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Editors Selection IGR 18-4

Surgical Treatment: Dual-blade Goniotomy vs. Ab-interno Trabeculotomy

Comment by Tanuj Dada & Shalini Mohan on:

112106 Randomised clinical trial for morphological changes of trabecular meshwork between Kahook dual-blade goniotomy and ab interno trabeculotomy with a microhook, Arimura S; Iwasaki K; Orii Y et al., Scientific reports, 2023; 13: 20783

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Arimura et al. have compared the postoperative cross-sectional incisional areas in trabecular meshwork (TM) between Kahook Dual Blade (KDB) goniotomy and ab interno trabeculotomy with a Microhook using AS-OCT. They randomized the recruited subjects into two groups of total 30 each, out of which 27 completed one year follow-up in the first group (KDB) and 25 in the second group (Microhook). An average of five measurement points set at 5° increments, contralateral to the temporal corneal incision were used for analysis. In addition to the primary outcome measure (incisional cross-sectional area in TM) the number of patients with unidentifiable incisional site was compared.

The authors reported that the mean cross-sectional area of the incision sites in TM was larger in the KDB group as compared to the Microhook group during all follow up visits (one week; p < 0.01, one month; p < 0.01, six months; p < 0.01, 12 months; p < 0.01). In addition, the number of patients with unidentified areas (indicating closure of the incision site) in the Microhook group was significantly higher than that in the KDB group between one and 12-month follow-up visits (one month; p = 0.02, six months; p = 0.03, 12 months; p = 0.02). The incisional site could be identified in 85% of patients in the KDB group at postoperative 12-month follow-up on ASOCT, while it was visible in only 48% of patients of the Microhook group.

However, this larger area at the incision site (KDB > Microhook) and visibility of the cleft, did not translate into better IOP outcomes as there was no statistically significant difference in IOP lowering or number of ocular hypotensive medications between the groups. The other important clinical finding was that the 12-month postoperative flare values were significantly higher in the KDB group than that in the Microhook group (p = 0.02).

The study highlights that the efficacy of IOP reduction might not depend on the incisional cross-sectional area of TM postoperatively

The study highlights that the efficacy of IOP reduction might not depend on the incisional cross-sectional area of TM postoperatively (up to one year). It is likely that aqueous may find its way to the Schlemm's canal (SC) through micro channels not visible on ASOCT and even a small circumference of opening in the TM may be sufficient for IOP lowering. However, a longer follow-up may have resulted in differential/worse IOP outcomes as rebound increase in IOP has been reported for the above procedures after a two-year follow-up period attributed to trabecular wound healing.¹

The present study provides insight into trabecular remodeling and repair following incisional MIGS procedures, but raises further questions regarding the correlation between trabecular opening size and IOP lowering.² What can be the possible reasons for this mismatch between trabecular opening vs IOP outcomes?

The study also underscores the current limitations of trabecular MIGS, a need for better understanding of the impact of trabecular injury and repair mechanisms, and development of pharmacological agents that target and inhibit wound healing in the TM after MIGS procedures.

It is important to understand that in addition to the wound healing in the inner wall of the SC, MIGS may induce injury to the outer wall of the canal and lead to fibrosis and closure of the collector channels and aqueous outflow pathways which are not visible on ASOCT. In fact, this may be more pronounced when a larger instrument like KDB is used as compared to the smaller tip of the Microhook. Dilating the canal with viscoelastic prior to cutting it may help to prevent injury to the outer wall of the SC.³ In addition, the higher long-term inflammation, indicated by higher flare (aqueous protein content) in the KDB group, may be an additional factor accentuating wound healing and worsening outflow obstruction in remaining/cut TM. Further, the pathological process which caused POAG in the first place with accumulation of extracellular debris /plaques on the TM also continues unabated and may move further downstream and directly impact the collector channels, increasing the intrascleral outflow resistance.

The study also underscores the current limitations of trabecular MIGS, a need for better understanding of the impact of trabecular injury and repair mechanisms, and development of pharmacological agents that target and inhibit wound healing in the TM after MIGS procedures.

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