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

Abstract

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 The intracranial optic nerve was retracted using a weight of 5, 10 or 50 g. The earliest change in VEP noted during retraction was a reduction in N50 wave amplitude.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

Measurement of optic nerve blood flow during dissection of parasellar tumors

“…VEPs can help to predict the final effects of trauma on visual function, while changes in amplitude and latency appear to represent an early indication of reversible damage to the visual system [18,41]. Our previous study supported the benefit of VEP monitoring during retraction of the optic nerve [1].…”

“…However, VEPs are susceptible to a variety of nonspecific influences, such as those of volatile anesthetics [1,40], blood pressure, and body temperature [31]. VEPs also vary in amplitude and latency both between patients and for an individual patient.…”

“…ONBF may be particularly impaired in cases where the tumor invades into the optic canal. We previously demonstrated the importance of optic canal unroofing under conditions requiring retraction of the optic nerve [1]. Optic nerve visual evoked potential (VEP) amplitude was reduced depending on intension of traction, while successful unroofing of the optic canal lengthened the duration of essentially intact VEP amplitude during traction.…”

Efficacy of Piezo-Electric Surgery for Optic Canal Unroofing: Our Experience with Two Cases

“…There are many studies in literature advocating extradural unroofing of the optic canal for early decompression of the optic nerve [1][2][3][4][5]. Conventional use of drills in deroofing has its own risks of injuring the optic nerve either by iatrogenic compression or by the heat produced during drilling.…”

“…The piezosurgery hand piece does not contain any rotating parts or drill attachments; thereby eliminating the risk of damage to surrounding important structures, making this device optimally precise & extremely reliable to be used in resecting bone in skull base surgeries [12]. For two decades ultrasonic vibrations have been used to cut tissue [1,16,17]. However, it is only in the last five -six years that experimental applications have been used routinely for standard clinical applications in many different fields of surgery [18][19][20].…”