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Editors Selection IGR 22-4

Basic Science: An implantable device for telemetric IOP monitoring

Comment by Arthur Sit on:

69389 Investigation of a novel implantable suprachoroidal pressure transducer for telemetric intraocular pressure monitoring, Mariacher S; Ebner M; Januschowski K et al., Experimental Eye Research, 2016; 151: 54-60

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Current clinical management of glaucoma patients typically involves measurement of intraocular pressure (IOP) every few months. However, IOP variability has been demonstrated to be a risk factor for glaucoma progression.^1-3 Characterizing IOP variability in individual patients is extremely difficult using traditional tonometers, which are designed for instantaneous ad hoc pressure measurements. Newer technologies, like the Triggerfish contact lens sensor (Sensimed AG, Lausanne, Switzerland), can characterize 24-hour IOP profiles. However, there are no clinically available devices that can potentially provide continuous IOP monitoring.

Mariacher et al. reported on a pre-clinical study to evaluate a novel implantable pressure monitoring system developed by Implandata of Hannover, Germany. A previous study in human subjects involved placement of similar devices in the ciliary sulcus.⁴ However, sulcus placement had limitations, including the need for concurrent cataract surgery and the risk of iris chafing. In the current study, a modified device was placed in the suprachoroidal space in the eyes of rabbits via a scleral incision. The pressure in the anterior chamber was adjusted using a fluid reservoir, and the readings from the implants were compared with those from an external portable pressure transducer.

It would clearly be impractical to require recalibration every few weeks

The authors reported that the readings from the telemetric pressure sensor implants closely matched the pressure set by the fluid reservoir (and measured by the external pressure transducer). While the difference between the implants and the anterior chamber pressure varied with the pressure level, Bland-Altman analysis indicated that the limits of agreement (± 95%) were within 4 mmHg at pressure levels up to 35 mmHg. This comparison was performed five times over 30 weeks post-operatively (one, four, eight, 12 and 30 weeks), with a total of six implants.

However, an important caveat to the results is that the telemetric IOP was recalibrated at each time point by calculating the mean difference between the implant and the external pressure transducer readings at a fluid column level of 20 mmHg. The correction factors seemed to vary by unpredictable amounts, with a change of more than 40 mmHg between time points in one sensor. The reasons for this variability were unclear, and there was much greater variability in some implants. One possibility noted by the authors was that fibrous tissue forming around the implant could potentially create mechanical stress in the area of the pressure sensors, leading to anomalous readings. One of the six sensors stopped working entirely after one week.

Another issue is the long-term safety of the implants. The authors noted evidence of device migration in one of the animals. As well, as noted in the paper, potential scarring may negatively impact future glaucoma filtering surgeries as the implant requires a conjunctival dissection and scleral incision.

Despite these limitations, this study represents important pioneering work in the field of wireless implantable IOP sensors. However, future work is clearly needed to determine if the correction factors stabilize enough over time to be clinically useful, since it would clearly be impractical to require recalibration every few weeks. As well, the issues of longevity and safety are significant. I look forward to future research that will hopefully address these important issues.

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