Retinal hemorrhage thresholds for Q-switched neodymium-Yag laser exposures.

Thresholds for retinal vitreal and contained hemorrhages were determined for 1064 nm laser light at 30-nsec and 4-nsec pulsewidths. Rhesus monkeys received graded exposures from a neodymium-yag laser onto either the macular or extramacular region of the retina. Contained hemorrhages appeared as concentric ring structures with white punctate centers. The vitreal hemorrhage was characterized by the presence of choroidal blood in the vitreal chamber at the exposure site. The 30-nsec contained hemorrhage threshold (ED50) was 1.7 mJ on the macula and 2.1 mJ for an extramacular exposure. The 30-nsec vitreal hemorrhage macular threshold was 2.3 mJ, and the extramacular threshold was 6.6 mJ. The threshold for the 4-nsec pulsewidths to produce a hemorrhage (vitreal or contained) on the retina (macula or extramacular) was 340 microJ.

Ocular effects of pulsed Nd laser radiation: variation of threshold with pulsewidth.

This study of retinal damage thresholds in the rhesus monkey eye investigated the effects of Nd:YAG laser radiation at four pulsewidths; 4, 30, and 200 ns, and 10 microseconds. The thresholds for induction of minimal ophthalmoscopically visible lesions for the four pulsewidths were 158, 326, 170, and 425 microJ, respectively, incident at the eye in single-pulse exposures. The data are interpreted to imply a flat trend of threshold with pulsewidth over the range of pulsewidths examined, in agreement with maximum permissible exposures quoted in existing laser safety standards. This finding is in contrast with the hypothesis of an anomalous trend of increasing threshold with decreasing pulsewidth which was suggested for the nanosec-microsec pulsewidth range based on the sparse data base previously available.

Visual evoked potential correlates of laser flashblindness in rhesus monkeys. II. Doubled-neodymium laser flashes.

The effects of Q-switched (20-ns) doubled-neodymium (Nd-2) laser flashes upon the visual evoked potential (VEP) of two rhesus monkeys were studied and compared with the effects of 100-ms argon laser flashes described previously. VEP's were recorded under barbiturate anesthesia using bipolar electrodes chronically implanted in foveal striate cortex, and were elicited by 6-Hz phase-reversing gratings. The parameters which were investigated included the retinal area and energy level of the flash, and the wavelength, spatial frequency, and contrast of the test grating. The effects produced by the Q-switched pulses at 50% of the maximum permissible exposure (MPE) were smaller in magnitude than those produced by the longer duration argon flashes. The reduced effects of the Nd-2 pulses were attributed to the lower energy contained in these flashes, as dictated by the MPE standards. In other respects, the Nd-2 flash effects closely paralleled the argon effects. The results of the various experiments suggest that VEP decrements produced by long and short flashes are similar at equal energy levels, despite the fact that the shorter flashes are believed to produce considerably less pigment bleaching.

Visual evoked potential correlates of laser flashblindness in rhesus monkeys I. Argon laser flashes.

The visual evoked potential (VEP) in four rhesus monkeys was used to assess the transient loss of visual function resulting from single 100-ms argon laser flashes (476.5 and 514.5 nm) whose energy levels did not exceed the maximum permissible exposure (MPE). VEP's were elicited by high-contrast square-wave test gratings which were phase-reversed at a frequency of 6 Hz, and were recorded using bipolar electrodes implanted in the foveal projection region of area 17. The parameters which were investigated included (1) flash size (focused vs. expanded), (2) position of the electrode's receptive field relative to the position of the flash (0, 1.5, 3.0, and 4.5-deg separation), (3) flash exposure level (50, 5.0, and 0.5% of the MPE), (4) peak wavelength of the test grating (454, 540, and 630 nm), and (5) spatial frequency of the test grating (1.0, 4.0, 6.0, and 12.0 c/deg). The results of the flash-size experiment revealed that the expanded flash, whose retinal diameter was approximately 750 microns, eliminated or severely attenuated the VEP for a longer duration than did the focused flash and also resulted in a more gradual recovery function. The combined results of the flash position and energy level experiments indicated that the effective energy of the focused flash declined rapidly beyond 1.5 deg, but still approximated 4% of its maximum value as far as 4.5 deg from its center. Few, if any, wavelength-specific effects were observed after exposure to either the 476.5- or 514.5-nm flashes, even when the energy of the flashes was reduced to a small fraction of the MPE. Finally, the flash effect was considerably longer in duration for the 12.0 c/deg grating relative to the low and intermediate frequency gratings. In general, the findings suggest that the focused and expanded argon laser flashes produce a VEP suppression whose time course and other characteristics correlate well with those associated with behaviorally observed flashblindness in humans after exposure to intense noncoherent flashes.

Approximate visual axis projection for the rhesus monkey using a funduscope and alignment laser.

A technique was designed to align the rhesus monkey visual axis with a predetermined fixation point on a visual stimulus screen using a helium-neon laser and funduscope.

Corneal damage induced by pulsed CO2 laser radiation.

Corneal damage thresholds were determined in Dutch Belted rabbits following CO2 laser radiation with pulse widths of 1.7, 25 and 250 ns. The threshold values for corneal clouding found for the 3 cases were 0.33, 0.54 and 0.18 J/cm2, respectively. These results are in accord with expectations based on a thermal-damage mechanism, with no evidence of acoustic shock or other mechanisms contributing to the observed damage for the exposure parameters used in this study. The data support the suggested maximum permissible exposure levels for short-pulse far-infrared laser radiation as quoted in current laser safety standards.

Additivity and repair of actinic retinal lesions.

Extensive exposures to light of intensities insufficient to produce thermal damage can still result in retinal damage via nonthermal mechanisms. In this work, the additivity and the repair rate for this actinic damage were measured. Rhesus monkey retinas were exposed to 458 nm light from an argon-ion laser at a dose equivalent to half the threshold retinal irradiance. After prescribed time intervals, the retinal sites were re-exposed to determine the split-dose threshold. This threshold is related to the single-dose threshold through the additivity, which in turn is dependent on the time between exposures. The observed recovery of tissue could be fitted by a single exponential with a time constant of 4 days. This result is incorporated in an analytic expression for the cumulative effect of repeated doses.

Multiple-pulse laser retinal damage thresholds.

The additivity from multiple exposures of the retina to laser light sources is investigated with regard to empirical and theoretical models. Prior data indicate additivity consistent with a total-on-time formulation in the case of point source conditions but not for extended source conditions. Thermal model calculations agree with the extended source results, but only agree with point-source data when the pulse repetition rate is higher than 10 Hz. The anomaly at low pulse repetition rates can still be interpreted in terms of a thermal damage mechanism if spatial variations are considered.