Corneal Biomechanics
Before we talk about intraocular pressure, let me say a word about corneal thickness and corneal biomechanics. We now measure corneal thickness with a test called pachymetry. Normal corneal thickness is about 530 microns (500 microns = ½ mm). Many of the instruments that measure eye pressure make certain assumptions about corneal thickness and other corneal properties such as viscoelasticity.
Through OHTS (ocular hypertension treatment study) we learned that corneal thickness is a risk factor for glaucoma. Patients with thin corneas have a greater chance of progressing to glaucoma than those with average or thick corneas.
There were two theories that arose to account for this. One is that the actual measurement in patients with thin corneas is off and underestimates the actual pressure. The other theory is that patients with thin corneas might have some underlying defect in their collagen structure.
There is an area which supports the nerve called the lamina cribosa that is also composed of collagen, just like the cornea. Perhaps the same factor that thins out the cornea also makes the lamina cribosa more fragile and less likely to support the optic nerve (increasing the likelihood of optic nerve cupping. In any case, patients with thick corneas are less at risk for glaucoma; those with thin corneas at greater risk. Eye pressure is measured in units called millimeters of mercury. Normal eye pressure has a range of 10 to 20. (Compare this to normal blood pressure of 120/80.) Interestingly eye pressure does vary 1 to 3 millimeters during systole and diastole. However in most clinic practices an average pressure is determined.
Eye pressures vary during the course of the day. This is called diurnal variation. Pressures tend to be higher at night, when sleeping and while the body is supine. If you have glaucoma it is important to be on treatment that is effective for these night time pressure increases. Not all glaucoma medications are equally effective in the evening.
There are several different ways to measure eye pressure, sometime referred to as intraocular pressure or IOP.
Non-contact tonometry
This is known as the air puff test or NCT. A puff is air is blown against your cornea; the instrument measures how much pressure is required to flatten the cornea a certain amount. It does not require anesthetic drops to perform. It is a particular popular way to check pressures since it won’t affect eyeglass measurements that might be taken later during your visit. The newest NCT instruments give reasonably accurate measurements.
Tonopen
This instrument is portable and about the size of a pencil. An anesthetic drop is placed in the eye to numb it. The tip of the tonopen (covered with sterile disposable latex) touches the cornea. Multiple measurements are taken with a pressure transducer and then averaged. It is considered very reproducible and reasonably accurate. It does however make certain assumptions about corneal thickness and elasticity and therefore may under-estimate the pressure.
Applanation
Sometimes referred to as Goldmann Applanation Tonometry after its Swiss inventor, this is the most commonly used pressure-measuring device. A yellow anesthetic drop is placed in the eye. Then a tiny measuring prism illuminated with cobalt blue is used and touches or applanates the center of the cornea. The doctor will turn a dial bringing up two semi-circles and this indicates what the pressure is.
When the original studies where done by Dr. Hans Goldman, he used eyes that had corneal thickness of about 540 microns and made certain assumptions about the elasticity of the cornea. In patients that have thicker or thinner corneas, or corneas that are more or less elastic than average, Goldman applanation may significantly over or under-estimate intraocular pressures.
The role of non-pressure related factors such as corneal thickness, optic canal support, ocular blood flow, genetics,, are less well understood than intraocular pressure in terms of their ability to cause glaucomatous optic nerve damage.