Objectives: To determine the sensitivity of intraoperative monitoring in neurosurgical operations using somatosensory evoked potentials and to identify reasons for false negative findings and possible settings with an increased risk for monitoring failure.
Methods: SEP monitoring of 658 neurosurgical operations was analysed. The target of monitoring was the function of a hemisphere in 251 cases, the brain stem in 198 cases, and the spinal cord in 209 cases.
Results: In 27 cases (4.1%), monitoring was classified as false negative. Further analysis showed that five of these patients had experienced delayed neurological damage. Among the remaining 22 false negative cases, 14 had a minor neurological deficit and eight had severe neurological damage. Overall sensitivity and negative predictive value of SEP monitoring was 79% and 96%, respectively. For the detection of severe neurological damage the corresponding figures were 91% and 98%. Sensitivity of monitoring varied depending on the target of monitoring and the type of lesion. Monitoring was less likely to detect neurological damage in surgery for infratentorial tumours with brain stem compression, small lesions of the motor cortex, and small vessel damage during aneurysm surgery.
Conclusions: SEP monitoring has acceptable sensitivity for detecting neurological damage during different neurosurgical procedures. Distinct settings with an increased risk of monitoring failure can be identified. In these cases measures to enhance the sensitivity of monitoring should be considered.