Volume 8, Number 2
Spring, 1999
The HRT: A Better Way
of Examining the Optic Nerve in Glaucoma?
By Jeffrey Henderer, MD
Ken Parker, PhD
Glaucoma specialists at Wills and elsewhere
are always looking for better ways to determine if an individual
definitely has glaucoma and, if so, what kind of glaucoma it is.
It is only when they have answers to these questions that they
can suggest the best treatment for a particular individual. The
only way they can get answers to these questions is to actually
examine the optic nerve to see if and how it has been damaged.
Measuring Optic Nerve
Damage
The typical optic nerve damage
that occurs in glaucoma is known as "cupping." As the cells making
up the nerve die, due at least in part to a pressure inside the
eye that is too great for that particular eye to tolerate, they
die and disappear. When sufficient numbers of these cells are
gone, they leave behind a small "crater" or "cup" in the nerve.
A portion of the nerve then appears to have been "scooped out."
So one important thing doctors look for when they examine the
optic nerve is the presence and extent of the "cup," how deep
and wide it is.
Glaucoma specialists can
get a good idea of the amount of cupping in an optic nerve by
looking at it with an instrument known as an ophthalmoscope. They
can get an idea of whether the cupping is remaining stable or
worsening by taking a series of photographs over time. But these
methods have an important limitation. They can only suggest how
big the cup is in the same way that an ordinary aerial photograph
of a crater could can give us only a rough idea of how deep the
crater is. We can get a much better idea of the depth of the cup
or crater by taking a stereoscopic photograph. This would allow
us actually to measure just how much the optic nerve has been
damaged.
The HRT and How It Works
Glaucoma Service doctors
are now examining patients with an instrument that can give more
detailed information about the 3-dimensional structure of the
cup -- the Heidelberg Retina Tomograph (HRT). The HRT uses a special
laser to take 3-dimensional photographs of the optic nerve and
surrounding retina.
This laser, which is not
powerful enough to harm the eye, is first focused on the surface
of the optic nerve and captures that image. Then it is focused
on the layer just below the surface and captures that image. The
HRT continues to take images of deeper and deeper layers until
the desired depth has been reached. Finally, the instrument takes
all these pictures of the layers and puts them together to form
a 3-dimentional image of the entire optic nerve.
You can imagine your optic
nerve as a stack of pancakes and you are looking at the stack
from above. First, you can only see the top cake. An ordinary
photograph taken from the same angle of course also would capture
only the top pancake. In order to see or photograph the next pancake,
we would have to remove the top cake. But using laser light, we
have only to change the focus from the top cake to the cake just
below it.
The HRT takes 32 layer-by-layer
pictures from the surface of the optic nerve to from 0.5 mm to
4.0 mm deep into the ocular structures. The computer then piles
all the slices together in a reconstructed paper printout that
looks like a map drawn to represent the hills and valleys of a
geographical area. By color coding areas of elevation and depression,
the HRT provides a two-dimensional representation of what the
original, three-dimensional, stack looks like.
The HRT image can be used
to compute things such as the area of the optic disc (the part
of the optic nerve at the back of the eye), the volume of the
cup, and the area of the rim around the cup as well. These numbers
can then be used in two ways. First, they might show measurements
different enough from normal to help in diagnosing glaucoma. As
changes in the optic nerve are often the first sign of glaucoma
and can precede visual field changes, one might be able to diagnose
the disease earlier. Second, the measurements can be followed
over time by taking a series of tests - much like taking a series
of visual fields. Changes in depth are then computed.Various changes
might indicate a worsening or mprovement in the disease.
Problems with the HRT
Despite the apparent advantages
of images obtained with the HRT, like any new method of observation,
several other factors need to be considered. Is it easy to do?
Is it accurate? Is it any better than current techniques?
One problem is that, even
though the test only takes a couple seconds to perform, any patient
movement (including moving the eye, blinking, or moving the head)
will disrupt the laser's path, impairing the quality of the image.
Likewise, if the patient does not focus on the same location from
test to test, the angle of the image will change, and that will
affect the measurements greatly.
Second, the images created
by the HRT must be reproducible. That is, different images taken
at about the same time should be nearly identical. Limited information
available to date suggests that they are. But the range of "normal"
optic nerves is sufficiently broad that finding measurements that
definitively indicate "early glaucoma" has been very difficult.
No test is 100% accurate at distinguishing normal from abnormal,
and the HRT is no exception. Several studies have been done applying
various formulas to the optic nerve picture, and in the right
setting the machine can separate a "normal" optic nerve from a
"glaucoma" optic nerve with reasonable accuracy. But, again, the
variability among individual eyes is so great that it sometimes
remains difficult to know for sure from an HRT image whether the
optic nerve is in fact damaged. This is why comparison for change
is so important.
Third, the machine has not
been used long enough to prove that it is any better than if your
doctor took a series of optic nerve photos and carefully examined
them. However, the HRT may provide more objective comparison over
time, and research at Wills is ongoing on this subject.
Instruments for Measuring
Blood Flow in the Eye
In addition to imaging the
optic nerve, several instruments have been developed to measure
the blood flow to various portions of the eye. The idea is that
in addition to increased pressure within the eye decreased blood
flow can also result in damage to optic nerve cells. One such
instrument, closely related to the HRT, is the Heidelberg Retinal
Flowmeter, or HRF. The HRF is similar to the HRT in that both
employ a scanning laser to acquire images. The HRF however, is
used to look at the blood flow in the small capillaries near the
optic nerve head. This is done by detecting changes in the frequency
of sound reflected by flowing blood. The velocity of flow that
is measured has been reported to be altered in glaucoma. Note
that this is not total blood flow, just the velocity. In addition,
the area of the retina that is being measured may not have much
to do with the blood supply to the optic nerve. But despite these
limitations, we are investigating the HRF as a potential test
in evaluating patients with glaucoma.
Another imaging instrument
we are investigating is the Ocular Blood Flow Analyzer. Ocular
blood flow has been reported to be reduced in glaucoma patients
with normal intraocular pressure. This analyzer is a small device
that uses light to calculate the choroidal blood flow of the eye.
It can also be used to measure intraocular pressure simultaneously.
This instrument is being used in several clinical studies to determine
the pattern of blood flow throughout the day in patients undergoing
a Glaucoma Service Diagnostic Laboratory evaluation and before
and after glaucoma surgery in patients to see if blood flow has
changed.
Despite their limitations,
these instruments clearly are steps ahead in diagnosing and managing
glaucoma. The Glaucoma Service feels that these devices, and even
more so the next generation models, have great potential. We are
conducting studies to learn how they can be best be used to detect
and prevent further damage from glaucoma.
Board Meeting Stresses Stronger Ties Between
the Foundation and Wills Eye Hospital
D. McWilliams Kessler, Executive Director of
Wills Eye Hospital, and new Wills Director of Development Joseph
Leive, met with the Foundation Board of Trustees at a March 5th
meeting in the Wills auditorium.
Mr. Kessler, citing the strong financial position
of the Hospital, outlined administrative steps being taken to
establish a major research component at Wills, including the development
of a business plan. He explained that considerations of space
for research activities will be important in planning the Hospital's
proposed move to new facilities designed to meet projected needs.
Mr. Leive outlined the Hospital's growing development
initiatives. He noted that significant emphasis was being placed
on the identification of major prospects from all the Clinical
Services. Mr. Leive noted that funds being solicited are being
earmarked primarily for research. In this regard he explained
that the consolidation of the existing Service foundations, of
which the Glaucoma Service Foundation is one, will be an important
factor in realizing these fundraising goals.
Other items:
-
Executive Director Ken Parker, PhD, reported
that the amount of unrestricted funds raised by the 1998 Annual
Fund campaign doubled as compared with the previous year --
from $70,223.52 to $192.776.39 -- and the total number of
such gifts increased 43% -- from 968 to 1386. He also noted
that an audit of the Foundation [now completed] is being conducted
by Howe, Keller & Hunter, P.C.
- Board member Ms. Adele Greene announced she is organizing
a Volunteer Corps to assist in Foundation research activities.
[Note: Any readers interested in volunteering may contact Ms.
Greene 215-928-0759 for further details.]
- Efforts to establish telemedicine capabilities at the Hospital
are moving apace. The ability of the Hospital to share its expertise
with those who cannot physically come to the Hospital, explained
Glaucoma Service staff member Dr. Richard Wilson, will be a
major component of the "new Wills."
- Assistant Research Director Dr. Jonathan Myers reported that
at a "research symposium" held in January board member Ms. Mary
Chatterton joined him and several other doctors on the Service
to plan research in optic nerve imaging, new surgical techniques,
genetics, and neuroprotection.
- Board member Ms. Nettie Taylor announced that the glaucoma
screening activities of the Foundation are continuing in area
churches in a program organized by her in conjunction with the
Penn Towne Chapter of the Links (a national African-American
service organization) and Merck & Co. Dr. Parker noted that
screenings also continue to be conducted at area senior citizens
centers under the auspices of the Philadelphia Corporation for
Aging.
- Dr. Parker reported that the Glaucoma Patient Support Group,
now in its third year, meeting at the Hospital on Sunday afternoons
once a month, typically has 40-50 participants at each session.
Dr. Spaeth ended the meeting by reminding the
Board and visitors that all the research and education activities
of the Foundation focus ultimately on the individual patient.
Also participating in the meeting were Board
Chair Mr. S. Stoney Simons, Glaucoma Service staff members Drs.
L. Jay Katz, Anne Chan, and E. Sivalingam, and Board members,
Mr. Louis Esposito, Mr. and Mrs. Francesco Bonmartini, Mr Stephen
Harmelin, Mr. Stanley Tuttleman, and Mr. and Mrs. Alexander Urban.
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Board members Nettie
Taylor (left) and Adele Green (right) with Foundation President
Dr. George Spaeth.
Photo by Jamie
Nicholl. |
|
Wills Eye Hospital
Director of Development Joseph Leive (left) with Glaucoma
Service physician Dr. Richard Wilson.
Photo by Jamie
Nicholl. |
|
Foundation Assistant
Research Director Dr. Jonathan Myers (left) with Board member
Mary Chatterton.
Photo by Jamie Nicholl. |
Drs. Werner and Ortiz
Join the Glaucoma Service Staff
 Dr.
Elliot Werner, currently Professor of Ophthalmology at Hahnemann
University, completed a Fellowship in Glaucoma with University
of British Columbia investigator Stephen Drance. He received the
American Academy of Ophthalmology Honor Award in 1993. He brings
to the Wills staff particular expertise in techniques for evaluating
visual field, optic disc, and nerve fiber layer changes in glaucoma.
Dr. Joseph Ortiz, currently
on the Attending Staff of the Neumann Medical Center in Philadelphia,
completed a Fellowship in Glaucoma under Peter Watson in Cambridge,
England, and was Chief of the Glaucoma Service at Cooper Hospital/University
Medical Center of the University of Medicine & Dentistry of New
Jersey for 9 years.
Doctor, Instrument, Patient
Dr. Marlene Moster (left) with
her patient Harry Marshall at the HRT instrument.
Photo by Jamie Nicholl.
The Searchlight apologizes
for the duplicate copies of the previous issue received by several
readers. We believe we have eliminated most if not all of these,
but if you have received more than one copy of this issue, please
let Foundation assistant Aimee Woods know, phone (215-928-3487)
or click
here to contact us.
Meeting the Challenge of Glaucoma through
Education and Research
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