Vedran Deletis

Brain stem mapping : Neurophysiological localization of motor nuclei on the floor of the fourth ventricle

We have improved upon a brain stem mapping technique that can be used to locate cranial motor nuclei on the floor of the fourth ventricle. This technique helped to intraoperatively locate the facial colliculus and the motor nuclei of cranial nerves IX/X and XII in 14 patients undergoing removal of brain stem tumors. The motor nuclei of these cranial nerves are usually located relative to specific anatomic landmarks on the ventricular floor. These landmarks were not evident in most patients studied because of the distorting effects of the tumor. Different points of the floor of the fourth ventricle were electrically stimulated while electromyographic responses were recorded with electrodes inserted in the orbicularis oculi and orbicularis oris muscles, the lateral posterior pharyngeal wall, and the intrinsic muscles of tongue. Mapping was performed before and after tumor resection. The technique was found to be useful for locating cranial motor nuclei before tumor resection. It enabled surgeons to avoid damaging the nuclei when entering the brain stem. This technique, however, has certain limitations. Because this is a mapping technique, not a monitoring technique, continuous monitoring during tumor resection was unavailable. Also, the presence of a muscle response after tumor resection did not always indicate preservation of immediate postoperative function (7 days after surgery) because damage to the corticobulbar tract and/or the underlying neural circuitry was not detectable by mapping. Further research is needed to determine the point of stimulation in the functional motor circuitry and the relationship between intraoperative recordings and postoperative function in the long term.

Facilitation of motor evoked potentials by somatosensory afferent stimulation

The effect of an electrically induced peripheral afferent volley upon electrical and magnetic motor evoked potentials (MEPs) from muscles of the upper and lower extremities was studied in 16 healthy volunteers. A standard conditioning-test (C-T) paradigm was employed whereby the test stimulus (transcranial electric or magnetic) was applied at random time intervals, from 10 msec prior to 90 msec after the conditioning stimulus (peripheral nerve stimulus). MEP amplitude facilitation was observed for the majority of the upper extremity muscles tested at two distinct periods, one occurring at short, and the other at long C-T intervals. This bimodal trend of MEP facilitation was found to be equally as prominent in the lower extremity muscles tested. The period of short C-T interval facilitation is consistent with modifications in the spinal excitability of the segmental motoneuron pool. On the other hand, the period of long C-T interval facilitation is suggested to be due to alterations in excitability of the motor cortex as a result of the arrival of the orthodromic sensory volley. Although most pronounced in muscles innervated by the nerve to which the conditioning stimulus was applied, this bimodal facilitatory effect was also observed in adjacent muscles not innervated by the stimulated nerve. Qualitatively, the conditioned MEPs from the upper and lower extremities responded similarly to both electrical and magnetic trans-cranial stimulation. In addition, our study demonstrates that the C-T paradigm has potential for use in the assessment of spinal and cortical sensorimotor integration by providing quantitative information which cannot be obtained through isolated assessment of sensory and/or motor pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

Intraoperative Monitoring of the Dorsal Sacral Roots

Abstract In 31 children (age, 2-17 years) and 1 adult, individual dorsal root action potentials (DRAPs) from the S1-S3 roots were recorded intraoperatively after electrical stimulation of the dorsal penile or clitoral nerves, in preparation for surgery within the cauda equina. In most patients, pudendal afferent activity was present in S2 and S3 bilaterally; in some, the afferent activity was confined to a single root bilaterally, and in one, to a single root on one side. Dorsal root action potentials of small amplitude were recorded from S1 in 15 patients, although in no patient was S1 the primary carrier of these afferents. No lesion of the roots or rootlets carrying significant afferent activity was created during the rhizotomy, and no dysfunction in micturition resulted. We propose that the neurophysiological identification of roots and rootlets carrying afferent activity from the penile or clitoral nerves allows for rhizotomy of the S2 roots with the least possible risk of postoperative micturition and sexual dysfunction.

Spinal cord and nerve root monitoring in spine surgery and related procedures

&NA; Intensive research in the field of intraoperative neurophysiologic monitoring has been performed directed at finding reliable stimulating and recording techniques and adequate anesthetic regimes applicable to spinal procedures. The aim is a comprehensive monitoring not only of afferent and efferent spinal cord pathways but also of sensory and motor nerve roots and cauda equina fibers. Conventional somatosensory evoked potentials (SEPs) are complemented by motor evoked potentials, dermatomal sensory evoked potentials, spinal cord evoked potentials, evoked electromyography, sensory and motor fiber mapping of the cauda equina, bulbocavernosus reflex testing, and neurogenic evoked potentials. Apart from describing the essentials of these techniques and their indications and limitations, this article deals with the influence of anesthetic management on the production and interpretation of evoked potentials.

Functional anatomic relationship between brain-stem tumors and cranial motor nuclei.

OBJECTIVEnTo identify patterns of cranial motor nuclei (CMN) displacement in cases of intramedullary brain stem tumor, using neurophysiological mapping of motor nuclei on the floor of the fourth ventricle.nnnMETHODSnRelationships between tumors and CMN were reviewed in 18 patients with brain stem tumors (seven pontine, nine medullary, and two pontomedullary tumors) and 2 with cervicomedullary junction spinal cord tumors. CMN VII, IX/X, and XII were mapped by applying electrical stimuli over the surgically exposed fourth ventricular floor through a handheld probe and recording electrical activity in the appropriate cranial muscles.nnnRESULTSnTumors distorted the anatomic location of CMN in repetitive patterns according to tumor site. Three patterns were identified as follows: Type 1, CMN located around the tumor on the floor of fourth ventricle; Type 2, one or more CMN located ventrally to the tumor; Type 3, CMN in original anatomic position. Six of seven patients with pontine tumors showed the Type 1 pattern. Seven of nine patients with medullary tumors showed Type 2, and the other two showed Type 1. Both patients with pontomedullary tumors showed Type 2. One patient with a cervicomedullary junction spinal cord tumor showed Type 1 and the other Type 3, depending on the tumor extension into the fourth ventricle.nnnCONCLUSIONnPontine tumors push the CMN to around the tumor edge, suggesting that precise localization of CMN before tumor resection is necessary to avoid their damage. Medullary tumors grow more exophytically and compress the CMN ventrally. Understanding patterns of CMN displacement can help surgeons establish the surgical plan, minimize risks, and enable safer surgery of brain stem tumors.

Intraoperative monitoring of spinal cord function using motor evoked potentials via transcutaneous epidural electrode during anterior cervical spinal surgery

Because false-positive results are not infrequent when monitoring somatosensory evoked potentials during surgery, monitoring of motor evoked potentials (MEPs) has been proposed and successfully used during the removal of spinal cord tumors. However, this often requires direct visual placement of an epidural electrode after a laminectomy. We evaluated the use of MEPs, recorded via a transcutaneously placed epidural electrode, to monitor motor pathway functional integrity during surgery on the anterior cervical spine. Sixteen patients underwent anterior cervical vertebral decompression and fusion for cervical myelopathy and/or radiculopathy. Before surgery, an epidural monitoring electrode was placed transcutaneously at the midthoracic level and was used to record MEPs after transcranial cortical electrical stimulation. Electrode placement was successful in all patients but one, and satisfactory baseline spinal MEPs were obtained except for one patient who had cerebral palsy with significant motor dysfunction. Patients showed no significant changes in spinal MEPs during surgery, and all had baseline or better motor function postoperatively. None had complications from epidural electrode placement or electrical stimulation. We conclude that motor pathways can be monitored safely during anterior cervical spinal surgery using spinal MEPs recorded via a transcutaneously placed epidural electrode, that MEP preservation during surgery correlates with good postoperative motor function, and that cerebral palsy patients may possess too few functional motor fibers to allow MEP recording.

Motor evoked potentials during brain surgery

In order to obtain a robust method for intraoperative monitoring of motor pathways, different stimulation patterns to elicit muscle motor evoked potentials (MEPs) were studied during neurosurgical procedures in 3 patients. MEPs were recorded by a catheter electrode in the subdural space and/or by needle electrodes in limb muscles. For stimulation single pulses and trains consisting of two to five pulses were used. Muscle MEN were only obtained after trains of at least 3 stimuli while single/double stimuli were inefficient. Simultaneous subdural recordings showed that single and double stimuli only elicited D-waves, whereas trains of 3 or more stimuli generated I-waves, as well. We propose that train stimulation can overcome the depressive effects of anesthesia on cortical motoneurons.

Inclusion of the s2 dorsal rootlets in functional posterior rhizotomy for spasticity in children with cerebral palsy

Many neurosurgeons have made a practice of sectioning the S2 dorsal roots during selective posterior rhizotomy for the treatment of spasticity in children with cerebral palsy, but the efficacy of this treatment has not previously been proven. S2 afferents are involved in reflex arcs of the plantar flexors (PFs), so that S2 lesioning should in theory reduce PF spasticity. To test this assumption, we determined the frequency of postoperative residual spasticity in the PFs when S2 lesioning was or was not performed. We assessed 85 children for whom 6-month follow-up was available. Functional rhizotomy from L2-S1 was performed on 13 of them (26 legs with PF spasticity) and from L2-S2 on 72 (141 legs with PF spasticity). Rootlets were lesioned if there was an abnormal response to intraoperative electrical stimulation. In 20 patients, lesioning of the S2 rootlets was assisted by the pudendal neurogram, a technique previously shown to prevent bladder dysfunction during sectioning of the sacral roots. When S2 roots were excluded from the lesioning process, residual PF spasticity was detected in 35% of the legs that had it preoperatively, leaving 5 (38%) of 13 children with functionally impairing spasticity. When S2 roots were included, 6% of legs that had PF spasticity retained it postoperatively (P < 0.001), leaving 8 (11%) of 72 patients with functionally limiting spasticity (P < 0.05). Thus, the addition of the S2 roots to the procedure resulted in an 81% reduction in the number of legs with residual PF spasticity and a 71% reduction in the number of patients with residual PF spasticity.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient paraplegia revealed by intraoperative neurophysiological monitoring: Was it caused by the epidural anesthetic or an epidural hematoma?

IMPLICATIONSnOur case report describes the electrophysiological features associated with the development of a spinal epidural hematoma during surgery of the lumbar spine. It stresses the importance of the evaluation of nonsurgical factors, which can potentially affect intraoperative evoked potentials; in this case, epidural local anesthetic or epidural hematoma.

The Use of Motor Evoked Potentials in the Diagnosis of Psychogenic Quadriparesis

We present a case illustrating the usefulness of motor evoked potentials (MEPs) in differentiating psychogenic from organic postoperative paralysis. Discussed is a 12-year-old girl who underwent surgery for the repair of a recurrent syringomyelia. On the 6 day after a proximal revision of her syringoperitoneal shunt she returned to the hospital with deep quadriparesis, bowel and bladder incontinence, and complaining of severe headache. An MRI scan showed the syrinx to be collapsed, and removal of the shunt had no impact on her clinical symptoms. Repeat somatosensory evoked potentials (SEPs) showed no change in comparison to those obtained at the end of her preceding surgery. MEPs were normal for the lower extremities, a finding which is inconsistent with a severe upper motor neuron lesion. She was diagnosed with psychogenic paralysis, and fully recovered within 1 month. We propose that a neurophysiological evaluation including MEPs is useful in the differential diagnosis of psychogenic and organic motor weakness.