The generally accepted normal mean eye pressure (intraocular pressure - IOP) value is 15.5 mmHg with a standard deviation of 2.5 mmHg. The normal pressure ranges from 10 to 21 mmHg. The eye pressure has a non-gaussian distribution with a skew towards higher pressures (meaning in many people pressure above the 21 mmHg level is not abnormal). Despite this, the value of 21 mmHg is accepted as a good, practical & simplistic way to separate the 'normal eye pressure' (less than or equal to 21) from 'abnormal or high eye pressure' (more than 22).

The distribution of eye pressure in glaucoma patients was analyzed in the Baltimore Eye Survey. The likelihood that an eye with glaucoma will have a pressure of more than 22 mmHg was 8.6 times more than the likelihood of it having a pressure of less than 21 mmHg. It is also well recognized that some eyes undergo extensive glaucoma damage even when the pressure is within the 'normal' range, while others suffer no damage with pressure well above 21 mmHg.
The intraocular pressure (IOP) is measured using applanation tonometry (Goldmann tonometer, Air-puff tonometer or Pneumotonometer). Goldmann applanation tonometry is the standard for routine measurements of eye pressure.

Eye pressure increases temporarily because of wearing tight neckties, caffeine intake, yoga positions where one stands on the head (upside down) and isometric exercises (weight lifting). The unanswered question is whether these intermittent bursts of elevated eye pressure lead to pathological problems especially in those who already have glaucoma. In one study, eye pressure increased from a mean of 13 mmHg to a mean of 28 mmHg with maximum isometric contraction (weightlifting) Reference: Neurol Res. 1999;21:243-6. In addition, if you are nervous when the eye pressure is being measured by the eye doctor and you hold your breath (instead of breathing normally), then the eye pressure readings may be erroneously measured higher than your true eye pressure.

The Ocular Hypertension Treatment Study (OHTS) showed central corneal thickness (CCT) to be a powerful predictor of development of glaucoma. Eyes with corneal thickness of 555 microns or less (i.e. eyes with relatively thin corneas) had a threefold greater risk of developing glaucoma than those who had corneal thickness of more than 588 microns. The implication was that a corneal thickness of less than 555 microns should be viewed as a risk factor for development of glaucoma.

Taken together, high eye pressure (more than 24 mmHg) and a cornea thinner than 555 microns may form the basis of starting treatment, the aim being to reduce the likelihood of developing glaucoma in the future. This rationale has stimulated immense interest and debate regarding the importance of corneal thickness in glaucoma management.

There are 2 main issues:

  ● Is Corneal thickness an independent risk factor for development of glaucoma? Does the fact that one has a thin cornea increase the risk of developing glaucoma in of itself - much the same way as having positive family history of glaucoma in of itself puts you at-risk for developing glaucoma?
There is no evidence as of yet to implicate corneal thickness as an 'independent' risk factor.

  ● Does the thickness of cornea influence the eye pressure measurement i.e. eye pressure gets measured differently (higher or lower) depending upon corneal thickness (thicker or thinner)?
There is ample evidence that thicker corneas cause falsely higher eye pressure readings and thinner corneas cause falsely lower eye pressure readings.

Ocular hypertension (OHT) diagnosis is made when a patient has consistently elevated eye pressure but no other sign consistent with glaucoma development. Because the major diagnostic criterion for OHT is based on the measurement of eye pressure, any variable that can affect eye pressure measurement could lead to an erroneous diagnosis of OHT. The most universally applied method of measuring eye pressure is Goldmann applanation tonometry. When originally devised, it was believed that significant variations in corneal thickness were uncommon and a corneal thickness of 520 microns was assumed in calibration of the applanation tonometer. As clinical measurement of corneal thickness has become widely available, several studies have found a wide variation in the thickness of cornea between individuals. Distribution analysis of central corneal thickness measurements revealed that the largest cluster of African American patients had around 520 to 540 microns, whereas the largest cluster of Caucasian patients had between 580 and 600 microns corneal thickness.

A considerable number of patients diagnosed as having ocular hypertension may simply have thicker than average corneas that result in an overestimation of what is likely a normal, true eye pressure. As a consequence, patients with ocular hypertension with thicker corneas may be at a much lower risk for glaucoma development than previously recognized. In contrast, thinner corneas result in an underestimation of the true eye pressure. Therefore even if the eye pressure is within the normal range, if the cornea is thin, that eye pressure should be viewed with suspicion. Indeed patients with 'Normal-Tension Glaucoma' have thinner corneas.

Glaucoma Risk

Thick Cornea + Normal eye pressure--
Thick Cornea + High eye pressure +

Thin Cornea + Normal eye pressure+/-
Thin Cornea + High eye pressure++++

Average Cornea + Normal eye pressure--
Average Cornea + High eye pressure++

As is evident from the above discussion, corneal thickness measurement (pachymetry) seems to be an important test as one can then assess the true risk of developing glaucoma for any given eye pressure.

Corneal thickness, LASIK and Glaucoma
The postoperative eye pressure readings after corneal refractive surgery for myopia as well as hyperopia is lower than the preoperative measurement. The reduced eye pressure after excimer laser refractive surgery is due to false low pressure reading by Goldmann applanation tonometry due to a thinner post operative cornea rather than a real decrease in eye pressure due to LASIK. In contrast to Goldmann applanation tonometry, pneumotonometry measures the eye pressure more reliably after LASIK.
False low eye pressure readings pose the risk of delaying the diagnosis of future glaucoma in patients who undergo refractive surgery. The inability to recognize glaucoma early may result in irreversible vision loss in patients who are glaucoma suspects and those who are steroid responders. Patients with ocular hypertension with falsely low IOP may not be subjected to the same degree of glaucoma suspicion and testing as they otherwise would have received.

Measuring Corneal thickness
Several methods are available to obtain reliable and reproducible measurements of corneal thickness. The most commonly used approach is ultrasonic pachymetry. Ultrasonic pachymetry is an efficient and accurate way to measure corneal thickness; however, the probe must touch the corneal surface and topical anesthesia is thus required. Its accuracy is dependent on the perpendicularity of the probe's application to the cornea and reproducibility relies on precise probe placement on the corneal center. It may be difficult to accurately locate the same point of measurement in serial examinations.
Over the past few years, new instruments have been developed to measure corneal thickness. The Orbscan II corneal topography system (Bausch & Lomb) is an optical scanning-slit instrument that provides topographic analysis and pachymetric measurements of the cornea. Scanning-slit topography requires the patient to fixate for 1.0 to 1.5 seconds. The SP-2000P specular microscope (Topcon Corp.) is a noncontact optical instrument that provides pachymetric measurements and specular microscopy simultaneously. The central corneal thickness measurements are higher with Orbscan than with ultrasonic pachymetry. This disparity between instruments can result from their distinct methodologies. The noncontact Orbscan system measures the hydrated mucous component of the tear film over the cornea; contact ultrasonic pachymetry does not. Thus, Orbscan readings are higher than ultrasonic readings and require the use of the acoustic equivalent correction factor (0.92). If the cornea is unusually thick (>600 microns) or thin (less than 500 microns) as measured with ultrasonic pachymetry, then an Orbscan could be performed to confirm the measurements.

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