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C CLINICAL RESEARCH

Clinical Recommendations for IOP measurement:
• Applanation tonometry remains the gold standard to measure IOP in individuals with or at risk of
developing glaucoma.
• Since a 24-hour diurnal IOP curve is not practical in most clinical settings, the best compromise is to get
4 to 6 IOP readings at different times of the day over several visits. At least 2 of these readings should be
done as early in the morning as feasible in order to attempt to capture IOP as close to the presumed high
point as possible.
• A modified diurnal may be practical for some clinics: measurements of IOP are taken every two hours
during office hours beginning as early as is feasible in the morning.

24-HOUR IOP MONITORING
It is well known that IOP fluctuates throughout the day, typically being higher in the early morning before
decreasing gradually throughout the day to its low point in the early evening. 104,105 One of the main limita-
tions of the current gold standard GAT is the inability to obtain a measurement throughout this diurnal pe-
riod. Glaucomatous eyes show a slightly different pattern of circadian IOP fluctuations: higher fluctuations
in 24-hour monitoring and a greater nocturnal IOP rise when compared to those without glaucoma. 106,107,108
Ideally, a 24-hour diurnal IOP measurement would be obtained for everyone at risk of glaucoma; however,
at this time, this is impractical in most clinical situations. 24-hour continuous monitoring is likely more
efficacious than a modified in-office diurnal assessment with GAT. Devices such as the Triggerfish (Sen-
simed) that use contact lens sensors to obtain 24-hour continuous measurements have been shown to have
good tolerability, safety and reproducibility in those with and without glaucoma. 109-111 This device does not
measure IOP directly, and its output cannot be calibrated into mmHg. Nevertheless, studies have shown
that the measurements with this device correlate strongly with tonometry. 110,112,113 The clinical availability of
ambulatory devices (perhaps including the i-Care tonometer for home use) will address a large unmet need
in managing glaucoma and will provide better understanding of the impact of diurnal IOP fluctuations. In
the meantime, it is recommended that multiple IOP measurements are obtained at various times of the day
to characterize what the IOP profile might look like pre-treatment.

Contemporary Medical Management Considerations for 24-hour IOP monitoring:
The prostaglandin analogues have been shown to reduce IOP over the 24-hour cycle, representing another
benefit of this class of medications. Fixed-combination (FC) medications have also consistently demon-
strated IOP lowering over the 24-hour period. The CAI medications have shown better IOP control through
the overnight hours than brimonidine, both alone and in FC with timolol. Indeed, these are important
medical considerations when selecting appropriate therapies for glaucoma management.

PACHYMETRY
Pachymetry is a measurement of central corneal thickness (CCT). The effects of CCT on GAT under- or over-esti-
mating IOP are well known. The GAT assumes a corneal thickness of around 520μm, which was felt to be the aver-
age value when the tonometer was developed. It is now known, however, that CCT varies dramatically across the
114
population. Several attempts have been made to develop a correction factor to adjust IOP measurements based on
CCT. 115-117 These nomograms are no longer considered valid or useful in the consideration of an individual patient’s
management since the relationship between CCT and IOP is likely too complex to characterize with a simple calcu-
lation. 102,118 Rather than adjusting or correcting IOP for CCT, both IOP and CCT should be recorded in the record as
the absolute value measured. Elevated IOP and thin CCT are considered significant risk factors for the development
of POAG.

16 CANADIAN JOURNAL of OPTOMETRY | REVUE CANADIENNE D’OPTOMÉTRIE VOL. 79 SUPPLEMENT 1, 2017