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To help you better understand the
following discussions on specific glaucoma topics you may need to first read
the FAQs on glaucoma found at
www.eye.com.ph/glaucoma.htm.
This information is provided for
your convenience and to help you understand your condition or procedure. It
is not meant to serve as a substitute for a discussion with your doctor
about the specifics of your condition, treatment, or procedure.
Intraocular Pressure and
Intraocular Pressure Measurement
“Normal” Intraocular Pressure
One of the main risk
factors for the development of glaucoma is a relatively increased
intraocular pressure (IOP). The normal level of IOP varies from person to
person. The statistically “normal” IOP is 16.5 mmHg (millimeters of mercury)
with pressures above 20 mmHg considered statistically “elevated”. These
values were obtained by averaging the IOP results of thousands of normal
individuals with a wide range of pressures (from 10 to 35). For example, if
a person with a usual IOP of 9 mmHg were to later have an IOP of 19 mmHg
that would be a significant elevation even though it’s still a statistically
normal IOP. Thus, a person may have an IOP that is increased relative
to their normal IOP but is still not increased compared to the rest of the
population (absolute increase). Ophthalmologists always keep this in mind
when making the diagnosis of glaucoma and deciding what IOP to aim for when
treating glaucoma patients.
Intraocular Pressure Fluctuation
The actual level of IOP is
not the only aspect of IOP that matters in glaucoma. Our IOP normally
fluctuates throughout the day. Glaucoma patients tend to have a wider daily
fluctuation in their IOP (difference between peak IOP and trough IOP). This
is called the diurnal variation. The IOP reading taken at the
doctor’s office is the measurement for that specific moment only and does
not indicate what the IOP levels have been the rest of the day. This is why
glaucoma patients or glaucoma suspects are sometimes asked to have several
IOP measurements taken in a single day. This is procedure is called
phasing or diurnals.
Intraocular Pressure Lowering
Intraocular pressure is not
the only risk factor for glaucoma but it is the one that ophthalmologists
(and some glaucoma patients, too) are most preoccupied with. This is because
IOP is the only risk factor that we are able to modify. We have many drugs
and treatments that can lower IOP but there are no treatments (yet!) that
can decrease the effects of aging or that can change a person’s genetic
make-up. (Indeed, gene therapy may already be available for certain diseases
but, unfortunately, research has not yet even uncovered all of the genes
that predispose to the various types of glaucoma.) Evidence has shown that
lowering intraocular pressure and decreasing intraocular pressure
fluctuation are effective for preventing glaucoma progression.
Intraocular Pressure Measurement
Intraocular pressure can be
measured using various instruments. The current gold standard instrument for
measuring IOP is the Goldmann applanation tonometer (GAT or AT). It
is called the gold standard because it gives the most accurate, reliable,
and reproducible measurements. It measures IOP by flattening an area of the
cornea and measuring the amount of pressure needed to flatten that area of
the cornea. Because the instrument needs to come into contact with the eye
your doctor will first put anesthetic drops to numb your eye and fluorescein
dye drops to make your tear film more visible during the measurement
process. The other instruments for measuring IOP work in different ways and
have varying degrees of accuracy. There are promising new IOP-measuring
instruments being studied and being introduced into the market and one of
those instruments may one day replace the AT as the gold standard for
measuring IOP.
Central Corneal Thickness and Pachymetry
Most methods of IOP
measurement including AT can be affected by corneal thickness. An unusually
thick or thin cornea can cause the IOP measurements to be erroneously high
or erroneously low, respectively. A central corneal thickness outside if the
normal range has also been associated with a difference in the risk of
glaucoma or glaucoma progression. Those with thinner corneas seem to have a
slightly higher risk of having glaucoma. If your doctor suspects that your
corneal thickness will have a role in the management of your glaucoma you
may be asked have your corneal thickness measured. This simple test is
called pachymetry and there are different kinds of instruments that
can be used. The most commonly available method involves lightly touching an
ultrasonic probe to the anesthetized cornea for a few seconds.
References:
Kahn HA et al. The
Framingham eye study. 1. Outline and major prevalence findings. American
Journal of Epidemiology 1977; 106:17.
Heijl et al. Reduction of
intraocular pressure and glaucoma progression: Results from the Early
Manifest Glaucoma Trial (EMGT). Archives of Ophthalmology 2003; 121:48-56.
Asrani et al. Large diurnal
fluctuations in intraocular pressure are an independent risk factor in
patients with glaucoma. Journal of Glaucoma 2000; 9:134-142.
The AGIS Investigators. The
Advanced Glaucoma Intervention Study (AGIS): 7. The relationship between
control of intraocular pressure and visual field deterioration. American
Journal of Ophthalmology 2000; 130:429-440.
Collaborative Normal
Tension Glaucoma Study Group. Comparison of glaucomatous progression between
untreated patients with normal tension glaucoma and patients with
therapeutically reduced intraocular pressures. American Journal of
Ophthalmology 1998; 126:487-97.
Kass et al. The Ocular
Hypertension Treatment Study: A randomized trial determines that topical
ocular hypotensive medication delays or prevents the onset of primary
open-angle glaucoma. Archives of Ophthalmology 2002; 120:701-713.
Ritch R, Shields MB, Krupin
T (Eds). The Glaucomas, 2nd Edition. St. Louis, Missouri, USA,
1996, Mosby-Year Book, Inc.
Epstein DL,
Allingham RR, Schuman JS (Eds).
Chandler and Grant’s
Glaucoma, 4th Edition. Baltimore, Maryland, USA, 1997, Williams &
Wilkins. |
