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C CLINICAL RESEARCH

Diabetes Mellitus:
• There are conflicting reports in the literature around the association between glaucoma and diabetes
mellitus. The unexpected findings in the OHTS suggested that subjects with self-reported diabetes
actually had lower risk of progression to POAG, which would mean that diabetes was not selected as a
predictive factor. However, a meta-analysis published in the American Academy of Ophthalmology Journal
in 2015 concluded that diabetes, duration of diabetes and elevated fasting blood glucose levels were all
associated with a significantly increased risk of glaucoma. They also found that diabetes and elevated
fasting blood glucose were associated with a slightly higher IOP. 27
Vascular Dysregulation
• Migraine and Raynaud syndrome are two conditions that have been identified as risk factors for the
development and progression of glaucoma. It is hypothesized that these conditions might be related to impaired
autoregulation of blood flow to the optic nerve, subjecting the tissue to hypoxia and reperfusion injury. 81

Sleep Apnea
• Studies have shown an association between the presence of sleep apnea and POAG. It is not yet known
82
what the exact clinical significance of this association is and what, if any, impact treating sleep apnea has
on slowing the progression of glaucoma.

Clinical Recommendation for risk factor analysis:
• Once a patient has been identified as being at risk for glaucoma, a thorough investigation of all risk factors
should be undertaken to help identify individuals at greater risk of glaucoma and assist in developing a
targeted approach to management.

TONOMETRY
Assessment of IOP is a critical part of the glaucoma examination. Goldmann applanation tonometry (GAT) remains the
gold standard for IOP measurement and should be used for those patients in whom a glaucoma risk profile has been iden-
tified. 95,96 Hand-held applanation tonometers (e.g. Perkins) have been shown to be comparable to GAT and may be useful
to measure IOP in those patients who may be unable to sustain positioning in the slit-lamp biomicroscope. 97,98

Non-contact tonometry (NCT), i-Care Tonometer, and Tono-Pen are often reliable alternatives. While these show
reasonable agreement with GAT in the normal IOP range, they are less accurate and show disparity with GAT at
high IOP levels. 99,100

Two tonometers, the Pascal Dynamic Contour Tonometer (DCT; Swiss Microtechnology AG, Port, Switzerland)
®
®
and the Ocular Response Analyzer (ORA; Reichert Corporation; New York, USA), have been developed in an at-
tempt to overcome the impact of corneal biomechanics. The DCT is a modified type of applanation tonometer. The
measurement principle is based on contour matching, which assumes that if the eye were enclosed by a contoured,
tight-fitting shell, the forces generated by IOP would act on the shell wall. Replacing part of the shell wall with a
curved pressure sensor would enable measurement of these forces and therefore the IOP. 100-102 The ORA is a NCT
that measures dynamic aspects of corneal deformation using an air pulse to cause two (inward and outward) corne-
al applanations. There are four measurements obtained by the ORA: 1) an estimate of Goldmann IOP, 2) an estimate
of IOP after correction for corneal biomechanical properties, 3) corneal hysteresis, and 4) corneal resistance factor.
While the measurements obtained on both of these instruments may be an addition to your glaucoma tool kit, they
do not replace the IOP measurement obtained using GAT. 100,101

Factors that influence tonometry measurements include: 103
• Central corneal thickness greater or lesser than average
• Corneal hysteresis
• Squeezing eyelids, holding breath, obesity or straining to reach slit-lamp
• Corneal scarring or corneal irregularity
• Elevating eye >15 degrees
• Excessive or inadequate amount of fluorescein
• Inaccurate calibration
• Repeated tonometry
• Observer bias

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