corneal thickness in primary care—should we all measure it?
TRANSCRIPT
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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’’.
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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.
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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)
(S. Shah)