Sensor yields 24-hour IOP record

Continuous monitoring of IOP has long been a goal of ophthalmologists seeking more effective methods of managing glaucoma. A recent study of one such device for this task, a contact lens-based ocular telemetry sensor (Sensimed Triggerfish Sensor, Sensimed AG, Lausanne, Switzerland), demonstrated that it was safe and tolerable and showed fair reproducibility of IOP patterns.

Dr Kaweh Mansouri, MPH, highlighted results at the annual meeting of the Association for Research in Vision and Ophthalmology. The sensor, which has been approved by European regulatory authorities but is not yet approved for sale in the US, recently underwent testing at the Hamilton Glaucoma Center, Department of Ophthalmology, University of California, San Diego, California, USA, for safety, tolerability and pattern reproducibility.

Analysis of the data from the sensors showed that 38 of the 40 subjects had some sort of adverse event, although most were mild events such as blurred vision (82.5%), conjunctival hyperaemia (80%), or superficial punctate keratitis (15%). All symptoms disappeared within 24 hours of lens removal.

On a visual analogue scale of 1 to 100, the average patient self-report was 27.2 for the first session with the sensor and 23 for the second (p = 0.54). Complete data from both sessions were available for 34 patients, Dr Mansouri noted.

Pattern reproducibility

One of the key questions surrounding 24-hour IOP monitoring is the reproducibility of the patterns. IOP measurements are highly dynamic and may be affected by physical, psychological, and environmental factors. Studies using other techniques, such as Goldmann tonometry, have had varying results.

In Dr Mansouri's study, the overall correlation between the two monitoring sessions was 0.59; it was 0.63 for patients using glaucoma medication and 0.51 for those who were not. These r values indicate a moderate degree of short-term reproducibility.

"The patterns for the two monitoring sessions were quite similar," Dr Mansouri said. He added that almost 70% of the individuals in the study had a clear and repeatable nocturnal pattern, while 30% had a flat or no discernible pattern.

Data were also analysed by examining the patterns from particular time periods. During the period shortly after patients went to bed (1 hour before to 2 hours into sleep) the r value was 0.71; in contrast, it was 0.49 for the period of 1 hour before going to bed to the end of the sleep period.

"The initial sleep period is usually more monotonous [because] patients tend to wake up later to go to the restroom, and so on, so this period is presumed to be less affected by different positional artefacts," Dr Mansouri explained.