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Tonometry readings correlate with central cornea IOP
Measuring IOP in peripheral cornea after refractive surgery can help manage IOP spikes.
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IOP measurements were statistically different between the central cornea and the four limbal positions when using a hand-held tonometer, according to a study. The greatest IOP correlation was between the central cornea and the temporal limbus.
“To the best of our knowledge, this is the first study in the literature to compare IOP measurements of the central cornea with the peripheral cornea in living eyes,” Tanuj Dada, MD, a professor of ophthalmology at Rejendra Prasad Center for Ophthalmic Sciences, India, told Ocular Surgery News.
Using the Tono-Pen Avia applanation tonometer (Reichert), Dada and colleagues compared IOP measurements in 51 glaucomatous eyes in a cross-sectional observational study. The IOP in the peripheral portion of the cornea was, on average, 0.3 mm Hg to 0.9 mm Hg higher than in the central cornea.
The study targeted corneas with no history of corneal disease. Patient criteria included corneal astigmatism of less than 1.5 D, a lack of recent contact lens wear and central corneal thickness ranging from 480 µm to 550 µm.
Tanuj Dada
Deriving IOP
The study authors pondered whether they could use the peripheral part of the cornea to measure IOP derived from the central cornea in a patient with central corneal scarring.
“In India, about 7 million to 8 million people suffer from corneal blindness,” Dada said. “Glaucoma is seen in many cases of corneal opacity. However, estimation of IOP in these patients is difficult, due to the presence of central corneal opacification. This opacity leads to changes in corneal thickness and elasticity. Unfortunately, any technique to measure IOP over the area of opacity provides erroneous readings. But in many of these patients, a peripheral part of the cornea remains clear.”
The Tono-Pen was chosen because of its small, 1-mm diameter applanating tip, which favors its eccentric placement near the limbus for IOP measurement and gives an average of 10 readings, Dada said.
“The Tono-Pen is also much easier to use than a Goldmann applanation tonometer in eyes with central corneal pathology,” he said.
IOP was measured first in the corneal center, which had a mean IOP of 16.59 mm Hg, followed by measurements of the temporal, nasal, superior and inferior limbus.
The correlation coefficient for limbal IOP in relation to central IOP was 0.884 for the temporal limbus, 0.865 for the nasal limbus, 0.811 for the superior limbus and 0.753 for the inferior limbus.
“Such a good correlation between central and peripheral corneal IOP was surprising,” Dada said. “The study results are also exciting, as a number of pathologies affect the central cornea. Additionally, after refractive surgery, the central cornea is affected, while the peripheral part remains intact. In all these conditions, IOP can be measured accurately at the peripheral cornea.”
Increasing accuracy
To increase the accuracy of IOP measurements with the Tono-Pen in this patient population, Dada recommended that the probe be kept perpendicular to the corneal surface. The patient should also refrain from exerting undue pressure or squeezing the eye. Additionally, Dada suggests taking 10 readings in order to capture the best results.
The Tono-Pen correlates well with the gold-standard Goldmann applanation tonometer and can be used in all types of glaucoma patients, both children and adults, Dada said.
“The device can also measure IOP in eyes with corneal disease. It gives a good estimate of the IOP, within 1 mm Hg of the actual IOP,” he said.
Regression equations mentioned in the published study can increase accuracy, according to Dada.
Dada said the Tono-Pen is particularly promising in patients undergoing refractive surgery, such as LASIK.
“The clinician should measure the IOP in both the central and peripheral part of the cornea before surgery, and then use the peripheral part of the cornea for IOP measurements after surgery,” he said. “This will be of great help if the patient develops IOP spikes or glaucoma at any time during the postoperative period.”
The Tono-Pen is also applicable to other corneal surgeries performed on the central cornea where the peripheral corneal rim is left intact, such as keratoplasty.
The study authors are currently involved in a study using the Tono-Pen to measure peripheral corneal IOP in patients undergoing penetrating keratoplasty. – by Bob Kronemyer