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    Tonometer can provide accurate IOP levels during screenings for glaucoma, physician says

    Because of precise measurements, risk threshold should be lowered to 18 mm Hg

    Berlin—IOP remains one of the primary risk factors for glaucoma, and the only modifiable one. Traditionally, 21 mm Hg has been considered a "risk threshold," although the validity and usefulness of this value has been debated heavily in recent years, said Mitsugu Shimmyo, MD, who spoke at the European Society for Cataract and Refractive Surgeons annual meeting. He said the threshold of IOP measurement for glaucoma screening detection should be lowered to 18 mm Hg because many more patients can be accurately captured in screening examinations, as demonstrated in a recent study.

    Dr. Shimmyo
    "Using traditional IOP measurement of 21 mm Hg as a threshold of pressure for detecting glaucoma, in my opinion, involved complicated issues," said Dr. Shimmyo, assistant professor of clinical ophthalmology at New York Medical College, and head author of the study. "In using this traditional threshold, ophthalmologists have had to consider different diseases such as glaucomatous optic neuropathies with low measurements, leading to the diagnoses of 'low-tension' glaucoma, 'normal-tension' glaucoma, and 'high-tension' glaucoma.

    "In reality, I believe that these conditions may belong to the spectrum of the same disease, whether the pressure is measured higher or lower than 21 mm Hg," he continued. "Though considered by some as unconventional, we simply would need to drop the threshold to 18 mm Hg instead of the current 21 mm Hg."

    Goldmann applanation tonometry (GAT) is considered to be the gold standard in measuring IOP. The accuracy of measurements is less than optimal, however, because they are influenced by the biomechanical properties of the cornea, including corneal thickness, corneal stiffness, and the viscoelasticity of the cornea. A newer tonometer (Ocular Response Analyzer [ORA], Reichert) measures IOP compensated for corneal effect (IOPcc) and biomechanical properties of the cornea, offering more precise measurements, Dr. Shimmyo said.

    Cross-sectional review

    He conducted a cross-sectional observational review of GAT and the newer device's parameters of one eye randomly selected from 733 normal patients and 257 patients with glaucoma, and compared the sensitivity and specificity of detecting glaucoma using GAT and IOPcc.

    In the study, glaucoma was defined by the presence of morphologic changes of the optic nerves and neurofiber layers with or without visual field changes. The sensitivity and specificity of diagnosing glaucoma was calculated at different threshold levels of GAT and IOPcc. Normal-tension glaucoma (NTG) was defined by GAT as lower than 21 mm Hg. Central corneal thickness (CCT), GAT, IOPcc, corneal hysteresis (CH), and corneal resistance factor (CRF) were compared.

    Results showed that the mean GAT and mean IOPcc were similar in normal eyes and those with glaucoma. In eyes with NTG, however, the mean IOPcc of 19.8 ▒ 3.41 mm Hg was higher by 5.4 mm Hg than the mean GAT of 14.4 ▒ 3.37 mm Hg. The eyes with NTG showed a significantly thinner CCT of 513 ▒ 39 Ám, a lower CH of 5.96 ▒ 1.09 mm Hg, and a lower CRF of 6.31 ▒ 1.14 mm Hg when compared with the means of normal CCT of 541 ▒ 33 Ám, normal CH of 10.60 ▒ 1.54 mm Hg, and normal CRF of 10.15 ▒ 1.60 mm Hg.

    'Rule of law'

    The GAT measurement at 21 mm Hg always has been accepted as the threshold pressure, and students of ophthalmology in the past believed that this was the rule of law, Dr. Shimmyo said. The definition of glaucoma has changed, however, because of not only the elevated pressure but also the presence of glaucomatous optic neuropathy.

    "This study shows that when using the GAT tension of 21 mm Hg as a threshold of high or low pressure, only 47% of the eyes with glaucoma could be captured," he said. "However, the IOPcc 18 mm Hg allowed us to capture 85% of eyes with glaucoma."

    Many eyes with a stiff cornea without glaucomatous optic neuropathy may register GAT pressures as high as 30 mm Hg, and because of this, a category of ocular hypertension was proposed. The same eyes, however, register low pressures using measurements from the newer tonometer. According to Dr. Shimmyo, pressure may still be the pathophysiologic mechanism of glaucoma; however, the presence of optic neuropathy remains as the definition of glaucoma.

    "We treat patients with normal-tension glaucoma by lowering the pressure," he said. "Why do we lower it? Because it works and the [newer device] seems to explain why. I am not arguing against the existence of normal-tension glaucoma, but I think that the number of patients with true normal-tension glaucoma seems to be much smaller than what we previously thought."

    Dr. Shimmyo said that he would like to see the facts found in this scientific study be reproduced and scrutinized by others so that they can make their own conclusions. Ophthalmologists should be open-minded in the new ideas, new concepts, and instruments, he said.

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