The digital aqueous humor outflow meter: an alternative tool for screening of the human eye outflow facility

Abstract

Purpose: To develop, characterize, and validate a prototype digital aqueous humor outflow tonographer (DAHOM).

Material and methods: The DAHOM was developed, characterized, and validated in three phases. Phase 1 involved construction of the sensor. This was broadly based on the fundamental design of a typical Schiotz tonographer with a series of improvements, including corneal indentation, which was converted to an electrical signal via a linear variable differential transducer, an analog signal which was converted to digital via ADC circuitry, and digital data acquisition and processing which was made possible by a serial port interface. Phase 2 comprised development of software for automated assessment of the outflow facility. Automated outflow facility assessment incorporated a series of fundamental improvements in comparison with traditional techniques, including software-based filtering of ripple noise and extreme variations, rigidity impact analysis, and evaluation of the impact of patient age, central corneal thickness, and ocular axial length. Phase 3 comprised characterization and validation of DAHOM, for which we developed an experimental setup using porcine cadaver eyes. DAHOM's repeatability was evaluated by means of Cronbach's alpha and intraclass correlation coefficient. The level of agreement with a standard Schiotz tonographer was evaluated by means of paired t-tests and Bland-Altman analysis in human eyes.

Results: The experimental setup provided the necessary data for the characterization of DAHOM. A fourth order polynomial equation provided excellent fit (R square >0.999). DAHOM demonstrated high repeatability (Cronbach's alpha ≥0.997; intraclass correlation coefficient ≥0.987) and an adequate level of agreement with a standard Schiotz tonographer.

Conclusions: This study presents the development, characterization, and validation of a prototype digital tonographer. DAHOM demonstrates high repeatability and a sufficient level of agreement with a typical Schiotz tonographer, while its digital design remedies known vulnerabilities of conventional tonographers.

Keywords: aqueous humor; glaucoma; outflow facility; pressure; tonography.

Figure 1

Digital outflow meter, including linear variable differential transducer and signal conditioner circuit. The digital outflow transducer with the contact tip (indentation piston), the base (center), and the linear variable differential transducer sensor (right). Connector cables are used for power supply and data transfer to the DAQ board. The signal conditioning board is responsible for sinusoidal wave generation, signal rectification, and phase-sensitive demodulation processing.

Figure 2

Data acquisition board with analog to digital converter and serial communication with a PC. Electronic circuitry with data acquisition, analog to digital converter, and RS 232 communication interface.

Figure 3

Data filtering and tonographic polynomial fit. Data as recorded in real time by the data acquisition system (left graph) and data post-study analysis with digital filtering (red line) and polynomial fit (green line, right graph).

Figure 4

Outflow facility windows software based on Java language. Outflow facility calculation software, designed in Java programming language with Windows^® interface environment. Mandatory “inputs” are designed to measure the outflow facility, while “optional” inputs are designed to measure the ocular rigidity factor. “Corrected” outputs represent the outflow facility and rigidity measurements, corrected by the “optional” inputs. Correlation is a predictive algorithm of the severity of the condition from 0 to 3 for normal to acute angle closure, respectively.

Figure 5

Laboratory setup for the characterization of the digital outflow meter. Laboratory setup for the calibration of the system. The balanced solution (BSS) container is used for intraocular pressure regulation, the micropressure sensor is used for intracameral real-time intraocular pressure measurements, while the digital outflow meter performs tonography. All data are collected by the PC and analyzed.

Figure 6

Experimental setup. Experimental setup in porcine cadaver eyes using a microelectromechanic intracameral pressure sensor.

Figure 7

Indentation versus intraocular pressure intracameral readings from the microelectronic silicon-based pressure sensor. Objective intraocular pressure versus indentation measurements in an experimental setup using extracted porcine cadaver eyes. Intraocular pressure measurements attained by a micropressure sensor, connected to the anterior chamber of the eye, while the indentation measurements attained by the digital outflow meter.

Figure 8

Bland-Altman plot showing interdevice difference plotted against mean measurements for each eye. Bland-Altman plot showing interdevice difference plotted against mean measurements for each eye. Dotted line, zero line. Blue solid line, mean difference and boundaries of the 95% limits of agreement.