Spectral domain optical coherence tomography imaging of the aqueous outflow structures in normal participants of the EPIC-Norfolk Eye Study

Purpose To measure the dimensions of aqueous outflow structures and to investigate associations with intraocular pressure (IOP) and ocular biometry parameters in an older British population. Method Fifty-two participants from the European Prospective Investigation of Cancer-Norfolk eye study underwent imaging using the Heidelberg Spectralis optical coherence tomographer with an anterior segment module. Pseudophakic participants and those known or suspected to have glaucoma were excluded, leaving 46 participants for analysis. Schwalbe's line (SL), scleral spur (SS), Schlemm's canal (SC) diameter and the trabecular meshwork cross-sectional area (TM cross-sectional area (CSA)) were identified and traced using ImageJ software. IOP was measured using the Ocular Response Analyser. Ocular biometry was measured by partial coherence interferometry. Results The mean (SD) subject age was 65.7 years (5.6). The SL and SS were identifiable in all nasal and temporal scans. The mean SL–SS distance was 800 μm (104) nasally and 808 μm (102) temporally. Repeatability of SS–SL, SS–SC, SC and TM CSA was good to excellent, and reproducibility fair to good. Nasal SL–SS distance was inversely associated with anterior chamber depth (ACD) (p=0.007, −116 μm per mm ACD, R2=0.18). Multiple linear regression showed nasal TM CSA was significantly associated with age and IOP (age: p=0.025, 0.007 mm2 per decade of age; IOP: p=0.029, −0.0012 mm2 per mm Hg, R2=0.23). Conclusions Aqueous outflow structures can be measured by optical coherence tomography, and their dimensions vary significantly with ocular biometric characteristics and IOP. Further investigation is required to determine associations between outflow structure sizes in different populations and pathologies, including ocular hypertension and glaucoma.