Analogs of microgravity: the function of Schlemm's canal, intraocular pressure and autonomic nervous during the head-down tilt test in healthy subjects

AIM:To evaluate the ocular outcomes and to elucidate possible mechanisms underlying intraocular pressure(IOP)change following the head-down tilt(HDT)test.METHODS:The study included 21 participants at the Department of Ophthalmology of Tongji Hospital.Subjects received the test of I-care tonometry,enhanced depth imaging optical coherence tomography and heart rate variability(HRV)analysis before and after 15 min HDT test.The lumen area of Schlemm’s canal(SCAR),IOP,HRV were calculated.RESULTS:IOP increased significantly after 20°head down position from 14.0±3.0 to 17.0±3.3 mm Hg(P<0.001).SCAR decreased from 13449.0±5454.9μm^(2) at sitting condition to 9576.6±4130.9μm^(2) post 15 min HDT test.High frequency(HF)indices increased significantly from 1462±865 Hz at baseline to 2128±824 Hz.Heart rate(HR)decreased significantly from 76±11.48 to 70±11.52 bpm after the HDT.The linear regression analysis showed that the difference of HF and SCAR significantly correlated with each other during the HDT(R^(2)=20%,P=0.04).CONCLUSION:These outcomes perform the first evidence of the activation of autonomic nervous system of HDT may cause the collapse of Schlemm’s canal lumen,which in turn leading to the increased IOP.