The Sensimed Triggerfish monitors IOP changes over 24-hour periods. It is not approved for sale.
Dr Mariem Abdou
Nova Southeastern University College of Optometry, Florida USA
Glaucoma is an optic neuropathy characterised as damage to the optic nerve with loss of the ganglion cells and nerve fibre layer, resulting in permanent vision loss.1,2
It is the second leading cause of blindness worldwide,3 with nearly 70 million individuals suffering from glaucoma.4 Risk factors include age, ethnicity, family history and elevated intraocular pressure (IOP).
Glaucoma was historically and incorrectly defined as increased IOP, which is considered higher than 21 mmHg. We now understand that elevated IOP is not the hallmark feature of glaucoma because pressures below 21 mmHg result in glaucoma in a number of cases, and pressures above 21 mmHg do not always lead to glaucoma.4 About 33-50 per cent of patients with glaucomatous damage have a normal IOP measured at the first visit.1 However, being the only modifiable element in this disease, the only known management protocol for glaucoma is to adjust the intraocular pressures to an appropriate target level via medicinal, laser or surgical techniques in order to reduce progression of the disease.2,4 Several randomised, controlled large-scale studies have confirmed the efficacy of lowering IOP for the treatment of glaucoma.5
The Early Manifest Glaucoma Trial saw that an increase in intraocular pressures by even 1 mmHg portended an 11 per cent risk of disease progression; therefore, it is critical to obtain completely accurate measurements of IOP at each visit.4
Goldmann applanation tonometry
Unlike other medical devices, Goldmann applanation tonometry (GAT) was developed and has remained unchanged for more than a century. It measures the IOP at the flattened, central region of the cornea using the Imbert-Fick Law. We know this is not ideal and factors such as corneal thickness, scleral rigidity, and biomechanical properties including tear film and other substances can affect the IOP reading performed via GAT.2,3,4
According to the Ocular Hypertension Treatment Study, corneal thickness should be taken into consideration as an independent risk factor for progression of ocular hypertension to glaucoma. This study suggests that when the corneal thickness is high, the IOP is likely to be overestimated, whereas a thinner corneal thickness indicates an underestimation of the IOP measurement. Subjects in this study with the thinnest central corneal thickness were three times more likely to develop glaucoma than the subjects with a thicker central corneal thickness.6
In addition to these factors, intraocular pressures follow a variable circadian rhythm. IOP fluctuations in a normal individual can be up to 4-5 mmHg, whereas glaucoma patients tend to have much higher fluctuations.2,3,4 Fluctuations in IOP can be due to a number of factors including body position and time of day or night.1 Therefore, a single measurement of IOP does not provide enough data to diagnose or treat a patient because it excludes potential peaks and fluctuation of the IOP.7
Studies that measure IOP over a 24-hour period have shown that measurements taken during regular office hours are not true representations of the IOP because two-thirds of the measurements were greatest during nocturnal periods when the individual was in the supine position4 and generally peak in the early morning.7 Nighttime IOP can be measured only if a patient is hospitalised or in a sleep laboratory, but this can be costly and cause stress-related artifacts in the measurements so it is not done routinely.2
Diurnal tension curve
Intraocular pressures are frequently obtained during regular business hours to develop a diurnal tension curve (DTC). The DTC is collected via four to five IOP measurements, two hours apart, to acquire as much variability in the IOP at different times of the day as is practical. Although serial IOP measurements in-office still remain the most efficient way to identify peaks in pressures and guide treatment decisions,6 the DTC can serve only as a summary of the IOP pattern because a complete 24-hour analysis is needed to reveal higher peaks and larger variations in IOP.3
Studies have demonstrated that the diurnal pattern of the IOP tends to be repeatable in untreated glaucoma patients. According to Katavisto, 80 per cent of glaucoma patients displayed consistency in their diurnal curve, yet Wilensky found this to be true in only 28 per cent of ocular hypertensive patients and 34 per cent of open angle glaucoma patients.8
Realini found that the diurnal IOP is highly variable and not repeatable day-to-day or several weeks apart, thus limiting the information gained from in-office studies.9,10 In studies comparing the DTC over time for ocular hypertensive patients who have developed glaucoma versus ocular hypertensive patients who did not progress, the subjects who converted to glaucoma had IOP patterns similar to those of glaucomatous eyes when compared to the controls.7 Additional studies reveal that treatment may regulate the IOP pattern in glaucoma so that there is less fluctuation.8
Contact lens sensor measurements
The likely highly variable diurnal IOP curve and long-term inconsistency of the IOP variation make in-office measurement a ‘snap-shot’ only of what is happening to a particular patient on a given day. Perhaps more information can be gained by ‘continuous measurement of IOP’, similar to a Holter monitor used to gauge heart electrical activity throughout the day.
The Sensimed Triggerfish is a contact lens sensor (CLS) for 24-hour monitoring of IOP in clinical studies. CLS measurements may be of practical use for detection of sleep-induced IOP changes as well as being able to obtain a true understanding of IOP circadian variations in both short- and long-term situations.11 Unfortunately, this devise is not currently clinically available but may be a future option to give a more complete assessment of IOP.
Although there are shortcomings by measuring IOP with the GAT technique and only during regular clinic hours, this still remains the standard in managing glaucoma patients today. Due to the diurnal variability of IOP, multiple measurements should be obtained during different clinic hours to obtain an understanding of diurnal tension curve in the least cumbersome manner. With these data, an appropriate target level and response to treatment can be better evaluated in an effort to delay or prevent the progression of glaucoma.
1. Ontario HQ. Diurnal tension curves for assessing the development or progression of glaucoma: An evidence-based analysis. Ontario Health Technology Assessment Series 2011; 11: 2: 1-45.
2. Mansouri K. The road ahead to continuous 24-hour intraocular pressure monitoring in glaucoma. J Ophthal Vis Res 2014; 9: 2: 260-268.
3. De Smedt S. Noninvasive intraocular pressure monitoring: current insights. Clin Ophthalmol 2015; 9: 1385-1393.
4. Mansouri K, Weinreb RN. Continuous 24 hour intraocular pressure monitoring for glaucoma with a contact lens sensor: time for a paradigm change. Swiss Medical Weekly 2012.
5. Huchzermeyer C, Reulbach U, Horn F et al. Longitudinal stability of the diurnal rhythm of intraocular pressure in subjects with healthy eyes, ocular hypertension and pigment dispersion syndrome. BMC Ophthalmol 2014; 14: 122.
6. Pensyl D, Sullivan-Mee M, Torres-Monte M et al. Combining corneal hysteresis with central corneal thickness and intraocular pressure for glaucoma risk assessment. Eye 2012; 26: 1349-1356.
7. Grippo TM, Liu JHK, Zebardest N et al. Twenty-four-hour pattern of intraocular pressure in untreated patients with ocular hypertension. Invest Ophth Vis Sci 2013; 54: 1: 512-517.
8. Hatanaka M, Babic M, Junior RS. Twenty-four-hour repeatability of diurnal intraocular pressure patterns in glaucomatous and ocular hypertensive individuals. Clinics 2011; 66: 7: 1235-1236.
9. Realini T, Weinreb RN, Wisniewski SR. Short-term repeatability of diurnal intraocular pressure patterns in glaucomatous individuals. Ophthalmology 2011; 118: 1: 47-51.
10. Realini T, Weinreb RN, Wisniewski SR. Diurnal intraocular pressure patterns are not repeatable in the short term in healthy individuals. Ophthalmology 2010; 117: 9: 1700-1704.
11. Mansouri K, Weinreb RN, Liu JHK. Efficacy of a contact lens sensor for monitoring 24-H intraocular pressure related patterns. PLoS One 2015; 10: 5: 1-14.
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