Newsletter Tip of the Month |
Publishing date: February 2009
The Science behind the Tip
Variability in central corneal thickness (CCT) is a potent confounder of applanation tonometry techniques such as Goldmann applanation tonometry (GAT)(1) and non-contact or air puff tonometry (NCT)(2). GAT most accurately reflects intraocular pressure (IOP) when CCT is 520 µm(1). At this time there is no accepted algorithm for correcting IOP measurements for deviations from this value, but it is obvious that low CCT values can result in gross underestimation of IOP.
Clinical implications are not minor. If pachymetry is not adopted as part of the routine eye exam, elevated IOP can be missed in eyes with a thin cornea, and the diagnosis of glaucoma delayed. It is important to examine the optic disc even when IOP is "normal", and to test visual fields in case of a suspect disc. If glaucoma damage is documented, pachymetry can avoid misclassification of patients as having normal tension glaucoma.
Myopic individuals having had LASIK are particularly at risk of having undetected glaucoma. GAT and NCT will grossly underestimate IOP in LASIK-thinned corneas of a mere 400 µm. LASIK-altered corneal curvature and rigidity contribute to erroneously low IOP readings(3). Furthermore, detecting glaucomatous damage is often difficult in highly myopic discs. Considering that, in addition, myopia is an independent glaucoma risk factor(4), caution is warranted in post-LASIK patients.
Contributor: Ann Hoste, Antwerp
Co-editors: John Thygesen and Ann Hoste
Peer reviewers: Roger Hitchings and Anders Heijl
References
1. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol
(Copenh). 1975;53:34-43.
2. Tonnu PA, Ho T, Newson T, et al. The influence of central corneal thickness and age on intraocular pressure
measured by pneumotonometry, non-contact tonometry, ... . Br J Ophthalmol. 2005;89:851-4.
3. Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement:
quantitative analysis. J Cataract Refract Surg. 2005;31:146-55.
4. Mitchell P, Hourihan F, Sandbach J, Wang JJ. The relationship between glaucoma and myopia: the Blue
Mountains Eye Study. Ophthalmology. 1999;106:2010-5.
Tip Reviewer: Roger Hitchings
Tip Editors: Ann Hoste, John Salmon and John Thygesen
