Is Goldmann Tonometry Passé?
A Rodrigues Figueiredo, MD
We can begin our argumentation by quoting two unarguably renowned and consensually accepted sources:
In their 4th and most recent edition (June 2014), the European Glaucoma Society Guidelines state that the most frequently used instrument and the current gold standard is the slit-lamp mounted Goldmann applanation tonometer (GAT)1.
The 2010 Preferred Practice Patterns Guidelines from the American Academy of Ophthalmology state that: Intraocular pressure (IOP) is measured in each eye, preferably by Goldmann applanation tonometry2...
Of course we all know the inaccuracies of the GAT, which were clearly evidenced by the OHTS in 2001, and the clinical importance which knowledge of central corneal thickness has gained since then3: thin corneas show lower GAT values and thicker corneas overestimate the real IOP. However, corneal curvature and biomechanics (elasticity) also change GAT IOP measures significantly. Therefore, two important IOP measurement technologies with innovative characteristics have emerged:
Dynamic contour tonometry (DCT): in theory, it is independent of corneal thickness; moreover, it is quite attractive in that it allows the measurement of ocular pulse amplitude (OPA), the first parameter related to ocular perfusion that can be routinely measured;
Ocular response analyzer (ORA): the first device to provide data on corneal biomechanics (hysteresis).
The truth is we do not use these devices in routine clinical practice. Why not?
- Hysteresis and OPA have not become clinically relevant
- They are lengthier examinations and require more collaboration from patients.
Whichever the reasons, the fact is that such methods (as well as other more recent ones) have failed to fill the gap left by a reduced trust in GAT. In practical terms, this has resulted in the widespread use of non-contact tonometry (NCT) or puff tonometers.
In my opinion, this is not a step forward: NCT is more practical because it is easier to use, but less precise (higher intratest and intertest variability)4. Some studies suggest that corneal thickness could influence puff tonometry more than GAT, especially with higher central thickness values5.
So, if GAT is passé... what is the future? How shall we respond to the challenges posed by corneal modifiers such as Lasik, corneal rings or crosslinking?
Let's leave the trees for a moment and look at the woods.
Maybe we are excessively concerned with our IOP measurement error margin. Routinely performed only a few times a year in each patient (every 6 or 3 months?). It could even be more frequent in case of need, but always during consultation hours and never during the night, often with no attention to therapeutic peaks.
Unlike other chronic disease biomarkers (glycosylated hemoglobin for diabetes; 24-hour ambulatory study for hypertension), our IOP assessment is still too random. What we need is to obtain pressure profiles:
- Mean IOP
- Pressure peaks
- And last but not least, nocturnal behaviour.
Then, perhaps, GAT will really be passé.