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Tonometry and Intraocular Pressure – Where are we Now?
Professor and Chairman of Ophthalmology, Department of Ophthalmology, Hospital Clínico San Carlos; Universidad Complutense de Madrid, Instituto de
Investigación Sanitaria del Hospital Clínico San Carlos (IdISSC), Madrid, Spain; Cooperative Research Network on Age-Related Ocular Pathology, Visual and Life
Quality, Instituto de Salud Carlos III, Madrid, Spain
Abstract Goldmann applanation tonometry has been the gold standard for measuring intraocular pressure for many years. However, it has
some limitations, including the effects of several ocular variables such as axial length, curvature, rigidity and corneal thickness on the
measurements. These limitations have prompted the development of new tonometers.
Keywords Glaucoma, intraocular pressure, tonometry, rebound tonometer, dynamic contour tonometer, ocular response analyzer
Disclosure: Julian García-Feijoo has received Clinical Trials/Research Support to his institution from SENSIMED and Tiolat. No funding was received in the publication of this
article. This article is a short opinion piece and has not been submitted to external peer reviewers but was reviewed by the advisory board before publication.
Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any noncommercial use, distribution, adaptation,
and reproduction provided the original author(s) and source are given appropriate credit.
Received: 14 July 2016 Pulished Online: 4 August 2016 Citation: European Ophthalmic Review, 2016;10(1):22–4
Correspondence: Julian García-Feijoo, Paseo de San Francisco de Sales 23, Casa 2,10B, 28003 Madrid, Spain. E: firstname.lastname@example.org
Glaucoma is an optic neuropathy, and although considered a
multifactorial disease, it is the level of intraocular pressure (IOP)
that is the main known risk factor for development and progression.
There are other risk factors that can modify the effect of IOP on
the development of glaucoma, but currently the main objective of
glaucoma management is to preserve visual function by achieving
a stable and continuous IOP decrease. Therefore, even if there are
other factors to consider in glaucoma besides IOP, its assessment
plays a fundamental role in the evaluation of the risk of conversion
Goldmann applanation tonometry (GAT) is the gold standard for measuring
IOP and most probably will continue to be so into the future. However,
its limitations were obvious from the start and include the substantial
effects of several eye variables such as axial length, curvature, rigidity
and corneal thickness. Also, in some eyes, especially those with corneal
problems (such as diseases, trauma, haze, corneal opacities, leucomas,
etc.) and surgery, taking measurements is not easy.
These limitations have prompted the development of new tonometers
to improve the measurement of IOP. However, other pending questions
exist, such as, which aspects of the IOP should be considered, and how
do they interact with other factors? It has been clearly established that
single snapshot IOP measurements do not provide enough information
on the dynamic behaviour of the IOP. In fact, glaucoma is a 24-hour
disease and the IOP exerts its effect on the ocular structures (including
the optic nerve head, trabecular meshwork, etc.) all day long, on a
daily basis. So, not only do we need to consider IOP variables such as
fluctuation, peak pressure, IOP range, but also how these variables may
impact eyes with different biomechanical and structural properties
and also its complex interaction with other pressures (intracranial,
translaminar, blood pressure, etc.).
22 Therefore, we need to start looking at IOP in a different way. IOP is
a dynamic variable that fluctuates throughout a 24-hour period, yet
patient follow-up is generally based on isolated IOP readings. It is true
that we can adequately assess and manage the majority of glaucoma
patients with single IOP measurements using GAT. However, one
could question, is this really good enough, or could we do any
better? The main challenges include: obtaining IOP measurements
not influenced by the corneal properties; to develop technologies
to measure the circadian IOP in clinical practice; and to understand
better the complex interactions between IOP, other pressures and
ocular biomechanics so we can effectively use the most relevant
parameters to characterise the disease and improve patient care.
Rebound tonometry (Icare ® and Icare ® PRO; iCare, Oy, Helsinki, Finland),
is based on the induction rebound principle, but measurements
obtained with it are affected by corneal thickness. 1,2 However, a
clear advantage of the Iare PRO over the other four iCare models is
that measurements may be taken with the patient lying or sitting/
standing. Also the small probe size of these tonometers facilitates
its use in children and in eyes with corneal abnormalities and make
it easier to measure IOP after corneal grafts, corneal rings, etc., as
we can select the best area to measure. Therefore, although it is not
a breakthrough, it addresses some practical problems of applanation
tonometry. However, we also have to consider the increased cost of
having to use disposable probes.
Dynamic contour tonometer (DCT), or Pascal tonometer (SMT Swiss
Microtechnology AG, Port, Switzerland) has a 7 mm diameter concave-
surface probe that adapts to the cornea’s contour and does not alter
its shape or curvature. An electronic pressure sensor embedded in the
tonometer’s concave probe surface enables the direct measurement
of transcorneal pressure. Such measurements are, at least a priori,
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