May 2007
Volume 48, Issue 13
ARVO Annual Meeting Abstract  |   May 2007
Iris Stiffening Following Drug Stimulation
Author Affiliations & Notes
  • J. E. Whitcomb
    University of Minnesota, Minneapolis, Minnesota
    Department of Mechanical Engineering,
  • V. A. Barnett
    University of Minnesota, Minneapolis, Minnesota
    Department of Physiology,
  • T. W. Olsen
    University of Minnesota, Minneapolis, Minnesota
    Department of Ophthalmology,
  • V. H. Barocas
    University of Minnesota, Minneapolis, Minnesota
    Department of Biomedical Engineering,
  • Footnotes
    Commercial Relationships J.E. Whitcomb, None; V.A. Barnett, None; T.W. Olsen, None; V.H. Barocas, None.
  • Footnotes
    Support NIH Grant EY015795
Investigative Ophthalmology & Visual Science May 2007, Vol.48, 3155. doi:
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    • Get Citation

      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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      © ARVO (1962-2015); The Authors (2016-present)

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Purpose:: To quantify how the elastic modulus (i.e., the stiffness) of the iris changes following stimulation by pilocarpine, phenylephrine, and tropicamide.

Methods:: 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.

Results:: 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.

Conclusions:: 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.

Keywords: iris • anterior segment • stress response 

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