In vivo measurement of iridial circulation using laser speckle phenomenon

To evaluate the use of the laser speckle phenomenon for noninvasive in vivo consecutive measurement of the iridial circulation.

A pigmented rabbit iris was illuminated using a diode laser, and the normalized blur of the resulting laser speckle pattern, NBiris, was determined as a quantitative index of blood velocity in the iridial tissue. The authors compared data on positional variation, reproducibility, and correlation to iridial blood velocity derived with this technique with the blood flow rate simultaneously determined by the microsphere technique. They also evaluated the effects on iridial circulation of ocular perfusion pressure (OPP) change, rectus muscle excisions, and instillation of topical timolol or betaxolol.

The NBiris increased gradually from the pupil margin to the periphery; the coefficient of variation of NBiris was lowest at the center of this area. The coefficient of reproducibility of two NBiris measurements at 5-minute intervals was 8.8%; at 24-hour intervals, it was 14.1%. The NBiris correlated well with the microsphere technique measurements of blood flow rate at several intraocular pressures (IOP) (r = 0.61, P = 0.0002, n = 40) and with the comparison of preinstillation and postinstillation unoprostone (r = 0.93, P = 0.0068, n = 8). The NBiris decreased with OPP reduction, decreased temporarily after excision of the superior or inferior rectus, and showed no significant change after excision of the medial or lateral rectus. Instillation of timolol caused a significant decrease in IOP but did not significantly change the NBiris. Topically applied betaxolol decreased IOP and increased NBiris at 2.5 hours after instillation in an ipsilateral eye.

The laser speckle method permits noninvasive, semiquantitative, consecutive measurement of the iridial circulation, with reasonable reproducibility.