A handheld, automated applanation tonometer has delivered accurate intraocular pressure (IOP) measurements in human donor eyes, suggesting a possible route to more portable and objective glaucoma assessment.
Goldmann applanation tonometry (GAT) remains the clinical reference standard for measuring IOP, but it depends on a slit lamp, restricts patient positioning, and requires the operator to align fluorescein mires subjectively. The technique also has a substantial learning curve, creating opportunities for measurement variability and observer bias.
Researchers from the University of Utah and Duke Eye Center, North Carolina, designed a prototype intended to retain the principles of applanation tonometry while automating its most subjective component. Rather than varying the force until a predefined area is flattened, the device applies a constant force and calculates IOP from the resulting applanation area.
The handheld unit incorporates a custom, medical-grade polymethyl methacrylate tip matching the dimensions of a standard GAT prism, but without its internal split prism. A spring, lever, pivot, and counterweight mechanism delivers a nominal force of 2.5 gram-force in both upright and supine orientations. A built-in camera records the fluorescein-stained applanation image under blue LED illumination.
Software then analyzes the recording frame by frame. It identifies the tonometer tip and applanation mire, fits an ellipse to the flattened area, removes unsuitable frames and outliers, and converts the median measured area into an IOP value.
To validate the system, the researchers tested 10 fresh human cadaveric eyes provided by the Miracles in Sight Eye Bank. Two eyes were excluded because corneal edema prevented clear mire detection, leaving eight eyes for analysis. The globes were mounted in a mannequin head and cannulated, allowing IOP to be set manometrically from 5 to 45 mm Hg in 5-mm Hg increments.
All 64 recordings from the eight included eyes were successfully processed and then converted into IOP measurements. Across the full pressure range, the device’s mean difference from the set IOP was −1.2 mm Hg, with a mean absolute difference of 3.1 mm Hg.
Performance improved in the clinically important 10–30 mm Hg range. Here, the mean difference was 0.7±1.7 mm Hg and the mean absolute difference was 1.4±1.1 mm Hg, with an R² of 0.95. As shown in the study, the prototype tended to overestimate pressures below 10 mm Hg and underestimate those above 30 mm Hg.
The study authors do acknowledge that the small, ex vivo study cannot reproduce patient movement or fully represent living corneal biomechanics. Corneal thickness and biomechanical data were also unavailable for this study, and clinical studies must now evaluate performance across diverse corneas, alongside usability, calibration stability, cost, and patient acceptance.
However, the study does offer up an interesting alternative to standard glaucoma assessment: by combining portability with automated image analysis, the prototype could eventually reduce operator-dependent error while enabling applanation measurements in both seated and supine patients.
