Prospective Comparative Analysis of 4 Different Intraocular Pressure Measurement Techniques and Their Effects on Pressure Readings
We thank Szumny and colleagues for their interest in our paper, “Prospective Comparative Analysis of 4 Different Intraocular Pressure Measurement Techniques and Their Effects on Pressure Readings.”1 We appreciate the opportunity to respond to their questions and comments regarding the methods and results of our study.
With respect to the order of intraocular pressure (IOP) measurement techniques performed, our study followed the same sequence of techniques for each eye, namely Goldmann applanation tonometry (GAT) without fluorescein, then GAT with fluorescein paper strip, then GAT with fluorescein droplet, and finally Tono-Pen Avia (TPA). We acknowledge that the diameter of the circular area of applanation utilized by the TPA is considerably smaller than the 3.06 mm of the GAT, and the hypothesis that having TPA follow 3 consecutive GAT measurements might affect TPA’s accuracy is reasonable. However, one would think that if such a systematic effect was present and caused spurious TPA results, TPA IOP measurements should have been artificially lowered for any of the many postulated reasons well described in the literature.2 We instead found TPA measurements to be significantly higher. Szumny and colleagues’ suggestion of performing TPA measurement first would therefore likely have yielded an even more dramatic lack of measurement agreement between TPA and GAT than that found in our study. Their separate suggestion of randomizing the order of measurement techniques for each eye is reasonable, and future studies could certainly follow such a methodology.
Regarding the time interval between IOP measurements, we waited 5 minutes before each subsequent measurement in our study. The shortest possible time interval between applanations without artificially lowering subsequent measurements is a topic of some debate in the literature,3–6 however, the 5-minute interval has been shown to be an acceptable approach.3,4,6
Concerning worsening ocular irritation as subsequent measurements were performed leading to increased tear production resulting in thicker GAT mires and spuriously high readings, none of our patients complained of any pain or discomfort before, during, or after any of the applanations performed. In addition, all attempts were made to ensure as uniform a tear film lake volume as clinically possible in all patients across all measurements, with gentle blotting of excess tears performed before each measurement with uniform technique by the same member of the research team throughout.
Following GAT measurement with topical proparacaine hydrochloride anesthesia only, GAT measurement was repeated with fluorescein paper strip application and then repeated again with a proparacaine hydrochloride/fluorescein solution droplet. Szumny and colleagues suggested that perhaps excessive fluorescein concentration from these 2 applications caused a chemical reaction that led to the anesthetic becoming less water soluble and therefore less effective, leading to increased irritation with reflex tearing and thicker GAT mires with spuriously high readings. In addition to patients not complaining of discomfort and our efforts to ensure uniform tear volume as mentioned above, the fluorescein from the first application (paper strip) was allowed to wash out of the eye, indicated by a clear tear film on slit-lamp examination, before the second application (solution droplet) and subsequent GAT measurement. We feel this would have prevented excessive fluorescein concentration from occurring.
In conclusion, our study demonstrated a lack of clinically acceptable IOP measurement agreement between 4 of the most commonly utilized clinical methods of monitoring longitudinal IOP in patients with chronic ophthalmological disease processes such as glaucoma, and treatment decisions with significant morbidity are routinely based upon their measurements. We note the methodological aspects of our study raised for discussion and would again like to thank Szumny and colleagues for their observations, interest in our work, and ideas raised for future study in this important area.