Optic nerve dysfunction during gravity inversion. Pattern reversal visual evoked potentials
T. R. Friberg and G. Sanborn
The intraocular pressure (IOP) in humans approximately doubles when a
head-down, or inverted, position is assumed. Simultaneously, the ophthalmic
artery pressure increases by an even greater absolute magnitude. We
recorded pattern reversal visual evoked potentials (PRVEPs) in 16 subjects
in both the inverted and upright positions. In ten of the 16 subjects, we
performed an additional experiment. On a different day, we obtained control
PRVEPs, and then, using a scleral suction cup, we artificially increased
the IOPs to the same values reached when the subjects were previously
inverted. We found that the PRVEP amplitudes were significantly reduced in
both experiments compared with corresponding control recordings, without
changes in latency or wave form. Furthermore, the degree of amplitude
reduction was even greater during gravity inversion than with suction cup
elevation of IOP. The effects of IOP on optic nerve function as measured by
the PRVEP appear to be dependent more on the IOP than on vascular
perfusion. We also concluded that gravity inversion activities pose
potential risks to the eyes.
