Acute Stress Increases Intraocular Pressure in Nonhuman Primates

Effect of Ambient Lighting on Intraocular Pressure Rhythms in Rats

Purpose

The purpose of this study was to determine the effects of ambient lighting on intraocular pressure (IOP) rhythmicity and variability.

Methods

IOP was continuously recorded by wireless telemetry from rats under light/dark (LD), dark/light (DL), asymmetric (6L18D and 18D6L), constant dark (DD), and constant light (LL) cycles. In some DD experiments, 1-hour light pulses were presented at varying times. IOP rhythmicity and variability were respectively quantified via cosinor analysis and peak detection algorithms that identified transient and sustained fluctuations.

Results

Rat IOP peaked at night and troughed during the day with LD amplitude of 8.7 ± 3.4 mm Hg. Rhythmicity persisted in DD and LL with a free-running period of 24.1 ± 0.3 and 25.2 ± 0.4 hours, respectively. Peak-to-trough amplitude was approximately 60% smaller in LL, often disappeared after 1 to 2 weeks as daytime IOP drifted 2.6 ± 1.5 mm Hg higher, and returned to approximately 60% larger in LD. Rhythmicity was similarly impacted but resynchronized to DL over 4 to 6 days. Rhythmicity was unaltered by short photoperiods (6L18D), but the nocturnal IOP elevation was markedly shortened by long photoperiods (18L6D) and temporarily lowered to daytime levels by light pulses during the subjective night. Transient and sustained event rate, amplitude, interval, and energy content were nearly identical in LD, DD, and LL.

Conclusions

Aqueous humor dynamics of rat eyes are intrinsically configured to set IOP at daytime levels. Circadian clock input modulates these dynamics to elevate IOP at night. Light at night blocks this input, sending IOP back to daytime levels. Effects of abnormal lighting on IOP rhythmicity may contribute to pressure-related ocular neuropathies.

Effect of "365 Breathing Technique" on Intraocular Pressure and Autonomic Functions in Patients With Glaucoma: A Randomized Controlled Trial

PRCIS

Patients with primary open angle glaucoma were advised to follow the "365 breathing technique" for 6 weeks in addition to their pharmacological glaucoma treatment. It helped to reduce intraocular pressure (IOP), stress biomarker-cortisol, and improve autonomic dysfunction.

OBJECTIVE

To study the effect of the "365 breathing technique" on IOP, autonomic functions, and stress biomarkers in patients with primary open angle glaucoma.

METHODS

In this randomized, controlled, interventional trial, after randomization, 40 patients in the intervention group followed "365 breathing" (three times a day, breathing rate: 6 cycles/min for 5 min) in addition to their pharmacological glaucoma treatment and 40 patients in the control group continued only with their pharmacological glaucoma treatment. IOP, serum cortisol, heart rate variability (HRV), and heart rate response to deep breathing test (DBT) were recorded at preintervention and 6 weeks postintervention.

RESULTS

The mean IOP, serum cortisol, parameters of the HRV test, and DBT were comparable between the two groups at baseline. At the 6-week follow-up, in the intervention group, mean IOP was significantly lower (16.09 ± 2.24 vs 18.38 ± 1.58 mm Hg, P = 0.03) and serum cortisol were significantly lower (13.20 ± 3.11 vs 14.95 ± 2.60 mcg/dL, P = 0.038) compared with the control group. In the HRV test, time domain analysis showed a significant difference in the root mean square of the successive difference between RR interval values between both groups at 6 weeks ( P = 0.015) pointing towards higher postintervention parasympathetic activation in the intervention group. In frequency domain analysis (HRV test), the ratio of the low-frequency component to the high-frequency component was significantly lower in the intervention group at 6 weeks (1.65 vs 1.79, P = 0.019) indicating a shift in sympathovagal balance towards greater vagal modulation.There was a significant increase in delta heart rate ( P = 0.019) and expiratory:inspiratory ratio ( P = 0.011) in the intervention group at 6 weeks when compared with baseline values, indicating improved parasympathetic reactivity to DBT.

CONCLUSION

"365 breathing" technique can reduce IOP and serum cortisol, and improve autonomic dysfunction in patients with glaucoma.

IOP and glaucoma damage: The essential role of optic nerve head and retinal mechanosensors

There are many unanswered questions on the relation of intraocular pressure to glaucoma development and progression. IOP itself cannot be distilled to a single, unifying value, because IOP level varies over time, differs depending on ocular location, and can be affected by method of measurement. Ultimately, IOP level creates mechanical strain that affects axonal function at the optic nerve head which causes local extracellular matrix remodeling and retinal ganglion cell death - hallmarks of glaucoma and the cause of glaucomatous vision loss. Extracellular tissue strain at the ONH and lamina cribrosa is regionally variable and differs in magnitude and location between healthy and glaucomatous eyes. The ultimate targets of IOP-induced tissue strain in glaucoma are retinal ganglion cell axons at the optic nerve head and the cells that support axonal function (astrocytes, the neurovascular unit, microglia, and fibroblasts). These cells sense tissue strain through a series of signals that originate at the cell membrane and alter cytoskeletal organization, migration, differentiation, gene transcription, and proliferation. The proteins that translate mechanical stimuli into molecular signals act as band-pass filters - sensing some stimuli while ignoring others - and cellular responses to stimuli can differ based on cell type and differentiation state. Therefore, to fully understand the IOP signals that are relevant to glaucoma, it is necessary to understand the ultimate cellular targets of IOP-induced mechanical stimuli and their ability to sense, ignore, and translate these signals into cellular actions.

Ocular examination findings and selected ophthalmic diagnostic tests in African green monkeys (Chlorocebus aethiops sabaeus)

OBJECTIVE

To document ocular lesions and establish ophthalmic diagnostic test reference values in a colony of African green monkeys (Chlorocebus aethiops sabaeus).

ANIMALS STUDIED

Fifty one geriatric (GAGM, 19-30 years old), 10 adult (AAGM, 5-9 years old) and 10 juvenile (JAGM, <2 years old) African green monkeys housed in a single Caribbean research colony.

PROCEDURES

Ocular biomicroscopy, indirect fundoscopy, Schirmer tear test (STT), rebound tonometry (TonoVet®) and corneal fluorescein staining were performed. Mixed ANCOVA tests were performed to compare STT and IOP between groups.

RESULTS

Common ocular lesions in GAGM included vitreal degeneration (27/51, 51/102 eyes) and cataracts (21/51, 32/102 eyes). Vitreal degeneration was also common in AAGM (8/10, 16/20 eyes) and infrequent in JAGM (3/10, 6/20 eyes). Cataracts were not present in any JAGM or AAGM. All eyes in all three groups had perilimbal corneal pigmentation and faint lace-like anterior corneal stromal opacification. Median (range) STT values were 16.0 (18) mm/min in GAGM. Mean (SD) STT values were 14.2 (4.6) mm/min in AAGM, and 8.9 (3.4) mm/min in JAGM. Median (range) IOP values were 16.5 (27) mmHg in GAGM. Mean (SD) IOP values were 18.0 (2.8) mmHg in AAGM, and 14.1 (2.2) mmHg in JAGM. JAGM had significantly lower STT and IOP values compared to AAGM (p = .0449, .0057, respectively) and GAGM (p = .0002, .0130, respectively).

CONCLUSIONS

Spontaneous ocular lesions were common in geriatric monkeys in this research colony. IOP and STT values were lower in juvenile African green monkeys relative to adult or geriatric animals.

Characterization of intraocular pressure variability in conscious rats

Elevation of mean intraocular pressure (IOP) has long been recognized as a leading risk factor for glaucoma. Less is known about the possible contribution of moment-to-moment variations in IOP to disease development and progression due to limitations of tonometry, the prevailing method of IOP measurement. Tonometry provides good estimates of mean IOP but not IOP variance. The aim of this study was to quantitatively characterize IOP variability via round-the-clock IOP telemetry in conscious unrestrained rats. The anterior chamber of one eye was implanted with a microcannula connected to a wireless backpack telemetry system, and IOP data were collected every 4 s for one week. The cannula was then repositioned under the conjunctiva, and control data were collected for an additional week. IOP statistics were computed in 30-min intervals over a 24-h period and averaged across days. All animals exhibited a diurnal variation in mean IOP, while deviations about the mean were independent of time of day. Correlation analysis of the deviations revealed transient and sustained components, which were respectively extracted from IOP records using an event detection algorithm. The amplitude and interval distributions of transient and sustained events were characterized, and their energy content was estimated based on outflow tissue resistance of rat eyes. Transient IOP events occurred ∼231 times per day and were typically ≤5 mmHg in amplitude and 2-8 min in duration, while sustained IOP events occurred ∼16 times per day and were typically ≤5 mmHg in amplitude and 20-60 min in duration. Both persisted but were greatly reduced in control recordings, implying minor contamination of IOP data by motion-induced telemetry noise. Sustained events were also often synchronous across implanted animals, indicating that they were driven by autonomic startle and stress responses or other physiological processes activated by sensory signals in the animal housing environment. Not surprisingly, the total daily fluidic energy applied to resistive outflow pathways was determined primarily by basal IOP level. Nevertheless, transient and sustained fluctuations collectively contributed 6% and diurnal fluctuations contributed 9% to daily IOP energy. It is therefore important to consider the cumulative impact of biomechanical stress that IOP fluctuations apply over time to ocular tissues.

Comparing the effects of intraocular pressure and tear production measurements in horses in two different environments: Horse stable and medical barn

BACKGROUND

To date, there are no studies on the impact of two distinct environments-one familiar to the horse and another unfamiliar-on intraocular pressure (IOP) and tear production.

OBJECTIVES

To compare the measured IOP and tear production values in horses between a horse stable and a medical barn.

STUDY DESIGN

Cross-over.

METHODS

Thirty healthy male Arabian horses, aged 6.88 ± 3.34 years were used. IOP and tear production measurements were assessed in both the horse stable and the medical barn, with a paired Student's t-test and Bland-Altman analysis conducted for comparison and agreement, respectively.

RESULTS

A significant increase in IOP was observed in the medical barn (34.2 ± 6.8 mmHg) compared with the horse stable (29.5 ± 7.2 mmHg, p = 0.02). However, no statistically significant difference in tear production was found between horse stable (22.1 ± 2.8 mm/min) and medical barn (23.6 ± 3.4 mm/min) (p = 0.09). The standard error of the slope was 0.36 for the IOP measured in the medical barn, indicating a difference of -4.7 mmHg compared with the IOP measured in the horse stable (p = 0.02). The bias was fitted to y = -7.9350 + 0.1003x. The standard error of the slope was 0.39 for the tear production measured in the medical barn, indicating a difference of -1.5 mm/min compared with the tear production measured in the horse stable (p = 0.09). The bias was fitted to y = 6.1530 + -0.3367x.

MAIN LIMITATIONS

The absence of horses with ocular disorders and an assessment of the potential impact of transportation.

CONCLUSIONS

A notable increase in IOP was observed in the medical barn compared with the horse stable, while tear production exhibited no significant variance between the two environments. The Bland-Altman analysis highlighted a discrepancy in IOP measurements in the horse stable, emphasising the potential influence of the environment on ocular parameters in horses.

Diurnal Variation and Effects of Dilation and Sedation on Intraocular Pressure in Infant Rhesus Monkeys

PURPOSE

Intraocular pressure (IOP) is an important factor in numerous ocular conditions and research areas, including eye growth and myopia. In infant monkeys, IOP is typically measured under anesthesia. This study aimed to establish a method for awake IOP measurement in infant rhesus monkeys, determine diurnal variation, and assess the effects of dilation and sedation.

METHODS

Awake IOP (iCare TonoVet) was measured every 2 h from 7:30 am to 5:30 pm to assess potential diurnal variations in infant rhesus monkeys (age 3 weeks, n = 11). The following day, and every 2 weeks to age 15 weeks, IOP was measured under three conditions: (1) awake, (2) awake and dilated (tropicamide 0.5%), and (3) sedated (ketamine and acepromazine) and dilated. Intraclass correlation coefficient (ICC) was used to determine intersession repeatability, and repeated measures. ANOVA was used to determine effects of age and condition.

RESULTS

At age 3 weeks, mean (±SEM) awake IOP was 15.4 ± 0.6 and 15.2 ± 0.7 mmHg for right and left eyes, respectively (p=.59). The ICC between sessions was 0.63[-0.5 to 0.9], with a mean difference of 2.2 ± 0.3 mmHg. Diurnal IOP from 7:30 am to 5:30 pm showed no significant variation (p=.65). From 3 to 15 weeks of age, there was a significant effect of age (p=.01) and condition (p<.001). Across ages, IOP was 17.8 ± 0.7 mmHg while awake and undilated, 18.4 ± 0.2 mmHg awake and dilated, and 11.0 ± 0.3 mmHg after sedation and dilation.

CONCLUSIONS

Awake IOP measurement was feasible in young rhesus monkeys. No significant diurnal variations in IOP were observed between 7:30 am and 5:30 pm at age 3 weeks. In awake monkeys, IOP was slightly higher after mydriasis and considerably lower after sedation. Findings show that IOP under ketamine/acepromazine anesthesia is significantly different than awake IOP in young rhesus monkeys.

A portable feedback-controlled pump for monitoring eye outflow facility in conscious rats

Intraocular pressure (IOP) is heavily influenced by the resistance of trabecular outflow pathways through which most of the aqueous humor produced by the eye continuously drains. The standard method of quantifying outflow resistance and other aspects of ocular fluid dynamics is eye cannulation, which allows for direct measurement and manipulation of IOP and flow in animal models. Since the method is invasive, indirect techniques that are slower and less accurate must be used for chronological studies. A novel technology is introduced that can autonomously measure outflow facility in conscious rats multiple times a day. A smart portable micropump infuses fluid into the eye through a permanently-implanted cannula and dynamically adjusts flow rate using a unique proportional feedback algorithm that sets IOP to a target level, even though IOP fluctuates erratically in awake free-moving animals. Pressure-flow data collected by the system from anesthetized rats were validated against intraocular recordings with commercial pressure and flow sensors. System and sensor estimates of outflow facility were indistinguishable, averaging 23 ± 3 nl·min-1·mmHg-1 across animals (n = 11). Pressure-flow data were then collected round-the-clock for several days from conscious rats, while outflow facility was measured every few hours. A significant diurnal facility rhythm was observed in every animal (n = 4), with mean daytime level of 22 ± 10 nl·min-1·mmHg-1 and mean nighttime level of 15 ± 7 nl·min-1·mmHg-1. The rhythm correlated with diurnal changes in IOP and likely contributed prominently to those changes based on the day-night swing in facility magnitude. Hence, the portable smart pump offers a unique tool for repeated long-term monitoring of outflow facility and other possible parameters of ocular health. It could also be useful in animal glaucoma studies for reversibly inducing acute or chronic ocular hypertension without explicitly damaging trabecular outflow pathways.

Ocular Hypertension Results in Hypoxia within Glia and Neurons throughout the Visual Projection

The magnitude and duration of hypoxia after ocular hypertension (OHT) has been a matter of debate due to the lack of tools to accurately report hypoxia. In this study, we established a topography of hypoxia in the visual pathway by inducing OHT in mice that express a fusion protein comprised of the oxygen-dependent degradation (ODD) domain of HIF-1α and a tamoxifen-inducible Cre recombinase (CreERT2) driven by a ubiquitous CAG promoter. After tamoxifen administration, tdTomato expression would be driven in cells that contain stabilized HIF-1α. Intraocular pressure (IOP) and visual evoked potential (VEP) were measured after OHT at 3, 14, and 28 days (d) to evaluate hypoxia induction. Immunolabeling of hypoxic cell types in the retina and optic nerve (ON) was performed, as well as retinal ganglion cell (RGC) and axon number quantification at each time point (6 h, 3 d, 14 d, 28 d). IOP elevation and VEP decrease were detected 3 d after OHT, which preceded RGC soma and axon loss at 14 and 28 d after OHT. Hypoxia was detected primarily in Müller glia in the retina, and microglia and astrocytes in the ON and optic nerve head (ONH). Hypoxia-induced factor (HIF-α) regulates the expression of glucose transporters 1 and 3 (GLUT1, 3) to support neuronal metabolic demand. Significant increases in GLUT1 and 3 proteins were observed in the retina and ON after OHT. Interestingly, neurons and endothelial cells within the superior colliculus in the brain also experienced hypoxia after OHT as determined by tdTomato expression. The highest intensity labeling for hypoxia was detected in the ONH. Initiation of OHT resulted in significant hypoxia that did not immediately resolve, with low-level hypoxia apparent out to 14 and 28 d, suggesting that continued hypoxia contributes to glaucoma progression. Restricted hypoxia in retinal neurons after OHT suggests a hypoxia management role for glia.

Repeatability, reproducibility, and agreement of three tonometers for measuring intraocular pressure in rabbits

The aim of this study was to evaluate repeatability, reproducibility, and agreement of three commonly used tonometers in animal research (TonoLab, TonoVet, and TonoPEN AVIA) in a cohort of 24 rabbits. Additionally, the impact of sedation on IOP was investigated in 21 New Zealand White rabbits with the TonoVet tonometer. Repeatability was determined using the coefficient of variation (CoV) for two observers. For the TonoLab (6.55%) and TonoVet (6.38%) the CoV was lower than for the TonoPEN AVIA (10.88%). The reproducibility was highest for the TonoVet (0.2 ± 3.3 mmHg), followed by the TonoLab (0 ± 12.89 mmHg) and lowest for the TonoPEN AVIA (− 1.48 ± 10.3 mmHg). The TonoLab and TonoVet showed the highest agreement (r = 0.85, R² = 0.73). After sedation, a significant IOP reduction (often > 25%) was observed. Our results show that among the three tonometers tested, the TonoVet tonometer is best for use in rabbits while the TonoLab should be avoided. The impact of sedation on IOP was substantial and should be taken into account during experimentation.

Influence of mental stress on intraocular pressure and visual field testing: is there a white coat syndrome in glaucoma?

PURPOSE

To investigate the effects of mental stress stimulus on intraocular pressure (IOP) measurement and visual field (VF) testing.

METHODS

Patients with open angle glaucoma underwent a baseline IOP measurement and VF testing. Afterwards, they completed a computerized mental stress test (Stroop test) which is a known standardized method to induce mental stress. After test completion, patients underwent a second IOP measurement and VF testing.

RESULTS

Seventy-two eyes of 36 patients were enrolled. Mean age was 67.0 ± 9.5 years (range 47-84 years). The mean baseline IOP was 15.0 mmHg, and after the Stroop mental stress test, IOP increased to 16.0 mm Hg (P < 0.001). There was a trend towards significant mean deviation decreased from -6.9 dB to -8.0 dB (P = 0.054, t-test) following the stress test. This difference became significant using the Wilcoxon nonparametric test (P = 0.008). Correlation was found between glaucoma severity and change in IOP (P = 0.02) and PSD (P < 0.01).

CONCLUSIONS

We found a small but statistically significant increase in IOP and a trend towards deterioration of visual field mean deviation, following a short mental stress test. Patients with more severe glaucoma showed more pronounced changes. Our results suggest that mental stress could affect IOP measurement in the clinic.

Effects of acute stress, general anesthetics, tonometry, and temperature on intraocular pressure in rats

Intraocular pressure (IOP) is important for eye health as abnormal levels can led to ocular tissue damage. IOP is typically estimated by tonometry, which only provides snapshots of pressure history. Tonometry also requires subject cooperation and corneal contact that may influence IOP readings. The aim of this research was to investigate IOP dynamics of conscious animals in response to stressors, common anesthetics, tonometry, and temperature manipulations. An eye of male Brown-Norway rats was implanted with a fluid-filled cannula connected to a wireless telemetry system that records IOP continuously. Stress effects were examined by restricting animal movements. Anesthetic effects were examined by varying isoflurane concentration or injecting a bolus of ketamine. Tonometry effects were examined using applanation and rebound tonometers. Temperature effects were examined by exposing anesthetized and conscious animals to warm or cool surfaces. Telemetry recordings revealed that IOP fluctuates spontaneously by several mmHg, even in idle and anesthetized animals. Environmental disturbances also caused transient IOP fluctuations that were synchronous in recorded animals and could last over a half hour. Animal immobilization produced a rapid sustained elevation of IOP that was blocked by anesthetics, whereas little-to-no IOP change was detected in isoflurane- or ketamine-anesthetized animals if body temperature (BT) was maintained. IOP and BT decreased precipitously when heat support was not provided and were highly correlated during surface temperature manipulations. Surface temperature had no impact on IOP of conscious animals. IOP increased slightly during applanation tonometry but not rebound tonometry. The results show that IOP is dynamically modulated by internal and external factors that can activate rapidly and last long beyond the initiating event. Wireless telemetry indicates that animal interaction induces startle and stress responses that raise IOP. Anesthesia blocks these responses, which allows for better tonometry estimates of resting IOP provided that BT is controlled.

Comparison of extraocular and intraocular pressure transducers for measurement of transient intraocular pressure fluctuations using continuous wireless telemetry

The optimal approach for continuous measurement of intraocular pressure (IOP), including pressure transducer location and measurement frequency, is currently unknown. This study assessed the capability of extraocular (EO) and intraocular (IO) pressure transducers, using different IOP sampling rates and duty cycles, to characterize IOP dynamics. Transient IOP fluctuations were measured and quantified in 7 eyes of 4 male rhesus macaques (NHPs) using the Konigsberg EO system (continuous at 500 Hz), 12 eyes of 8 NHPs with the Stellar EO system and 16 eyes of 12 NHPs with the Stellar IO system (both measure at 200 Hz for 15 s of every 150 s period). IOP transducers were calibrated bi-weekly via anterior chamber manometry. Linear mixed effects models assessed the differences in the hourly transient IOP impulse, and transient IOP fluctuation frequency and magnitude between systems and transducer placements (EO versus IO). All systems measured 8000-12,000 and 5000-6500 transient IOP fluctuations per hour > 0.6 mmHg, representing 8-16% and 4-8% of the total IOP energy the eye must withstand during waking and sleeping hours, respectively. Differences between sampling frequency/duty cycle and transducer placement were statistically significant (p < 0.05) but the effect sizes were small and clinically insignificant. IOP dynamics can be accurately captured by sampling IOP at 200 Hz on a 10% duty cycle using either IO or EO transducers.

Intra-Subject Variability and Diurnal Cycle of Ocular Perfusion Pressure as Characterized by Continuous Telemetry in Nonhuman Primates

Purpose

To characterize ocular perfusion pressure (OPP) fluctuations with continuous telemetry over 24-hour periods across multiple days in nonhuman primates (NHPs) to test the hypotheses that OPP differs among NHPs and that the diurnal cycle of OPP is characterized by low OPP during sleep.

Methods

We have developed and validated two implantable radiotelemetry systems that allow continuous measurement of intraocular pressure (IOP), arterial blood pressure (BP), and OPP up to 500 Hz. OPP was measured unilaterally in 12 male NHPs for periods of 38 to 412 days. IOP transducers were calibrated directly via anterior chamber manometry, and OPP was calculated continuously as central retinal artery BP minus IOP. OPP data were corrected for signal drift between calibrations and averaged hourly.

Results

OPP varied widely among animals, with daily averages ranging from ∼47 to 65 mm Hg. In eight of 12 NHPs, OPP was significantly lower during sleep compared to waking hours. In three animals, the diurnal cycle was reversed and OPP was significantly higher during sleep (P < 0.05), and one NHP showed no diurnal cycle. Day-to-day OPP variability within NHPs was the largest source of overall OPP variability, even larger than the differences between NHPs. Average daily OPP showed an unexplained ∼32-day cyclic pattern in most NHPs.

Conclusions

Average OPP varied widely and exhibited differing diurnal cycles in NHPs, a finding that matches those of prior patient studies and indicates that OPP studies in the NHP model are appropriate. Infrequent snapshot measurements of either IOP or BP are insufficient to capture true IOP, BP, and OPP and their fluctuations.

Effect of Anesthesia on Intraocular Pressure Measured With Continuous Wireless Telemetry in Nonhuman Primates

Purpose

To compare the effects of both injectable anesthesia (ketamine/dexmedetomidine versus ketamine/xylazine) and inhalant anesthesia (isoflurane) on IOP using continuous, bilateral IOP telemetry in nonhuman primates (NHP).

Methods

Bilateral IOP was recorded continuously using a proven implantable telemetry system in five different sessions at least 2 weeks apart in four male rhesus macaques under two conditions: ketamine (3 mg/kg) with dexmedetomidine (50 μg/kg) or ketamine with xylazine (0.5 mg/kg) for induction, both followed by isoflurane for maintenance. IOP transducers were calibrated via anterior chamber manometry. Bilateral IOP was averaged over 2 minutes after injectable anesthetic induction and again after isoflurane inhalant had stabilized the anesthetic plane, then compared to baseline IOP measurements acquired immediately prior to anesthesia (both before and after initial human contact).

Results

When compared to pre-contact baseline measurements, ketamine/dexmedetomidine injectable anesthesia lowers IOP by 1.5 mm Hg on average (P < 0.05), but IOP did not change with ketamine/xylazine anesthesia. IOP returned to baseline levels shortly after isoflurane gas anesthesia was initiated. However, injectable anesthesia lowered IOP by an average of 5.4 mm Hg when compared to that measured after initial human contact (P < 0.01).

Conclusions

Anesthetic effects on IOP are generally small when compared to precontact baseline but much larger when compared to IOP measures taken after human contact, indicating that IOP is temporarily elevated due to acute stress (similar to a "white coat effect") and then decreased with anesthetic relaxation. Anesthetic induction with ketamine/xylazine and maintenance with isoflurane gas should be used when IOP is measured postanesthesia.