Excerpts
From: The International Glaucoma
Review, Volume 9-2, 2007
(Tenth in a Series)
Undiagnosed Glaucoma
- 50% of the glaucomas in a society are undiagnosed.
- There is a perception that disease progression is slow
if not very slow, with many patients never becoming aware
of their visual loss, so that some patients may receive treatment
unnecessarily.
Highlights from the World Glaucoma Congress’s IOP Consensus
Report
- Goldmann applanation tonometry (GAT) is still the gold
standard; however, its position is being reassessed. Although
unproven, several new tonometers offer the potential for
more accurate IOP readings. To optimize GAT measurements,
routine assessment of calibration is recommended.
- There is strong evidence to support higher mean IOP as
a significant risk factor for the development and the progression
of glaucoma. However, there is insufficient evidence to support
24-hour IOP fluctuation or IOP variation over periods longer
than 24 hours as a risk factor for either development or
progression.
Notes from the Pan-American Glaucoma Society Congress Ophthalmology
- The presence of dry eye should be suspected in every patient
being administered glaucoma therapy.
- Sleep apnea, as a risk factor for worsening in glaucoma,
must be carefully investigated in normal tension glaucoma
suspects.
Notes from the Canadian Glaucoma Society Meeting
- The Canadian Glaucoma Study followed 258 on the left from
1994-2005 to evaluate risk factors for progression of primary
open-angle glaucoma (POAG). Significant factors included
the presence of anticardiolipin antibiotics, female gender,
increasing age (4% increase per year) and higher IOP (20%
increase per mmHg). Factors previously associated with progression
that were not found to be significant in this study included
diabetes, hypertension, and a history of cardiovascular disease.
M & T Commentary
Every study has its own unique findings; some can be corroborated,
others cannot. This particular study found an association
with anticardiolipin antibodies (which are associated with
coagulopathies), which is certainly unique and may merit
further investigation. It did add to the growing weight of
evidence that diabetes is NOT associated with glaucoma.
- Canadian national and provincial data regarding glaucoma
laser and surgical procedure rates from 1999-2005 were analyzed.
Laser trabeculoplasty rates were found to have doubled since
2002, coinciding with the introduction of selective laser
trabeculoplasty.
M & T Commentary
People who purchase expensive laser technology tend to use
it. There is consensus that ALT and SLT perform virtually
identically, so this rise in use may translate more to “investment
recovery” than to “clinical indication.”
Goldmann Applanation Tonometry (GAT) and the Importance of
Calibration
A variety of studies suggests that under ideal clinical conditions
(i.e., during the course of a controlled research trial), intraobserver
reproducibility of GAT is about is about ± 2.5mmHg; inter-observer
reproducibility is lower. Nonetheless, GAT remains clinically
useful and widely deployed in clinical practice – GAT isn’t
going to be replaced any time soon.
The GAT is a remarkably simple device that can go out of calibration
due to corrosion, gumming of lubricants and damage to the internal
mechanism. The end-user cannot ‘re-calibrate’ a GAT, but an
end-user can verify whether the tonometer is within factory
specifications using the calibration bar shipped with the device.
How often is this being done? Not often enough, it would seem.
So, before we try to interpret our GAT measurements up or down
a few mmHg because of thick or thin corneas, we should at least
make sure that the underlying measurement is sound.
Medical Therapy and Supine IOP in Normal Tension Glaucoma
The magnitude of the supine rise in intraocular pressure in
eyes with normal tension glaucoma correlates with visual
field progression. Does medical therapy blunt this supine
rise? To address this important question, the investigators
(Kiuchi et al.) have conducted a well-designed crossover
study evaluating the supine rise in intraocular pressure
before and after a one-month treatment with timolol, latanoprost,
and brinzolamide in 24 subjects with newly-diagnosed normal
tension glaucoma. Interestingly, they found no appreciable
blunting of the supine IOP rise with any of the medications:
the supine IOP rise was approximately 3mmHg before treatment
and after a one-month course of each medication. Postural
changes in intraocular pressure have been recognized for
decades. While healthy optic nerves can apparently tolerate
this posture-driven rise, glaucomatous optic nerves may not.
The pathophysiology of normal tension glaucoma is intriguing
and elusive, but a role for IOP reduction has been established
by the Collaborative Normal-Tension Glaucoma Study. And while
the clinical implications of postural intraocular pressure
elevations in eyes with normal tension glaucoma have not
been established, it is troubling that medications may not
protect the optic nerve from supine IOP elevations in these
eyes.
Gingko Biloba and Retinal Blood Flow
No significant effects on retinal perfusion were found during
a three-hour window post ingestion when compared to placebo.
Due to the widespread popularity and use of over-the-counter
medical supplements such as gingko biloba, it is imperative
that we understand the effects of these medications on ocular
circulation.
M & T Commentary
While a hot topic over the past decade, Gingko does not appear
to contribute meaningfully to the glaucomatous optic neuropathy
combative armamentarium.
Effects of Marijuana on IOP
Marijuana lowers IOP for only two to three hours, making it
highly impractical in most patients, and furthermore in the
majority of patients the IOP lowering effect is lost in continued
dosing.
Structural Assessment
Optic disc photographs represent a reference standard for structural
detection of progression, but they cannot be used to quantify
progression. Furthermore, their reading can be subjective
and there are inter-observer variations. The findings strongly
suggest the need to substitute optic disc stereophotographs
with objective imaging techniques.
M & T Commentary
To say that disc photographs “cannot” be used to quantify progress
is a bit too strong, for they certainly can. However, most
doctors do not carefully study and compare such photos in a
manner to be quantifiably precise. For general clinical care,
we agree that “objective imaging” instruments are probably
a more practical methodology to observe for progression.
Fundamental Assessment
The relationship between progressive structural and functional
injury has been the subject of intensive research, and current
knowledge indicates a poor correlation between the two strategies
Effects of Cataract Surgery on Intraocular Pressure
Clinically useful findings were reported that revealed an added
IOP-lowering benefit from cataract surgery. In this four-year
retrospective study conducted on 226 eyes from 151 patients
having undergone uncomplicated clear corneal phacoemulsification
by a single surgeon, there was:
- a significant post-surgical IOP reduction, which was maintained
over four years;
- no significant long-term reduction in IOP-lowering medications,
however in patients with glaucoma.
Corneal Properties and IOP Measurement
The effect of corneal biomechanical properties on the accuracy
of IOP measurement has been gaining increasing interest.
Corneal hysteresis (CH) determined by the Ocular Response
Analyzer (Reichert, Inc.) is a measurement of this effect,
based on the corneal biomechanical response to a rapid indentation
of the cornea caused by an air puff.
One poster presented by the a group of investigators reported
a small, but significant correlation between CH and visual
field severity. In 107 glaucoma patients, it was found that
CH was significantly lower in glaucoma patients than in ‘normal’
subjects. This suggests that:
- CH may be associated with the ability of the optic nerve
head (ONH) to tolerate elevated IOP;
- low CH may be a risk factor for disease progression.
Following the theme of corneal biomechanics, investigators
described the relationship between the anterior segment parameters
and optic nerve deformity. These study authors found that both
CH and central corneal thickness (CCT) were significantly lower
in the glaucoma group than in the non-glaucoma group. However,
among the anterior segment parameters measured, only CH was
significantly correlated with ONH deformation during IOP elevation.
These findings suggest that:
- CH may represent an essential biochemical property of the
eye that reflects the flexibility of the lamina cribrosa;
- a low CH suggests a ‘softer’ eye that is less capable of
absorbing energy at elevated IOP.
M & T Commentary
The concept of corneal hysteresis is indeed intriguing, and
with a few more years of research and a more patient-friendly
way of determining this parameter, CH may bring an additional,
more important aspect to the comprehensive glaucoma evaluation
– just as corneal pachymetry did.
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