Ocular pulsation

Contact lens sensor detects changes after canaloplasty procedure

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Photo of Roibeard O’hEineachain

Changes in variation of ocular pulsation amplitude detected with the Triggerfish® Sensor (SENSIMED) appear to correlate with the short- and long-term efficacy of canaloplasty procedures, according to Anna Byszewska MD, Military Institute of Medicine, Warsaw, Poland. The study was designed and conducted under the direction of Prof Marek Rękas, Ophthalmology Department of Military Institute of Medicine, Warsaw, Poland, and thanks to cooperation with Prof D Robert Iskander and Dr Monika Danielewska from the Wrocław University of Science and Technology, Department of Biomedical Engineering, who analysed the data. The results of this study were recently published in IOVS (Assessing Efficacy of Canaloplasty Using Continuous 24-Hour Monitoring of Ocular Dimensional Changes. Rekas M, Danielewska ME, Byszewska A, Petz K, Wierzbowska J, Wierzbowski R, Iskander DR. Invest Ophthalmol Vis Sci. 2016 May 1;57(6):2533–42. DOI: 10.1167/iovs.16-19185, PMID: 27159443). In a study involving 10 eyes of 10 patients who underwent canaloplasty, the short-term variance over 30 seconds of Triggerfish signals and long-term variance over 24 hours both diminished after three months significantly and this effect persisted for 12 months. The reductions in amplitude variation appeared to correlate with a 31% change in intraocular pressure (IOP) during the same period, said Dr Byszewska. “The Triggerfish signal is a dynamic examination which can show us novel aspects of canaloplasty efficacy,” she said. She noted that the Triggerfish device consists of a contact lens embedded with a measuring system and a chip. It measures dimensional changes in the eyeball in the clear cornea. Data is transferred to an antenna in the form of a patch around the eye, which is attached to a recorder. The data can then be easily downloaded to a computer for analysis. “The Triggerfish does not measure IOP, it measures the dimensional, volumetric changes of the cornea. It is the pulsation of the eye rather than IOP. Ocular pulsation has a circadian rhythm. It is lower during the day and higher during the night. We know that after penetrating procedures this rhythm goes flat. What we wanted to see is what occurs after non-penetrating procedures, such as canaloplasty,” Dr Byszewska explained. The patients in the study included six women and four men with a mean age of 69 years. Nine of the patients had primary open-angle glaucoma and one had pseudoexfoliative glaucoma. In addition to standard examinations, the Warsaw investigators fit patients with the Triggerfish device for 24 hours together with continuous electrocardiogram (ECG) holter to investigate the influence of heart activity on the ocular pulse rhythm. They carried out 24-hour examinations at three time points, prior to surgery and after washout of anti-glaucoma medication, and then after three and 12 months. They analysed raw Triggerfish signals to find the best sine-wave fit. Two parameters were considered: the amplitude of the fitted sine-wave and the overall root mean square (RMS). Statistical analysis included standard descriptive statistics and paired t-test. The researchers found statistically significant differences between the pre-op and three-month post-op for both the amplitude RMS and the sine-wave amplitude, as well as between the pre-op and 12-month post-op results. They also performed a spectral analysis of the signal in both the time domain and frequency domain. They found that there were two repeated peaks of 0.25Hz frequency and 1.0Hz frequency. This could be due to respiratory rate and pulse rate as seen on the Triggerfish signal. However, those peaks were only detectable at night. During the day the signals were too noisy, due to eye blinking and eye movement. Dr Byszewska noted that the mean IOP was reduced to 14.2mmHg at 12 months on a mean of 0.1 anti-glaucoma medications, compared to the mean IOP of 20.6mmHg measured preoperatively following washout. However, the relationship between the changes detected and IOP has yet to be fully elucidated. “Perhaps the reduction in IOP is dampening the ocular pulse some way. But it is not the same as IOP and it is not so simple to explain. Also the circadian rhythm of pulsation over 24 hours is lower after canaloplasty and this change persists at 12 months. Therefore, it is a long-term change and could be a criterion for progression or non-progression, but we still don’t know,” she said. “The results are very interesting. We are continuing our research under the direction of Prof Rękas, so that the analyses can be performed on a much larger population of glaucoma patients.” Anna Byszewska: 
ania.byszewska@gmail.com