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Page 6 of 7 Wiens et al. Mini-invasive Surg 2021;5:8 I http://dx.doi.org/10.20517/2574-1225.2020.105

As shown, eyes presenting with a reticular patterned episcleral venous plexus are good candidates for
procedures like the hemi-GATT that target a larger sector of Schlemm’s canal. Not only was there a
reduction in IOP, but there was also an elimination of medication burden associated with glaucoma.
Furthermore, since the subject presented with a diffuse reticular pattern, one might have been tempted to
perform a GATT to target the entire drainage system circumference. However, previous preliminary results
comparing the success rates for the GATT and hemi-GATT show no significant difference (success rate of
[9]
74% for GATT and 70% hemi-GATT) . Herein, the hemi-GATT was shown to enhance drainage in both
the superior and inferior sectors while preserving the superior 180-degrees of Schlemm’s canal to enable
future angular surgery, if needed.

When determining the approach to performing a hemi-GATT, a surgeon must choose the sector to
target. Due to the ease of surgical access, the nasal quadrant is most commonly favored as the location for
Schlemm’s canal surgeries. However, surgical ease is not the sole reason for this target. The nasal quadrant
[2]
is also the location of the highest density of collector channels . With 25-35 collector channels per eye, one
desires to target as many of these as possible when performing a hemi-GATT to maximize conventional
[3]
aqueous flow . To exit the eye via the conventional pathway, aqueous fluid must pass through the collector
channels for subsequent drainage into the deep venous scleral plexus, the mid scleral plexus and the
[3]
episcleral plexus to finally reach the conjunctival veins . While no clinically established marker has yet
been proven to conclusively predict the likelihood of success with canal-based MIGS procedures, growing
[3,6]
evidence supports the presence of an EVFW as a marker of surgical success . Research has shown that
the ability to elicit a pronounced EVFW with diffuse blanching of the visible vessels correlates with the
[3]
need for fewer medications and lower postoperative IOP . It is theorized that eyes with a positive EVFW
must have patent collector channels and a downstream outflow system for the infused BSS to blanch the
[3]
episcleral vessels . As demonstrated here, the inferior 180-degrees incorporating the infra-nasal quadrant
serves as an optimal location for hemi-GATT to best enhance aqueous outflow. Despite leaving the
superior 180-degrees of the eye untreated, diffuse episcleral venous blanching in this area was observed
with BSS infusion. In some patients, treatment of a section of Schlemm’s canal may be sufficient to achieve
the desired surgical outcome.

In conclusion, a hemi-GATT targeting the inferior 180-degrees of Schlemm’s canal is a MIGS procedure
that is applicable to eyes demonstrating a reticular pattern episcleral venous plexus. The ability to elicit
a pronounced EVFW post-hemi-GATT that was seen in both the superior untreated sector and the
inferior treated sector, indicates patency of the collector channels and enhanced aqueous outflow via the
conventional outflow pathway. Further work may help determine the ideal glaucoma surgical procedure
based on a patient’s particular episcleral venous pattern.

DECLARATIONS
Authors’ contributions
Made substantial contributions to conception and design of the study, and performed data analysis and
interpretation: Wiens J, Gooi M, Schlenker M, Gooi TL, Wentzell D, Gooi P

Availability of data and materials
Not applicable.

Financial support and sponsorship
None.