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J. E. Whitcomb, V. A. Barnett, T. W. Olsen, V. H. Barocas; Iris Stiffening Following Drug Stimulation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3155.
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To quantify how the elastic modulus (i.e., the stiffness) of the iris changes following stimulation by pilocarpine, phenylephrine, and tropicamide.
Irides (n = 20) were dissected from porcine eyes within 4 hr post mortem and mounted on a mechanical testing system. The samples were stretched up to 30% strain in the radial direction, and the modulus was calculated from the linear portion of the stress-strain curve. One of the three drugs (n = 6 or 7) of interest was then added (80 ug/ml) to the bath surrounding the tissue, and the experiment was repeated. Experiments were performed on intact irides and on strips cut from the iris.
Changes in pupil diameter of free-floating samples and in isometric force of mounted samples confirmed that the tissue was responsive to the drugs. For the strips, the untreated iris modulus was 4.0±0.9 kPa (mean ± s.d., n = 20), and the treated iris modulus was 7.7±2.0 (pilocarpine, n = 7), 6.9±2.2 (phenylephrine, n = 6), and 8.4±1.7 (tropicamide, n = 7). Whole irides (n = 10 total) gave similar trends but values approximately 25% higher, presumably because the shape of the intact iris makes the stress and strain fields inhomogeneous within the sample.
Although pilocarpine, phenylephrine, and tropicamide work by different mechanisms, all three gave similar results - an increase in modulus of roughly two times. We conclude that in most normal situations, the iris stiffness varies within about a factor of two range, remaining quite compliant at all pupil diameters.
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