Intraocular pressure measurements using vibration based non-contact tonometry

Glaucoma is a group of diseases of the optic nerve that can lead to blindness and is generally characterized by an increase of intraocular pressure (IOP). Early detection of glaucoma is important because, in many cases, the disease can be successfully treated in its early stages. Monitoring of the IOP is the most common way to detect glaucoma. In this study, the correlation between intraocular pressure (IOP) and the vibration characteristics of the eye is investigated through a series of finite element and experimental modal analyses. Cadaver pig eyes were used in this investigation, due to their similarity in size and structural properties to human eyes. Finite element analysis was performed using a three dimensional shell model to examine the correlation between IOP and vibration characteristics of eyes. Experimental modal analysis was then carried out using a hammer and an accelerometer to acquire the vibration parameters by performing the spectral analysis. The IOP of eye was controlled and gradually changed using a manometer system. The comparison between the finite element model and the experimental results shows a good correlation: an increase in IOP also increased the natural frequencies. Therefore, vibration characteristics can be used to estimate a change in IOP. The correlations between different physical parameters of the eye and IOP were also investigated through a series of regression analyses.;Based on the premise that IOP is correlated to natural frequency, a non-contact vibration based tonometer is designed and tested. A design analysis identified three critical systems in a vibration based tonometer: the excitation of the eye, the measurement of the input signal, and the measurement of the output signal. This approach is applied to develop a non-contact EMA technique for tonometry purposes. Proof of concept of this non-contact EMA technique was explored by exciting a cantilever beam using a collimated air impulse controlled by a solenoid valve. The reflected airwave from the beam surface was measured by a microphone array. The experimental tests demonstrate the effectiveness of the proposed methodology to both accurately and cost-effectively measure structural dynamics in translational and rotational degrees. This method is also assessed on cadaver pig eyes at different IOPs and showed a good correlation between the dynamics and IOPs. This study provides a basis for the development of a vibration based portable tonometer for which patients can self-monitor their IOPs to prevent vision loss.