1999) Mechanical characterization of the bovine iris J Biomech 32,999-1003[Medline][Order article via Infotrieve]
-
Kaufman, PL (1989) Pressure-dependent outflow Ritch, R Shields, MB Krupin, T eds. The Glaucomas ,219-240 CV Mosby Co St. Louis.
-
Maurice, DM (1987) Flow of water between aqueous and vitreous compartments in the rabbit eye Am J Physiol 252,F104-F108[Abstract/Free Full Text]
-
Smith, SD (1991) Measurement of the rate of aqueous humor flow Yale J Biol Med 64,89-102[Medline][Order article via Infotrieve]
-
Croft, MA, Kaufman, PL, Crawford, KS, Neider, MW, Glasser, A, Bito, LZ (1998) Accommodation dynamics in aging rhesus monkeys Am J Physiol 275,R1885-R1897[Abstract/Free Full Text]
-
Koretz, JF, Handelman, GH, Brown, NP (1984) Analysis of human crystalline lens curvature as a function of accommodative state and age Vision Res 24,1141-1151[Medline][Order article via Infotrieve]
-
Shum, PJ-T, Ko, L-S, Ng, C-L, Lin, S-L. (1993) A biometric study of ocular changes during accommodation Am J Ophthalmol 115,76-81[Medline][Order article via Infotrieve]
-
Drexler, W, Baumgartner, A, Findl, O, Hitzenberger, CK, Fercher, AF (1997) Biometric investigation of changes in the anterior eye segment during accommodation Vision Res 37,2789-2800[Medline][Order article via Infotrieve]
-
Cook, CA, Koretz, JF (1991) Acquisition of the curves of the human crystalline lens from slit lamp images: an application of the Hough transform Appl Opt 30,2088-2099
-
Ibi, K. (1997) Characteristics of dynamic accommodation responses: comparison between the dominant and non-dominant eyes Ophthalmic Physiol Opt 17,44-54[Medline][Order article via Infotrieve]
-
Attaway SW, Hendrickson BA, Plimpton SJ, et al. Parallel contact
detection algorithm for transient solid dynamics simulations using
Proto3D. Presented at ASME International Mechanical Engineering
Congress and Exposition. Atlanta, GA. 1996.
-
Bron, AJ, Tripathi, RC, Tripathi, BJ (1997) Wolff’s Anatomy of the Eye and Orbit 8th ed. Chapman & Hall Medical London.
-
Pavlin, CJ, Foster, FS (1998) Ultrasound biomicroscopy: high-frequency ultrasound imaging of the eye at microscopic resolution Radiol Clin North Am 36,1047-1058[Medline][Order article via Infotrieve]
-
Potash, SD, Tello, C, Liebmann, J, Ritch, R. (1994) Ultrasound biomicroscopy in pigment dispersion syndrome Ophthalmology 101,332-339[Medline][Order article via Infotrieve]
-
Blake, J, Horgan, T, Carroll, P, Stokes, J, Fitzpatrick, P. (1995) Effect of accommodation of the lens on ocular pressure Ir J Med Sci 164,269-270[Medline][Order article via Infotrieve]
-
Aggarwala, KR (1995) On the short-term variability of measurements of intraocular pressure Optom Vis Sci 72,753-755[Medline][Order article via Infotrieve]
-
Koretz, JF, Kaufman, PL, Neider, MW, Goeckner, PA (1989) Accommodation and presbyopia in the human eye. 1: evaluation of in vivo measurement techniques Appl Opt 28,1097-1102
-
Farrar, SM, Shields, MB (1993) Current concepts in pigmentary glaucoma Surv Ophthalmol 37,233-252[Medline][Order article via Infotrieve]
-
Liebmann, J, Tello, C, Chew, S-J, Cohen, H, Ritch, R. (1995) Prevention of blinking alters iris configuration in pigment dispersion syndrome and in normal eyes Ophthalmology 102,446-455[Medline][Order article via Infotrieve]
-
Gottanka, J, Johnson, D, Grehn, F, Lütjen-Drecoll, E. (2001) Why is intraocular pressure elevated in pigmentary glaucoma [ARVO Abstract]? Invest Ophthalmol Vis Sci 42,S145Abstract nr 776
-
Robinson, CH, Nopanitaya, W, McPherson, SD (1981) Pigmentary glaucoma: an ultrastructural study Ann Ophthalmol 13,49-51[Medline][Order article via Infotrieve]
-
Murphy, CG, Johnson, M, Alvarado, JA (1992) Juxtacanalicular tissue in pigmentary and primary open angle glaucoma: the hydrodynamic role of pigment and other constituents Arch Ophthalmol 110,1779-1785[Abstract]
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F. N. Kanadani, S. Dorairaj, A. M. Langlieb, W. A. Shihadeh, C. Tello, J. M. Liebmann, and R. Ritch
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Arch Ophthalmol,
November 1, 2006;
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Abstract
Introduction
