The effect of adrenergic and cholinergic agents on anterior scleral thickness and Schlemm's canal in young healthy adults

Abstract

 

Purpose

 

To investigate the effect of 2% homatropine and 2.5% phenylephrine on anterior scleral thickness and Schlemm’s canal (SC) area in young healthy subjects using anterior segment spectral-domain optical coherence tomography (SD-OCT).

 

Methods

 

Fourteen subjects (mean age 27.9 ± 4 years) participated in this randomized, single-blind, placebo controlled study. Measurements of anterior scleral thickness and SC area were derived from SD-OCT (SOCT-Copernicus HR) images of the temporal anterior segment, before, 30 and 60 minutes following a single administration of either 2% homatropine, 2.5% phenylephrine or placebo. The pre- and post-instillation OCT images were manually segmented by a masked observer. Anterior sclera thickness was measured 1.75 mm posterior to Schwalbe’s line and included estimates of both episcleral (EpiST) and scleral thickness (ST) (Figure1).

 

Results

 

Homatropine resulted in a small but significant thickening of the anterior sclera and episclera at 30 and 60 minutes after instillation (mean change of 9.1 ± 3.5 μm and 7.5 ± 4 μm (episclera) and 4.4 ± 2.1 μm and 1.7 ± 3.1 μm (sclera); all p<0.01). Conversely , phenylephrine resulted in a thinning of the sclera and episclera (mean decrease of - 9.5 + 5.6 μm and - 15.4 ± 6.5 μm (episclera) and - 4.4 ± 2.5 μm and - 2.8 ± 2.5 μm (sclera); all p< 0.01). Homatropine caused an increase in the SC area, with a mean increase of 2175 ± 650 μm2 and 2615 ± 592 μm2 (both p< 0.0001) after 30 and 60 minutes respectively. No significant changes in SC area were found after either 2.5% phenylephrine or placebo at all examination points (p> 0.05).

 

Conclusions

 

Homatropine led to an increase in the anterior EpiST and ST along with an increase in the area of SC, whereas phenylephrine caused a small thinning of both EpiST and ST, but no significant change in the area of SC. The changes in the ST and EpiST are likely related to differences in the cholinergic-nitrergic and adrenergic input to the vasculature of the anterior segment, while the enlargement of SC with homatropine implies the involvement of a cholinergic-nitrergic pathway in modulation of the aqueous drainage system.

 

 

Representative cross-sectional OCT image of the temporal sclera illustrating the scanning protocol and analysis procedure.

 

 

Example of the increase in Schlemm’s canal area over time after homatropine administration in a representative subject.

 

Keywords: 421 anterior segment • 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)