Monitoring visual evoked potentials during retraction of the canine optic nerve: protective effect of unroofing the optic canal.

To evaluate the effects of unroofing the optic canal during retraction of the optic nerve, the authors monitored changes in visual evoked potentials (VEPs) stimulated by a light-emitting diode in the canine model. At rest, an early VEP wave was reliably observed with an amplitude of 8.2 +/- 0.6 microV and a latency of 51.5 +/- 0.7 msec; this wave was named N50. The intracranial optic nerve was retracted using a weight of 5, 10 or 50 g. The earliest change in VEP noted during retraction was a reduction in N50 wave amplitude. The length of time required until N50 amplitude decreased to 50% of the control group (T50) was 10.7 +/- 1.8 minutes with a weight of 5 g, 4.9 +/- 0.7 minutes with 10 g, and 2.9 +/- 0.4 minutes with 50 g, with statistically significant differences between the groups. Retraction of the optic nerve with all weights finally resulted in the disappearance of the N50 wave. The amplitude of the N50 wave recovered fully to control size when retraction was released immediately after the wave disappeared. The time course of amplitude recovery did not differ significantly between groups. Unroofing the optic canal prolonged the T50 during retraction significantly to 20.7 +/- 2.9 minutes with a weight of 5 g, 18.9 +/- 4.2 with 10 g, and 9.0 +/- 2.4 with 50 g. These results demonstrate that unroofing the optic canal can protect the optic nerve from damage during operations that require optic nerve retraction.