Tonometer enables IOP measurement in patients without use of anesthesia

Eugene, OR-A new instrument for measuring IOP (iCare Tonometer, Tiolat Oy; available from Rhein Medical) seems to be a satisfactory alternative method of measuring IOP that seems to correlate well with Goldmann applanation tonometry, the gold standard of IOP measurement. The tonometer is especially useful in obtaining IOP measurements in patients who may be uncooperative or those with physical limitations regarding the use of applanation tonometry and does not require anesthesia because the patient barely feels the probe touching the cornea, according to Richard S. Hoffman, MD.

“The IOP values obtained with the tonometer are strikingly close to those obtained with applanation tonometry,” he said, after using the instrument in his clinical practice for about 2 months. Dr. Hoffman is associate clinical professor of ophthalmology, Casey Eye Institute, Oregon Health and Science University, and in private practice in Eugene, OR.

This is in contrast to the applanation tonometer (Tono-Pen, Reichert Ophthalmic) that which he uses as a screening device and to obtain a rough estimate of IOP values when a precise value is not necessary, such as in a patient without glaucoma who has undergone cataract surgery. The downside of the previous instrument is that the values do not always correlate with applanation tonometry. IOP measurements can deviate by as much as 5 mm Hg in some patients, he said. The applanation tonometer is advantageous for obtaining IOPs intraoperatively and in children who are anesthetized.

The manufacturer reported that a comparison between the applanation tonometer and the new device showed that there was a mean 0.5 mm Hg difference (standard deviation, ±3.4 mm Hg) between the two instruments.

How the instrument works

The battery-powered instrument, which works on the new principle of induction-based rebound, is held between 4 to 8 mm from the patient’s cornea with the patient in the sitting position. If the actual distance from the cornea is not within that range or the tonometer is not horizontal to the floor, the clinician receives an error message. With the instrument held in place by an adjustable forehead support to obtain the correct distance from the cornea, the device shoots toward the cornea a small fine, light, plastic probe with a round bulb that is 1 mm wide. The IOP is determined by the speed with which the bulb hits the cornea and bounces back. Six measurements are taken and the average of the six is the IOP value.

Pros and cons

No anesthesia is needed to measure the IOP because the bulb has split-second contact with the cornea. Dr. Hoffman described the sensation as that of a hair touching the eye. Patients are not resistant to undergoing six measurements in his experience. Cross contamination between patients is eliminated by the use of the disposable probes.

The tonometer facilitates obtaining IOP measurements in obese patients, patients with Down’s syndrome, and in patients with dementia.

Dr. Hoffman recounted his experience with a 58-year-old patient severely affected by Down’s syndrome and keratoconus. The patient was barely cooperative with ophthalmic examinations and Dr. Hoffman had never been able to obtain an IOP value with the patient wake. Because of the small size of the probe, the patient’s eye does not have to be open widely in order to obtain an IOP value. In the patient under discussion, he tolerated the IOP measurement with the tonometer well and for the first time Dr. Hoffman was able to obtain a measurement with the patient in the clinic.

Dr. Hoffman said the probes could possibly be reused but that this is not the recommended practice. Another possibility is that the new tonometer not be used routinely but be reserved for patients in whom obtaining an accurate IOP value is mandatory, such as patients diagnosed with glaucoma who are uncooperative for applanation tonometry.

Another disadvantage is that the instrument cannot be used to record the IOP when a patient is supine, as is the case, for example, with patients who are undergoing surgery or in infant exams under anesthesia. Despite these drawbacks, Dr. Hoffman expressed enthusiasm about this product.

“I am very excited to have a manual tonometer that I believe has the accuracy of Goldmann applanation tonometry,” he said. “This new tonometer can be used in uncooperative and demented patients in whom obtaining an accurate applanation tonometry reading is not possible, and also in wheelchair-bound patients who are physically challenged and cannot reach the slit lamp.”