Intraocular pressure and ocular perfusion pressure in myopes during 21 min head-down rest

Myopic eyes show structural differences from normal eyes and may respond differently to microgravity, increasing the risk for possible development of ocular hypertension and glaucoma on orbit. In this experiment we used head-down rest (HDR) at an angle of 15 degrees to produce hydrostatic changes similar to acute exposure to microgravity.

There were 65 subjects (129 eyes) who were divided into groups characterized by refraction: emmetropes (N = 46; refraction error between -0.99 D and +0.10 D), low myopes (N = 39; > or = -1.0 D to < -3.0 D), and moderate myopes (N = 44; > or = -3.0 D to < -6.0 D). Each subject was studied resting in a horizontal position and after 1, 6, 11, 16, and 21 min of HDR. Measured variables included systolic and diastolic blood pressure (SBP and DBP, respectively), intraocular pressure (IOP), and ocular perfusion pressure (OPP).

The mean values of IOP increased significantly in all eyes during HDR, with IOP peaking at 6 min. Compared to emmetropes and low myopes, moderate myopes showed a significantly greater increase in IOP and higher peak values for IOP (18.6, 18.7, and 19.8 mmHg for emmetropes, low, and moderate myopes, respectively). Mean values of OPP in moderate myopes were significantly lower than in emmetropes and low myopes during HDR. Compared with baseline, mean SBP and DBP decreased obviously in emmetropes during HDR, while changes were minimal in the other groups.

Abnormal auto-regulation of ocular blood pressure in myopes of moderate and greater severity may pose a risk factor for developing ocular hypertension and possibly glaucoma when exposed to microgravity. HDR may offer a method of screening candidates for spaceflight for this risk prior to microgravity exposure.