GLAUCOMA IMAGE ANALYZERS IN PERSPECTIVE
There is now a cacophony of rhetoric on the whole issue of
“structural and functional” assessment as it relates to the
glaucoma diagnostic evaluation. Very simply, scanning devices,
such as the GDx-VCC, HRT, OCT, and RTA assess the structure
of the optic nerve and/or retinal nerve fiber tissues, whereas
visual field testing assesses the functional integrity of the
optic nerve and visual pathway.
The big question is, when does one rely more on structural
indices, and when does one rely more on functional indices?
There is a lot of overlap and a wide range of opinions regarding
this issue. Generally, it is felt that structural assessment
has its forte in early detection, whereas once there is a repeatable
visual field defect, it is probably best to follow these patients
with visual field studies every 6 to 12 months. Of course,
if a patient progresses to a “repeatable” visual field defect,
reducing the IOP to a pachymetry-adjusted IOP of 12 to 14mmHg
needs to be considered. The Advanced Glaucoma Intervention
Study demonstrated a halt to measurable progression in disease
when the IOP was held to about 12mmHg or lower. Many such
patients can achieve this through medical therapy; however,
many may require a trabeculectomy to achieve such a low target.
It is becoming clearer that the Frequency Doubling Technology
and Short Wavelength Automated Perimetry may be able to detect
visual field loss earlier than standard (white-on-white) automated
perimetry (SAP). Which then will be more sensitive for earlier
detection - a structural scan or one of these enhanced perimetric
techniques? This distinction is not fully understood at this
time, and even if it were, individual variability would always
be a potential confounder to any general consensus.
The glaucoma scanning technologies bring another piece to
the diagnostic puzzle, and we are now comfortable with recommending
that all optometrists acquire one of them as soon as it is
feasible. They are, however, of maximum diagnostic value when
used within the context of a comprehensive glaucoma evaluation.
Used as a single piece of diagnostic data, their false-positive
and false-negative results could cause the naïve clinician
to either overtreat or undertreat many patients; similar to
applanation tonometric readings that are not pachymetrically
modified.
The emergence of increasingly sophisticated diagnostic technologies
is always welcome. However, the old analogy comes to mind
of watching a dog chase a car; what is he going to do with
it if he ever catches it? In a similar vein, it is crucial
that we have a keen understanding of how to integrate any new
and evolving technology in a manner that enhances patient care.
There are four, high-profile, image analyzer instruments available,
and all of them can bring valuable adjunctive data into the
decision-making milieu. Unfortunately, these instruments are
sometimes promoted with excessive zeal (by both the companies
and lecturers). That over-exuberance has the potential for
compromised patient care when relatively naïve doctors confer
too much weight to the data obtained with any of these devices.
All scanning devices have limits to their sensitivity and specificity.
Therefore, both the structural and functional data obtained
can be either immensely helpful or non-contributory, and at
times, even misleading. It is the astute clinician who has
the wisdom to look at all the clinical findings in proper
perspective who provides excellent patient care. This is why
we feel compelled to share the following perspective with you,
our colleagues.
Just as the prostaglandins ushered in a new level of excitement
in the treatment of glaucoma, the retina nerve fiber layer
analyzers have ushered in a new level of excitement in the
diagnosis of glaucoma. While these are indeed exciting new
additions to our diagnostic armamentaria, they must be used
appropriately. The GDx-VCC, HRT, OCT, and RTA all have unique
strengths and weaknesses, and all can bring greater diagnostic
strength to the diagnostic work-up. We feel the GDx-VCC is
the overall best-valued instrument at this time, and the one
we use in our clinical care. We urge every optometrist to
acquire one of these image analyzers, but exercise sound clinical
judgment when integrating the data generated from these devices.
We urge you to carefully consider the following quotes and
commentary so that you can integrate these technologies into
your practices APPROPRIATELY.
- “The Gold Standard for diagnosing glaucoma is a well-trained
clinician applying observational skills in the examination
of the optic nerve, and interpreting examination findings in
the clinical context of a particular patient. Computerized
imaging technology cannot substitute for sound clinical skills
and judgment in the diagnosis of glaucoma.”
“Structural changes in the optic nerve head may precede visual
field abnormalities in early glaucoma. These changes may be
subtle and manifest themselves over many years. Therefore,
careful documentation of optic nerve structure at the time
glaucoma is initially suspected will be extremely valuable
in subsequently confirming or excluding a diagnosis of glaucoma
during a patient’s lifetime.”
“No instrument utilizing computerized imaging technology currently
exists that has a sensitivity and specificity required to function
as a useful screening tool for glaucoma.”
“Documentation of optic nerve structure is of limited use
in advanced glaucoma. The diagnosis of advanced glaucomatous
optic neuropathy is easily made by the trained observer, and
does not require confirmation by optic nerve imaging studies.
Also, small changes in optic nerve topography are difficult
to measure in the presence of advanced neuroretinal rim loss.
Functional assessment of the optic nerve head with visual field
testing is more appropriate to assess disease progression in
advanced glaucoma.”
Reference
Tanaka, G. H., Is Scanning Laser Imaging of the Optic Disc
a Substitute For Stereo Disc Photography?, Glaucoma Subspecialty
Day, American Academy of Ophthalmology, November 2003.
M & T Commentary
These four points nicely place this evolving technology in
appropriate clinical perspective. We urge you to acquire
a scanning device, and use it wisely in evaluating all of
your glaucoma suspects, as well as those with mild to moderate
glaucoma.
- “While OCT provides an objective measurement of nerve fiber
layer structure, its clinical use in the early detection and
follow-up of glaucoma patients requires additional clinical
evaluation. This author does not currently use this information
to make important therapeutic decisions in patients at present.
Additional clinical correlation with existing techniques and
prospective evaluations are required.”
Reference
Jaffe, G. J., and Caprioli, J., Perspective: Optical Coherence
Tomography to Detect and Manage Retinal Disease and Glaucoma,
AJO, January 2004.
M & T Commentary
This same exact statement applies to all scanning technologies
and all visual field technologies. The evaluation of a patient
suspected of having glaucoma is a complex, cerebral integration
of numerous factors. These include age, race, family history,
IOP, diurnal profile, central corneal thickness, optic nerve
head status, gonioscopy, and visual field studies. One must
also consider the patient’s interest in being treated, ability
to purchase eyedrops, instill eyedrops, willingness to return
for follow-up care, etc. Scanning image results are one
component of this comprehensive evaluation, not a litmus
test for glaucoma.
- “Regarding this entire category of devices, Dr. Ritch cautions
that doctors shouldn’t depend on them too much. ‘I heard about
a woman in her 40’s whose pressures, discs, and visual fields
were normal. She was treated with medications, laser, and
nearly a trabeculectomy because of a defect found on a GDx
test,’ “ he says. ‘This is what you have to worry about, people
using these instruments as gospel. They’re not. They just
supply more data for you to fit into the picture.’ Dr. Weinreb
agrees. ‘It’s important to place these tests in the context
of an accurate and thorough clinical examination. Basing treatment
on information gleaned only from a single instrument test would
be a misuse of the technology. At the current time, they are
meant to enhance an informed clinical examination, not to replace
it.’ “
Reference
New Avenues in Glaucoma Diagnostics, Ophthalmology Management,
August 2003.
M & T Commentary
Drs. Robert Ritch and Robert Weinreb are distinguished, internationally
recognized, glaucoma experts, and we completely agree with
their perspective. As said earlier, we are thrilled to have
our GDx-VCC, and appreciate its value as a COMPONENT of our
diagnostic evaluation.
- “I think it’s extremely critical for ophthalmologists (M & Tx
and optometric physicians) to have one of these devices in
their offices,” said David S. Greenfield, M.D., associate professor,
Bascom-Palmer Eye Institute, University of Miami, Florida.
“These devices have the ability to quantitatively evaluate
structures that could never before be quantitatively measured
in a very user-friendly fashion. While none of these technologies
will replace visual field testing, their role is to provide
additional, adjunctive information about structure that is
complementary to the information generated with visual field
testing. Most glaucoma patients and eyes suspected to have
early glaucomatous damage will benefit from having baseline
and serial testing with ocular imaging.”
M & T Commentary
Dr. Greenfield is yet another highly regarded glaucoma expert,
and indeed, has great expertise in imaging technology.
“The HRT, GDx, and OCT can HELP (emphasis added) determine
which glaucoma patients require treatment and which can be
monitored without treatment.”
M & T Commentary
The degree of this “help” will be relative to the numerous
other factors in the diagnostic evaluation, i.e., is the
diagnosis pretty clear-cut, or more borderline. Obviously,
the more borderline the diagnostic call, the keener one will
have to look at all of the diagnostic data.
Reference
Incorporate Optic Nerve Head Imaging Into Your Practice, EyeWorld,
December 2003.
- “Measurements of optic disc stereometric parameters by HRT
is highly reproducible. However, the use of retinal nerve
fiber-related parameters should be taken cautiously.”
Reference
Miglior, S. et al, Intraobserver and Interobserver Reproducibility
in the Evaluation of Optic Disc Stereometric Parameter by
Heidelberg Retina Tomograph, Ophthalmology, June 2002.
- “With structural tests, we have to recognize that there is
no perfect instrument. There wasn’t one in 1994, there isn’t
one in 2003, and there won’t be one in 2010. Every instrument,
whether objective or subjective, has advantages and disadvantages.”
Reference
Weinreb, R. N, Weighing the Cost of Glaucoma Progression, Ophthalmology
Management, August 2003.
- “In this study, HRT-based classification of ‘Glaucoma’ or
‘Normal’ was moderately sensitive, but not very specific when
compared with the clinical impression as the Gold Standard.
Clinicians should not rely on the HRT diagnosis alone, but
should use it to supplement the impression based on an eye
examination and other ancillary tests.”
Reference
Kesen, M. R. et al, The Heidelberg Retina Tomagraph Versus
Clinical Impression in the Diagnosis of Glaucoma, AJO, May
2002.
M & T Commentary
While this article examined the HRT, its conclusions are equally
valid for all of these image analyzers. By the way, these
technologies are exactly that - “image analyzers.” However,
you - the doctor - are the “patient analyzer;” please bear
this critical distinction in mind!
- “There are two challenges to assessing the structure of the
optic nerve. The first is distinguishing between a normal
and a glaucomatous optic nerve at a single point in time at
any severity of disease. This is obviously more difficult
in mild disease. However, the Gold Standard of performing
this task is the well-trained clinician. The ability of current
imaging technology to allow the clinician to diagnosis glaucoma
correctly on a single examination is not perfect, and its usefulness
in this regard is of secondary importance. The more important
challenge is detecting glaucoma progression over time. Scanning
laser imaging of the optic nerve enhances, but does not replace
good clinical skills in diagnosing glaucoma. On a cautionary
note, it is unsound clinical practice to base treatment decisions
on a single-imaging study, and we must interpret imaging data
in a clinical context.”
Reference
Tanaka, G. H., Stereo Disc Imaging Still Gold Standard for
Glaucoma Imaging, Ophthalmology Times, 15 January 2004.
- We had the distinct privilege to attend the first global Association
of International Glaucoma Society’s consensus meeting on “Structure
and Function in the Management of Glaucoma” in San Diego, November
13-14, 2003. The Optometric Glaucoma Society, of which we
are members, is one of sixteen member societies of this worldwide
body of glaucoma experts. The following quote from page 90
of their printed program says:
“There was general agreement that the current literature does
not provide the requisite evidence to validate any of these
imaging instruments for widespread, routine, clinical use,
as the techniques have not been shown to be better than standard
clinical testing, or a dilated examination from a trained clinician.
However, in the hands of an experienced clinician who understands
the strengths and limitations of the instruments, information
may be helpful in many clinical situations. Unfortunately,
it is unlikely that general ophthalmologists (M & Txand
probably optometric physicians, as well) examine the optic
disc and/or nerve fiber layer with the same degree of expertise
as a glaucoma sub-specialist. In a recent survey of 395 US
managed care patients, only 53% of patients received an optic
disc photograph or drawing at their initial visit. ‘x it can
be argued that imaging can be recommended as a routine clinical
tool, as it will increase the assessment of the optic disc
and retinal nerve fiber layer (RNFL) in the management of glaucoma.
However, x it is important to consider whether the cost of
possible misinterpretation of results from imaging instruments
to diagnosis glaucoma (and the possible overtreatment due to
false positives) outweighs the benefits of providing optic
disc and RNFL information to the general ophthalmologist and
optometrist who would otherwise not assess the optic disc and
RNFL of their glaucoma patients.’ “
- “Scanning computerized ophthalmic diagnostic imaging (SCODI)
plays its least prominent role in advanced glaucomas. By this
stage, patients have definite evidence of perimetric damage.
“It’s still reasonable to utilize SCODI at baseline and again
annually. The results of SCODI at this juncture are used not
to alter the course of therapy, but rather to further bolster
the case for stability or progression.”
Reference
Rabinowitz, A., Our Glaucoma Management Series Examines the
Expanding Role of These Technologies, How They’re Being Used,
and How to Document for Reimbursement, Part 2. Ophthalmology
Management, May 2004.
M & T Commentary
We note how Dr. Rabinowitz stresses how these are not used,
“to alter the course of therapy,” but rather, he views them
as an adjunct to determining whether the patient’s advanced
glaucoma is stable or progressing. We certainly agree with
his perspective, and hold this to be true even in cases of
less advanced disease. The forte of image analyzers is likely
their ability to detect change over time. The new Humphrey
“Glaucoma Progression Analysis” software is likewise a major
advancement for determining visual field deterioration over
time.
- “Each instrument appears to have unique strengths and limitations
yet they all provide adjunctive information to the current
gold standards; standard automated perimetry and stereo disc
photography.
“Dr. Garway-Heath emphasized that physician judgment remains
key in evaluating the output of these imaging devices and in
incorporating that into the diagnosis and management of individual
patients.
“ ‘My own feeling is, it’s important that generalists not
take the reports of these imaging modalities and view them
as gospel,’ he said. ‘Doctors need to use their intelligence
in interpreting these reports, because there is a danger of
making decisions without looking at all the evidence. The
imaging reports need to be placed in context of the other clinical
information. It’s then that these instruments will have something
to offer beyond photos and visual fields.’
“Dr. Higginbotham agrees. She said that glaucoma is a big,
gray area and all the data must be considered.”
Reference
Scerra, Chet, et al, Several Options Now Available For Imaging
Glaucoma. Ophthalmology Times, June 1, 2004
Lastly, I (RKT) would like to relate a personal experience
of how not to compromise your patients quality of life (the
ultimate goal, by the way).
The good man that married my wife and I more than fifteen
years ago went to his longstanding family optometrist who had
recently acquired a scanning device. As occasionally happens,
the device led the OD to believe this 58-year-old male had
glaucoma, i.e., a “false positive” result. Even though all
other diagnostic parameters were within normal limits, this
optometrist conferred upon this good man the diagnosis of glaucoma
and started him on a topical, pressure-lowering medicine.
This pastor called me long distance that very evening, terrified
that he might go blind. How truly sad. I had him see an OD
friend of mine in his same local area for a second opinion,
and he was found to be completely normal via the Gold Standard
of a competent, comprehensive eye examination.
This is a prime example of how some patients are being abused
through personal incompetence; the technology can be helpful
when used by a competent doctor who truly understands how to
integrate and interpret the clinical data in the context of
a comprehensive assessment. So, again, by all means, purchase
one of these devices and use it with sound clinical judgment.
As said before, our instrument of choice at this time is the
GDx-VCC.
GDx-VCC printout of glaucoma suspect patient referred in for
a 2nd opinion. Asymmetric cupping is easily sean. The nerve
fiber index (NFI) numbers are 23-OD and 26-OS, well within
normal limits. The nerve fiber layer maps at the bottom demonstrate
normal "double hump" distributions of NFL thickness.
The GDx-VCC and all other clinical findings were compatible.
Because of the large ONH cupping, she will be followed annually.
|
