New optical device for functional studies of the optic nerve head

Purpose
To develop a confocal ocular laser Doppler device for the recording of functional optic nerve head (ONH) blood flow responses to various physiological stimuli.

Method
The flowmeter consists of a self-aligned confocal system, a fundus illumination (green light) and CCD based observation unit, a target fixation system and a CCD camera to observe the pupil. CCD allows monitoring of the position of the probing laser beam at the pupil to insure constant entrance point in successive measurements of ONH blood flow. Doppler signal is digitized and a dedicated software operating on a portable computer calculates the relative velocity, volume and flux of the red blood cells in the ONH microcirculation. Synchronization of theses parameters with the heart pulse allows determination of the flow pulsatility. Flicker stimulation of the macular area can be performed at 2 wavelengths. Measurements were performed at rest in each of 29 normal subjects to determine the variability of ONH blood flow parameters. Preliminary responses of these parameters to the breathing of various gases were obtained.

Results
The variability of blood flow obtained from 3 successive measurements from the same site ranged from 0.5 to 33% (mean 11%, n = 29). " e breathing of carbogen (95% O2, 5% CO2) for 5 min by subjects with optimal target fi xation induced an increase of ONH blood fl ow of 28 +/- 16 (sd) % (p = 0.03, n = 11) in accordance with previously published data.

Conclusion
The new confocal device combined with monitoring of the entrance site of the laser beam at the pupil provides laser Doppler blood fl ow data at the ONH that show a variability that makes the device potentially useful for physiological and clinical investigations.