Reproducibility and clinical evaluation of rebound tonometry. Invest Ophthalmol Vis Sci

PURPOSE. To establish the reproducibility of a rebound tonometer in humans and the effect of corneal thickness on measurements, comparing it with Goldmann applanation tonometer. METHODS. In a first study designed to examine the reliability of the RBT, three experienced ophthalmologists undertook three consecutive intraocular pressure (IOP) measurements in 12 eyes of 12 normal subjects. A cross-sectional study was then performed to compare measurements obtained using the two tonometers in 147 eyes of 85 patients with ocular hypertension or glaucoma. RESULTS. Intraobserver coefficients of correlation obtained in the reproducibility study were 0.82, 0.73, and 0.87. Interobserver correlation was 0.82. There was a good correlation between IOP readings obtained by the RBT and the GAT (r ϭ 0.865, P Ͻ 0.0001). RBT readings were consistently higher than GAT measurements (median difference, 1.8 Ϯ 2.8 mm Hg). A Bland-Altman plot indicated the 95% limits of agreement between the two methods were Ϫ3.7 to 7.3 mm Hg (slope ϭ Ϫ0.022, P ϭ 0.618). Using RBT, the point that best discriminated between patients with an IOP Յ 21 mm Hg and those with Ͼ21 mm Hg, as determined by the GAT was Ͼ23 mm Hg (sensitivity, 70.5%; specificity, 95.1%). In terms of pachymetry, the two tonometers behaved in a similar way, with correlation observed between IOP measurements and central corneal thickness. CONCLUSIONS. Rebound tonometry is a reproducible method of determining IOP in humans. In general, it tends to overestimate IOP compared with Goldmann applanation tonometry. 1 It is also the only risk factor on which we can efficiently act by using drugs, laser, or surgery, and therefore efforts to find a tool capable of accurately and reproducibly measuring IOP have been constant for more than a century. A recently developed method of establishing IOP, rebound tonometry, has been used mainly in experimental models of glaucoma for noninvasive pressure measurements in animals. This method has provided good results in ease of use and the precision and reproducibility of its results. 2-5 The rebound method has also led to the marketing of a handheld tonometer for use in humans (ICare; Tiolat Oy, Helsinki, Finland). The rebound tonometer (RBT) is an assembly of two coils coaxial to a probe shaft that bounce a magnetized probe off the cornea and detect the deceleration of the probe caused by the eye. A moving magnet within a coil induces changes in the voltage at the two ends of the coil generating a magnetic field with a given voltage, which is detected by the tonometer sensor. The voltage produced is proportional to the probe speed. Of all the variables linked to the probe's movement, the inverse of its deceleration speed seems to correlate best with IOP. 2 The probes used by the tonometer are disposable and are 24 mm long and weigh 11 mg. The probe tip has a 1-mmdiameter plastic cover, to minimize corneal damage. To date, there have been no attempts to evaluate the reproducibility and reliability of the measurements provided by this instrument in humans. The present study was thus designed to evaluate the use of rebound tonometry in clinical practice, by comparing the reproducibility of the measures obtained to those provided by conventional applanation tonometry, and by examining the effect of corneal thickness on IOPs determined using the ICare RBT. METHODS The study was divided into two parts. In the first, we determined the reliability of the RBT by three consecutive measurements in each of 12 eyes of 12 normal subjects conducted by three experienced ophthalmologists, leaving a 5-minute interval between examinations. Intra-and interobserver coefficients of variation were estimated, and the number of attempts needed to obtain a valid measure was recorded. In the second part of the study, we compared the RBT and the Goldmann applanation tonometer (GAT; Haag-Streit, Köniz, Switzerland) by performing a cross-sectional study of 85 consecutive patients (with ocular hypertension and glaucoma) examined at the Department of Glaucoma, Hospital Clínico San Carlos (Madrid, Spain). Eyes with normal corneas and no history of previous ocular trauma or surgery were evaluated. Informed consent according to the tenets of the Declaration of Helsinki was obtained from each patient. In all patients, central corneal thickness was determined by ultrasound pachymetry (Dicon P55; Paradigm Medical Industries Inc., Salt Lake City, UT). Intraocular pressure was then determined with both the RBT and the GAT. Patients were randomly divided into two groups. Those in group 1 were first examined with the GAT, to obtain the mean of three measurements for the analysis, and the same procedure was repeated with the RBT, to obtain a further three measurements. The patients in the second group were examined with the two tonometers in reverse order. All IOP measurements were made by the same examiner, who was blind to the readings obtained with the first tonometer