Excerpts
From: The International Glaucoma
Review, Volume 9-4, 2008
(Twelveth in a Series)
Angle Assessment: Gonioscopy and Illumination
- Sadly, ophthalmologists perform gonioscopy less frequently
than they should. Moreover, when gonioscopy is performed
the technique might not be ideal. Too often, gonioscopy is
performed in a brightly lit room employing a diffuse bright
beam of light. It has long been felt that excessive illumination
could artificially open the iridocorneal angle, making the
examiner miss people at risk for pupillary block angle-closure
glaucoma.
- Ophthalmologists who perform gonioscopy in a bright room
or with a slit lamp entering the pupil risk failing to identify
occludable iridocorneal angles.
M & T Commentary
Obviously, these statements hold equal truth for optometrists.
The Prevalence of Angle-closure Glaucoma
- Angle-closure glaucoma (PACG) has become increasingly recognized
as a major cause of preventable blindness in the world, due
largely to high prevalence rates in the very populous Asian
countries.
Updates for the GDx
- Evaluation of the retinal nerve fiber layer is important
for the clinical management of glaucoma. Scanning laser polarimetry
provides an indirect, objective quantification of the nerve
fiber layer by measuring the amount of retardation, or phase
shift of polarized light as it passes through the birefringent
RNFL. The GDx VCC, designed to neutralize the influence of
corneal birefringence, has been shown to have better diagnostic
accuracy than the original scanning laser polarimeter that
assumed the same fixed magnitude and axis of corneal polarization
for each eye. Unfortunately, atypical birefringence patterns
(ABPs) appear in a proportion of GDx VCC scans. Scanning
polarimetry with enhanced corneal compensation (GDx ECC)
was introduced to improve the RNFL measures by reducing the
prevalence of ABPs. It should be noted that GDx ECC obtained
scans cannot be used interchangeably with GDx VCC scans.
- The results of this recent study evaluating five RNFL parameters
are consistent with other recent publications that highlight
the advantages of GDx ECC over GDx VCC. Specifically, these
studies indicate that compared to RNFL measurements with
GDx VCC, GDx ECC RNFL measures show 1) improved diagnostic
accuracy of GDx ECC particularly in eyes with ABPs; and 2)
stronger correlation to visual field damage.
M & T Commentary
It is difficult to know with certainty, but it does appear
that all GDx uses should upgrade to the “ECC” when it becomes
available.
Incidence Open-angle Glaucoma (OAG) and Antihypertensives
- Müskens et al. Report an investigation of the relationship
of antihypertensive medications and incident open-angle glaucoma.
The authors found that calcium channel blockers were associated
with a statistically significant increased risk of incident
glaucoma. There was a trend for the use of systemic beta-blockers
to be protective but this did not reach statistical significance
even in the multivariate analysis.
- This study does not support the avoidance of beta-blockers
or diuretics in glaucoma or the use of calcium channel blockers
for the treatment of glaucoma.
- The fact that systemic antihypertensive agents were so
weakly related to incident glaucoma is encouraging in terms
of the treatment of systemic hypertension being unlikely
to interact in a meaningful way with regard to the development
of glaucoma.
M & T Commentary
This is good information since many of our glaucoma patients
are on oral antihypertensive medicines.
CAIs and Corneal Effects
- Carbonic anhydrase (CA) isoenzymes II and IV play an important
role in the pump function of the endothelium, keeping the
cornea in a relatively steady state of dehydration.
- Dorzolamide is a potent inhibitor of CA II. In normal eyes,
both original regulatory safety data and in subsequent studies,
dorzolamide exhibited little in the way of corneal toxicity.
There are, however, numerous anecdotal and published reports
of irreversible corneal decompensation during dorzolamide
therapy in some patients.
- Data suggests that patients with preexisting endothelial
disease are more susceptible to the adverse corneal effects
of topical carbonic anhydrase inhibitors (CAIs).
- The glaucoma clinician should pay more attention to the
corneal endothelium, and in patients with compromised endothelia
(i.e., Fuchs’ dystrophy, bullous keratopathy, penetrating
keratoplasty), consider other drugs before topical CAIs.
Preservatives and the Cornea
- Many reports, in humans, animal or cell models confirmed
that mild symptoms or signs may account for subclinical inflammatory
changes impairing the tear film, eyelids, conjunctiva or
cornea. One major component responsible for those findings
is the preservative benzalkonium chloride (BAK) which is
cytotoxic and has detergent properties.
- These effects were more prominent with prostaglandins and
benzalkonium at 0.02% concentration. Although experimentally
performed in animal eyes, they showed that preserved drugs
and prostaglandins may very quickly stimulate stress-induced
signals in the ocular surface epithelia and they support
similar studies in humans, especially in patients treated
over the long term. Such data are quite consistent with previous
reports showing overexpression of inflammation- or apoptosis-related
markers in impression cytology specimens or conjunctival
biopsies. This is a new example illustrating the involvement
of the ocular surface in glaucoma and the noxious role of
preservatives in corneal and conjunctival epithelia.
Consensus on Intraocular Pressure
Every ophthalmologist (M & T: …and optometrist) should
know the consensus outcomes.
- They are the solid global basis for the management of the
glaucoma patients.
- Please read them carefully and implement them in your practice.
From time to time IGR will publish some of the consensus statements
as part of the World Glaucoma Association (WGA) promulgation
strategy.
Central Corneal Thickness
- The extent to which central corneal thickness (CCT) contributes
to the measurement error (in relation to the other factors)
in individual patients under various conditions has yet to
be established.
Calibration of Goldmann Applanation Tonometry (GAT)
- Currently there are no data to support a specific frequency
of calibration verification for GAT.
- The frequency for verification of GAT calibration of at
least twice a year is suggested.
Goldmann Applanation Tonometry for Contact Lens Wearers
- Contact lens wearing patients should have tonometry performed
after having been awake, without contact lenses, for at least
two hours for contact-lens-induced and diurnal corneal edema
to resolve.
Intraocular Pressure
- IOP is more variable in glaucomatous than in healthy eyes,
but both 24-hour IOP fluctuation and IOP variation over periods
longer than 24 hours tends to be correlated with mean IOP.
- There is currently insufficient evidence to support 24-hour
IOP fluctuation as a risk factor for glaucoma development
or progression.
- Diurnal IOP is generally highest after awakening and decreases
during the day-time period.
M & T Commentary
It appears that the virtue of obtaining multiple IOP checks
at various times is primarily to search for the peak IOP,
not necessarily to characterize the diurnal curve pattern.
- Posture is an important variable in the measurement of
IOP; IOP in the sitting position is generally lower than
in the standing position.
Target IOP
- The determination of a target IOP is based upon consideration
of the amount of glaucoma damage, the IOP at which the damage
has occurred, the life expectancy of the patient, and other
factors including status of the fellow eye and family history
of severe glaucoma.
- At present the target IOP is estimated and cannot be determined
with any certainty in a particular patient.
- There is no validated algorithm for the determination of
a target IOP. This does not, however, negate its use in clinical
practice.
- The use of target IOP in glaucoma requires periodic reevaluation.
This entails examination of the optic nerve and assessment
of visual function to detect glaucomatous progression, the
effect of the therapy upon the patient’s quality of life,
and whether the patient has developed any new systemic or
ocular conditions that might affect the risk/benefit ratio
of therapy.
- During the reevaluation, it is essential to determine whether
the IOP target is appropriate and should not be changed.
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