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Digital Goldmann tonometer demonstrates reproducibility
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Goldmann applanation tonometry is considered the gold standard for assessing IOP measurements in a typical clinical practice, but it is limited by certain corneal diseases, the influence of central corneal thickness and corneal curvature, and inter- and intra-observer variability.
A digital version of the Goldmann applanation tonometer has been developed in an effort to diminish variability in IOP readings. The AT900D tonometer (Haag-Streit) employs a scale that includes tenths of 1 mm Hg.
A study in the Journal of Glaucoma examined the reproducibility of the digital tonometer and compared its readings to those derived from conventional analog tonometry.
“We found that the AT900D was a reliable option for the measurement of IOP and with a reproducibility that is comparable to that of the analog version of the instrument,” co-author José M. Martínez-de-la-Casa, MD, PhD, a professor of ophthalmology at Hospital Clinico San Carlos in Madrid, Spain, said.
José M. Martínez-de-la-Casa
In 2010, Martínez-de-la-Casa began using the AT900D digital Goldmann tonometer (DGT) in his department.
“We needed a tonometer to measure IOP in patients enrolled in multiple clinical trials performed at our center,” he said.
First, the researchers carried out a study examining reproducibility, comparing DGT to conventional analog Goldmann tonometer (AGT).
“Coefficients of variation determined for each of the three sessions were used to establish the reproducibility of DGT and AGT,” Martínez-de-la-Casa said.
The mean differences recorded (AGT – DGT) were 0.1, 0.3 and –0.1 for the three sessions, which were spaced 48 hours apart.
“For all three sessions, coefficients of variation were lower for DGT than for AGT,” he said.
This difference was significant for session two (3.55% vs. 4.84%) and session three (3.50% vs. 5.04%).
The differences in coefficients of variation found in session one (3.99% vs. 4.87%) could even be considered clinically significant, Martínez-de-la-Casa said. With a large sample size, that significance would have been reached, according to the study authors.
Several factors are known to affect the validity of AGT readings, Martínez-de-la-Casa said. Besides corneal thickness and curvature, influential factors include the concentration of fluorescein, the hypotensive effect of consecutive measures, accommodation and different viewing positions.
“Obviously, DGT has not been able to improve any of these limitations, since it is based on the same physical principle, the Imbert-Fick law. However, other sources of error have been noted that could effectively be improved by the introduction of DGT,” he said.
For example, although DGT is unable to automatically calibrate, an indicator lights up when the calibration error is beyond the accepted limits (+2.5 mm Hg), he said.
The hedgehog effect — a preference by the examiner for certain digits with an analog scale — is also avoided with DGT, according to Martínez-de-la-Casa.
“[The] decimal scale provides a single reading; thus, the examiner no longer has to subjectively interpret the result,” he said.
Likewise, excessive or insufficient applanation during measurements is avoided with DGT because a red light-emitting diode indicator turns green when the correct applanation is applied. – by Bob Kronemyer
Reference:
- Morales-Fernandez L, Martínez-de-la-Casa JM, Garcia-Feijoo J, et al. Reproducibility of the new Goldmann AT900D digital tonometer. J Glaucoma. 2012;21(3):186-188.
For more information:
- Jose M. Martínez-de-la-Casa, MD, PhD, can be reached at Hospital Clinico San Carlos, Martin Lagos sn, 28240 Madrid, Spain; email: martinezcasa@ya.com.
- Disclosure: Martínez-de-la-Casa has no relevant financial disclosures.