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Editors Selection IGR 17-3
Surgical Treatment: Drainage Devices
Comment by Kouros Nouri-Mahdavi on:
65891 Risk factors for the hypertensive phase after implantation of a glaucoma drainage device, Jung KI; Park CK, Acta Ophthalmologica, 2016; 94: e260-e267
Jung and Park present the results of a prospective study exploring the risk factors for development of hypertensive phase (HTP) after placement of a model FP-7 Ahmed Glaucoma Valve in a group of 128 eyes of 128 patients. They also explored the relationship between the risk factors and bleb wall parameters (thickness and relative density of the bleb wall) on anterior segment OCT (AS-OCT) in 38 eyes.
Longer axial length (> 25 mm) was found to be the strongest predictor of an HTP after surgery. Incidence of HTP was found to be very high in both high myopia and non-high myopia groups (95% vs. 73%, respectively). They reported a thinner capsule on AS-OCT to be associated with high myopia. Eyes with an HTP were found to have similar success rates as compared to those without.
The finding of an association between a thicker wall bleb with better IOP control is not consistent with our understanding of bleb remodeling as reported on histological studies by Dr. Molteno's group and other investigators
The reported findings are interesting but need to be considered in the context of the study. By defining the HTP as uncontrolled IOP at the three-month point, probably milder degrees of HTP were excluded, in which the IOP was normalized by medical treatment by the third month. The number of prior surgeries was not provided in the two groups. I wonder if highly myopic eyes had a higher rate of prior conjunctival surgeries leading to a more prominent HTP in this group. The success criteria used were lenient overall (IOP ≤ 21 mmHg and 20% reduction of IOP) and the very small number of failures may have led to the lack of a difference between success rates in the two groups (eyes with and without HTN or eyes with and without high myopia) on the Kaplan-Meier curves.
The reported relationship between the AL and development of HTP or bleb wall thickness is new and these findings will need to be confirmed in future studies. Some of the reported relationships might have been driven by outlier points (for example refer to Fig. 5).
The authors bring up an interesting theory about why the HTP may be more common in highly myopic eyes. They argue that based on animal studies, myopic sclera (and possibly Tenon's capsule) has less proteoglycans and hence has less water content and therefore, may be less impermeable to water movement. However, the finding of an association between a thicker wall bleb with better IOP control is not consistent with our understanding of bleb remodeling as reported on histological studies by Dr. Molteno's group and other investigators.
In summary, the association between high myopia and the higher incidence of a hypertensive phase is an unexpected finding that needs further exploration. Drs. Jung and Park bring up an interesting hypothesis that will need to be confirmed in future studies. In the meantime, it may be prudent for clinicians to watch for and treat the hypertensive phase more aggressively in patients with high myopia.
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