Research based on OCT imaging shows that, when the human body is upside down, the choroid can expand and the anterior chamber can become more shallow, a study published in the British Journal of Ophthalmology shows. 

Researchers sought to determine whether dynamic changes in choroidal thickness caused shallowing of the anterior chamber. They found that changing from sitting to an upside-down position results in choroidal thickening, anterior chamber shallowing and IOP elevation with reduced choroidal blood flow. They say this data provides evidence about the relationship between choroid expansion and shallowing of the anterior chamber, which may be of relevance for the pathogenesis of angle closure.

The team used 34 healthy participants to determine this. Researchers established baselines for their IOPs, choroidal thicknesses and anterior chambers using OCT. The participants were then placed upside down in an inversion machine for 1.5 minutes. The researchers measured IOP elevation and reviewed OCT images after being upside down and after 15 minutes of rest.

They evaluated 68 eyes from 34 subjects. After being upside down, researchers found a significant increase in choroidal thickness from 226.39µm±52.44µm to 238.34µm±54.84µm. The choroidal flow index also decreased to 0.3004±0.0190 from a baseline of 0.3357±0.0251. The subjects experienced a decrease in anterior chamber depth, from 3.21mm±0.22 mm to 3.13mm±0.21 mm and angle opening distance at 500µm from the scleral spur (which changed from 0.65mm±0.24mm to 0.58mm±0.20mm). 

Li F, Li H, Yang J, et al. Upside-down position leads to choroidal expansion and anterior chamber shallowing: OCT study. Br J Ophthalmol. September 13, 2019. [Epub ahead of print].