Eye Surgery Procedures & Treatment
Talking to Your Eye Doctor About Glaucoma
Exams and Tests for Glaucoma
An examination of the eye may be used to diagnose glaucoma. However, checking the intraocular pressure alone (tonometry) is not enough because eye pressure changes. The doctor will need to examine the inside of the eye by looking through the pupil, often while the pupil is dilated. Additional tests are used to detect glaucoma.
Remember that the hallmark of glaucoma of any type is pressure in the eye sufficient to damage the optic nerve. Usually this means IOP that is elevated above the range of 8-22, but it also can mean that the vascular supply or the supporting structures or the optic nerve make it more susceptible to damage at “normal” IOP.
Usually the doctor will perform a complete examination of the eyes.
Tests may include:
Visual Acuity Visual field measurement. This measures the subjective functioning of the nerve fiber layer and can show characteristic loss of the peripheral vision. It can clearly indicate the presence of glaucoma in a person who is a good test taker (specificity), but it may not detect glaucoma until there is already considerable damage (not sensitive). It is also subject to a number of potential sources of error, like patient reliability and attentiveness, or problems of the eye like cataract or macular degeneration not related to glaucoma.Pupillary reflex response, which will not be equal if one eye is more damaged from glaucoma than the other.
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Intraocular pressure (IOP) measurement by tonometry. The normal range is 8-22 mmHg (millimeters of mercury). There are several instruments used to measure the pressure in the eye, and often more than one is used to check the accuracy of the reading. Also, the doctor may check the IOP at different times of day at different visits, or several times over the course of a day (serial tonometry). The IOP can be falsely high if the person is squeezing his eyes shut, or holding his breath, or has a thicker than average cornea.
Corneal thickness (pachymetry) is measured to determine the effect of thickness and therefore compliance on the measured IOP. This concern arose with the widespread use of refractive surgery, which thins down the cornea. A thinner cornea has greater compliance and might seem to have a lower IOP than what is true. Conversely, a thicker cornea might seem to have a higher IOP than true.
Gonioscopy uses a special lens to see the outflow channels of the angle between the iris and the cornea, like a dentist’s mirror. The lens touches the eye, so the patient has an anesthetic drop placed in the eye first.
Slit lamp examination. This instrument is a microscope with a linear beam of light used to evaluate the front of the eye, and, with an additional lens, the magnified and 3 dimensional image of the optic nerve.
Optic nerve photographs documents the effect of pressure constantly pushing on the nerve, or “cupping”. The normal cup-to-disc ratio is 20-60%, but there are many variations in the appearance of the optic nerve and its appearance must be evaluated in the context of the person’s history, symptoms and examination. Talk to your ophthalmologist or eye doctor more about this.
Optic nerve fiber analysis (instruments include the OCT, HRT or GDx currently). No matter what the look or the shape of the optic nerve, the numbers of nerve fibers coming into the eye and fanning out under the retina tend to fall within a range for a given age of patient. So, these instruments measure the thickness of the nerve fiber and give a statistical analysis of the patient’s nerve fiber layer vs. their peers. It adds to the information obtained by the visual field test and does not depend on the patient’s response. It can be followed over time, and some instruments allow a digitalized measurement of the cupping and contour of the optic nerve.
