glaucoma is an optic neuropathy characterized by the death of retinal ganglion cells ( rgc ) which results in a loss of vision . increased intraocular pressure ( iop ) ocular manometry is considered to be the gold standard for determining iop as this technique measures the actual iop . however , this is an invasive procedure used experimentally and involves cannulation of the anterior chamber to measure iop with either a digital instrument or a column of water or mercury . tonometry measures iop using devices called tonometers which use different techniques to evaluate corneal tension induced by iop . tonometers can be classified according to whether they come into contact with the cornea , if they are portable or not , and the method used for iop measurement . these methods include indentation , applanation , and , more recently , the rebound technique . applanation tonometry is based on the principle that the force required to flatten a certain area of a spherical surface is the same as the pressure inside that sphere . later , some handheld applanation tonometers appeared which allowed the examination of bedridden patients and children . in veterinary medicine , portable contact applanation tonometers are most commonly used   among which the tonopen xl is the most popular , especially in horse and cattle . in equine ophthalmology , indentation tonometry with the schiotz tonometer is not possible due to the thickness of the cornea and the inability to position the head of the awake animal horizontally . this device uses a goldmann prism ( 3 mm double prism ) that is adjusted during tonometry to form fluorescein semicircles from a small blue light source . thus , the large divisions of the scale represent grams and the small divisions 0.2 g , with examinations typically started with the scale at 1 . in humans , readings measured on this scale are multiplied by 10 to determine the tension in mmhg . in a recent study of dogs and cats , it is approximately the same , in other words , it is also multiplied by 10 . while measurement accuracy depends on the experience of the operator , the perkins tonometer is approximately 3 to 5 times cheaper than the tonopen xl and tonovet . the studies using the perkins tonometer have been conducted on dogs , cats , and rabbits . however , use of this tonometer has still not been reported in horses , and there has only been one cattle study that used this tonometer . one compared the effectiveness and accuracy of 3 tonometers , tonopen xl , perkins and ocular blood flow pneumatonometer . the other compared ability of the the perkins , tonopen xl , and tonovet tonometers to measure iop . therefore , the objective of this study was to evaluate and validate the accuracy of the perkins handheld applanation tonometer for measuring iop in horses and cattle . a pilot study previously authorized by the ethical committee was conducted to evaluate the use of invasive manometry in horses and cattle . the procedure was performed after bilateral auriculopalpebral nerve block and topical anesthesia had been administered , mainly due to the position the horizontal position of the head when the animals were awake and to measure actual iop without the influence of general anesthesia . the procedure was well - tolerated by both horses and cows and deemed to be safe for performing manometry without causing discomfort or pain in the animals . twenty eyes from ten adult horses and cattle were obtained for the postmortem study from a slaughterhouse ( king meat ) and bom mart ( brazil ) , respectively . both eyes in all animals were left within the orbits , and the heads of the animals were obtained immediately after sacrifice and transported in refrigerated boxes . , 10 adult conscious horses and cattle were obtained from the veterinary teaching farm of unoeste . only normal eyes were used in this study as determined by ophthalmic examination ( direct ophthalmoscopy , pupillary light reflex , schirmer tear test , and fluorescein test ) . the ophthalmic exams and perkins tonometry were always performed by the same observer to prevent inter - observer variability . after the procedure , the animals were returned to pasture at the unoste veterinary teaching farm . for the postmortem study ( fig . 1 ) , each eye was cannulated with a 23-gauge scalp vein needle ( embramac , brazil ) through the cornea about 1 to 2 mm from the limbus in the supero - lateral quadrant . cyanoacrylate glue ( superbond ; loctite , brazil ) was applied around the needle to prevent leakage of the aqueous humor . the stopcock was connected to a reservoir ( syringe ) containing 10 ml of a physiological saline solution ( fresenius kabi , brazil ) and to an aneroid manometer ( missouri , brazil ) , which was set at the zero position relative to the center of the eye . the calibration curve for manometry versus tonometry was determined by artificially increasing the iop in 5 mmhg increments up to 50 mmhg in tetracaine 1% the open stopcock mode . prior to taking the tonometer reading , one drop of 1% fluorescein ( allergan , brazil ) three readings were taken with the perkins tonometer ( clement clarke , uk ) and the mean value was calculated . for the in vivo study ( figs . 2 and 3 ) , the horses were led into a solid - sided restraint cage with a bar in front and a bar or solid gate behind . if necessary , the horse 's head was positioned above the level of the heart for measuring iop with the perkins tonometer and manometer by using a twitch on the upper lip . the procedures to measure iop in the horses and cattle always started at 8.00 a.m. and ended at 10.00 a.m. the cattle were guided into a funnel corral and then restrained with a neck yoke . bilateral auriculopalpebral nerve blocks with 10 ml of 2% lidocaine hcl ( hipolabor , brazil ) and bilateral topical anesthesia with two drops of tetracaine 1% ( anestesico ; allergan , brazil ) were applied to all animals . to avoid transmission of infectious ocular diseases , the prism from the perkins tonometer was removed and washed in physiological saline solution after each use . it was then submerged for 10 min in a solution of 3% hydrogen peroxide , washed again in a physiological saline solution , and then dried with sterile gauze . soon after , cyanoacrylate glue was applied with a 25  7 hypodermic needle ( injex , brazil ) at the corneal puncture site to protect against fluid leakage . after the procedure , the animals were given one drop of chloramphenicol eye drops ( visalmin ; bunker , brazil ) , and diclofenac sodium eye drops , ( still ; allergan , brazil ) twice daily for 1 week . the mean iop values from ocular manometry and tonometry in the postmortem study were used to create a calibration curve . perkins tonometer readings taken during the in vivo study were converted to iop in mmhg using this calibration curve , and were analyzed by student 's t - tests . linear regression analysis was performed to analyze the relationship between the postmortem manometry and tonometry iop measurements , and the correlation coefficient ( r ) was calculated . a p - value < 0.05 was considered to indicate statistical significance . the correlation coefficients ( r ) between the data from manometry and the perkins tonometer were 0.866 in horses and 0.864 in cattle . for the postmortem study , the corresponding linear regression was y = 0.082x + 0.268 for horses , and y = 0.082 + 0.290 for cattle 4 ) . in the in vivo study , the iop values measured by the manometer were 25.1  2.9 mmhg ( range , 19.0~30.0 mmhg ) in horses ( table 1 ) and 19.7  1.2 mmhg ( range , 18.0~22.0 ) in cattle ( table 2 ) . in the in vivo study , the mean iop measured by the perkins tonometer was 2.2  0.3 ( range 1.8~2.6 mmhg ) in horses . this value corresponded to 23.4  3.2 mmhg ( range , 18.6~28.4 mmhg ) according to our calibration curve ( table 1 ) . in cattle ( table 2 ) , the mean iop measured by the perkins tonometer was 1.8  0.1 ( range 1.6~2.0 mmhg ) and corresponded to 18.8  1.7 mmhg ( range 15.9~20.8 mmhg ) according to our calibration curve . a recent study of using the perkins tonometer on dogs and cats   spurred our interest in applying this tonometer to other species . aside from the reduced cost of the perkins tonometer , the advantages of this procedure observed in that study were increased accuracy , easy maintenance of the tonometer , and the ability to calibrate and disinfect the prism . a number of disadvantages were also associated with the perkins tonometer including its narrow reading range ( with an upper limit of 50 mmhg ) and the longer training time necessary for its correct use . in the present study , identical advantages and disadvantages were observed with the use of the perkins tonometer in horses and cattle . there was a strong correlation between the iop values obtained by direct ocular manometry and the perkins tonometer in horses and cattle . the correlation coefficients for the perkins tonometer in horses ( r = 0.866 ) and cattle ( r = 0.864 ) were very good . iop readings taken by the perkins tonometer are multiplied by 10 in humans , and a recently study demonstrated that the same factor should be used for dogs and cats . based on our calibration curves for horses ( y = 0.082x + 0.268 ) and cattle ( y = 0.082x + 0.290 ) , we approximated the equation and multiplied perkins tonometer reading by a factor of 10 for both horses and cattle . we therefore recommend calculating the mean of three readings and multiply this by a factor 10 when using the perkins tonometer . in our in vivo study , the mean iop values calculated with the calibration curve from the perkins tonometer data for horses ( 23.4  3.2 mmhg ; range , 18.6~28.4 mmhg ) were very similar to the iop directly measured by manometry ( 25.1  2.9 mmhg ; range , 19.0~30.0 ) . the iop values calculated from the perkins tonometer data were also similar to those measured by the rebound ( 22.1  5.9 mmhg ; range , 10~34 mmhg ) , tonopen ( 23.3  6.89 mmhg ) , and mackay - marg ( 23.5  6.1 mmhg ) tonometers . the mean iop values we calculated for cattle from the perkins tonometer data and calibration curve ( 18.8  1.7 mmhg ; range , 15.9~20.8 mmhg ) were also very similar to those directly measured by manometry ( 19.7  1.2 mmhg ; range , 18.0~22.0 mmhg ) . the calculated perkins tonometry iop values from our study were also close to those reported by gerometta et al .   that were measured using the same tonometer ( right eye : 16.1  1.0 mmhg ; range , 14.3~17.2 mmhg , and left eye : 16.5  1.2 mmhg ; range , 14.3~18.2 mmhg ) . however , the iop values from our study were lower than those reported by gum et al . , in holstein and jersey cows , using the mackay - marg tonometer were 27.5  4.8 mmhg ( range 16~39 mmhg ) , mackay - marg tonometer were 28.2  4.6 mmhg ( range 19~39 mmhg ) and tonopen xl tonometer were 26.9  6.7 mmhg ( range 16~42 mmhg ) . we chose to use an aneroid manometer instead of an electronic monitor or mercury column manometer used in other reports   because the aneroid manometer has been shown to be an accurate , practical and low - cost option for taking experimental iop measurements . the aneroid manometer is also easier to position at the correct height in relation to the eye . we did not compare the perkins tonometer with other tonometers in horses and cattle in this study . therefore , more research is needed to compare the accuracy of the perkins tonometer with that of other tonometers . compared to other tonometers , the perkins tonometer requires a longer training time to be used correctly in a clinical setting . nevertheless , this tonometer has the distinct advantages of relatively high accuracy and low cost . the results of our experiment demonstrated that the perkins handheld tonometer could be an additional method for accurately measuring iop in equine and bovine eyes in the future .