T. Aung
Intraocular pressure is a major risk factor for glaucoma development and a primary
risk factor for glaucoma progression (Gordon et al., 2002; Heijl et al., 2002; AGIS
Investigators, 2000). At present, IOP-lowering remains the only proven method of
delaying glaucoma progression. Measurement of IOP is therefore fundamental to risk
assessment and patient management. Current limitations of measuring IOP include
circadian patterns of IOP, and the precision and accuracy of IOP measurements.
The
IOP fluctuates within the 24-hour period, with diurnal IOP generally highest after
awakening, decreasing during the daytime and rising during the nocturnal/sleep period.
Patients with glaucoma show greater diurnal fluctuations compared with healthy
controls (Fig. 1) (Liu et al., 2003). Office-visit measurements are insufficient
to assess IOP peak, mean or degree of IOP fluctuation, capturing mere snapshots
of true IOP over time. Peak and mean IOP are important parameters affecting glaucoma
progression, although it is unclear whether short-term IOP fluctuation affects
progression (Nouri-Mahdavi et al., 2004; Bengtsson et al., 2007; Asrani et al.,
2000). Twenty-four-hour IOP monitoring can reveal higher peaks and wider fluctuation
of IOP values than those detected during typical office or clinic hours (Barkana
et al., 2006).
Fig. 1. Twenty-four-hour IOP variability in glaucoma patients and healthy
controls. (Liu et al. Invest Ophthalmol Vis Sci 2003;44:1588-1590. Copyright
© Association for Research in Vision and Ophthalmology.)
Measurement precision using tonometry is affected by observer-related
factors, including intra- and inter-observer variability, and ocular and patient-related
factors. Precision can be reported in terms of coefficient of repeatability (intra-observation
variation), and the mean difference and 95% limits of agreement for inter-observer
and interinstrument differences. Data suggest that Goldmann tonometry has lower
measurement variability (Tonnu et al., 2005; Kotecha, 2005). Corneal astigmatism,
tear fi lm, posture, eyelid squeezing, valsalva, contact lens wear and corneal refractive
surgery can all impact on the precision of IOP measurements.
Accuracy of IOP measurements
refers to how close tonometry measurements are to true IOP. Central corneal thickness
(CCT) confounds tonometry accuracy, affecting all currently available forms of tonometry
in different ways. On average, greater CCT results in overestimation of IOP measured
by applanation tonometry. In clinical practice, CCT is measured in most patients
with glaucoma, although the contribution of CCT to IOP error in the individual patient
is unknown. Correction nomograms that adjust Goldmann IOP measurement based solely
on CCT are neither valid nor useful in clinical practice (AIGS Consensus Statement
4, 2007). More data are needed to establish if reduced CCT has an independent biological
role in the causation and progression of glaucoma. Several biomechanical properties
of the cornea are relevant to IOP. For example, corneal elasticity is predicted
to have a greater effect on IOP than CCT and adds another layer of complexity to
IOP measurement (Liu et al., 2005). Viscoelasticity affects IOP measurement accuracy:
different tonometry techniques load the cornea at different rates, no adequate data
exist to quantify corneal viscoelasticity, and there is no technique to measure
it in vivo (Fig. 2) (Kotecha et al., 2006).
Fig. 2. Biomechanical properties of the cornea. (Reproduced with permission
from T. Aung.)
Future prospects for improving the precision
and accuracy of IOP measurements include the development of improved tonometers,
such as the Dynamic Contour Tonometer and Ocular Response Analyzer
(Fig. 3), as well as prospects for continuous IOP monitoring
(Detry-Morel, 2007). Advances in microsensors, nanosensors and
biocompatible materials may facilitate continuous IOP monitoring.
More distant advances using even smaller devices may allow IOP
at the optic nerve head to be monitored.
Fig. 3. Advances in tonometry technology. (Kotecha et al. Invest Ophthalmol
Vis Sci 2006;47:5337-5347. Copyright © Association for Research in Vision
and Ophthalmology.)
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