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Theories take shape to unravel mystery of presbyopia development
in the human eye
Stefanie Petrou-Binder MD in Berlin
SEVERAL theories are taking shape which attempt to unravel the mystery
of how presbyopia develops in the human eye.
As research intensifies, a unanimous explanation for the occurrence
of pseudophakic pseudoaccommodation in presbyopic patients who have
received IOLs within the framework of cataract surgery looks increasingly
likely.
"The precise mechanisms behind pseudophakic pseudoaccommodation
are still unclear. It may be a combination of ciliary muscle contraction
with the consequent shifting of the lens, or a discrepancy between
the spherical aberration of the IOL and the natural lens,"
German specialist Felix Gora MD said.
Dr Gora and colleagues presented a study on pseudophakic pseudoaccommodation
at the annual meeting of the German Society of Ophthalmology in
Berlin. The researchers studied 20 eyes in 17 patients, 12 of whom
were female and five male. The patients ranged in age from 27 to
81, with an average of 57 years.
All the study participants were cataract patients, who were scheduled
for surgery at the clinic. The investigators excluded from their
study any patients who had previously undergone intraocular surgery.
Patients with a history of perforating traumas, less than 0.5 visual
acuity, anatomical or pathological lens findings (pseudoexfoliating
lens, phacodonesis or zonula defects, for example) and ocular disease
like diabetic retinopathy and chronic uveitis were excluded.
The investigators examined all patients immediately after and again
four weeks following cataract surgery. They measured the postoperative
refraction, near vision and accommodation.
Dr Gora measured visual acuity postoperatively with EDTRS tables
from six, four, two and one metre distances. He measured near vision
with Birkhäuser-reading tables from a distance of 35cm and
the accommodation width using an accommodation metre and the length
of the bulbus with the IOLMaster.
The average near vision of the post-cataract study patients was
0.28 ± 0.15. The average focal point distance was 0.42 ±
0.10.
The researchers noted a linear relationship and a statistically
significant correlation between near vision and the focal point
distance values. They reported no correlation between axial length
and near vision or axial length and focal point distance.
This study corroborates the outcome of previous studies which have
reported a pseudophakic pseudoaccommodation following the implantation
of a regular IOL.
Although the extent has varied from study to study, the mean pseudoaccommodation
of 2.49 ± 0.60 D measured in this study lies within the range
of values established in previous trials.
In a study which included 39 eyes, Nakazawa measured a 2.01 ±
0.95 D pseudoaccommodation. Similarly, a study conducted by Wang
showed an average 1.53 ± 0.59 D pseudoaccommodation he measured
in 73 post-cataract IOL-implanted eyes. By contrast, another study
showed as little as 0.5 D pseudophakic accommodation in 20 eyes.
According to Dr Gora, the divergent pseudoaccommodation values measured
in different studies can be explained by the implementation of various
examination methods.
He pointed out that pseudoaccommodation may also result from a patient’s
own subjective account in the near vision examinations and focal
point measurements.
"While IOL implantation seeks to improve visual acuity, the
removal of the natural crystalline lens often results in a concomitant
‘pseudoaccommodation’, allowing patients to accommodate
again, usually to a small degree.
"The physiological pupillary constriction, spherical aberration
and corneal and lenticular multifocality can be included as sources
of error. Accommodation as a function of the bulbar length could
not be proven in our study. Further objective trials that are not
dependent on the investigator or examination method must be done
to establish the exact cause of pseudophakic pseudoaccommodation,"
Dr Gora said.
While this trial and others substantiated the occurrence of pseudophakic
pseudoaccommodation itself, continuing studies performed by Mainz-based
ophthalmologist Paul-Rolf Preussner MD seek to elucidate its cause.
"Most pseudophakic pseudoaccommodation can be explained by
spherical aberration and a small fraction by uncorrected astigmatism.
The effects differ for the various IOL types on the market,"
Dr Preussner said.
He examined cases of pseudophakic pseudoaccommodation in his clinic
using OKULIX, which is a software package he developed measuring
single rays on all optical surfaces of the eye.
He explained that numerical ray-tracing does not rely on Gaussian
optics (the basis of IOL calculations), as these do not account
for spherical aberrations, and therefore do not realistically represent
the actual eye.
Dr Preussner’s research revealed that the spherical aberration
is zero for an ideal IOL. He noted small effects, on the other hand,
for aspherical IOLs such as the Pharmacia Tecnis.
"We have learned that we can even design IOLs that would allow
a very high pseudoaccommodation, for instance 3.0 D, but visual
acuity is reduced in these. In general, the amount of pseudoaccommodation
is inverse to the quality of mesopic contrast vision with best correcting
glasses," he said.
Felix
Gora MD
Regensburg University Eye Clinic, Regensburg, Germany
Email: gora@eye-regensburg.de
Paul-Rolf Preussner MD
University Clinic Mainz, Germany
Email: pr.preussner@uni-mainz.de
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