Anterior-posterior transcranial ultrasound to measure cranial oscillations.

BACKGROUND: We aimed to provide information on whether or not the correlation between body tilt and the pulse amplitude of transcranial ultrasonic time-of-flight waveform can be observed in the anterior-posterior skull direction. Also, we asked the question whether or not the skull pulsation can be detected since the cranial bones involved are thicker. METHODS: The experimental model of body tilt that alters intracranial pressure by shifting body fluid headward was employed. Transcranial ultrasound waveforms were examined in 15 healthy volunteers positioned at five tilt angles of +30 degrees, 0 degrees, -30 degrees, -60 degrees, and -90 degrees from the horizontal body position. A pulse-echo transducer was placed on the middle forehead and ultrasound waveforms were recorded. Synchronized variations in the ultrasonic time-of-flight with heartbeats were monitored using the pulsed phase locked loop technique for the output voltage of the ultrasound transducer. Simultaneous effects of body tilt on cardiovascular parameters were also evaluated. RESULTS: Pulse amplitudes of ultrasonic time-of-flight waveforms were found to vary with body tilt. Repeated-measures ANOVA and regression analysis showed a negative correlation between body tilt angle and pulse amplitude. The regression line has the equation: pulse amplitude = (1.158-0.01023 x tilt angle) x 10(-4) voltage. There was no such relationship between head-down body tilt and altered mean blood pressure or heart rate. CONCLUSION: An increase in the pulse amplitude of the anterior-posterior transcranial ultrasonic time-of-flight waveform can be detected when the head-down body tilt angle increases.

Relative Stability of Regional Facial and Ocular Temperature Measurements in Healthy Individuals.

Purpose: Non-contact measurement of facial temperature using infrared thermography has been used for mass screening of body temperature during a pandemic. We investigated the relative stability of temperature measurement in different facial regions of healthy individuals. Methods: Twenty healthy subjects underwent two experiments. In the first experiment, subjects washed their faces with a 20°C wet towel for 1 minute. Temperature changes compared to baseline in the forehead, cornea, inner canthus, and outer canthus were determined using an infrared camera for 10 minutes. In the second experiment, lubricating eye drops at 20°C were instilled over one eye. Temperature changes in the same regions of interest were monitored for 5 minutes. Results: Baseline temperatures before face washing in the forehead and cornea, inner canthus, and outer canthus of the right eye were 33.4°C ± 0.8°C (mean ± SD), 33.3°C ± 0.8°C, 34.3°C ± 0.7°C, and 32.8°C ± 0.7°C, respectively. Reductions in temperature due to face washing were most significant for the forehead and least significant for the cornea. One minute after face washing, the corresponding changes were -2.8°C ± 0.6°C, -0.3°C ± 0.6°C, -0.6°C ± 0.7°C, and -0.9°C ± 0.7°C for the forehead, cornea, inner canthus, and outer canthus, respectively. After administering the eye drops, no significant temperature changes were observed. Conclusions: When facial temperature was exogenously cooled, the cornea had the most stable temperature readings. Translational Relevance: When using infrared thermography to screen facial temperature, the measurement of corneal temperature is probably a better representative if the stability of temperature readings is critical.

Asymmetry of 24-hour intraocular pressure reduction by topical ocular hypotensive medications in fellow eyes.

PURPOSE: A core assumption for the 1-eye therapeutic trial of ocular hypotensive medications is the symmetrical reduction of intraocular pressure (IOP) in paired eyes. This assumption was evaluated for 24-hour IOP reduction in patients who underwent monotherapy or adjunctive therapy. DESIGN: Database study. PARTICIPANTS: Patients 41 to 79 years of age with primary open-angle glaucoma or ocular hypertension. METHODS: Twenty-four-hour IOP data from the paired eyes of patients undergoing bilateral monotherapy (n = 66) of latanoprost, travoprost, timolol, or brimonidine or bilateral adjunctive therapy (n = 52) with brinzolamide or timolol added to latanoprost monotherapy were analyzed retrospectively. Measurements of IOP were obtained every 2 hours in a sleep laboratory before and after at least 4-week drug treatments. Strengths of association for single-pair IOP reductions and average IOP reductions in the paired eyes during the office-hour, diurnal, nocturnal, and 24-hour periods and in different body positions were analyzed. MAIN OUTCOME MEASURES: Variance for the difference, percentage distribution of large absolute difference, and coefficient of determination (r(2)) in the paired IOP reductions. RESULTS: The standard deviations for the differences in single-pair IOP reductions from the means were larger than 2.5 mmHg for all periods and body positions under monotherapy and adjunctive therapy. Absolute differences in single-pair IOP reductions of the cutoff thresholds of 3 and 2 mmHg or more occurred in more than 20% and 36% cases, respectively. Corresponding coefficients of determination were 0.240 to 0.374 with monotherapy and 0.215 to 0.381 with adjunctive therapy. When the average differences in the paired IOP reductions were analyzed for a specific period and posture, the standard deviations for the differences in the paired IOP reductions and the percentage distributions of large absolute differences were reduced, and most coefficients of determination were improved. CONCLUSIONS: There is only a weak association between the right- and left-eye responses to IOP-lowering monotherapy or adjunctive therapy during a 24-hour period when single-pair IOP data are considered. Considering the averages of multiple paired IOP responses can improve the strength of the association. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

The Relationship Between Plasma Tetrahydrocannabinol Levels and Intraocular Pressure in Healthy Adult Subjects.

BACKGROUND: Δ9-tetrahydrocannabinol (THC) has been shown to decreased intraocular pressure (IOP). This project aims to define the relationship between plasma THC levels and IOP in healthy adult subjects. METHODS: Eleven healthy subjects received a single dose of inhaled cannabis that was self-administered in negative pressure rooms. Measurements of IOP and plasma THC levels were taken at baseline and every 30 min for 1 h and afterwards every hour for 4 h. IOP reduction and percent change in IOP over time were calculated. Linear regression models were used to measure the relationship between IOP and plasma THC levels. Two line linear regression models with F-tests were used to detect change points in the regression. Then, Pearson correlations were computed based on the change point. RESULTS: Twenty-two eyes met inclusion criteria. The average peak percentage decrease in IOP was 16% at 60 min. Percent IOP reduction as well as total IOP reduction demonstrated a negative correlation with THC plasma levels showing r-values of -0.81 and -0.70, respectively. F-tests revealed a change point in the regression for plasma levels >20 ng/ml. For levels >20 ng/ml, the correlation coefficients changed significantly with r-values of 0.21 and 0.29 (p < 0.01). CONCLUSION: Plasma THC levels are significantly correlated with IOP reduction up to plasma levels of 20 ng/ml. Plasma levels >20 ng/ml were not correlated with further decrease in IOP. More research is needed to determine the efficacy of THC in reducing IOP for eyes with ocular hypertension and glaucoma.

Intraocular pressure and choroidal thickness respond differently to lower body negative pressure during spaceflight.

Spaceflight-associated neuro-ocular syndrome (SANS) develops during long-duration (>1 mo) spaceflight presumably because of chronic exposure to a headward fluid shift that occurs in weightlessness. We aimed to determine whether reversing this headward fluid shift with acute application of lower body negative pressure (LBNP) can influence outcome measures at the eye. Intraocular pressure (IOP) and subfoveal choroidal thickness were therefore evaluated by tonometry and optical coherence tomography (OCT), respectively, in 14 International Space Station crewmembers before flight in the seated, supine, and 15° head-down tilt (HDT) postures and during spaceflight, without and with application of 25 mmHg LBNP. IOP in the preflight seated posture was 14.4 mmHg (95% CI, 13.5-15.2 mmHg), and spaceflight elevated this value by 1.3 mmHg (95% CI, 0.7-1.8 mmHg, P < 0.001). Acute exposure to LBNP during spaceflight reduced IOP to 14.2 mmHg (95% CI, 13.4-15.0 mmHg), which was equivalent to that of the seated posture (P > 0.99), indicating that venous fluid redistribution by LBNP can influence ocular outcome variables during spaceflight. Choroidal thickness during spaceflight (374 µm, 95% CI, 325-423 µm) increased by 35 µm (95% CI, 25-45 µm, P < 0.001), compared with the preflight seated posture (339 µm, 95% CI, 289-388 µm). Acute use of LBNP during spaceflight did not affect choroidal thickness (381 µm, 95% CI, 331-430 µm, P = 0.99). The finding that transmission of reduced venous pressure by LBNP did not decrease choroidal thickness suggests that engorgement of this tissue during spaceflight may reflect changes that are secondary to the chronic cerebral venous congestion associated with spaceflight.NEW & NOTEWORTHY Spaceflight induces a chronic headward fluid shift that is believed to underlie ocular changes observed in astronauts. The present study demonstrates, for the first time, that reversing this headward fluid shift via application of lower body negative pressure (LBNP) during spaceflight may alter the ocular venous system, as evidenced by a decrease in intraocular pressure. This finding indicates that LBNP has the potential to be an effective countermeasure against the headward fluid shift during spaceflight, which may then be beneficial in preventing or reversing associated ocular changes.

Diurnal and nocturnal effects of brimonidine monotherapy on intraocular pressure.

PURPOSE: To investigate the effect of brimonidine monotherapy on intraocular pressure (IOP) during the nocturnal/sleep period. DESIGN: Prospective, open-label experimental study. PARTICIPANTS: Fifteen patients with newly diagnosed open-angle glaucoma or ocular hypertension (ages, 46-72 years). METHODS: Baseline data of 24-hour IOP in untreated patients were collected in a sleep laboratory. Measurements of IOP were taken using a pneumatonometer every 2 hours in the sitting and supine body positions during the 16-hour diurnal/wake period and in the supine position during the 8-hour nocturnal/sleep period. Patients were treated afterward with 0.1% brimonidine 3 times per day for 4 weeks, and 24-hour IOP data were collected under the same laboratory conditions. MAIN OUTCOME MEASURES: Diurnal and nocturnal IOP means under the brimonidine treatment were compared with the baseline. RESULTS: The diurnal IOP mean was significantly lower under the brimonidine treatment than the baseline IOP in both the sitting and supine positions. There was no statistically significant change in IOP under the brimonidine treatment from the baseline during the nocturnal period. CONCLUSIONS: Although 0.1% brimonidine monotherapy significantly lowered IOP during the diurnal/wake period, it did not significantly lower IOP during the nocturnal/sleep period. FINANCIAL DISCLOSURE(S): Proprietary or commercial disclosure may be found after the references.

Correlation Between Office-Hour and Peak Nocturnal Intraocular Pressure in Patients Treated with Prostaglandin Analogs.

PURPOSE: To test the hypothesis that the correlation between office-hour intraocular pressure (IOP) and peak nocturnal IOP is weakened after using a prostaglandin analog. DESIGN: Before-and-after study. METHODS: Twenty-four-hour IOP data obtained in a sleep laboratory of 51 patients (22 patients with open-angle glaucoma and 29 patients with ocular hypertension) were reviewed. Patients had no IOP-lowering medication upon study entry and were then treated with prostaglandin monotherapy for 4 weeks. Measurements of IOP were taken every 2 hours in the sitting and supine positions during the diurnal/wake period (7:30 AM-9:30 PM) and in the supine position during the nocturnal/sleep period (11:30 PM-5:30 AM). Individual and average IOP readings during office hours (9:30 AM-3:30 PM) and peak IOP during the nocturnal/sleep hours were analyzed using the Pearson correlation coefficient and linear regression. RESULTS: There were statistically significant correlations for all the paired variables for the analyses. Average office-hour IOP had a higher correlation with peak nocturnal IOP than individual office-hour IOP. After the treatment with prostaglandin analog, the correlation between average office-hour IOP and nocturnal peak IOP in the sitting position (r = 0.373) and the supine position (r = 0.386) were reduced from the sitting baseline (r = 0.517) and the supine baseline (r = 0.573) in right eyes. Similar change patterns appeared in left eyes. CONCLUSION: There is a correlation between office-hour IOP reading and peak nocturnal IOP under no IOP-lowering treatment as well as under prostaglandin monotherapy. The strength of correlation was weaker under the treatment compared with baseline.

Comparing diurnal and nocturnal effects of brinzolamide and timolol on intraocular pressure in patients receiving latanoprost monotherapy.

PURPOSE: To compare the diurnal and nocturnal effects of brinzolamide and timolol on intraocular pressure (IOP) in patients already receiving monotherapy with latanoprost. DESIGN: Prospective, open-label, and crossover clinical trial. PARTICIPANTS: Twenty-six patients with glaucoma or ocular hypertension (ages, 44-79 years) who were receiving treatment with 0.005% latanoprost once every evening. METHODS: Baseline data of 24-hour IOP were collected in a sleep laboratory while patients were receiving latanoprost monotherapy. Measurements were taken every 2 hours in the sitting and supine positions during the 16-hour diurnal/wake period and in a supine position during the 8-hour nocturnal/sleep period. Patients were randomly assigned to receive an add-on treatment with either 1% brinzolamide 3 times per day or 0.5% timolol gel forming solution once every morning for 8 weeks, and then crossed over to receive the other add-on treatment. At the end of each add-on treatment period, 24-hour IOP data were collected. MAIN OUTCOME MEASURES: Diurnal and nocturnal IOP means were compared among the baseline, the brinzolamide add-on treatment, and the timolol add-on treatment. RESULTS: During the diurnal period, the mean IOP under brinzolamide or timolol add-on treatment was significantly lower than the baseline IOP in both the sitting and supine positions. There was no statistical difference between the 2 add-on treatments. During the nocturnal period, the supine IOP under brinzolamide add-on treatment was significantly lower than both the baseline and the timolol add-on treatment. There was no difference in nocturnal IOP between the timolol add-on treatment and the baseline. CONCLUSIONS: In patients already receiving the latanoprost monotherapy, adding brinzolamide or timolol significantly reduced IOP during the diurnal period. However, only the brinzolamide add-on treatment had an IOP-lowering efficacy during the nocturnal period.

Relationship of the 24-hour pattern of intraocular pressure with optic disc appearance in primary open-angle glaucoma.

PURPOSE: To study the relationship between the 24-hour pattern of intraocular pressure (IOP) with optic disc appearance in primary open-angle glaucoma (POAG) patients. DESIGN: Observational clinical study. PARTICIPANTS: Seventy-five eyes of 45 POAG patients. METHODS: Patients underwent 24-hour IOP assessment in a sleep laboratory. Two observers classified the optic disc appearance for each eye as either concentric or nonconcentric. The IOP measurements were obtained with the subjects in the supine and sitting positions during the diurnal period and in the supine position during the nocturnal period. The mean, peak, and trough IOP and IOP range (peak through trough) were calculated for the office-hour period (9 am to 4 pm), the diurnal period (7 am to 11 pm), the nocturnal period (11 pm to 7 am), and the 24-hour period. Further, the difference in supine and sitting IOP during the diurnal periods was calculated, and generalized estimating equations were used to compare IOP measurements in both groups. MAIN OUTCOME MEASURES: Diurnal and nocturnal IOP measurements. RESULTS: Forty eyes were classified as having concentric optic disc appearance and 35 eyes as having nonconcentric optic disc appearance. The mean nocturnal IOP was significantly greater in the concentric group (mean+/-standard deviation [SD], 24.0+/-3.8 mmHg) compared with the nonconcentric group (mean+/-SD, 21.9+/-1.9 mmHg; P = 0.004). Most IOP peaks of patients with the concentric optic disc appearance occurred during the nocturnal period, as opposed to the diurnal period of patients with the nonconcentric optic disc appearance. CONCLUSIONS: Concentric optic disc appearance may be associated with higher nocturnal IOP compared with nonconcentric optic disc appearance. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Intraocular pressure measurements throughout the 24 h.

PURPOSE OF REVIEW: This review summarizes the relationship of 24 h intraocular pressure (IOP) on the management of glaucoma. RECENT FINDINGS: The 24 h IOP pattern demonstrates nocturnal elevation in the majority of individuals. Prostaglandin analogs and carbonic anhydrase inhibitors lower both diurnal and nocturnal IOPs. Timolol monotherapy and timolol add-on treatment to a prostaglandin analog does not lower IOP during the nocturnal period. Laser trabeculoplasty can reduce nocturnal IOP elevation in medically treated glaucoma patients, even in those without significant reduction of diurnal IOP. Though both IOP and central corneal thickness display a 24 h rhythm with peaks during the nocturnal period, there is no correlation between central corneal thickness and 24 h IOP variation in normals and glaucoma patients. Corneal biomechanical properties (corneal hysteresis and corneal resistance factor) remain relatively stable during the 24 h period and are not associated with 24 h IOP fluctuation. SUMMARY: Antiglaucoma therapies differ in their ability to lower IOP throughout the 24 h day. The 24 h IOP pattern is independent of central corneal thickness, corneal hysteresis and corneal resistance factor.

Telemetric monitoring of 24 h intraocular pressure in conscious and freely moving C57BL/6J and CBA/CaJ mice.

PURPOSE: To study change patterns of 24 h intraocular pressure (IOP) in conscious and freely moving mice using telemetry. METHODS: Adult C57BL/6J and CBA/CaJ mice were entrained to a standard 12 h light and 12 h dark cycle. A telemetric pressure transmitter was implanted subcutaneously on the upper back of each light-dark entrained mouse, and the pressure catheter tip was inserted into the vitreous chamber. Broadcasted IOP data were received at 120 Hz. Means of 2 min IOP were recorded every 5 min for 4-13 days to generate the 24 h IOP pattern in each mouse strain. The pattern of IOP in the C57BL/6J strain was also determined under an acute constant dark condition for 24 h. RESULTS: There were distinct patterns of 24 h IOP in the C57BL/6J and CBA/CaJ mouse strains. Under the standard light-dark condition, IOP was higher during the dark period than the light period in both strains. Elevation in IOP from the light period to the dark period was significantly smaller in the CBA/CaJ strain (1.6+/-1.7 mmHg, mean+/- standard deviation (SD), n=21) than in the C57BL/6J strain (3.4+/-2.5 mmHg, n=20). The 24 h IOP pattern in the C57BL/6J strain persisted under an acute constant dark condition (n=8). CONCLUSIONS: Distinct change patterns of 24 h IOP appeared in these two mouse strains. Although mean IOP during the dark period was significantly higher than the light period in both strains, the magnitudes of light-dark IOP elevation differed. The 24 h IOP change pattern can be driven endogenously in the absence of light.

Direct effect of light on 24-h variation of aqueous humor protein concentration in Sprague-Dawley rats.

Sprague-Dawley rats 10-12 weeks of age were entrained to a standard light-dark cycle with lights turned on at 6 am and off at 6 pm. Variations of 24-h aqueous humor protein concentration were determined. Samples were taken every 4h (N=10-14) under the standard light-dark condition at 8 pm, midnight, 4 am, 8 am, noon, and 4 pm. Under an acute constant dark condition, when lights were not turned on at 6 am, samples were collected at 8 am, noon, 4 pm, and 8 pm. Aqueous humor protein concentrations under the standard light-dark condition were found in the range of 0.305+/-0.115 mg/ml (mean+/-SD, N=10) at midnight to 1.505+/-0.342 mg/ml (N=14) at noon. The 3 light-phase protein concentrations were each higher than the 3 dark-phase concentrations. Aqueous humor protein concentrations at 8 am, noon, and 4 pm under the acute constant dark condition were each higher than the concentrations at 8 pm (after both 2h and 26 h in the dark), midnight, and 4 am, demonstrating an endogenously driven 24-h pattern. At 8 am, noon, and 4 pm, protein concentrations were 56-147% higher when exposed to light. Intraocular pressure (IOP) was monitored using telemetry in separate groups of light-dark entrained rats under the standard light-dark condition and the acute constant dark condition. The 24-h IOP pattern was inverse to the 24-h pattern of aqueous humor protein concentration under the standard light-dark condition, and this IOP pattern was not altered by the acute constant dark condition. In conclusion, an endogenously driven 24-h variation of aqueous humor protein concentration occurred in Sprague-Dawley rats with higher concentrations during the light-phase than the dark-phase. This endogenous pattern of protein concentration was accentuated by a direct effect of light, which was unrelated to the 24-h pattern of IOP.

Association of central corneal thickness and 24-hour intraocular pressure fluctuation.

PURPOSE: To evaluate the association between office-hour central corneal thickness (CCT) and 24-hour intraocular pressure (IOP) fluctuation in patients with glaucoma. DESIGN: Observational case-control study. METHODS: Measurements of IOP were obtained every 2 hours during a 24-hour period from 52 untreated glaucoma patients and 29 age-matched normal control subjects housed in a sleep laboratory. Habitual IOP measurements were obtained using a pneumatonometer in the sitting positions during the diurnal/wake period (7 AM to 11 PM) and in the supine position during the nocturnal/sleep period (11 PM to 7 AM). CCT was measured in all subjects using ultrasound pachymetry once during office hours. The association between IOP fluctuation (peak IOP-trough IOP) during the 24-hour period and the office-hour CCT was assessed in both glaucoma patients and healthy age-matched controls using Spearman rank order correlation. RESULTS: There was no statistically significant correlation between IOP fluctuation and CCT in glaucomatous (P=0.405) and normal subjects (P=0.456). CONCLUSIONS: Twenty-four-hour IOP fluctuations were not correlated with single CCT measurements taken during office hours in glaucoma patients.

Effect of laser trabeculoplasty on nocturnal intraocular pressure in medically treated glaucoma patients.

PURPOSE: To evaluate the effects of laser trabeculoplasty on 24-hour intraocular pressure (IOP) in a group of medically treated open-angle glaucoma patients. DESIGN: Prospective experimental study. PARTICIPANTS: Eighteen open-angle glaucoma patients. METHODS: Laser trabeculoplasty (180 degrees ) was performed on 28 eyes of 18 glaucoma patients. Twenty-four-hour IOP data were collected in a sleep laboratory before and 45 to 80 days after the procedure. Measurements of sitting and supine IOP were taken during the 16-hour diurnal/wake period, and measurements of supine IOP were taken during the 8-hour nocturnal/sleep period in 2-hour intervals. MAIN OUTCOME MEASURES: Changes in the mean, peak, and range of IOP during the office-hour, diurnal, nocturnal, and 24-hour periods. RESULTS: Compared with the baselines, changes in the mean, peak, and range of IOP were not significant during the office-hour period and during the diurnal period in either the sitting or the supine position. The mean, peak, and range of IOP were reduced significantly during the nocturnal period in the supine position. Mean and peak 24-hour IOP were reduced significantly in the habitual body positions (sitting during the diurnal period and supine during the nocturnal period). The reduction of mean 24-hour IOP in the supine position also was significant. CONCLUSIONS: In this group of medically treated open-angle glaucoma patients, laser trabeculoplasty reduced IOP more consistently during the nocturnal period than during the diurnal period.

Resistance to blood flow in the rabbit ophthalmic artery after topical treatment with timolol.

PURPOSE: The aim of this study was to investigate the resistance to blood flow in the ophthalmic artery of rabbits receiving topical treatment with timolol. METHODS: Eight (8) New Zealand albino rabbits received 20 mul of timolol treatment (vehicle, 0.1%, 0.33%, 1%, and 3.3%) on the right eye. Blood-flow velocity in the ophthalmic artery was determined in the treated eye using color Doppler imaging (CDI) with a 12-MHz linear ultrasound transducer prior to the treatment and at 0.5, 1, 1.5, 2, and 3 h after the treatment. Intraocular pressure (IOP) was measured in both eyes, using a pneumatonometer at the same time points. Pourcelot's resistive index of blood flow was calculated, using the peak systolic velocity and the end diastolic velocity. A control experiment was performed with CDI obtained from the right eye when the left eye was treated with 1% timolol. RESULTS: In the eye treated with 1% and 3.3% timolol, a dose-dependent increase in the resistive index of blood flow occurred in the ophthalmic artery. No change in the resistive index occurred when the contralateral eye was treated with 1% timolol. Changes of IOP were not different between the two eyes under all the experimental conditions. Timolol, at all concentrations, caused a significant reduction of heart rate. A similar reduction of heart rate occurred when either eye was treated with 1% timolol. CONCLUSIONS: Topical treatment with timolol in rabbits can increase the resistance to blood flow in the ophthalmic artery. This effect is caused by a mechanism local to the eye and is not dependent on an IOP change.

Sympathetic activities influence blood-flow velocity and resistance in the rabbit ophthalmic artery.

PURPOSE: The aim of this study was to investigate sympathetic influences on blood-flow velocity and resistance in the rabbit ophthalmic artery during the transition period from the light to the dark phase. METHODS: Eight (8) New Zealand albino rabbits were entrained to a daily 12-h light and 12-h dark cycle. Blood-flow velocities in the ophthalmic artery were determined at -2 (baseline), 0, 2, and 4 h after the onset of darkness. Pulsed and color Doppler images of the ophthalmic artery were recorded using a 12-MHz linear ultrasound transducer. Resistive index of blood flow was calculated using the peak systolic velocity and the end diastolic velocity. Measurements of intraocular pressure (IOP) were taken, using a pneumatonometer at the same time points. Blood-flow velocity, resistive index, and IOP were also determined in the same rabbits after surgical decentralization of the ocular sympathetic nerves. RESULTS: Compared with the baseline at -2 h, a significant increase in the resistive index of blood flow in the ophthalmic artery occurred at 2 and 4 h after the onset of darkness. Parallel elevations of IOP were observed. After the sympathetic decentralization, peak systolic and end diastolic velocities decreased and resistive indexes increased from the presurgical values in the ophthalmic artery. In the postsurgical rabbits, elevation of IOP was absent during the light-dark transition period. There was also no time-dependent increase of resistive index in the ophthalmic artery. CONCLUSIONS: During the light-dark transition period, resistance to blood flow in the rabbit ophthalmic artery increases in parallel to IOP elevation. The removal of sympathetic activities decreases blood-flow velocity, but increases blood-flow resistance. Like the endogenous elevation of IOP, the time-dependent increase of resistance to blood flow in the rabbit ophthalmic artery during the light-dark transition period depends upon, at least partially, ocular sympathetic activities.

Effect of 24-hour corneal biomechanical changes on intraocular pressure measurement.

PURPOSE: To study 24-hour changes of corneal biomechanical properties and their influences on measurement of intraocular pressure (IOP). METHODS: Fifteen healthy young volunteers (age range, 20-25 years) were housed for 1 day in a sleep laboratory. Sitting and supine central corneal thickness (CCT) were measured every 2 hours with an ultrasonic pachymeter. Sitting IOP and corneal hysteresis, an indicator of viscoelasticity, were measured with a noncontact tonometer. RESULTS: There were consistent 24-hour variations of CCT and IOP for the group. Nocturnal mean CCT and nocturnal mean IOP were significantly higher than the diurnal mean CCT and diurnal mean IOP, respectively. The peak CCT occurred at 1:30 to 5:30 AM and the trough CCT at 1:30 PM. The peak IOP occurred at 5:30 AM and the trough IOP at 9:30 PM. Cosine fits of each subject's 24-hour CCT and IOP data showed synchronized rhythms. The phase timing of 24-hour CCT rhythm was significantly earlier than the phase timing of 24-hour IOP rhythm. Twenty-four-hour variation of corneal hysteresis was inconsistent and cosine fits of 24-hour data of corneal hysteresis did not display a 24-hour rhythm. CONCLUSIONS: In healthy young adults, CCT was thicker, and IOP was higher during the nocturnal period than during the diurnal period. Nocturnal peak CCT occurred a few hours earlier than did nocturnal peak IOP. The twenty-four-hour change in corneal viscoelasticity was not significant. There was no evidence that the 24-hour change in IOP was due to the change in corneal biomechanical properties.

Circadian variation of mouse aqueous humor protein.

PURPOSE: The first aim was to document the 24 h pattern of total protein concentration in aqueous humor of the C57BL/6J mouse strain under various environmental light-dark conditions. The second aim was to determine the diurnal/nocturnal change of aqueous humor protein concentration in the CBA/CaJ mouse strain reported to show a unique absence of diurnal/nocturnal elevation of intraocular pressure (IOP). METHODS: Mice of both strains were entrained to a daily 12 h diurnal/light (6 AM to 6 PM) and 12 h nocturnal/dark cycle. Total protein concentrations of aqueous humor were determined using specimens collected from C57BL/6J mice every 4 h under standard light-dark, acute constant dark, and acute constant light conditions. Aqueous humor protein concentrations in CBA/CaJ mice were determined at three diurnal times and three nocturnal times. RESULTS: The C57BL/6J mouse strain showed a higher aqueous humor protein concentration during the diurnal period than during the nocturnal period under standard light-dark conditions. This pattern persisted under acute constant dark and was modulated under acute constant light. The CBA/CaJ mouse strain showed a similar diurnal/nocturnal decrease of total aqueous humor protein concentration. CONCLUSIONS: Endogenous circadian variation of aqueous humor protein concentration occurs in the C57BL/6J mouse strain. The nocturnal concentration is significantly lower than the diurnal concentration. A similar diurnal/nocturnal decrease of aqueous humor protein concentration occurs in the CBA/CaJ mouse strain, indicating that this change of aqueous humor protein is unrelated to the diurnal/nocturnal pattern of IOP.

Asymmetry of right versus left intraocular pressures over 24 hours in glaucoma patients.

PURPOSE: To assess the asymmetry of 24-hour intraocular pressures (IOPs) between right and left eyes of untreated open-angle glaucoma (OAG) patients, and its influence on the monocular therapeutic trial. DESIGN: Experimental study with human patients. PARTICIPANTS: Forty-one subjects (40-78 years old) with untreated OAG. METHODS: Subjects were housed in a sleep laboratory for 24 hours. Intraocular pressures of both eyes were measured with a pneumatonometer every 2 hours with the patient in the sitting and supine positions from 7 am to 11 pm and in the supine position only from 11 pm to 7 am. Mean, peak, and trough IOPs were compared in right versus left eyes. The strength of association between right and left IOPs was assessed using coefficients of determination (R2). Validity of the monocular therapeutic trial was assessed by examining residual values from 2 linear regression models of right versus left IOP. MAIN OUTCOME MEASURES: Strength of association between IOPs of right and left eyes, and residual values from linear models of IOP. RESULTS: No statistically significant difference was found between mean, peak, or trough IOPs of right and left eyes over a 24-hour period. The strength of association for mean IOP was only moderate (R(2) = 0.421-0.623). Residual values of > or =3 mmHg were found in 14.0%+/-12.0% (mean +/- standard deviation) of IOP measurements for a symmetric linear regression model, and 8.5%+/-10.6% of IOP measurements for a best-fit linear regression model over 24 hours. CONCLUSIONS: In this group of untreated glaucoma patients, the strength of association between the right and left eye mean IOPs is only moderate. Residual values of > or =3 mmHg were more common using a symmetric model than with a best-fit model. The current method of performing monocular therapeutic trials in glaucoma patients is unreliable, but it may be possible to improve reliability by using several IOP measurements obtained at different times of the day instead of a single office measurement.