corneal thickness in primary care—should we all measure it?

3
Letter to the Editor Corneal thickness in primary care—Should we all measure it? Keywords: Cornea; CCT; Thickness; IOP; Intraocular pressure; Primary care; Glaucoma; Ocular hypertension; Reichert; Pascal; DCT; ORA; Dynamic Contour Tonometry; Ocular Response Analyser; Corneal resistance factor; Corneal hysteresis 1. Background Intraocular pressure (IOP) is the only proven modifiable risk factor to reduce the rate of progression of glaucomatous optic neuropathy. The Ocular Hypertention Treatment Study (OHTS) further showed that reducing IOP in patients with ocular hypertension (OHT) reduces the long term risk of developing glaucoma [1]. Therefore, accurate measurement of IOP is crucial to manage patients at risk of developing glaucoma and also to assess and monitor those who have the disease. It is becoming increasingly evident that measuring IOP using conventional techniques of applanation can give inaccurate readings. This is especially so in patients in whom the central corneal thickness (CCT) is significantly deviated from normal [2–4]. Population studies have shown that CCT varies significantly amongst populations as well as within individuals. The Rotterdam study [5] showed a mean CCT of 537 mm but with a wide range (427–620 mm). Other studies have demonstrated significant racial variation with African-Caribbean eyes prone to having lower CCT values [6–10]. Recent work has shown that OHT patients with lower CCT values have an increased risk of developing glaucoma [11] and patients with glaucoma who have lower CCT values present significantly later with their disease [6,12]. It is postulated that the association between reduced CCT and advanced glaucoma may be partly due to inaccuracies in correctly measuring IOP. However, there is also the suggestion that CCT measurements are surrogate measures of posterior scleral thickness and of the lamina cribrosa through which all ganglion cell axons pass. The thinner the posterior sclera and lamina cribrosa, the more susceptible the optic nerve head is to changes in IOP [13]. 2. Correction factors to provide a more accurate figure for IOP One suggestion is to correct for the effect of CCT on measured IOP by using correction factors derived from large scale population based studies. The Rotterdam study [5] suggested that for every 10 mm increase in CCT, the measured IOP increased by 0.19 mmHg. Bron et al. [14] and Shah et al. [15] have suggested different values of 0.32 and 0.11 mmHg, respectively for a 10 mm increase in CCT. The fact that the different studies identify dissimilar correction factors suggests that this is still an unresolved issue. However, in the absence of other available methods, it may be better to correct the IOP with the CCT values in mind if other clinical variables are equivocal. For instance, Doughty and Zaman [16] showed by a meta- analysis of 300 data sets that a 10% change in CCT could result in a 3.4 mmHg difference in IOP. Therefore, a patient with an applanation IOP of 20 and a CCT of 450 mm (approx. 20% below normal) without other discernible corneal pathology could potentially have a true IOP of 27 mmHg. Conversely, a patient with the same measured IOP but a CCT of 650 mm could have a true IOP of 13 mmHg. It is however, important to bear in mind that the correction factors are derived from linear trend lines that are observed within the population group as a whole. These trend lines are not necessarily accurate for the individual patient and that is why doubt exists as to whether GAT IOP measurements can be corrected in such a linear fashion to derive true IOP for individual patients. It is also becoming clearer that CCT is not the only corneal variable to vary within individuals within a population even if the cornea looks ‘‘normal’’. www.elsevier.com/locate/clae Available online at www.sciencedirect.com Contact Lens & Anterior Eye 31 (2008) 109–111 1367-0484/$ – see front matter # 2008 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.clae.2008.01.004

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Page 1: Corneal thickness in primary care—Should we all measure it?

www.elsevier.com/locate/clae

Available online at www.sciencedirect.com

Contact Lens & Anterior Eye 31 (2008) 109–111

Letter to the Editor

Corneal thickness in primary care—Should we all measure it?

Keywords: Cornea; CCT; Thickness; IOP; Intraocular pressure; Primary care; Glaucoma; Ocular hypertension; Reichert; Pascal; DCT; ORA; Dynamic

Contour Tonometry; Ocular Response Analyser; Corneal resistance factor; Corneal hysteresis

1. Background

Intraocular pressure (IOP) is the only proven modifiable

risk factor to reduce the rate of progression of glaucomatous

optic neuropathy. The Ocular Hypertention Treatment Study

(OHTS) further showed that reducing IOP in patients with

ocular hypertension (OHT) reduces the long term risk of

developing glaucoma [1]. Therefore, accurate measurement

of IOP is crucial to manage patients at risk of developing

glaucoma and also to assess and monitor those who have the

disease.

It is becoming increasingly evident that measuring IOP

using conventional techniques of applanation can give

inaccurate readings. This is especially so in patients in

whom the central corneal thickness (CCT) is significantly

deviated from normal [2–4]. Population studies have shown

that CCT varies significantly amongst populations as well as

within individuals. The Rotterdam study [5] showed a mean

CCT of 537 mm but with a wide range (427–620 mm). Other

studies have demonstrated significant racial variation with

African-Caribbean eyes prone to having lower CCT values

[6–10].

Recent work has shown that OHT patients with lower

CCT values have an increased risk of developing

glaucoma [11] and patients with glaucoma who have

lower CCT values present significantly later with their

disease [6,12]. It is postulated that the association between

reduced CCT and advanced glaucoma may be partly due

to inaccuracies in correctly measuring IOP. However,

there is also the suggestion that CCT measurements are

surrogate measures of posterior scleral thickness and of

the lamina cribrosa through which all ganglion cell axons

pass. The thinner the posterior sclera and lamina cribrosa,

the more susceptible the optic nerve head is to changes in

IOP [13].

1367-0484/$ – see front matter # 2008 British Contact Lens Association. Publi

doi:10.1016/j.clae.2008.01.004

2. Correction factors to provide a more accurate

figure for IOP

One suggestion is to correct for the effect of CCT on

measured IOP by using correction factors derived from

large scale population based studies. The Rotterdam study

[5] suggested that for every 10 mm increase in CCT, the

measured IOP increased by 0.19 mmHg. Bron et al. [14]

and Shah et al. [15] have suggested different values of 0.32

and 0.11 mmHg, respectively for a 10 mm increase in CCT.

The fact that the different studies identify dissimilar

correction factors suggests that this is still an unresolved

issue.

However, in the absence of other available methods, it

may be better to correct the IOP with the CCT values in

mind if other clinical variables are equivocal. For

instance, Doughty and Zaman [16] showed by a meta-

analysis of 300 data sets that a 10% change in CCT could

result in a 3.4 mmHg difference in IOP. Therefore, a

patient with an applanation IOP of 20 and a CCT of

450 mm (approx. 20% below normal) without other

discernible corneal pathology could potentially have a

true IOP of 27 mmHg. Conversely, a patient with the same

measured IOP but a CCT of 650 mm could have a true IOP

of 13 mmHg.

It is however, important to bear in mind that the

correction factors are derived from linear trend lines that are

observed within the population group as a whole. These

trend lines are not necessarily accurate for the individual

patient and that is why doubt exists as to whether GAT IOP

measurements can be corrected in such a linear fashion to

derive true IOP for individual patients. It is also becoming

clearer that CCT is not the only corneal variable to vary

within individuals within a population even if the cornea

looks ‘‘normal’’.

shed by Elsevier Ltd. All rights reserved.

Page 2: Corneal thickness in primary care—Should we all measure it?

Letter to the Editor / Contact Lens & Anterior Eye 31 (2008) 109–111110

3. Other approaches to measure IOP

3.1. Reichert Ocular Response Analyser

In addition to variation in corneal thickness between

individuals, it is becoming apparent that there is also

variation in the biomechanical properties of the individual

cornea. The Reichert Ocular Response Analyser (Reichert

Inc., Buffalo Town, USA) uses the principle of bi-directional

applanation of the cornea to derive corneal hysterisis (CH)

and corneal resistance factor (CRF) in addition to IOP and

CCT. These new measurements are claimed to be measures

of the individual corneal rigidity and may allow for

individual IOP correction.

Shah et al. [17] have found that CH, CCT, CRF are

correlated with one another but that the correlation is only

moderate. This suggests that CH and CRF are additional

measurements of the individual corneal rigidity and these

variables may be more useful when trying to correct IOP

measurements for altered ocular rigidity. More recent work

has shown that CH may be a better measure of the risk of

glaucoma progression [18].

3.2. Pascal Dynamic Contour Tonometry

Recent work has also concentrated on measuring IOP

independent of CCT. The Pascal Dynamic Contour

Tonometer (DCT) is one such device that utilises the

principle of contour matching to measure IOP. Investigators

have found that DCToverestimates IOP compared with GAT

by 1.0–2.0 mmHg [19]. DCT does seem to be less dependent

on CCT and several investigators have found only a weak

association of this technique with CCT [20,21] whilst others

have found no link with CCT [19]. An interesting study [22]

comparing IOP readings with DCT before and after myopic

refractive surgery correction found no significant change

whilst GAT IOP readings decreased significantly again

giving weight to the argument that this instrument is not

affected by changes in CCT.

The main drawbacks of DCT are that the sensor tip is

covered with a membrane and there may be problems with

the membrane fitting poorly or wrinkling. Bubbles of air can

get trapped between the tip and the eye which can give

erroneous results. This technique also requires more co-

operation from the patient than Goldmann applanation

tonometry as the reading can take several seconds during

which, the patient is required to keep their eye still.

Excessive tear film or a very dry eye can also affect the DCT

reading which means that it is not always possible to get a

reading in all patients.

3.3. What to measure in primary care?

It is becoming more apparent that corneal properties play

a significant role in the screening of populations for

glaucoma. The clinician should understand that IOP

measurement using currently widely used devices may

not be accurate. It is also possible that corneal properties

may reflect on the susceptibility of the individual to

progressive glaucoma independent of IOP. It would there-

fore seem prudent to measure other corneal parameters such

as CCT in patients at high risk of developing glaucoma or in

whom the extent of glaucomatous optic neuropathy does not

correlate with IOP findings.

Careful consideration is also needed should such

glaucoma suspects or patients with established glaucoma

wish to consider refractive surgery as the alteration of

corneal properties may result in further inaccuracies in IOP

measurements [23]. Contact lenses in this situation may

offer an alternative solution for the patient’s needs.

If a patient has a significantly reduced CCT, the clinician

needs to appreciate that the measured IOP cannot be relied

upon. For every 10 mm decrease in CCT, the applanation

IOP could be 0.11–0.49 mmHg higher than measured IOP

and if the patient also has co-existing corneal pathology such

as Keraoconus or Fuchs corneal dystrophy, the IOP could be

much higher. More recent data suggests that in patients with

no apparent corneal disease but reduced corneal resistance

factor as measured by DCT, the true IOP could also be

significantly higher.

Therefore, ocular hypertensive patients with reduced

CCT may need referral with lower measured IOP values

and similarly, glaucoma patients with reduced CCT may

not be adequately controlled despite good measured IOP.

Conversely, patients with high CCT values may not have

true ocular hypertension even if measured IOP is high and

they could possibly continue to be observed in a

community setting. Similarly, glaucoma patients with high

CCT may not need very low target IOP’s. However,

decisions like these on such patients can only be made with

a period of careful follow up and information from other

parameters.

References

[1] Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension

Treatment Study: baseline factors that predict the onset of primary

open-angle glaucoma. Arch Ophthalmol 2002;120:714–20.

[2] Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central

corneal thickness. Acta Ophthalmol (Copenhagen) 1975;53:34–43.

[3] Whitacre MM, Stein RA, Hassanein K. The effect of corneal thickness

on applanation tonometry. Am J Ophthalmol 1993;115:592–6.

[4] Johnson M, Kass MA, Moses RA, et al. Increased corneal thickness

simulating elevated intraocular pressure. Arch Ophthalmol 1978;96:

664–5.

[5] Wolf RCW, Klaver CCW, Vingerling JR, et al. Distribution of corneal

thickness and its association with intraocular pressure. The Rotterdam

Study. Am J Ophthalmol 1997;123:767–72.

[6] Herndon LW, Weizer JS, Stinnett SS. Central corneal thickness as a

risk factor for advanced glaucoma damage. Arch Ophthalmol

2004;122:17–21.

[7] Nemesure B, Wu SY, Hennis A, Leske MC. Corneal thickness and

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[8] Shimmyo M, Ross AJ, Moy A, Mostafavi R. Intraocular pressure,

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[9] La-Rosa FA, Gross RL, Orengo-Nania S. Central corneal thickness of

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[10] Hahn S, Azen S, Ying-Lai M, Varma R. Central corneal thickness in

Latinos. Invest Ophthalmol Vis Sci 2003;44:1508–12.

[11] Medeiros FA, Sample PA, Zangwill LM, et al. Corneal thickness as a

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[12] Kim WJ, Chen PP. Central corneal pachymetry and visual field

progression in patients with open-angle glaucoma. Ophthalmology

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[13] Jonas JB, Berenshtein E, Holbach L. Lamina cribrosa thickness and

spatial relationships between intraocular space and cerebrospinal fluid

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[14] Bron AM, Creuzot-Garcher C, Goudeau-Boutillon S, et al. Falsely

elevated intraocular pressure due to increased corneal thickness.

Graefes Arch Clin Exp Ophthalmol 1999;237:220–4.

[15] Shah S, Chattejee A, Mahai M, et al. Relationship between corneal

thickness and measured intraocular pressure in a general ophthalmol-

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[16] Doughty MJ, Zaman ML. Human corneal thickness and its impact on

intraocular pressure measures: a review and meta-analysis approach.

Surv Ophthalmol 2000;44:367–408.

[17] Shah S, Laiquzzaman M, Cunliffe I, et al. The use of the Reichert

ocular response analyzer to establish the relationship between ocular

hysteresis, corneal resistance factor and central corneal thickness in

normal eyes. Contact Lens Anterior Eye 2006 Dec;29(5):257–62.

[18] Congdon NG, Broman AT, Bandeen-Roche K, et al. Central corneal

thickness and corneal hysteresis associated with glaucoma damage.

Am J Ophthalmol 2006;141(May (5)):868–75.

[19] Kamppeter BA, Jonas JB. Dynamic contour tonometry for intraocular

pressure measurement. Am J Ophthalmol 2005;140:318–20.

[20] Ku JY, Danesh-Meyer HV, Craig JP, et al. Comparison of intraocular

pressure measured by Pascal dynamic contour tonometry and Gold-

mann applanation tonometry. Eye 2006;20:191–8.

[21] Kotecha A, White ET, Shewry JM, Garway-Heath DF. The relative

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metry and dynamic contour tonometry. Br J Ophthalmol 2005;89:

1572–5.

[22] Kaufmann C, Bachmann LM, Thiel MA. Intraocular pressure

measurements using dynamic contour tonometry after laser in situ

keratomileusis. Invest Ophthalmol Vis Sci 2003;44(9):3790–4.

[23] Chatterjee A, Shah S, Bessant DAR, Naroo SA, Doyle SJ. Reduction

in intraocular pressure after excimer laser photorefractive keratect-

omy: correlation with pre-treatment myopia. Ophthalmology

1997;104:355–9.

Shabbir Mohamed*

University Hospital Birmingham, Selly Oak,

Birmingham, B29 6QD, United Kingdom

Sunil Shah

Birmingham Heartlands and Solihull NHS Trust,

Bordesley Green East, Birmingham,

B9 5SS, United Kingdom

*Corresponding author. Tel.: +44 7796693060

E-mail addresses: [email protected]

(S. Mohamed)

[email protected]

(S. Shah)