No abstract

Hydraulic permeability is a topic of deep interest in biological materials because of its important role in a range of drug delivery-based therapies. The strong dependence of permeability on the geometry and topology of pore structure and the lack of detailed knowledge of these parameters in the case of brain tissue makes the study more challenging. Although theoretical models have been developed for hydraulic permeability, there is limited consensus on the validity of existing experimental evidence to complement these models. In the present study, we measure the permeability of white matter (WM) of fresh ovine brain tissue considering the localised heterogeneities in the medium using an infusion based experimental set up, iPerfusion. We measure the flow across different parts of the WM in response to applied pressures for a sample of specific dimensions and calculate the permeability from directly measured parameters. Furthermore, we directly probe the effect of anisotropy of the tissue on permeability by considering the directionality of tissue on the obtained values. Additionally, we investigate whether WM hydraulic permeability changes with post-mortem time. To our knowledge, this is the first report of experimental measurements of the localised WM permeability, showing the effect of axon directionality on permeability. This work provides a significant contribution to the successful development of intra-tumoural infusion-based technologies, such as convection-enhanced delivery (CED), which are based on the delivery of drugs directly by injection under positive pressure into the brain.

Reduction of intraocular pressure is the sole therapeutic target for glaucoma. Intraocular pressure is determined by the dynamics of aqueous humour secretion and outflow, which comprise several pressure-dependent and pressure-independent mechanisms. Accurately quantifying the components of aqueous humour dynamics is essential in understanding the pathology of glaucoma and the development of new treatments. To better characterise aqueous humour dynamics, we propose a method to directly measure pressure-independent aqueous humour flow. Using the iPerfusion system, we directly measure the flow into the eye when the pressure drop across the pressure-dependent pathways is eliminated. Using this approach we address i) the magnitude of pressure-independent flow in ex vivo eyes, ii) whether we can accurately measure an artificially imposed pressure-independent flow, and iii) whether the presence of a pressure-independent flow affects our ability to measure outflow facility. These studies are conducted in mice, which are a common animal model for aqueous humour dynamics. In eyes perfused with a single cannula, the average pressure-independent flow was 1 [−3, 5] nl/min (mean [95% confidence interval]) (N=6). Paired ex vivo eyes were then cannulated with two needles, connecting the eye to both iPerfusion and a syringe pump, which was used to impose a known pressure-independent flow of 120 nl/min into the experimental eye only. The measured pressure-independent flow was then 121 [117, 125] nl/min (N=7), indicating that the method could measure pressure-independent flow with high accuracy. Finally, we showed that the artificially imposed pressure-independent flow did not affect our ability to measure facility, provided that the pressure-dependence of facility and the true pressure-independent flow were accounted for. The present study provides a robust method for measurement of pressure-independent flow, and demonstrates the importance of accurately quantifying this parameter when investigating pressure-dependent flow or outflow facility.

Intraocular pressure (IOP) is not static, but rather oscillates by 2-3 mmHg due to cardiac pulsations in ocular blood volume known as the ocular pulse. The ocular pulse induces pulsatile shear stress in Schlemm's canal (SC). We hypothesize that the ocular pulse modulates outflow facility by stimulating shear-induced nitric oxide (NO) production by SC cells. We confirmed that living mice exhibit an ocular pulse with a peak-to-peak (pk-pk) amplitude of 0.5 mmHg under anaesthesia. Using iPerfusion, we measured outflow facility (flow/pressure) during alternating periods of steady or pulsatile IOP in both eyes of 16 cadaveric C57BL/6 mice (13-14 weeks). Eyes were retained in situ, with an applied mean pressure of 8 mmHg and 1.0 mmHg pk-pk pressure amplitude at 10 Hz to mimic the murine heart rate. One eye of each cadaver was perfused with 100 µM L-NAME to inhibit nitric oxide synthase, while the contralateral eye was perfused with vehicle. During the pulsatile period in the vehicle-treated eye, outflow facility increased by 16 [12, 20] % (p<0.001) relative to the facility measured during the preceding and subsequent steady periods. This effect was partly inhibited by L-NAME, where pressure pulsations increased outflow facility by 8% [4, 12] (p < 0.001). Thus, the ocular pulse causes an immediate increase in outflow facility in mice, with roughly one-half of the facility increase attributable to NO production. These studies reveal a dynamic component to outflow function that responds instantly to the ocular pulse and may be important for outflow regulation and IOP homeostasis.

To investigate aqueous humor dynamics in uveitic eyes. DESIGN: Cross-sectional study. PARTICIPANTS: Patients with recurrent (‡3 attacks) anterior uveitis (now quiescent) and being treated for glaucoma or ocular hypertension (OHT) (Group 1), previous recurrent anterior uveitis (‡3 attacks) without glaucoma or OHT (Group 2), and normal subjects with no ocular problems and IOP < 21 mm Hg at screening (control group; Group 3). METHODS: Patients had one-off measurements. Group 1 patients who were on antihypertensives were washed out for a 4-week period, prior to their study measurements. Main outcome measures were tonographic outflow facility, aqueous humor flow rate, and uveoscleral outflow. RESULTS: One hundred and one patients were screened between February 2014 and February 2017. Nine patients did not meet the inclusion criteria. Groups 1 and 3 each included 30 patients, and Group 2 included 32 patients. The mean intraocular pressure was higher in Group 1 compared to the others (25 ± 10.2 mm Hg in Group 1 vs 16 ± 2.7 mm Hg in Group 2 vs 16 ± 2.2 mm Hg in Group 3, P < .001). The tonographic outflow facility was lower in Group 1 compared to the others (0.18 ± 0.1 mL/min/mm Hg in Group 1 vs 0.25 ± 0.1 mL/min/mm Hg in Group 2 vs 0.27 ± 0.1 mL/ min/mm Hg in Group 3, P [ .005). However, aqueous humor flow rate was not statistically different (2.47 ± 0.9 mL/min in Group 1 vs 2.13 ± 0.9 mL/min in Group 2 vs 2.25 ± 0.7 mL/min in Group 3, P [ .3). There was also no significant difference in calculated uveoscleral outflow. CONCLUSION: This is the first aqueous humor dynamics study in patients with uveitic glaucoma/OHT and recurrent anterior uveitis compared with agematched controls. We have demonstrated that the elevated intraocular pressure seen in the uveitic glaucoma/OHT eyes (3-6 attacks) was due to reduced tonographic outflow facility. The aqueous humor flow rate was not detectibly different, nor did the calculated uveoscleral outflow demonstrate any discernible difference. However, the exact mechanism remains to be elucidated.