technique is advantageous in cases where neuromuscular blockade is required; however, continuous monitoring of nerve function is obviously extremely difficult, and a certain amount of inconvenience is added due to the need to introduce a hand-held device into the surgical field.

EMG monitoring of the facial nerve has also been an important part of procedures performed to relieve hemifacial spasm. Abnormal muscle response to stimulation of the appropriate branch of the facial nerve, which is typically seen in this patient population, disappears when the nerve is released from the offending vessel. This finding has been associated with good postoperative outcomes, while perseverance of an abnormal response parallels residual post-operative spasm.

Intraoperative monitoring of the other cranial nerves is also possible. Needle electrodes can be placed into any of the extraocular muscles, thereby monitoring cranial nerves III, IV and VI. The masseter or temporalis serve as recording sites for the motor division of the trigeminal nerve. Electrodes placed into the soft palate or posterior pharyngeal musculature can be used to monitor the function of cranial nerve IX. However, great care must be taken when stimulating these motor fibers, as some of them innervate the carotid body and stimulation may result in bradycardia or hypotension. Similarly, vagal monitoring, using electrodes placed endo-scopically into the vocal cords or cricothyroid muscle, may also result in marked bradycardia, arrythmias or alterations of blood pressure. The absence of vagal activity, as recorded in laryn-geal muscles, can be used to determine whether posterior pharyngeal recordings are attributable to cranial nerve IX rather than to nerve X. EMG recordings from either the trapezeus or sternocleidomastoid muscles will indicate activity of cranial nerve XI; however, stimulus intensity should be kept low to minimize the possibility of forceful jerking of the head with resulting trauma at the pin sites. Electrodes placed in the tongue allow for recording of evoked potentials from hypoglossal stimulation.

Visual Evoked Potentials

There has been interest in developing a reliable method of measuring and interpreting visual evoked potentials (VEPs) for use during procedures that could compromise elements of the visual pathway from the retina to the occipital cortex. Optic stimuli have been traditionally delivered through LED-emitting goggles or fitted contact lenses. There have been very few studies assessing the role of VEP monitoring in neurosurgical procedures, perhaps due to the enormous variation in observed waveform characteristics in conjunction with persistently unreliable recordings. Preliminary results have been mixed, but the overriding consensus suggests that the use of VEP monitoring is not justified as a technique owing to this extensive variability. It has been demonstrated that alterations in VEP have very poor sensitivity and specificity for the prediction of post-operative visual changes. Therefore, this technique remains primarily in the experimental arena.

Measurement of Cerebral Blood Flow

Two major methods have been devised for the purpose of primary quantification of CBF. As opposed to EEG or SSEP monitoring, which give a secondary glimpse of CBF by detecting physiological changes that occur owing to decreases in flow, direct measurement of flow would hypo-thetically provide more rapid and relevant intraoperative feedback. The first of these methods relies on measuring the clearance of an injectable tracer from brain tissue. The tracer that has been used for this purpose, due to its relative insolubility in water and rapid diffusion across the blood-brain barrier, is 133Xe. Typically, clearance is detected with a hand-held sensor placed over the region of interest following injection of the tracer into the ICA. A clearance curve is generated and the area under the curve is used for calculating CBF. The use of 133Xe for CBF measurement during CEA has been well described. Sundt has reported the greatest experience with regional CBF (rCBF) measurements during CEA [22]. It is primarily his analysis of nearly 2,200 monitored patients that supports shunt placement for flows of less than 18-20 ml/100 g/min. Others have questioned the efficacy of measuring rCBF during CEA. Zampella et al. performed EEG monitoring and rCBF measurements during 431 consecutive

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