Intraocular Pressure Check
All glaucoma patients are familiar with the “pressure
check” in which a drop of anesthetic is placed on the eyeball
and a device to measure the pressure is placed directly on the
eye. The gold standard for IOP measurement is the Goldmann tonometer,
which measures the force required to flatten a small area of the
central cornea. The accuracy of the Goldmann tonometer is well
established to within 1 to 2 mm Hg.
Gonioscopy
Gonioscopy is a test that allows the doctor to look at the angle
of the eye. The angle is located at the very edge of the cornea,
where the white of the eye joins the iris (colored part) and the
cornea. The angle cannot be seen by looking directly at the eye.
A special lens with a mirror, the goniolens, is required to actually
see the angle. The goniolens has to be placed directly on the
surface of the eye. Although an anesthetic drop is used, the test
can be a little unpleasant, but it is generally a pretty safe
procedure.
Cornea Thickness
The cornea is the clear covering of the eye,
like the crystal on a watch. The cornea covers the iris, the colored
part of the eye. It focuses much of the light onto the retina.
The thickness of the cornea is measured with a pachymeter. Some
pachymeters use light; the newer ones use ultrasound. The average
cornea is about 545 microns thick, a little more than half a millimeter.
The central corneal thickness apparently introduces an artifact
in the IOP measurement, so that in patients with thick corneas,
the real IOP is actually lower than the measured IOP. In thin
corneas, the opposite is true. The result is that people with
thin corneas actually have a higher real IOP than what is measured,
which puts them at a greater risk.
Visual Field Testing (Perimetry)
Visual field tests are designed to map a person’s
visual field, to document the level of peripheral vision. As most
glaucoma patients know, the eye not being tested is patched, your
chin is placed on a chin rest and with your eye being tested you
are asked to focus on a small yellow spot in front of you. The
machine itself looks like a large concave bowl. The test consists
basically of responding every time a flash of light is perceived,
all the while looking straight ahead. The ideal visual field test
would be easy to take, easy to administer, and 100% reliable.
We have no such test, but fortunately for everyone involved, recent
years have seen substantial improvements in all of these areas.
Especially welcome to the glaucoma patient are tests that are
faster and less tedious.
More information:
Understanding Visual Field
Testing
Visual Field Examples
SWAP (Short Wavelength Automated Perimetry)
SWAP stands for Short Wavelength Automated Perimetry.
The visual field is tested on a standard perimeter, but instead
of using a white spot on a white background, a blue spot is used
on a yellow background. There is some evidence that this type
of visual field testing can detect abnormalities at an earlier
stage. The test, however, is much longer than the standard test.
SWAP is most useful in very early cases with normal or questionable
standard visual fields.
HRT - Heidelberg Retina Tomograph
The HRT (Heidelberg Retina Tomograph) uses a
special laser to take 3-dimensional photographs of the optic nerve
and surrounding retina. This laser, which is not powerful enough
to harm the eye, is first focused on the surface of the optic
nerve and captures that image. Then it is focused on the layer
just below the surface and captures that image. The HRT continues
to take images of deeper and deeper layers until the desired depth
has been reached. Finally, the instrument takes all these pictures
of the layers and puts them together to form a 3-dimentional image
of the entire optic nerve.
More information:
The HRT: A Better Way of
Examining the Optic Nerve in Glaucoma?
GDx Nerve Fiber Analyzer
The GDx is a tool that uses laser to determine
the thickness of the nerve fiber layer. The nerve fiber layer
is important because it is the tissue of which the optic nerve
is made. Damage to the nerve layer could be a sign of glaucoma.
The test cannot distinguish glaucoma from other causes of nerve
fiber damage, such as a vein occlusion. Care must be taken when
interpreting the results.
OCT (Optical Coherence Tomography)
OCT (optical coherence tomography) is a device
that measures the thickness and contour of the different layers
of the retina. Like the GDX, it measures the thickness of the
nerve fiber layer. Studies have not shown conclusively that any
one of these techniques is significantly better than the others
for diagnosing glaucoma.
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