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‘False positive’ high IOP might be due to abnormally thick corneas, study shows
Some patients may have been needlessly treated for glaucoma.
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UDINE, Italy – Moderately high tonometric values should not necessarily be considered a sign of ocular hypertension. The readings should be recalculated and related to pachymetric and perimetric data, according to a surgeon here.
Corneal thickness has a significant influence on IOP, and if this factor is not taken into consideration, “we’ll end up treating a considerable percentage of patients for glaucoma who are not in fact affected by this disease,” said Paolo Brusini, MD, director of the Santa Maria della Misericordia Hospital Eye Clinic here.
In a study presented at a meeting of the Italian Ophthalmologic Society, Dr. Brusini demonstrated that the rate of “false positive” high IOP is as high as 41% when more extensive parameters of evaluation are applied.
Investigations
“It is well-known that Goldmann applanation tonometry provides reliable results only if corneal thickness is within the ‘normal’ range of 520 µm to 545 µm. If corneal thickness is above average, IOP is overestimated, and vice versa. This fact should always be considered when we make our diagnosis of glaucoma in patients whose only diagnostic sign is a moderate degree of ocular hypertension,” Dr. Brusini said. “They might not have high IOP, but just an unusually thick cornea.”
To evaluate this assumption, he tested 88 patients with moderately high IOP between 21 mm Hg and 25 mm Hg. Mean age was 63 years, ranging between 29 and 86. Only the right eye of each patient was considered.
All patients underwent Short Wavelength Automated Perimetry (SWAP, Zeiss-Humphrey Systems) and Frequency Doubling Technology (FDT, Zeiss-Humphrey Systems). Scanning laser polarimetry with GDx (Laser Diagnostic Technologies) was performed for nerve fiber analysis. Central corneal thickness was calculated by taking the lowest of five consecutive measurements by ultrasound pachymetry (Altair, Optikon 2000).
Putting values right
Patients were divided into three groups: those with thin corneas (less than 541 µm), medium corneas (between 541 µm and 590 µm), and thick corneas (more than 590 µm).
“We investigated the relationship between corneal thickness and functional alterations,” Dr. Brusini said. “Then we corrected IOP values according to the Doughty-Zaman formula, which suggests a variation of 0.46 mm Hg for every 10 µm of difference from the average corneal thickness of 545 µm.
“Based on this correction, we redistributed the patients into two groups, the first with IOP less than 21 mm Hg and the second with IOP equal to or greater than 21 mm Hg. Finally, we evaluated the percentage of structural and functional alterations in the two groups.”
Visual field and optic nerve tests
FDT showed significantly different results between the three groups.
“The percentage of visual field alterations was much higher in the group of patients with thin corneas (12 patients, 40%), while in the group of patients with thick corneas, visual field loss was quite rare (three patients, 13.6%),” Dr. Brusini said. A similar trend, though not statistically significant, was found with SWAP; six patients in the first group had visual field alterations, as compared to one patient in the group with thick corneas, he said.
Unexpectedly, GDx examination gave conflicting results. “Alterations were more common with thick corneas, which was surprising. This might be due to a double refraction of the cornea that is not compensated by the instrument when corneal thickness is above normal values. This hypothesis is being investigated in our clinic,” he said.
After patients were regrouped according to the Doughty-Zaman formula, the investigators discovered that 41% (36 patients) had final IOP less than 21 mm Hg. A significantly lower rate of visual field alterations was observed in connection with these patients, although results of GDx polarimetry conflicted with those of FDT and SWAP.
Appropriate treatment
“Our results led to a series of important conclusions,” Dr. Brusini said. “First of all, a high percentage of patients who are normally diagnosed with high IOP might simply have thicker-than-average corneas. With the means currently available, this has tended to result in an overestimation of IOP values. This hypothesis is confirmed by the fact that functional defects measured by nonconventional perimetry are less common in this group of patients.”
Another important consideration concerns patients with thin corneas (less than 540 µm).
“They should be considered high-risk cases and should be monitored with special attention, even in cases of moderately high IOP,” Dr. Brusini said.
Obviously, not all depends on the thickness of the cornea, but these findings may explain why, after years of high IOP measurements, some patients never develop optic nerve damage and visual field loss, he said.
“I am afraid we have been administering glaucoma treatment to patients who were perfectly healthy. I now perform all the examinations that I’ve mentioned in the study whenever I find patients with moderately high IOP,” he said.
“If results are in a normal range, I don’t treat them. I will just try to be aware of anything that might develop and ask these patients to come for regular visits. This is carried out with far less anxiety for all concerned.”
For Your Information:
- Paolo Brusini, MD, can be reached at U.O. di Oculistica, Azienda Ospedaliera Santa Maria della Misericordia, 33100 Udine, Italy; +(39) 0432-552743; fax: +(39) 0432-552741; e-mail: brusini@libero.it. Dr. Brusini has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.