Intraocular pressure variation with oculorotary muscle tension

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Abstract
Intraocular pressure changes have been measured in cats and rabbits in an attempt to determine mechanisms underlying the nature of the pressure rise accompanying increased tension of the oculorotary muscles. A quantitative relationship between peak lateral rectus muscle tension and peak intraocular pressure rise was determined following intravenous administration of graded doses of succinylcholine which produces oculorotary muscle contracture. The relationship was nonlinear, with a steep parabolic rise toward maximum tension. Compression of the globe, resulting from cocontracture, was demonstrated to contribute about 30% of the intraocular pressure rise; calculations based on a simple piston model agreed with this pressure component, which was linearly related to muscle tension. A larger measured pressure component, proportional to the square of the developed muscle tension, appeared due to distortion of the globe by muscle stress concentration creasing or indenting the sclera. Simulation of this component by tightening a string arching over the cornea produced intraocular pressure increases whose progressive magnitude matched the nonlinear pressure-tension relationship. The decrease of intraocular pressure from its peak followed a more rapid time course than did the decline in muscle tension, and exhibited an undershoot before ultimate return to base line. The magnitude and recovery time constant of the undershoot were consistent with a linear viscoelastic model involving expression of fluid from the globe.