A.J. Sit
Period office measurement of IOP pro vides an incomplete profile of the
complex IOP pattern. Approaches to significantly increasing the frequency
of IOP measurements may include three different strategies: self-tonometry,
temporary continuous measurement and permanent continuous measurement.
Self-tonometry is the easiest solution technically, but is difficult for
many patients and does not capture the nocturnal period. Temporary continuous
IOP monitoring would involve automated, non-invasive, ambulatory IOP measurements
over 24 to 48 hours. It would likely be contact lens-based, minimizing the
amount of patient effort required. Indications may include diagnosis and
assessment of therapy in ocular hypertension or mildto- moderate glaucoma.
Earlier studies have investigated the feasibility of using custom molded
silicone contact lenses to measure corneal deformation (Greene and Gilman,
1974) as well as passive radio telemetric devices for continual, non-invasive
monitoring of IOP (Cooper and Beale, 1977). More recently, Leonardi et al.
(2004) developed a soft contact lens with an embedded micro-fabricated strain
gauge allowing measurement of changes in corneal curvature associated with
variations in IOP (Fig. 4). This device has been able to wirelessly detect
artificially induced changes in IOP in humans over a ten minute interval
(Pitchon et al., 2008). However, no device suitable for clinical use has
been developed. Issues to be addressed include measurement noise due to
eye and contact lens movement, as well as contact lens discomfort or intolerance
with extended wear.
Fig. 4. A sensing contact lens designed for non-invasive IOP monitoring.
(Leonardi et al. Invest Ophthalmol Vis Sci 2004;45:3113-3117. Copyright
© Association for Research in Vision and Ophthalmology.)
In contrast to temporary monitoring, permanent continuous IOP monitoring
would utilize an implantable device powered by radio frequency or other
wireless power. Potential indications may include monitoring of IOP in advanced
glaucoma cases, and an IOP ‘alarm’ that would provide an alert if the pressure
increased above target. Implantable devices measuring pressure within
the intraocular environment would likely give the most reliable IOP data,
because they would bypass the problems associated with external measurements.
Several devices have been proposed for continuous monitoring of IOP, including
a scleral buckle with embedded strain gauge to measure scleral deformation,
intraocular lenses (IOL) with pressure sensors and on-board telemetry, and
pressure sensors for measurement of IOP at the choroid surface (Wolbarsht
et al., 1980; Svedbergh et al., 1992; Flower et al., 1982; Rizq et al.,
2001). Problems that need to be addressed with implantable microsensors
include the question of where to place the device (e.g., the anterior chamber
or the vitreous), safety (none have been tested in humans), long-term stability
and how to use the acquired data. Despite feasibility studies and long-term
permanent IOP monitoring data in animals having been completed (McLaren
et al., 1996), no continuous IOP monitor is available for clinical use and
concerns persist about surgical implantation of a permanent microelectronics
system in the eye.
Temporary continuous pressure monitoring may have greater applicability
in clinical practice due to its non-invasive nature. However, temporary
and permanent monitoring will likely be complementary and serve different
needs. Fundamental questions about IOP variability remain to be
answered, including: the clinical significance of the nocturnal IOP increase
and random fluctuations; the duration necessary for an IOP increase to be
clinically significant; the required frequency of monitoring and whether
the circadian pattern is conserved in individual patients; and if 24-hour
pressure monitoring is available, how treatment efficacy will be defined.
It is likely that most of these questions will require the development of
continuous IOP monitoring before they can be answered.
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