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

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✓ 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 µV 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 The intracranial optic nerve was retracted using a weight of 5, 10 or 50 g. The earliest change in VEP noted during retraction was a reduction in N50 wave amplitude.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.

Article Information

Address reprint requests to: Kiyoshi Saito, M.D., Department of Neurosurgery, Nagoya University School of Medicine, 65 Tsurumai, Showa-ku, Nagoya 466, Japan.

© AANS, except where prohibited by US copyright law.

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Figures

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    Drawings depicting the operating area. A: Retraction of the optic nerve with the roof of the optic canal intact. B: Retraction of the optic nerve after unroofing the optic canal.

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    Consecutive change in the visual evoked potentials (VEPs) during optic nerve retraction with a 10-g weight. Wave 1 indicates the preretraction control. A reliable and reproducible negative peak (N50) appeared at approximately 50 msec. In this dog the latency of the N50 lasted 47.0 msec and its amplitude was 5.5 µV. Waves 2, 3, and 4 show VEPs at 4, 8, and 12 minutes, respectively, after retraction began. Immediately after recording Wave 4, retraction was released. Waves 5, 6, and 7 show the VEPs 4, 10, and 18 minutes, respectively, after retraction was halted. In all cases, the N50 amplitude gradually decreased to zero during retraction and recovered to control size after retraction was discontinued.

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    Reduction of N50 amplitude by retraction of the optic nerve before unroofing. Data are expressed as a percentage of the control amplitude. There are statistically significant differences between 5 g and 10 g as well as between 5 g and 50 g (p < 0.05).

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    Reduction of N50 amplitude by optic nerve retraction using a 5-g (A), 10-g (B), or 50-g (C) weight, before and after unroofing the optic canal. After unroofing, the length of time required to reduce N50 amplitude was significantly longer at all three weights than that required before unroofing (p < 0.05).

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