Abstract
Objective—To determine whether an applanation tonometer and rebound tonometer can be used to detect similar intraocular pressure (IOP) measurements in eyes of dogs undergoing phacoemulsification.
Animals—24 dogs (40 eyes) undergoing elective phacoemulsification.
Procedures—IOP measurements were obtained from each eye by use of both the rebound tonometer and applanation tonometer. Central corneal thickness was measured by use of an ultrasonic pachymeter 3 hours before surgery and 2 and 24 hours after surgery. Statistical analysis was performed by use of paired t tests.
Results—Mean ± SD IOP 3 hours before surgery, 2 hours after surgery, and 24 hours after surgery was 11.9 ± 4.7 mm Hg, 15.5 ± 11.7 mm Hg, and 10.9 ± 6.7 mm Hg, respectively, as measured with the rebound tonometer and 12.2 ± 5.3 mm Hg, 15.7 ± 12.5 mm Hg, and 12.4 ± 5.4 mm Hg, respectively, as measured with the applanation tonometer. Measured IOP did not differ significantly between the 2 tonometers 3 hours before surgery and 2 hours after surgery, but measured IOP differed significantly between the tonometers 24 hours after surgery.
Conclusions and Clinical Relevance—Use of a rebound tonometer underestimated IOP, relative to results for use of an applanation tonometer, by 1.65 mm Hg in eyes 24 hours after phacoemulsification. Caution should be used when IOP measurements obtained with a rebound tonometer are in the high part of the reference range, and verification of these values with an applanation tonometer would be advised.
The measurement of IOP is an important diagnostic test during an ophthalmic examination. Accuracy of IOP measurements in ophthalmologically normal eyes, eyes with glaucoma or uveitis, eyes with corneal disease (eg, edema, fibrosis, or mineralization), and eyes on which surgery was recently performed has been evaluated extensively with the use of applanation tonometers. The Goldmann applanation tonometer has been considered the criterion-referenced standard for the assessment of IOP in eyes of humans.1 This tonometer has been calibrated to be accurate at a designated corneal thickness of 520 μm.2 Correction factors for IOP that are based on deviation of the CCT from 520 μm have been determined.3–5 The Goldmann applanation tonometer overestimates IOP in eyes with increased CCT6 and underestimates IOP in eyes with decreased CCT.7,8 The degree of deviation in IOP measurement depends on the cause of the increased CCT.9 A disadvantage of the Goldmann applanation tonometer is the large applanation tip, which is 7 mm in diameter and needs to be placed on an area of normal corneal tissue for an accurate IOP measurement.10 This tonometer is typically attached to a slit-lamp biomicroscope, which inhibits its portability and makes its use in veterinary medicine impractical.
A handheld applanation tonometer that is based on a principle similar to that of the MacKay-Marg tonometer has been the tonometer of choice for use in veterinary medicine. The MacKay-Marg tonometer has been considered the criterion-referenced standard tonometer in veterinary medicine11 but is currently no longer commercially available. The tip of a handheld applanation tonometer (3 mm in diameter) is smaller than that of the Goldmann applanation tonometer, which enables measurements on eyes with a small area of normal cornea.12 A disposable latex cover for the tip prevents transfer of bacteria and viruses and also keeps the tip clean. The applanation tonometer displays a mean value for 3 to 6 accepted readings on a liquid crystal display; it also displays a coefficient of variation.
A new commercially available tonometer is based on the principal of rebound technology. It determines IOP by propelling a replaceable, magnetized probe at the cornea at a set speed; it then measures the deceleration and converts this measurement to units of pressure (ie, mm Hg). The probe tip (1.0 mm in diameter) is smaller than that of any other commercially available tonometer.13 Use of this tonometer does not require topical anesthesia, and it is considered easier to use and results in less ocular discomfort.14 The design of this rebound tonometer is ideal for use in laboratory animal medicine, especially for mice and rats.13,15 To our knowledge, the effect of corneal disease, corneal thickness, and postoperative uveitis or glaucoma on IOP as measured by use of this rebound tonometer in dogs has not been determined.
The CCT can be measured via 2 mechanisms (ultrasonic pachymetry and optical coherence tomography). Ultrasonic pachymetry is considered the most accurate when used appropriately.16 Corneal thickness measurements need to be obtained directly on the center of the cornea because the cornea is thicker at the periphery, and the probe needs to be placed perpendicular to the corneal surface to provide an accurate measurement.17 Optical coherence tomography allows 2-D mapping of corneal thickness by use of noncontact, noninvasive cross-sectional visualization.17
The objective of the study reported here was to test the subjective impression, obtained via the experience of the authors, that use of a rebound tonometer typically overestimates IOP measurements in eyes after surgery, compared with values attained by use of an applanation tonometer. The null hypothesis was that no difference exists between IOP measured by use of an applanation tonometer or rebound tonometer in eyes of dogs after phacoemulsification.
Materials and Methods
Animals—Twenty-four dogs (40 eyes) on which elective cataract phacoemulsification was performed from June 2006 to July 2007 were included in the study. All dogs had a complete ophthalmic examination, which included slit-lamp biomicroscopya and indirect ophthalmoscopy.b Dogs with corneal disease were excluded from the study. All dogs included in the study had satisfactory results for a scotopic flash electroretinogram and ocular ultrasonography before surgery. Preanesthetic blood screening was required for all participants. Satisfactory regulation of blood glucose concentrations in diabetic dogs was required before surgery.
Ocular surgery—Phacoemulsification was performed by multiple surgeons by use of a 1-handed phacoemulsification technique. A 41-diopter rigid polymethylmethacrylate lens or a 41-diopter foldable acrylic lens was available for placement in the posterior chamber; selection was made on the basis of surgeon preference.
Study protocol—Data recorded for each patient included sex, age, breed, and diabetic status. The type of intraocular lens, if placed, was also recorded. The IOP was measured with both a rebound tonometerc and an applanation tonometer.d The IOPs were recorded 3 hours before surgery (baseline), 2 hours after surgery, and 24 hours after surgery. Time intervals were selected on the basis of another study18 in which investigators determined that the greatest increase in CCT was 1 day after phacoemulsification and because the highest incidence of postoperative ocular hypertension is during the first few hours after surgery.19 Increase in IOP was prevented by minimal pressure application around the neck and to the eyelids during measurements.
The CCT was measured by use of an ultrasonic pachymeter.e Five CCT measurements were obtained at each time point (3 hours before surgery, 2 hours after surgery, and 24 hours after surgery). The highest and lowest CCT measurements were discarded, and a mean value of the remaining 3 measurements was calculated and used for analysis.
The rebound tonometer was used to obtain an initial measurement before application of topical anesthetic necessary for measurement by use of the applanation tonometer and pachymeter. When surgery was performed on both eyes, IOP measurements for both eyes were used in the study. For dogs undergoing bilateral phacoemulsification, IOP measurements were performed on both eyes in random order.
The tonometers and pachymeter were calibrated in accordance with the manufacturer's recommendation. Proparacaine HCl (0.05%)f was used before IOP measurements were obtained with the applanation tonometer and before CCT measurements. All measurements were performed by the same investigator (ALTH) to eliminate interobserver error.
Statistical analysis—Mean and SD were calculated for each instrument used for the 3 time points. A paired Student t test was used to determine a zero-bias hypothesis, which allowed the bias to be assessed as the mean of the differences compared with zero. Values of P < 0.05 were considered significant.
Results
Neutered male dogs (n = 18) were overrepresented in the study group; 5 dogs were spayed females, and there was 1 sexually intact male dog. Dogs ranged from < 1 to 17 years of age (mean, 7.5 years). Sixteen of 24 dogs had bilateral surgery and 8 had unilateral surgery (5 right eyes and 3 left eyes). Breed distribution revealed an over-representation of Poodles (n = 6) and Poodle-crossbred dogs (2 Poodle–Cocker Spaniel crossbreds), compared with results for the general hospital population. Other breeds represented included Labrador Retriever (n = 4), Bichon Frise (3), mixed (3), West Highland White Terrier (2), Silky Terrier (1), Boston Terrier (1), Miniature Schnauzer (1), and Lhasa Apso (1). There were 9 diabetic dogs (18 eyes) and 1 hypothyroid dog (1 eye). Of the 40 eyes, 17 (42.5%) received a polymethylmethacrylate intraocular lens, 16 (40%) received a foldable acrylic intraocular lens, and 7 (17.5%) were left aphakic. An intraocular lens was not placed in aphakic eyes because of an extensive tear in the posterior capsule, subluxation of the lens capsule, or severe microphakia.
Mean ± SD, maximum, and minimum IOP measured with both tonometers at the 3 time points were summarized (Table 1). Of the 24 dogs, only 22 (92%) had measurements obtained 2 hours after surgery. Mean ± SD CCT for eyes that underwent surgery was 631 ± 64.9 μm 3 hours before surgery (baseline), 747 ± 77.6 μm 2 hours after surgery, and 773 ± 104.7 μm 24 hours after surgery. The presurgical CCT was higher than a value of 562 μm reported in another study.20 There was a large difference in CCT between nondiabetic and diabetic dogs. Nondiabetic dogs had a mean CCT of 615 μm, whereas diabetic dogs had a value of 650 μm. Although 615 μm is still elevated, compared with the value of 562 μm in that other study,20 a possible explanation may have been phacolytic uveitis that caused subclinical corneal edema.
Intraocular pressure measured by use of an applanation tonometer and rebound tonometer 3 hours before surgery, 2 hours after surgery, and 24 hours after surgery in 24 dogs (40 eyes) undergoing elective phacoemulsification.
| Time | Tonometer (mm Hg) | Mean ± SD IOP (mm Hg) | Maximum IOP (mm Hg) | Minimum IOP (mm Hg) | P value* |
|---|---|---|---|---|---|
| 3 hours before surgery | Applanation | 11.9 ± 4.7 | 25 | 4 | 0.612 |
| Rebound | 12.2 ± 5.2 | 25 | 4 | — | |
| 2 hours after surgery† | Applanation | 15.5 ± 11.6 | 47 | 1 | 0.878 |
| Rebound | 15.7 ± 12.4 | 50 | 1 | — | |
| 24 hours after surgery | Applanation | 10.9 ± 6.7 | 29 | 2 | 0.042 |
| Rebound | 12.2 ± 5.4 | 23 | 1 | — |
Differences in mean values between the applanation and rebound tonometers at each time point were analyzed by use of a paired t test; values were considered significant at P < 0.05.
Represents results for only 22 dogs.
— = Not applicable.
No significant difference was found between mean IOP measurements obtained with the applanation tonometer and rebound tonometer 3 hours before and 2 hours after phacoemulsification. However, 24 hours after surgery, IOP measured by use of the rebound tonometer was significantly (P = 0.042) underestimated (mean ± SD, 1.65 ± 4.95 mm Hg), compared with the mean value obtained with the applanation tonometer. Most (29/40 [72.5%]) of the differences in IOP between the applanation tonometer and rebound tonometer were ≤ 5 mm Hg.
Discussion
In the study reported here, IOP measurements obtained by use of a rebound tonometer and applanation tonometer were compared. In other studies, no significant difference in IOP measurements was found between an applanation tonometer and the MacKay-Marg tonometer in ophthalmologically normal eyes21,22 or eyes with uveitis, glaucoma, or noninflammatory intraocular disease.23 Investigators in 1 study24 reported a similar correlation of IOP measurements between the applanation tonometer and MacKay-Marg tonometer for eyes of humans that underwent keratoplasty or epikeratophakia or had scarred corneas. There was also a strong correlation between IOP measurements obtained with an applanation tonometer and via direct intracameral measurements.25 In 1 study,23 investigators determined a sensitivity of 93% and specificity of 91% for the applanation tonometer, compared with the MacKay-Marg tonometer. The applanation tonometer was found to be within the 95% limit of agreement with the Goldmann applanation tonometer in eyes of humans with ocular hypertension and glaucoma.26 Additionally, the applanation tonometer was affected least by the CCT, compared with results for the Goldmann applanation tonometer and a pneumotonometer, in ophthalmologically normal corneas of humans.27
Accuracy of the applanation tonometer used in the present study has been evaluated over the past 10 years.9–12,22–37 The applanation tonometer overestimates IOP in eyes of humans with low IOP (< 9 mm Hg) and underestimates IOP in eyes of humans with high IOP (> 30 mm Hg), although it corresponds closely with results for the Goldmann applanation tonometer at IOPs between 10 and 19 mm Hg.2,28 The applanation tonometer overestimates IOP when measurements are obtained over areas that include band keratopathy; over locations where cyanoacrylate adhesive has been applied10 in eyes of humans undergoing surgical procedures, including penetrating keratoplasty12,31 or epikeratophakia32; or in humans with Fuchs endothelial dystrophy.9 The applanation tonometer is accurate when used to measure IOPs in the eyes of dogs through 2 types of plano soft contact lenses when IOPs are < 30 mm Hg.33 Even though the applanation tonometer overestimates IOPs in eyes of humans, it has proven to be an accurate instrument for measuring IOPs, especially when values are within the reference range. Because the MacKay-Marg tonometer was not commercially available, the applanation tonometer was used as the criterion-referenced standard in the present study because the actual IOP could not be validated in these clinical cases.
The rebound tonometer used in the present study is an accurate instrument for use in measuring IOP in ophthalmologically normal eyes of dogs,34 horses,34 rats,35 and cats.36 Investigators in 1 study37 determined that measurements obtained with a rebound tonometer had good correlation with measurements obtained with the MacKay-Marg tonometer in glaucomatous eyes of dogs. Although the rebound tonometer underestimated IOP in ophthalmologically normal canine eyes in another study,38 the tonometer used in that study was calibrated for use in human eyes. In a study39 of eyes of humans with ocular hypertension, the rebound tonometer overestimated IOP, compared with results for the Goldmann applanation tonometer, and also was affected by the CCT.
In the present study, IOP measurements obtained with the rebound tonometer were similar to values obtained with the applanation tonometer in eyes evaluated at baseline and 2 hours after surgery. The rebound tonometer underestimated IOP in eyes 24 hours after surgery, compared with results for the applanation tonometer. Because the CCT was significantly greater in eyes 24 hours after surgery, compared with the CCT at baseline and 2 hours after surgery, this was suspected to be the cause of the underestimation. A possible explanation of the underestimation of IOP with the rebound tonometer would be a decrease in the return motion of the probe attributable to the change in corneal rigidity. The high CCT values obtained in the present study should not have affected measurements obtained by use of the applanation tonometer because increases in the CCT have been found to have the least effect on measurements obtained with the applanation tonometer.27 In future studies, researchers should take the elevated postoperative CCT values into consideration when evaluating a rebound tonometer because it is suspected that IOP is underestimated with increases in CCT. Of 40 eyes in the present study, 4 (10%) had an IOP > 30 mm Hg at 2 hours after surgery when measured with the applanation tonometer. The applanation tonometer typically underestimates IOP in eyes that have an IOP > 30 mm Hg; thus, the comparison of IOP measurements with the rebound tonometer may be affected in these eyes. Values > 30 mm Hg were not detected at either of the other 2 time points.
Mean ± SD underestimation of the IOP with the rebound tonometer, compared with the IOP measured with the applanation tonometer, at 24 hours after surgery was 1.65 ± 4.95 mm Hg. In veterinary medicine, this is considered a small variation. Considering that 29 of 40 (72.5%) differences between the applanation tonometer and rebound tonometer were ≤ 5 mm Hg, the rebound tonometer should be considered an accurate instrument for measurement of IOP in eyes that have undergone phacoemulsification. Caution should be used when IOP measurements obtained with a rebound tonometer are in the high part of the reference range, and verification of these values with an applanation tonometer is advised.
ABBREVIATIONS
| CCT | Central corneal thickness |
| IOP | Intraocular pressure |
SL15, Kowa Optimed Inc, Torrance, Calif.
Keeler Vantage, Keeler Instruments Inc, Boomall, Pa.
TonoVet tonometer, Tiolat Oy Lumic International, Baltimore, Md.
Tono-Pen VET tonometer, Medtronic Ophthalmic, Jacksonville, Fla.
Compuscan P ultrasonic pachymeter, 20 MHz, Storz Instrument Co, St Louis, Mo.
0.5% Proparacaine HCl, Falcon Pharmaceuticals, Fort Worth, Tex.
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