The authors of this study report their initial results using the Triggerfish wireless ocular telemetry sensor (OTS) (Sensimed AG, Lausanne, Switzerland) for continuous IOP monitoring in patients with open-angle glaucoma. The results of the study, the first to monitor IOP fluctuations over a 24-hour period without awakening subjects during the sleep period, demonstrate the feasibility of 24-hour IOP monitoring and good safety and tolerability of the device. The results also confirm previous studies showing nocturnal IOP elevations, which have been thought to be due to the increase in episcleral venous pressure associated with supine positioning.
This was a prospective, observational cohort of 15 patients. The OTS is a disposable silicone contact lens with an embedded micro-electromechanical system, which measures changes in corneal curvature induced by variations in IOP. An antenna, mounted around the eye, receives the data, which are then transmitted to a recorder. Measurements are taken every 600 seconds for a duration of 60 seconds, providing a total of 144 measurements over a 24-hour period.
Thirteen patients (87 percent) completed 24-hour IOP monitoring. One patient discontinued monitoring due to device intolerance. Incomplete recordings were obtained in a second patient due to technical malfunctioning of the device. In nine of the 13 patients (69 percent), the highest signals were recorded during the nocturnal period. No serious adverse events were recorded. Minor complications were corneal erosion (1X1 mm) in a patient with severe dry eye syndrome and four cases of superficial punctate keratitis. Patients rated their comfort during monitoring seven out of 10, with higher numbers signifying greater comfort. After 24-hour monitoring, therapy was changed in 11 patients (73 percent).
The authors conclude that this device has the potential to improve the clinical care of glaucoma patients in the same way that continuous blood pressure monitoring or home measurements of blood glucose levels have done for patients with high blood pressure or diabetes. However, they say that important questions still need to be answered, such as the effect of nighttime changes in corneal thickness and ocular movements on the precision of the device.
While this study greatly enhances our understanding of the diurnal variation in IOP, it is unclear how these results should affect clinical management. The researchers did not monitor body position during sleep periods. Some of the IOP elevations may be due to the subject lying on the side with the contact lens device so that the monitored eye was in a lower position than the heart.
Other factors that may be important in the pathogenesis of glaucomatous optic neuropathy, such as ocular perfusion pressure and cerebrospinal fluid pressure, may also exhibit nocturnal fluctuations that may compensate for or exacerbate IOP elevations. Blood pressure and cerebrospinal fluid pressure increase within the head when a person lies down due to the influence of gravity, just as episcleral venous pressure increases. However, in some individuals, blood pressure may dip during the early morning hours and thereby reduce ocular perfusion. Future studies that simultaneously monitor IOP and blood pressure may yield important information.