One of the most mysterious aspects of glaucoma management is the (in)accuracy of intraocular pressure measurement. The intraocular pressure is the treatment target of our therapy, and yet we have many difficulties estimating the pressure inside of the eye. We do not really measure the eye pressure, unless we are cannulating the eye and performing manometry. All of our other pressure assessments are estimates, and may be influenced by the equipment we use and the physical characteristics and behaviors of the cornea itself. Two fantastic articles recently published in JAMA Ophthalmology delve into this topic. The first article, by Wachtl and colleagues, reviewed 112 glaucoma patient records and sought to understand what influences the discordance between pressure measurements obtained using Pascal dynamic contour tonometry (DCT) measurements and pressure measurements obtained using Goldmann applanation tonometry (GAT). The authors considered a number of variables including patient age, central corneal thickness (CCT) and glaucoma disease severity. Interestingly, the authors found that differences between pressure measurements with DCT and GAT were greater in eyes with thinner corneas and in eyes with advanced glaucoma. What is even more interesting is that the application of a number of formulas to adjust GAT measurements for CCT led to greater discordance, on average. The conclusion from this article would seem to be that intraocular pressure estimation is fraught with some inaccuracy, but that correcting the measurement for CCT may lead to greater problems.
In a commentary on this topic, Dr James Brandt, the man responsible for bringing central corneal thickness into the Ocular Hypertension Treatment Study, explained the dilemma. He explains that corneal thickness can account for the greater incidence of glaucoma in African Americans, but that his work in the Ocular Hypertension Treatment Study demonstrated that a simple nomogram for adjusting GAT values based on CCT is not possible. He urges us to stop adjusting intraocular pressure measurements for CCT. Instead he argues for clinical vigilance, as much work remains to be done on this topic.
My own fascination with the cornea and glaucoma has led to an interest in the topic of corneal hysteresis, best described as a corneal behavior that represents the corneal viscoelasticity. Corneal thickness obviously tells you a little bit about the biomechanics of the cornea, just as a ruler can tell you a little bit about the strength of the supporting beams of a house. But biomechanics will tell you that a titanium supporting beam that is 4 inches wide will be stronger than a similarly sized supporting beam made up of balsa wood. A low corneal hysteresis has been found to be predictive of glaucoma progression and, interestingly, hysteresis influences the accuracy of GAT measurements as well. A low hysteresis is correlated with how much pressure reduction an eye will achieve from pressure-lowering drops,cataract surgery, and laser trabeculoplasty. This is where things get complicated. Having a low hysteresis is associated with glaucoma worsening, but it’s also associated with having a greater pressure reduction from laser or eyedrops. So in some cases, the treating physician will start a high-risk (low hysteresis) glaucoma patient on therapy, observe a great pressure reduction, and, perhaps, assume that the risk of glaucoma has been abated when in reality it remains high. Dr Brandt’s comment on clinical vigilance again rings true. The cornea, and intraocular pressure assessment are truly mysterious, and we need to remain vigilant and to assess risk and progression on a frequent basis to stay ahead of glaucoma.