Testing for Glaucoma
The eye requires a certain amount of pressure to keep it inflated and functioning properly.
If pressure in the eye gets too high that can damage the optic nerve and result is vision loss, which is what happens in glaucoma.
One of the tricky things about glaucoma is that there are no early symptoms. Unfortunately, vision lost from nerve damage cannot be restored.
The level of pressure within the eye is determined by a balance between fluid being pumped into the eye and fluid leaving through the filtering system.
In the above diagram the blue arrows show the circulation of aqueous fluid in the front of the eye. Aqueous is a watery liquid filtered from blood which is pumped into the eye by the Ciliary Body. It circulates around the lens and through the pupil to reach the filtering system, the Trabecular Meshwork. It exits through pores in the meshwork to collect in the Canal of Schlemm and then return to the bloodstream.
Causes of Raised Pressure:
- Open Angle: By far the most common cause is increased resistance to outflow by the pores of the filtering system.
- Narrow Angle: Less common is angle closure where the iris blocks access to the filtering system.
- Other: There are more mechanisms that are uncommon to rare and will not be covered here.
By whatever mechanism, when the pressure gets high enough to damage the nerve that is called glaucoma.
Testing for glaucoma requires:
- Measurement of pressure
- Evaluation of optic nerve health
An instrument that measures pressure is called a tonometer.
The photo shows a Goldmann tonometer in position in front of the eye about to measure pressure.
After instilling anesthetic drops the lighted blue tip is pressed gently against the cornea. The pressure reading is taken from the dial in the lower center of the picture.
Points about testing pressure:
- Intraocular pressure can vary significantly in a 24 hour period.
- Because of the way pressure is measured the thickness of the cornea affects the pressure reading. If there is any suspicion about pressure we measure corneal thickness and apply a correction factor.
- The amount of pressure required to damage the optic nerve is different for different people.
Corneal thickness varies in individuals just as height does. In the photo is a pachymeter, an instrument that uses ultrasound to measure the thickness of the cornea. The reading on the screen shows a reading of 554 microns (0.554 mm), approximately average central corneal thickness.
Evaluating the Health of the Optic Nerve
Nerve damage shows up as:
- Structural change: localized loss of rim tissue (usually)
- Function Loss: visual field loss
The photo and diagram show a normal optic nerve head with a full healthy rim of nerve tissue. A detailed look at the contour of the surface shows an even rim of nerve tissue with a depression in the center, called the “cup.” The rim is carefully inspected looking for localized loss of nerve tissue that would indicate glaucoma damage.
Optic Nerve with Glaucoma Damage
The photo shows an example of an optic nerve, from a left eye, where there is advanced thinning of the rim of nerve tissue. Compare to the photo and drawing above with a full even rim of nerve tissue.
With a machine called an HRT (Heidelberg Retinal Tomograph) we can get a detailed contour map of the nerve.
In the nerve above the thinning is obvious, but the HRT is very helpful in borderline cases and for looking for change over time.
This is part of the HRT printout for the nerve in the above photo.
On the left is a contour map. The blue-green is the rim. The red is the central cup.
On the right, the computer has divided the nerve into six sections and compared each section to the normal range for the patient’s age. The green checks indicate that section is within normal range. The yellow exclamation mark means borderline. The red X’s mean out of the normal range.
We always double check to confirm that the computer’s interpretation agrees with what we see looking at the nerve in the eye.
The other part of assessing glaucoma damage is to measure the function of the nerve by checking peripheral vision. One of the sneaky things about glaucoma is that it takes peripheral vision first where you are not likely to notice it. Only when it is far advanced does it affect central vision.
The above is part of a visual field printout for a left eye.
In looking at the visual field remember the image on the retina is upside down compared to the real world.
Where the lines cross is the center of vision. This is a left eye field so the normal blindspot is on the left side. On a gray-scale diagram a normal field should be almost white in the center and light gray at the edges. Darker gray and black shading indicate degree of damage, specifically, the darker the shade the further the tested point is from normal.
The dark area on the lower right side of the field represents moderate damage and it corresponds to thinning of the upper rim of the nerve, the yellow “!” on the HRT.
The darker area in the upper field represents advanced damage which is encroaching on central vision. It corresponds to thinning of the lower rim of the nerve, the red “X” on the HRT.
The Glaucoma Patient
All the findings are consistent in this patient with glaucoma. Now that we have confirmed damage our mission is to get the pressure low enough to prevent further damage. We start by picking a target pressure that experience has shown to be reasonable and use necessary measures, usually eye drops, to reach that level.
For example, if the original pressure at which damage occurred was 30, then we start drops with the initial aim of getting the pressure at or below 20.
Then we follow the nerve and fields over time to determine if damage is actually arrested. If follow-up testing shows no change then we have evidence that we have reached a satisfactory target pressure. If more damage is occurring then we need to set our target pressure lower. That can be achieved by adding medications (more drops), laser treatment, or surgery if necessary.
Consistent follow-up is the key to maintaining vision.
For more information on risk see OHTS Study.