INTRAOPERATIVE IMPROVEMENT OF SOMATOSENSORY-EVOKED POTENTIALS AND LOCAL SPINAL-CORD BLOOD-FLOW IN PATIENTS WITH SYRINGOMYELIA

This report is the first to correlate data concerning intraoperative s omatosensory evoked potentials (SSEPs) and local spinal cord blood flo w (1SCBF) in patients with syringomyelia. In a consecutive study, bila teral median nerve SSEPs were recorded intraoperatively in 13 patients undergoing a syrinx shunt to the posterior fossa cisterns (syringocis ternostomy). 1SCBF was measured in five of these patients using laser doppler flowmetry (LDF) calibrated in arbitrary units (Ati). SSEP reco rdings obtained 30 min after syrinx decompression demonstrated a sligh t but consistent reduction of N-20 latencies (mean change: 0.53 ms rig ht, p < 0.003; 0.58 ms left, p < 0.001) concurrent with a similar but less consistent increase of N-20 amplitudes (0.16 mV right, p = 0.256; 0.29 mV left, p = 0.03). Prior to shunting, LDF recordings from the s pinal cord overlying syrinxes revealed very low 1SCBF values in five o f five patients (mean LDF, 13.2 AU +/- 15.3 SD). Immediately after shu nting, there was a dramatic rise of 1SCBF (mean LDF, 241.2 AU +/- 106. 3 SD) associated with visualized hyperemia of the spinal cord and pial vessels. The 1SCBF fell to intermediate levels after 2 min (157.2 AU +/- 33.0 SD) and remained at these levels during the interval of recor ding (5 min). Hyperventilation testing in two patients prior to shunti ng revealed no change in 1SCBF consistent with a loss of CO2 vascular reactivity and a paradoxical increase of 1SCBF in one patient 5 min af ter shunting. Each patient in this study experienced neurological impr ovement in the immediate postoperative period associated with collapse or disappearance of the syrinx on magnetic resonance imaging scans. B ecause syrinx shunting results in an acute decompression of the disten ded spinal cord, it is possible that the rapid improvement of SSEPs re flects a relief of mechanical factors such as stretching and compressi on of nervous tissue, However, the LDF findings in this study suggest that distended spinal cord cavities are also capable of producing regi onal ischemia, A significant reduction of 1SCBF is a possible contribu ting cause of neurological injury and SSEP abnormalities. Intraoperati ve improvement of SSEPs and 1SCBF were found to correlate well with ne urological recovery following syringocisternostomy. Our results indica te that SSEP monitoring can provide useful information during surgical procedures for syringomyelia and that further experience with LDF mon itoring may provide insights into the pathophysiology of this conditio n.