The effect of increased intra-abdominal pressure on orbital subarachnoid space width and intraocular pressure

In accordance with the trans-lamina cribrosa pressure difference theory, decreasing the trans-lamina cribrosa pressure difference can re- lieve glaucomatous optic neuropathy. Increased intracranial pressure can also reduce optic nerve damage in glaucoma patients, and a safe, effective and noninvasive way to achieve this is by increasing the intra-abdominal pressure. The purpose of this study was to observe the changes in orbital subarachnoid space width and intraocular pressure at elevated intra-abdominal pressure. An inflatable abdominal belt was tied to each of 15 healthy volunteers, aged 22-30 years （12 females and 3 males）, at the navel level, without applying pressure to the abdomen, before they laid in the magnetic resonance imaging machine. The baseline orbital subarachnoid space width around the optic nerve was measured by magnetic resonance imaging at 1, 3, 9, and 15 mm behind the globe. The abdominal belt was inflated to increase the pressure to 40 mmHg （1 mmHg = 0.133 kPa）, then the orbital subarachnoid space width was measured every 10 minutes for 2 hours. After removal of the pressure, the measurement was repeated 10 and 20 minutes later. In a separate trial, the intraocular pressure was measured for all the subjects at the same time points, before, during and after elevated intra-abdominal pressure. Results showed that the baseline mean orbital subarachnoid space width was 0.88 ＋ 0.1 mm （range： 0.77-1.05 mm）, 0.77 ＋ 0.11 mm （range： 0.60-0.94 mm）, 0.70 ＋ 0.08 mm （range： 0.62-0.80 ram）, and 0.68 _＋ 0.08 mm （range： 0.57-0.77 mm） at 1, 3, 9, and 15 mm behind the globe, respectively. During the elevated intra-abdominal pressure, the orbital subarachnoid space width increased from the baseline and dilation of the optic nerve sheath was significant at 1, 3 and 9 mm behind the globe. After decompression of the abdominal pressure, the orbital subarachnoid space width normalized and returned to the baseline value. There was no significant difference in the intraocular pressure before, during and after the intra-abdominal pressure elevation. These results verified that the increased intra-abdominal pressure widens the orbital subarachnoid space in this acute trial, but does not alter the intraocular pressure, indicating that intraocular pressure is not affected by rapid increased in- tra-abdominal pressure. This study was registered in the Chinese Clinical Trial Registry （registration number： ChiCTR-ONRC-14004947）.

Longitudinal observation of intraocular pressure variations with acute altitude changes

BACKGROUND Higher intraocular pressure(IOP)is a major risk factor for developing glaucoma,and the leading cause of irreversible blindness worldwide.High altitude(HA)may be involved in IOP,but the reported results were conflicting.Ascent to HA directly by plane from low altitude regions is an acute,effortless exposure.However,the effects of such exposure to different altitudes on IOP have rarely been reported.AIM To investigate changes in IOP after rapid effortless exposure to HA in stages and compare it with systemic parameters.METHODS Fifty-eight healthy subjects(116 eyes)were divided into three groups:17 lowaltitude(LA)residents[44 m above sea level(ASL)],22 HA residents(2261 m ASL)and 19 very HA(VHA)residents(3750 m ASL).The LA group flew to HA first.Three days later,they flew with the HA group to VHA where both groups stayed for 2 d.Then,the LA group flew back to HA and stayed for 1 d before flying back to 44 m.IOP,oxygen saturation(SpO2)and pulse rate were measured.The linear mixed model was used to compare repeated measurements.RESULTS IOP in the LA group significantly decreased from 18.41±2.40 mmHg at 44 m to 13.60±3.68 mmHg at 2261 m ASL(P<0.001),and then to 11.85±2.48 mmHg at 3750 m ASL(P=0.036 compared to IOP at 2261 m ASL)and partially recovered to 13.47±2.57 mmHg upon return to 44 m.IOP in the LA group at HA and VHA was comparable to that in the local residents(12.2±2.4 mmHg for HA,11.5±1.8 mmHg for VHA).IOP was positively associated with SpO2 while negatively associated with pulse rate.CONCLUSION IOP in the LA group gradually reduced as altitude elevated in stages and became comparable to IOP in local residents.Hypoxia may be associated with IOP,which deserves further study.

Reproducibility of Perfusion Parameters of Optic Disc and Macula in Rhesus Monkeys by Optical Coherence Tomography Angiography

Background： Optical coherence tomography （OCT） angiography is a novel technique by which we can detect the local perfusion of fundus directly. The aim of this study was to evaluate the reproducibility of optic disc and macular flow perfusion parameters in rhesus monkeys using OCT angiography. Methods： Eighteen healthy monkeys （18 eyes） were subjected to optic disc and macula flow index measurements via a high-speed and high-resolution spectral-domain OCT XR Avanti with a split-spectrum amplitude de-correlation angiography algorithm. Right eye was imaged 3 times during the first examination and once during each of the two following examinations. The intra-visit and inter-visit intraclass correlation coefficients （ICCs） were both determined. Results： The average flow indices of the four optic disc area layers were 0.171 ± 0.009 （optic nerve head）, 0.015 ± 0.004 （vitreous）, 0.052 ±0.009 （radial peripapillary capillary）, and 0.167 ± 0.011 （choroid）. Average flow indices of the four macula area layers were 0,044 ± 0.011 （superficial retina）, 0.036 ± 0.011 （deep retina）, 0.016 ± 0.009 （outer retina）, and 0.155 ± 0.013 （choroid）. Intra-visit （1CC value： 0.821-0.954） and inter-visit （ICC value： 0.844±0.899） repeatability were both high. Conclusions： The study is about the reproducibility of optic disc and macular perfusion parameters as measured by OCT angiography in healthy rhesus monkeys. Flow index measurement reproducibility is high for both the optic disc and macula of normal monkey eyes. OCT angiography might be a useful technique to assess changes when examining monkeys with experimental ocular diseases.

Reduction on OFF-responses of Electroretinogram in Monkeys with Long-term High Intraocular Pressure

Background： There are ON- and OFF-pathways in the normal vertebrate retina. Short- and long-flash electroretinogram （ERG） are suitable methods to observe the function of ON- and OFF-pathways in vivo, respectively. It is clear that high intraocular pressure （IOP） might cause dysfunction of cone-dominated photopic negative response （PhNR） in monkeys with high lOP in ON-pathway. However, whether cone-dominated OFF-responses are also affected is less known. The aim of this study was to observe photopic OFF-responses of ERG in monkeys with high lOP. Methods： Nine monkeys were involved in the experiment from January 2006 to December 2016. High IOP was induced in the right eye by laser coagulation of the mid-trabecular meshwork in five monkeys. Six years after the laser coagulation, both short- and long-flash of the photopic ERG were recorded. Stimulus light was red flashes superimposed on a blue background. Four normal monkeys were examined under the same ERG protocols as controls. Paired t-test was used to compare the difference of each ERG parameter between the lasered eye and the fellow eye. Analysis of variance （ANOVA） with Tukey adjustment was adopted to calculate the differences among the lasered eye, the fellow eye, and the eyes of normal monkeys. Results： The mean amplitude of a-wave （11.73 ± 2.05） and PhNR （8.67 ± 2.44） in lasered eyes was significantly lower than that of a-wave （21.47 ±3.15） and PhNR （22.05 ± 3.42） in fellow eyes （P = 0.03 and P 0.01, respectively） in response to short flash. The mean amplitude old-wave （ 1.60 ± 0.59） and/-wave （3.13± 0.64） was significantly reduced in the lasered eyes than that of d-wave （4.01 ± 0.56） and/-wave （8.79 ± 1.75） in the fellow eyes （P =0.02 and P = 0.02, respectively） in response to long flash. Conclusions： Reduced OFF-responses are recorded in monkeys with high IOP when dysfunction ofphotoreceptor is involved. The reduced OFF-responses to long-flash stimulus show evidence of anomalous retinal circuitry in glaucomatous retinopathy.