Clinical and Experimental Ophthalmology

2005;

33

: 113–114

Editorial

_____________________________________________

Editorial

Intraocular pressure: relevant or redundant?

Recent findings have pressured ophthalmologists into think-ing again about the relationships between intraocularpressure (IOP), corneal thickness and proper glaucomamanagement. The question is: Do we care what the absoluteIOP is? and if so, when? In this issue of

Clinical and Experimen-tal Ophthalmology

, Cheng

et al.

use a new tonometer in searchof ‘real’ IOP in patients who have undergone refractivesurgery.

1

They sought to measure IOP with a method lessdependent upon corneal thickness change induced by laser

in situ

keratomileusis (LASIK). They measured 60 peoplewith the ‘pressure phosphene tonometer’ (PPT) that esti-mates IOP by pushing on the sclera through the eyelid,using the subjective perception of a ring of light by thepatient as the endpoint. Several other groups have deter-mined that the actual IOP determination by the PPT ispoorly correlated to Goldmann tonometric IOP, even whenperformed by medical personnel.

2–5

Cheng

et al

. did not seekto validate PPT, but used it to estimate IOP in the same eyebefore and after LASIK, independent of corneal thicknesschange. They found a minimal effect of LASIK on PPT-measured IOP, compared to a significant fall in applanation(about 4 mmHg). They confirmed that IOP measured byapplanation is lower after photorefractive surgery.

Accumulating data from LASIK procedures have chal-lenged the theory behind applanation tonometry. Goldmannassumed that applanation IOP was generally independent ofcorneal thickness in the range typically seen.

6

However, itis now recognized that many corneas are thin (or thick)enough to require a correction factor to be applied forestimation of true IOP. Furthermore, measured IOP afterLASIK may be altered by iatrogenic thinning of the centralcornea, altering the applanation zone, the corneal curvatureor Bowman’s membrane. A laser-modified cornea may havedifferent applanation properties compared with a virgincornea of the same thickness. As the myopes who undergorefractive procedures are at particular risk for open angleglaucoma (OAG), what does this mean for the assessment ofIOP and glaucoma?

Two studies have performed tonometry in surgically can-nulated eyes to estimate the effect of corneal thickness onmeasured IOP,

7,8

although no cannulation study has, as yet,reported the effect of LASIK-induced thinning on tonome-try. Several reports suggest that the measured IOP is lowerin proportion to the amount ablated,

9,10

a finding consistentwith the idea that corneal thinning should provide lessresistance to tonometer deformation, thereby resulting in adecrease in measured IOP but not in actual IOP. As evenminor thickness changes in the cornea can affect tonometry,

the PPT must be subject to even larger variabilities in eyelidand scleral thickness and ocular rigidity, due to the forceapplied to the eye.

But how relevant is knowledge of the ‘true’ IOP to themanagement of glaucoma? In most cases, it does not seemcritical. The definition of OAG does not now include aparticular level of IOP,

11

as prevalence studies have consist-ently found that up to half of those with OAG suffer damageat normal levels of IOP.

12,13

Instead, the diagnosis ofwhether one has OAG depends mainly on disc and fieldfindings, not the IOP level. The use of terms such as lowtension and high tension glaucoma has led to a falsedichotomy, based on an illusory magic number.

14

OAG is aheterogeneous disorder whose incident risk is actuallycontinuously associated with higher IOP. Rather than usean arbitrary IOP cut-off point to identify patients with OAG(which can lead to screening failures) the hunt for OAGshould depend on vigilant disc and nerve fibre layer exami-nations along with a low threshold for performing fieldtesting. Why should we be concerned over falsely low IOPreadings following refractive surgery? If we are (incorrectly)detecting OAG by looking for abnormal IOP, those withlow or laser-lowered IOP readings may be falsely assumednot to have OAG. With a rapidly growing number of peoplewho undergo refractive surgery, there could be an increasein missed OAG diagnoses.

Nor does therapy for OAG any longer depend on ‘normal-izing’ IOP. Baseline IOP is now estimated from the untreatedeye and a target IOP is then set for treatment, which istailored to each patient, and recorded in the chart for contin-ued reference. If refractive surgery is to be performed in apatient who has glaucoma or is a glaucoma suspect, a newbaseline of IOP readings needs to be established followingthe procedure and new target IOP needs to be determined.This can easily be done by measuring IOP twice before andtwice shortly after the procedure to determine the shift inmeasured IOP due to the ablation. It is clear therefore thatchanging the target IOP during glaucoma treatment is oftennecessary because of the effects of refractive surgery. Even ina newly diagnosed OAG patient without IOP recorded priorto LASIK, the present, untreated pressure can be used as thebaseline from which a target can be set for initial therapy.Although this target IOP value may appear rather low in suchsituations, a target pressure strategy should be adopted, as itis now known that a measured value of 8 mmHg in such aneye is not the actual IOP and is not unattainably low.

When IOP values are higher than average in a personwith a normal disc and field, it is useful to know the corneal

114 Editorial

thickness by pachymetry. Ocular hypertensive suspectswith thick corneas are less likely to develop OAG (and canbe told so), but they still requiring monitoring. Those withthin corneas might be at greater risk for developing glau-coma, even independent of the effect of thickness ontonometry, as suggested by the Ocular Hypertension Treat-ment Study.

15

The idea is intriguing—perhaps suggesting alink between thinner cornea and overall eye wall resistanceto deformation by IOP. This hypothesis was not corrobo-rated by the Early Manifest Glaucoma Trial, which did notfind that thin corneas were associated with glaucoma pro-gression.

16

As pointed out by Brandt,

17

the influence of thebehaviour of the corneoscleral shell on OAG is important,but its thickness may be only a partial surrogate for funda-mentally important characteristics such as the acute andchronic viscoelastic responses of ocular structures to stress.

In the near future, we may see an instrument thatmeasures IOP independent of corneal thickness, such as apressure monitor placed within an intraocular lens, provid-ing continuous IOP recordings by telemetry. Although eventhis device will have inherent measurement error, it wouldhelp to determine whether glaucoma progression moreclosely depends on the mean IOP or other characteristics(such as the magnitude of its spikes or other patterns offluctuation). Likewise, as more is discovered about theproperties of the corneoscleral shell, it may be that directmeasurement of its viscoelastic properties will provide prog-nostic information about which people are at greatest risk ofdeveloping OAG or undergoing progressive worsening.

Helen V Danesh-Meyer

FRANZCO

1

andHarry A Quigley

MD

2

1

Department of Ophthalmology, University of Auckland,Auckland, New Zealand and

2

Department of Ophthalmology,Wilmer Ophthalmological Institute, Baltimore,

Maryland, USA

R

EFERENCES

1. Cheng ACK, Leung DYL, Cheung EYY, Fan DSP, Law RWK,Lam DSC. Intraocular pressure measurement in patients withprevious LASIK surgery using pressure phosphene tonometer.

Clin Experiment Ophthalmol

2005;

33

: 153–7.2. Danesh-Meyer HV, Niederer R, Gaskin BJ, Gamble G.

Comparison of the Proview pressure phosphene tonometer

performed by the patient and examiner with the Goldmannapplanation tonometer.

Clin Experiment Ophthalmol

2004;

32

:29–32.

3. Li J, Herndon LW, Asrani SG, Stinnett S, Allingham RR.Clinical comparison of the Proview eye pressure monitor withthe Goldmann applanation tonometer and the TonoPen.

ArchOphthalmol

2004;

122

: 1117–21.4. Alvarez TL, Gollance SA, Thomas GA

et al.

The Proviewphosphene tonometer fails to measure ocular pressure accu-rately in clinical practice.

Ophthamology

2004;

111

: 1077–85.5. Chew GSM, Sanderson GF, Molteno ACB. The pressure

phosphene tonometer—a clinical evaluation.

Eye

2004;

11

:1–3 (Epub ahead of print).

6. Goldmann H, Schmidt T. [Applanation tonometry.]

Ophthal-mologica

1957;

134

: 221–42.7. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and

central corneal thickness.

Acta Ophthalmol (Copenh)

1975;

53

:34–43.

8. Whitacre MM, Stein RA, Hassanein K. The effect of cornealthickness on applanation tonometry.

Am J Ophthalmol

1993;

115

: 592–6.9. Phelan PS, McGhee CN, Bryce IG. Excimer laser PRK and

corticosteroid induced IOP elevation: the tip of an emergingiceberg?.

Br J Ophthalmol

1994;

78

: 802–3.10. Munger R, Hodge WG, Mintsioulis G, Agapitos PJ, Jackson WB,

Damji KF. Correction of intraocular pressure for changes incentral corneal thickness following photorefractive kera-tectomy.

Can J Ophthalmol

1998;

33

: 159–65.11. Foster PJ, Buhrmann R, Quigley HA, Johnson GJ. The defini-

tion and classification of glaucoma in prevalence surveys.

Br JOphthalmol

2002;

86

: 238–42.12. Tielsch JM, Katz J, Singh K

et al.

A population-based evalua-tion of glaucoma screening: The Baltimore Eye Survey.

Am JEpidemiol

1991;

134

: 1102–10.13. Weih LM, Nanjan M, McCarty CA, Taylor HR. Prevalence

and predictors of open-angle glaucoma. Results from theVisual Impairment Project.

Ophthalmology

2001;

108

: 1966–72.14. Wilson MR. The myth of ‘21’.

J Glaucoma

1997;

6

: 75–7.15. Gordon MO, Beiser JA, Brandt JD

et al.

The Ocular Hyper-tension Treatment Study: baseline factors that predict theonset of primary open-angle glaucoma.

Arch Ophthalmol

2002;

120

: 714–20.16. Leske MC, Heijl A, Hussein M

et al.

Factors for glaucomaprogression and the effect of treatment: the early manifestglaucoma trial.

Arch Ophthalmol

2003;

121

: 48–56.17. Brandt JD. Corneal thickness in glaucoma screening, diagnosis,

and management.

Curr Opinion Ophthalmol

2004;

15

: 85–9.