The Implications of an Ab Interno Versus Ab Externo Surgical Approach on Outflow Resistance of a Subconjunctival Drainage Device for Intraocular Pressure Control

Abstract

Purpose Minimally invasive glaucoma surgery (MIGS) devices that drain into the subconjunctival space can be inserted via an ab externo or ab interno approach. Limited experimental data exists as to the impact of either technique on intraocular pressure (IOP) control. We performed microfluidic studies by using ex vivo rabbit eyes to assess the effect of each approach on outflow resistance of a subconjunctival drainage device for IOP control. Methods A microfluidic experiment system was designed, consisting of a controlled reservoir of water connected to a pressure pump/flow sensor. The flow rate of water was fixed at 2 μl/min to simulate aqueous humor production. The pressure readings for each approach were recorded at a frequency of 1 Hz. A baseline reading was made before tube insertion into the eye (PEEK tube length set to aim for an initial outflow resistance of 5 to 10 mm Hg/μL/min) followed by measurements for a cumulative 2-ml volume entering the subconjunctival space. Results were adjusted for water viscosity at 37°C and reported as outflow resistance (mm Hg/μL/min ± standard error of mean). Results Outflow resistance via the ab interno approach was 90.4% higher than with the ab externo approach being measured at 0.80 ± 0.11 mm Hg/μL/min and 0.42 ± 0.05 mm Hg/μL/min, respectively. Bleb formation was observed to be less predictable with the ab interno approach. Conclusions The ab interno approach demonstrated greater outflow resistance and less predictable bleb formation than the ab externo approach. These results have implications for long-term IOP control and success depending on the approach to device insertion and could be an important consideration for future MIGS devices. Translational Relevance The effect of the ab interno versus ab externo approach of a MIGS device inserted into the subconjunctival space was assessed. The ab interno approach demonstrated greater outflow resistance and less predictable bleb formation that may have implications for the development of future MIGS devices.

A Tenon’s capsule/bulbar conjunctiva interface biomimetic to model fibrosis and local drug delivery

“…Compressed gels and porcine conjunctiva were incubated in 12 well plates in DMEM with 10% FBS, supplemented with drugs (test) or drug vehicles (control), and contraction was monitored for 21 days [ 9 ]. Doxycycline hyclate (Sigma-Aldrich, Gillingham, UK) was dissolved in water and used at the final concentration of 416 μM (as described in Li et al [ 19 ]), with the medium replaced with a fresh solution once a week. Ehop-016 and NSC23766 (both from Tocris Biosciences, Abingdon, UK) were resuspended in dimethyl sulfoxide, and added for 24 hours every seven days at final concentrations of 10 μM and 50 μM respectively.…”

“…Similarly, while models of drug diffusion and pharmacokinetics in the eye have been described, they often involve placing the drug-containing device in liquid medium to measure release, which does not suitably mimic drug diffusion in physiological tissues [ 18 ]. Indeed, the tissue itself may have a significant impact on flow, particularly when microimplants are used in different positions [ 19 ].…”

Critical Response to Ab Interno Versus Ab Externo Surgical Approach on Outflow Resistance of a Subconjunctival Drainage Device

“…We read with interest the recent paper by Lee et al 1 published in TVST entitled "The Implications of an Ab Interno Versus Ab Externo Surgical Approach on Outflow Resistance of a Subconjunctival Drainage Device for Intraocular Pressure Control. "…”

Author Response: Ab Interno versus Ab Externo Surgical Approach on Outflow Resistance of a Subconjunctival Drainage Device

“…Sheybani, Grover, and Fellman for their interest in our recent publication. 1 With regard to the comment that our title created confusion, our article was clearly designed to investigate the differences in outflow resistance in the subconjunctival space between devices inserted via an ab interno versus ab externo approach. We explicitly described the limitations of our laboratory study performed on ex vivo rabbit eyes.…”