Pressure applied to the eyeball increases the intraocular pressure and impairs retinal circulation, resulting in field loss and finally, ocular blackout. To quantitate this phenomenon, the plethysmographic goggle ophthalmodynamometer was used to vary pressures in normal healthy male patients, and the time onset of blackout was recorded. The results could be divided into the following three groups: (1) Induced pressures lower than that of the retinal artery diastolic pressure caused a transient field defect, but no ocular blackout. (2) At pressures between diastolic and systolic, as the induced pressure increased, the time onset to blackout decreased in a logarithmic manner. (3) At induced pressures systolic and above, the time onset until blackout remained constant. These results did not change with varying illumination.
Palena PV, Jaeger EA, Behrendt T, Duane TD. Quantitative Effect of Increased Intraocular Pressure on Blackout. Arch Ophthalmol. 1970;83(1):84-88. doi:10.1001/archopht.1970.00990030086016