Exploring literature-based definitions of hypotony following glaucoma filtration surgery and the impact on clinical outcomes.

PURPOSE: To highlight the variations in published definitions of hypotony and their impact on reported clinical outcomes. To propose a revised definition, focusing on clinically significant hypotony (CH).
METHODS: Literature review of hypotony definitions published between January 2010 and December 2015 was carried out. Numerical definitions for hypotony, its onset, duration and associated clinical signs were recorded. Each definition was applied to surgical outcomes data collected prospectively from a cohort of 300 glaucoma patients treated at a single centre. The sensitivity and specificity of each definition in identifying CH [defined as low intraocular pressure (IOP) with signs of maculopathy hypotony and choroidal detachment] were calculated.
RESULTS: A total of 128 eligible papers were identified, and 14 different definitions for hypotony were extracted. In 53 (41.4%), hypotony was not defined. In the remaining 75 (58.6%), the numerical definitions varied between 4 and 8 mmHg, and of these, 24 (32%) included the onset and duration of hypotony as part of the definition. Definition-dependent hypotony rates within the cohort varied between 1% and 59.3%. No statistical differences were found between the groups based on corneal thickness or axial length. Clinically significant hypotony (CH) was identified in 37 (12.3%), with large differences in the sensitivity and specificity among published definition.
CONCLUSION: Variations in defining postoperative hypotony can have a large impact on the reported success and failure rates among studies. There is a need for a more robust universal definition, focusing on clinically important signs, to allow better comparison between different treatment modalities.