Page 17 - CJO_F17_GLAUCOMA_SUPPLEMENT

MANAGING OPEN ANGLE GLAUCOMA

CCT can be measured using ultrasound or optical coherence tomography. Ultrasound pachymeters are easy to use,
portable and cost-effective instruments. Their accuracy is dependent on the probe being placed perpendicular to
the corneal surface. In the seminal studies that showed the importance of CCT in glaucoma, measurements were
taken with ultrasound pachymetry. Studies have shown that the measurements obtained through anterior seg-
119
ment OCT are generally in good agreement with those obtained through ultrasound, although OCT might under-
estimate CCT. 120,121

Clinical Recommendations for measurement of central corneal thickness:
• Pachymetry should be measured on each eye with the mean of three measurements recorded.
95
• CCT should be reassessed intermittently, as it may change over time and with the use of some
topical medications. 122

• When possible, CCT should be assessed using ultrasound pachymetry to be consistent with large
clinical trials.
96

CORNEAL BIOMECHANICS
Corneal tissue has both viscous and elastic properties to help absorb and dissipate applied energy. This
results in corneal biomechanical variables that not only impact the measurement of IOP but may also be
independent risk factors for glaucoma. The reason for this is still unknown but may be linked to the weaken-
ing and thinning of the lamina cribrosa that occurs as glaucoma progresses. Corneal hysteresis (CH) has
123
the most evidence supporting its role as a strong and independent risk factor for glaucoma. CH reflects the
viscous damping in the cornea as a measure of its ability to resist and repulse after absorbing an externally
applied force. It is calculated as the difference in non-contact air jet pressure producing two corneal appla-
nations, one inward and one outward. Studies have demonstrated lower CH in individuals with glaucoma as
compared individuals with ocular hypertension or without disease. Low CH has also been linked with risk
124
of progression and greater visual field loss in glaucoma. 125,126
There is currently only one instrument that measures CH, the Ocular Response Analyser (ORA). The ORA
is a non-contact tonometer that produces two corneal biomechanical values and two IOP measurements.
The two corneal biomechanical values are corneal hysteresis and corneal resistance factor (CRF). CRF is
calculated from CH through a linear combination of both inward and outward pressure and is considered
to be a measurement of corneal resistance independent of IOP. The first IOP measurement is an estimate
of Goldmann IOP and the second is an estimate of the IOP corrected for the two biomechanical properties.

Clinical Relevance: Despite the association between corneal hysteresis and glaucoma onset and progression,
there is still a paucity of clinical evidence to support adding CH measurement to the standard glaucoma work
up. In addition, neither IOP measurement on the ORA will replace GAT as gold standard. This means that
should the clinician decide to use the ORA in practice, they should do this in combination with obtaining IOP
by GAT. The ORA, however, may add valuable clinical insight into management. For example, when managing
a patient with high IOPs and seemingly normal optic nerve and fields, a clinician may feel more confident in
deferring treatment if the patient also has thick CCT and high CH. Conversely, in a patient with glaucoma that
appears to be progressing despite low IOP measurement, a more aggressive target pressure may be warranted,
especially in the presence of thin CCT and low CH.

Clinical Recommendation for Corneal Biomechanics:
• At the moment, obtaining corneal hysteresis and other corneal biomechanical measurements is
not standard of care in the glaucoma examination, but this topic should be followed closely as our
understanding of their clinical relevance evolves.

CANADIAN JOURNAL of OPTOMETRY | REVUE CANADIENNE D’OPTOMÉTRIE VOL. 79 SUPPLEMENT 1, 2017 17