The relationship between intraocular pressure and glaucoma: An evolving concept

Abstract

Intraocular pressure (IOP) is the most important modifiable risk factor for glaucoma and fluctuates considerably within patients over short and long time periods. Our field's understanding of IOP has evolved considerably in recent years, driven by tonometric technologies with increasing accuracy, reproducibility, and temporal resolution that have refined our knowledge regarding the relationship between IOP and glaucoma risk and pathogenesis. The goal of this article is to review the published literature pertinent to the following points: 1) the factors that determine IOP in physiologic and pathologic states; 2) technologies for measuring IOP; 3) scientific and clinical rationale for measuring diverse IOP metrics in patients with glaucoma; 4) the impact and shortcomings of current standard-of-care IOP monitoring approaches; 5) recommendations for approaches to IOP monitoring that could improve patient outcomes; and 6) research questions that must be answered to improve our understanding of how IOP contributes to disease progression. Retrospective and prospective data, including that from landmark clinical trials, document greater IOP fluctuations in glaucomatous than healthy eyes, tendencies for maximal daily IOP to occur outside of office hours, and, in addition to mean and maximal IOP, an association between IOP fluctuation and glaucoma progression that is independent of mean in-office IOP. Ambulatory IOP monitoring, measuring IOP outside of office hours and at different times of day and night, provides clinicians with discrete data that could improve patient outcomes. Eye care clinicians treating glaucoma based on isolated in-office IOP measurements may make treatment decisions without fully capturing the entire IOP profile of an individual. Data linking home blood pressure monitors and home glucose sensors to dramatically improved outcomes for patients with systemic hypertension and diabetes and will be reviewed as they pertain to the question of whether ambulatory tonometry is positioned to do the same for glaucoma management. Prospective randomized controlled studies are warranted to determine whether remote tonometry-based glaucoma management might reduce vision loss and improve patient outcomes.

Figure 1.. Blood supply to the anterior optic nerve, optic nerve head, and retina.

Inset demonstrates changes relevant to glaucoma, including excavation of the optic nerve head, posterior bowing of the lamina cribrosa, and impaired autoregulation leading to reduced blood flow.

Figure 2.. Aqueous humor dynamics.

Inset demonstrates high magnification view of the anterior chamber angle structures, as well as flow of aqueous humor (arrows) with drainage through the conventional (trabecular) and uveoscleral outflow pathways.

Figure 3.. Shiotz tonometry.

A) Tonometer in case with collection of weights, aside a conversion table. B) Shiotz tonometer applied to a patient’s eye.

Figure 4.. Goldmann applanation tonometry.

A) Patient positioned at slit lamp. B) Applanation of the cornea by tonometer. C) When the reading on the tonometer dial = IOP, the inner edges of the fluorescent arcs align. Separation indications dial (D) is underreading IOP and overlap indicates dial is overreading IOP.

Figure 5.. Perkins applanation tonometry.

A) The Perkins tonometer is a handheld applanating device. B) Use of Perkins tonometry in a supine patient.

Figure 6.. Tono-Pen.

Figure 7.. iCare rebound tonometry.

Figure 8.. iCare HOME2 self-tonometry.

Figure 9.. Pneumotonometry.

A) The pneumotonometer base is connected by tubing to the probe. B) Use of pneumotonometry on a patient. C) Results printout which demonstrates IOP over several seconds.

Figure 10.. Ocular response analyzer.

Figure 11.. Precision vs accuracy in IOP measurement.

A) Precise measurements are highly reproducible, while accurate measurements overall estimate the true mean with high fidelity. B) Tonometry that is precise but not accurate may not ascertain the true intraocular hydrostatic pressure, but will clearly demonstrate patterns of relative change. C) Tonometry that is accurate but not precise can inform relative change, but only when a sufficient number of measurements of averaged.

Figure 12.. IOP variability vs fluctuation

Figure 13.. Metrics describing IOP fluctuation.

Hypothetical IOP measurements from a patient over 4 days. Daily maximum IOP (red; note outlier measuremen in Day 1); Daily minimum IOP (yellow; note outlier measurement in Day 3). Calculation of the mean daily IOP maximum, minimum, and range protects against outliers (which may be spurious).