Intraocular pressure following four different intravenous sedation protocols in normal horses

Abstract

Background Intravenous sedation is frequently necessary for ophthalmic examination in horses. Common sedation protocols have not been directly compared in terms of relative intraocular pressure (IOP) reduction, duration of IOP reduction and time to maximum IOP reduction. Objectives To compare the effects of standing sedation protocols on IOP. Study design Randomised cross‐over experiment. Methods Twelve healthy horses received four intravenous sedation protocols with a 48 hours washout: 0.5 mg/kg xylazine and 0.01 mg/kg butorphanol (SED1); 10 µg/kg detomidine and 0.01 mg/kg of butorphanol (SED2); 10 µg/kg detomidine (SED3); 0.5 mg/kg xylazine (SED4). IOP was measured with rebound tonometry before sedation (Tpre) and 5, 10, 15, 30, 45 and 60 minutes post‐sedation (Tpost). Post‐sedation readings were taken with the head elevated to the Tpre position. Separately, IOP readings were also obtained following sedation with the head not elevated (TpostHeadDown). IOP values were compared using mixed ANOVA and ANCOVA models respectively with significance at P < .05. Results All protocols decreased IOP compared with baseline with greatest reduction at Tpost5. IOP at Tpre (mean ± SD) was 21.8 ± 4.4 mm Hg. At Tpost5, IOP was 16.3 ± 3.8 mm Hg (SED1), 14.5 ± 2.9 mm Hg (SED2), 17.1 ± 3.8 mm Hg (SED3) and 16.9 ± 4.2 mm Hg (SED4). SED2 Tpost5 IOP was lower than other treatments. Considering all time points following sedation, SED3 IOP readings were higher than other treatments. TpostHeadDown IOPs were higher than readings taken with the head elevated (P < .001). Main limitations Animals with ocular disease were not studied. No animals received mock sedation or equivalent. Conclusions A combination of detomidine and butorphanol causes greater IOP reduction 5 minutes following sedation than other commonly used sedation protocols. IOP reduction is less pronounced when detomidine is used alone. Consideration of head height is important when performing IOP measurements in horses.

Retrobulbar lidocaine injection via the supraorbital fossa is safe in adult horses but produces regionally variable periocular anaesthesia

“…There was no difference in IOP between control and treated eyes at B‐A and B‐S (Figure 3A). As expected, mean IOP significantly decreased with sedation, but the difference was not different between treated and control eyes ( P = .6) 24‐26 . A significant increase in IOP was observed between treated and control eyes at the 10‐minute time point (mean difference [95% CI]: 4.0 [0.8, 7.3] mm Hg; P = .02).…”

“…The effect of the SFRB on IOP was unknown prior to this study, and a volume of 10 mL was chosen based on original references and data from prior studies 4,6,28 . As expected, a rapid and significant IOP decrease was observed in both eyes following IV administration of detomidine 24‐26 . Following SFRB with 10 mL of lidocaine, IOP was statistically significantly higher in the treated eye from 10 to 60 minutes with a mean difference of 4.0‐6.0 mm Hg.…”

“…4,6,28 As expected, a rapid and significant IOP decrease was observed in both eyes following IV administration of detomidine. [24][25][26] Following SFRB with 10 mL of lidocaine, IOP was statistically significantly higher in the treated eye from 10 to 60 minutes with a mean difference of 4.0-6.0 mm Hg. A 6-mmHg change in IOP would be considered clinically insignificant and safe in normotensive and intact equine eyes.…”

“…As expected, mean IOP significantly decreased with sedation, but the difference was not different between treated and control eyes (P = .6). [24][25][26] A significant increase in IOP was observed between treated and control eyes at the 10-minute time point (mean difference [95% CI]:…”

Comparison of four tonometers in the measurement of intraocular pressure in healthy horses

“…Direct manometry was tested to validate the Mackay‐Marg tonometer in horses, Tono‐Pen tonometer in dogs and horses, 20 and Perkins tonometer in horses and cattle 8 . Other factors that can affect IOP measurement in horses include the position of the head, the use of auriculo‐palpebral block, sedatives, time of the day, and the type of tonometer 9,21–24 …”