Controlling your Eye Pressure (IOP):

Corneal Thickness

Before we talk about intraocular pressure, let me say a word about corneal thickness and corneal biomechanics. 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 the 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 was off and underestimated the actual pressure. So patients with thin corneas who presented with seemingly normal or low pressures actually have high pressures. 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 make the lamina cribosa more fragile and less likely to support the optic nerve (increasing likelihood of glaucoma).

In any case, we now measure corneal thickness with a test called pachymetry. Of all the different ways we have to measure eye pressure, only the Pascal Dynamic Tonometer seems to be independent of the corneal thickness artifact, as well as correcting also for difference in viscoelasticity. (See more below.) 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.

NCT     Non-contact tonometry

This is known as the air puff test.  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 in optometric offices since it won’t effect eyeglass measurements that might be taken later during your visit. It is however widely considered the least accurate way to determine intraocular pressure.

Tonopen

This instrument is portable and about the size of a large 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 over or under-estimate the pressure in certain situations.  If you tend to squeeze your eyelids this is a good way to check your IOP since it does not required a lot of patient cooperation.

Applanation

Sometimes referred to as Goldmann Applanation Tonometry after its 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 to line 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.

Pascal Dynamic Contour Tonometry (PDCT)

There is mounting evidence that Goldmann Applanation Tonometry (GAT) even with adjustments for pachymetry is inaccurate.

In short, PDCT takes a true IOP measurement independent of corneal thickness or biomechanics using a piezoelectric transducer without indenting the cornea.

Through the OHTS study, we’ve learned that central corneal thickness (CCT) is related to the risk of glaucoma. Patients with thin corneas carry a higher risk. Goldmann Applanation Tonometry is known to underestimate pressure in thin corneas.

Pachymetry was then added to our glaucoma workups. Correction factors were established to add or subtract to the GAP reading based the CCT. Unfortunately, as we are now learning, a linear correction of GAT based upon pachymetry may not only be the wrong magnitude, but also it may even be the wrong direction!

There are three potential sources of error for GAT. First, variation in corneal curvature which accounts for only one point of error. Second, corneal thickness, which accounts for between 2.0 to 2.5 mm of Hg error. And third, corneal biomechanics. Corneal biomechanics can account for HUGE errors up to 10 mm of Hg cadaver models (less in vivo). This the linear correction factors related to CCT and IOP are flawed since they do not take into account corneal biomechanics.

PDCT removes biomechanics and thickness from the measurements and is thus much more accurate than GAT. At our office currently we now use PDCT. Its truer pressure readings have been a tremendous help in diagnosing glaucoma earlier and in better management of patients, particularly those with thin corneas, post-LASIK patients and those with corneal abnormalities such as Pellucid marginal degeneration. The post LASIK patient in particular is a ticking time bomb since GAT will underestimate pressure by at least 4 points and those optic nerve head may be tilted and difficult to interpret the C/D ratio accurately.

An outstanding question is whether the Pascal is able to explain most of the previous cases of NTG (normal tension glaucoma) based upon the more accurate higher-pressure reading we are now recording. The role of non-pressure related factors such as lamina cribosa or TM structural issues, or optic nerve blood flow, may still be operative in NTG. Pascal will help answer these long-standing riddles in glaucoma early diagnosis and management.

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