Volume discrepancies in the orbit and the effect on the intra-orbital pressure. An experimental study in the monkey

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Abstract
Discrepancies between the volume of the bony orbital cavity and the volume of the intra-orbital soft tissue contents in case of orbital fracture, intra-orbital tumor, Graves' disease or during surgical intervention are likely to induce changes in intra-orbital pressure. In this animal model study rhesus monkeys (Macaca mulatta) were used to measure the retrobulbar pressure (RBP). The authors recorded RBP in monkeys with micro pressure transducers under various intraorbital volume or pressure conditions. The normal mean value of the RBP in monkeys proved to be 7 ±; 2 mm Hg. With a stepwise increase of pressure upon the eye, a proportionally increasing RBP was measured. An abrupt 10% increase of soft tissue volume by filling an intra-orbital balloon with water induced an immediate rise of RBP from 7 to 63 mm Hg and a significant protrusion. With the filled balloon in place, the high RBP diminished exponentially and was reduced to 32 mm Hg (50% decrease in pressure) in about 37 seconds. After five minutes the RBP had reached values of 15 ± 2 mm Hg. On emptying the balloon the RBP came back to the normal mean value of 7 ± 2 mm Hg. The constant and well regulated RBP will be of particular importance to keep the vascularization and function of the eye intact and the optic nerve within certain limits, during intraorbital volume or pressure alterations and will also stabilize the geographical position of the eye.

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