Wednesday, 13 July 2011
is a disease of the optic nerve most
likely due to a compromised blood supply.
The nerve can be damaged even more by elevated
intraocular pressure pushing on the
blood vessels supplying the nerve and causing
a further reduction in blood flow.
Intraocular pressure is maintained by a balance
between aqueous inflow and outflow. The
aqueous produced by the ciliary body passes
from the posterior chamber (the space behind
the iris) through the pupil into the anterior
chamber . It drains through the trabecular
meshwork and out of the eye through
the venous canal of Schlemm.
Glaucoma vs. glaucoma suspect :
Normal intraocular pressure is 10–20 mmHg
and should be measured at different times of
day as there is a diurnal rhythm. Pressure
greater than 28 mmHg should be treated to
prevent loss of vision. Treat pressures of 20–27
mmHg when there is loss of vision, damage to
the optic nerve, or a family history of glaucoma.
Patients with pressures of 20–27 mmHg
without these findings are called glaucoma
suspects and are followed, but not treated.
Infrequently, the optic nerve could be damaged
even when the pressure is less than 20
mmHg (normal tension glaucoma) and must
be further reduced to the low teens.
Several instruments can be used to indirectly
measure intraocular pressure by indenting
A Goldmann applanation tonometer is the most accurate. It is used in conjunction
with a slit lamp, and requires the use of anesthetic
drops and fluorescein dye.
A Schiötz tonometer is a portable instrument that indents the anesthetized
cornea and is used for bedside measurements.
blowing a puff of air at the eye. It is used by
technicians since it does not require eye drops
or corneal contact.
With all three instruments, the tonometric
pressure reading is only an estimate of the
real pressure. A thick cornea requires extra
force to indent and, therefore, gives a falsely
elevated reading, and the opposite is true
with thin corneas. To better approximate
the real pressure—especially in glaucoma
suspects where exactitude is important—an
ultrasonic pachymeter is used to measure
corneal thicknessA conversion factor for
corneal thickness then adjusts the tonometric reading upward or downward.