The effect of α1-adrenergic receptor antagonist tamsulosin (Flomax) on iris dilator smooth muscle anatomy

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To characterize and determine the effect of tamsulosin (Flomax) on the human iris dilator muscle anatomy.


Retrospective, case-control study.


This study comprised 51 cadaveric eyes from 27 patients (14 with a history of tamsulosin use and 13 control patients) who underwent autopsy at the Duke University Medical Center, Durham, North Carolina.


Patients' records were reviewed, and age, medical, surgical, and ocular history; gender; medications; and duration and dosage of tamsulosin were recorded. Specimens were sectioned through the pupillary axis in the horizontal meridian and reviewed by light microscopy. A morphometric analysis was performed to measure the maximum and minimum iris dilator muscle thickness and the iris stromal thickness (micrometers) at 6 points in each eye. All microscopic evaluations and measurements were performed by the same masked observer.

Main Outcome Measures

To determine whether there is a significant difference in the iris dilator muscle or stromal thickness in those patients receiving tamsulosin treatment compared with age-matched controls.


The mean iris dilator muscle thickness in the tamsulosin-treated group (6.53±1.99 μm) was significantly thinner compared with that of the control group (8.50±1.61 μm) (P=0.006). There was no difference in iris stromal thickness between the 2 groups (P=0.268). There was no direct relationship between duration of tamsulosin use and iris dilator muscle or stromal thickness. Statistical significance was maintained when the iris dilator muscle thickness was compared between the groups using history of diabetes and cataract extraction as separate variables. No difference was noted when comparing the iris stromal thickness using diabetes as a separate variable. However, stromal thickness was significantly different between the groups in pseudophakic eyes (P=0.005).


According to histologic examination of cadaver eyes, patients receiving tamsulosin treatment exhibited decreased iris dilator muscle thickness compared with control patients. There was no difference noted in the iris stromal thickness within the groups. We believe this finding may shed light on the pathophysiology of intraoperative floppy iris syndrome. Further studies need to be performed to assess the significance of this histologic finding.