Neurosurgical Monitoring of Steady-State Evoked Potentials

With Yonina Eldar, Gideon Inbar, Alon Sinai, Hillel Pratt and Menashe Zaaroor

Evoked potentials are the electrical potential generated by the activity of neurons in the brain, and recorded from a human or animal following presentation of a sensory stimulus, such as a light flash. When the stimulus is repetitive and fast enough, the evoked potentials create an interference pattern: a train of sinusoidal waves that remain constant with time, whose frequency matches the rate of stimulation (see the figure on the right, source: Chiappa, Evoked Potentials in Clinical Medicine, 1997). This phenomenon, known as Steady state evoked potentials (SSEP), is used in variety of clinical applications, including assessing auditory and visual acuity in infants and adults unable to provide reliable verbal responses, detection of cortical blindness, and Brain-Computer Interface.

Nowadays, Evoked Potentials monitoring is routine in surgical procedures placing sensory pathways at risk. Monitoring is typically performed by a neurophysiologist attending the surgery and provides means of identification of emerging neurologic impairment early enough before the damage becomes permanent, allowing corrective measures during the limited time-window during which damage is reversible. Intra-operative monitoring aids the surgeon in identifying neural tissue around and in a tumor, by manipulating it and observing whether the EP is altered. Such monitoring provides reassurance to the surgeon when indicating that complications are unlikely to have occurred. Since during most neurosurgical procedures the patient is anesthetized, Evoked Potentials provides the surgeon with relevant real time information about the patients' sensory functions.

It has been shown that the amplitude and phase of the SSEP can monitor the patient's functional state during a surgical brain procedure. The measurements are sensitive to changes in the relevant sensory system, respond to such changes within seconds, and do not produce false alarms in surgical procedures that do not affect the sensory pathway or the brain in general. In this project, we present a system for real-time monitoring of SSEP, whose high-level structure is described in the figure on right. During surgery, the patient's visual system is stimulated using light flashes from goggle-mounted LEDs, while EEG data is recorded by scalp electrodes. The core of the system is an algorithm that detects real time changes in the SSEP, and alarms the surgeon when a change has likely occurred.