Structural Limits of Orbital Compliance and Treatment Options for Pathologic Orbital Pressure

Purpose: The integrity of the orbit has a finite structural limit due to the compliance of its tissue. The authors investigate these limits to quantify them and inform the treatment of heightened ocular and orbital pressure. Methods: Cadaveric study with 12 orbits being volumized before randomization of treatment for pathologic levels of ocular and orbital pressure. First-line and second-line treatment was chosen randomly (lateral cantholysis, superior septolysis, inferior septolysis). Prior to treatment, IOP, orbital compartment pressure, and axial globe projection was measured after delivery of each 1cc aliquot and surgical treatment. Results: Orbital compartment pressure and IOP were well correlated (r = 0.99). The average reduction in IOP after treatment averaged a 56.2 mm Hg reduction in IOP. All treatments were statistically equivalent (p < 0.01). Loss of compliance (P_LOC) was determined when the mean plus 1 standard deviation of change in IOP/ml volume was achieved with simultaneous change in exophthalmometry of <0.5 mm change/ml added volume, indicating an acceleration in pressure in the face of a steady volume. This criteria was met for 11 of 12 orbits. The 12th orbit missed this threshold by 1 mm Hg in IOP. P_LOCoccurred at an average IOP of 43.0 mm Hg (±5.8 mm Hg, 90% CI) and after an average injection of 13 ml (±1.4 mm, 90% CI). Additionally, lateral cantholysis, superior septolysis, and inferior septolysis were statistically equivalent in reducing IOP after P_LOC. Conclusions: IOP and orbital compartment pressure are excellent proxies for each other in the authors' model. Orbital compliance is a mathematic phenomenon that can be quantified, as evident in this investigation. P_LOCcan inform timing for orbital decompressions in the presence of heightened IOP. Multiple procedures can be used to extinguish dangerously high orbital compartment pressure.