Continuous IOP measurement and control still evolving
When IOP is measured during office visits, or even once per day as is done in some studies, only a snapshot in the continuum of pressure changes is reflected, which can be misleading, according to one glaucoma specialist.
Continuous 24-hour pressure monitoring would be the only way to optimize glaucoma management through customization, but whether that is a realistic option remains a question, Frances Meier-Gibbons, MD, told Ocular Surgery News.
“We have implantable devices, but they are invasive, as a surgical procedure is needed. On the other hand, we have external, noninvasive devices, including contact lens sensors, but they are not reliable or at least difficult to interpret,” she said.
One study has shown that 52% to 69% of the high pressure readings are obtained outside of office hours, particularly during sleep.
“Hence, if you have measurements in the office, once every 3 or 4 months, just a couple of seconds, you miss 60% of the high eye pressure,” Meier-Gibbons said. “Another study shows that about 20% of the so-called normal tension glaucoma patients have higher pressure readings outside of office hours.”
Sleep and circadian rhythms
IOP follows a circadian rhythm, with the highest values occurring when the person is in a supine position at night and the maximum IOP lowering occurring during the day. Within this nyctohemeral cycle, fluctuations occur. If the fluctuations are irregular and large, they load and unload stress to the tissue, with consequent damage.
“There is ongoing discussion on whether fluctuations might be an independent risk factor for glaucoma progression. It is likely a matter of magnitude, but what is certain is that our way of measuring IOP is inadequate to provide information on fluctuations,” Meier-Gibbons said.
A possible way of mirroring night values is by measuring pressure in the supine position at the office. A study by Mosaed and colleagues found that the average values of supine IOP during office hours had the strongest correlation with peak nocturnal IOP and concluded that supine IOP measurements estimate peak nocturnal IOP better than sitting measurements.
“I can tell from my experience, when I measure IOP in the supine position and then take the patient to the slit lamp and measure IOP there, there is a remarkable difference of 3 mm Hg to 4 mm Hg,” Meier-Gibbons said. “With some patients I simulate the night environment in some ways by having them in the supine position in a dark room for about 1 hour and measuring IOP with the Perkins applanation tonometer. It is not exactly like night because you cannot imitate all the sympathomimetic reactions that occur during the night, but at least you can have a better guess.”
Meier-Gibbons said she takes three diurnal curve measurements during office hours, one at 8 a.m., one at noon and one at 6 p.m.
“If I suspect there is a high night pressure, I refer the patient to stay overnight at the hospital,” she said.
If diurnal curves vary by maximally 4 mm Hg to 5 mm Hg, the patient’s condition can be considered to be fairly stable, but greater variations indicate that glaucoma is likely to progress and it may be advisable to modify the therapy. Patients whose glaucoma progresses despite normal IOP values during the day should be observed and monitored closely. Once compliance issues are excluded, other potential causes should be considered, Meier-Gibbons said.
“They may have IOP peaks at night, but don’t forget to consider the low blood pressure at night. Low blood pressure coupled with higher IOP in the supine position can lead to pathological changes because the optic nerve is not getting sufficient blood replacement during the night,” Meier-Gibbons said.
Continuous measuring devices
Implantable devices for continuous IOP measurement are meant to be placed in the eye during cataract surgery.
The ARGOS study investigated the safety of the Argos-IO (Implandata Ophthalmic Products), a ring-shaped telemetric IOP sensor implanted in the ciliary sulcus after IOL implantation. IOP measurements were performed with a reader unit held in front of the eye. Patients were able to successfully measure IOP at home after receiving instruction, and measurements compared well with those of Goldman applanation tonometry. However, all patients suffered some degree of pupillary distortion and pigment dispersion, and some developed inflammatory reactions.
Newer approaches use microelectromechanical systems (MEMS) sensors to be implanted in the anterior chamber or the vitreous. They are still at the early stages of development but seem promising, according to Meier-Gibbons.
“MEMS are micro, and we will be able to inject them, getting reliable measurements from inside the eye,” she said.
MEMS technology has been utilized with contact lenses but has not shown convincing results so far, she said.
“There are studies saying the Triggerfish measures IOP, but this is not quite correct. It measures changes of the ocular surface at the corneoscleral border over 24 hours, but we don’t know how to relate these changes to IOP. The measurement unit is the millivolt, which is a unit of electrical potential, not pressure,” Meier-Gibbons said.
The Triggerfish (Sensimed) was approved in the U.S. in 2016, based on the studies of Mansouri and Weinreb. However, due to insufficient evidence, its use is currently not recommended by the European Glaucoma Society, she said.
Microfluidics systems are also under investigation and are aimed at evaluating IOP by measuring fluid displacement in micro-shaped channels inserted in the eye.
“They work as a manometric device and measure IOP without using external power but using the pressure itself,” Meier-Gibbons said.
She believes that microfluidics systems and MEMS have potential as a new generation of minimally invasive implants for the measurement of IOP and perhaps the treatment of glaucoma.
Rebound tonometry is another option that is easy to use by patients at home with portable devices such as the Icare Home tonometer.
“It is an interesting technology but not so widespread because some people like it and want to be involved, but others perceive measuring their own pressure as a psychological burden. Potentially, you can ask your patients to measure their IOP also at night but be aware that some of them might call you at 2 in the morning saying their pressure has gone up. If they are too much in charge, they might be difficult to handle,” Meier-Gibbons said.
Science and technology are evolving, but ophthalmology is not yet at the point of having the whole picture of how eye pressure varies within 24 hours day after day.
“It is one of our goals for future years, but it is a challenge in which many aspects, including psychological aspects, are involved,” Meier-Gibbons said. – by Michela Cimberle
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- Mosaed S, et al. Am J Ophthalmol. 2005;doi:10.1016/j.ajo.2004.09.062.
- Pronin S, et al. JAMA Ophthalmol. 2017;doi:10.1001/jamaophthalmol.2017.3151.
- For more information:
- Frances Meier-Gibbons, MD, can be reached at Eye Center Rapperswil, Merkurstr. 50, 8640 Rapperswil/SG, Switzerland; email: email@example.com.
Disclosure: Meier-Gibbons reports she is a consultant for Alcon, Allergan, Glaukos, Heidelberg, Novartis and Santen.