C. Strauss

Magnetic source imaging combined with image-guided frameless stereotaxy: A new method in surgery around the motor strip

OBJECTIVE In this study, information about the localization of the central sulcus obtained by magnetic source imaging (MSI) was intraoperatively translated to the brain, using frameless image-guided stereotaxy. In the past, the MSI results could be translated to the surgical space only by indirect methods (e.g., the comparison of the MSI results, displayed in surface renderings, with bony landmarks or blood vessels on the exposed brain surface). METHODS Somatosensory evoked fields were recorded with a MAGNES II biomagnetometer (Biomagnetic Technologies Inc., San Diego, CA). Using the single equivalent current dipole model, the localization of the somatosensory cortex was superimposed on magnetic resonance imaging with a self-developed contour fit program. The magnetic resonance image set containing the magnetoencephalographic dipole was then transferred to a frameless image-guided stereotactic system. Intraoperatively, the gyrus containing the dipole was identified as the postcentral gyrus, using neuronavigation, and the next anterior sulcus was regarded as the central sulcus. With intraoperative cortical recording of somatosensory evoked potentials, this assumption was verified in each case. RESULTS In all cases, the preoperatively assumed localization of the central sulcus and motor cortex with MSI agreed with the intraoperative identification of the central sulcus using the phase reversal technique. CONCLUSION The combined use of MSI and a frameless stereotactic system allows a fast orientation of eloquent brain areas during surgery. This may contribute to a safer and more radical surgery in lesions adjacent to the motor cortex.

Neurophysiologic monitoring in posterior fossa surgery. II. BAEP-waves I and V and preservation of hearing.

SummaryOf 135 cases operated upon for posterior fossa lesions 103 showed preoperative hearing. In 34 acoustic neurinomas 14 had postoperative initially preserved hearing, in 20 microvascular decompressions 19 had preserved postoperative hearing and in 49 other lesions 5 lost hearing. The relationship between preservation of hearing and the preservation or loss of brainstem auditory evoked potentials (BAEP) waves I and V in the three groups of namely: acoustic neurinomas, microvascular decompressions and other lesions are presented. It is noteworthy that only patients with preserved waves I or V are suitable candidates for intraoperative monitoring. The loss of wave V is usally associated with hearing loss (10 out of 13 cases). But hearing loss is also possible despite preservation of wave I (3 out of 60) or despite preservation of wave V (2 out of 68). The predictive value of the preservation of waves I and V is not an absolute one, but it strongly suggests preserved hearing postoperatively. The dilemma remains that once waves I or V are lost during surgery there is no certainty as to postoperative hearing. If wave V recovers after an initial loss, hearing is usually preserved but not in all cases. In wave I amplitude changes alone were more frequent than in wave V, where latency changes alone were more frequently observed. Particular surgical manoeuvres could be found to be often associated with a wave deterioration. Hearing preservation could never be achieved in patients who already preoperatively had no BAEP. It is concluded that BAEP monitoring is of great value in surgery for microvascular decompression and cerebello-pontine-angle (CPA) tumours with preserved hearing.

The prognostic value of intraoperative BAEP patterns in acoustic neurinoma surgery

Based on a consecutive series of 70 hearing patients with unilateral acoustic neurinomas and intraoperative monitoring of brain-stem auditory evoked potentials (BAEP), 4 dynamic BAEP patterns could be characterized. These patterns correspond with early and late postoperative hearing outcome. All patients with stable wave V (pattern 1) showed definite hearing preservation, all patients with irreversible abrupt loss of BAEP (pattern 2) lost their hearing, despite early hearing preservation in two cases. All patients with irreversible progressive loss of either wave I or wave V (pattern 3) eventually suffered from definite postoperative hearing loss, despite early hearing preservation in two cases. Those cases with intraoperative reversible loss of BAEP (pattern 4) showed variable short and long term hearing outcome. In 34% hearing was preserved, 44% suffered from postoperative hearing loss, the remaining 22% showed postoperative hearing fluctuation, either as a delayed hearing loss or as reversible hearing loss. Postoperative hearing fluctuation indicates anatomical and functional preservation of the cochlear nerve during surgery and is suggestive of a pathophysiological mechanism initiated during the surgical procedure and continuing thereafter. Patients at risk for delayed hearing loss can be identified during surgery by a characteristic BAEP pattern and may benefit from vasoactive treatment.

Preservation of hearing in large acoustic neurinomas following removal via suboccipito-lateral approach.

Summary A consecutive series of 61 patients with pre-operative hearing underwent surgical removal of a large acoustic neurinoma via the lateral suboccipital approach between 1984 and 1996. Brain-stem auditory evoked potentials (BAEP) were present in all cases before surgery and all patients underwent intra-operative monitoring of BAEP. The average tumour size including the portion within the meatus acusticus internus was 30, 5 mm (range 20 mm–49 mm). Complete tumour removal was achieved in all but three cases. In 43.1% of patients with complete tumour removal hearing was preserved initially after surgery. Delayed postoperative hearing loss was observed in 11 patients and hearing recovery in 2 patients leading to 27.5% definite hearing preservation. Hearing preservation was achieved in 37% of cases with tumour size between 20 mm–29 mm and in 23.5% of case with tumour size larger than 30 mm. These results indicate that even in large neurinoms hearing preservation should be attempted in all patients with documented pre-operative hearing and BAEP.

Neurophysiologic monitoring in posterior fossa surgery. I: Technical principles, applicability and limitations

SummaryIn 135 cases of posterior fossa surgery almost exclusively in the cerebellopontine angle (CPA) intraoperative monitoring of brainstem acoustic evoked potentials (BAEP) and partly somatosensory evoked potentials (SEP) was performed. The series consisted of 20 microvascular decompressions, 63 acoustic neurinomas, 7 vascular lesions and 45 other space occupying lesions, mostly in the CPA. BAEP monitoring alone was employed in 76 cases, combined BAEP und SEP monitoring less frequently. The technique of anaesthesia and intraoperative monitoring is presented in detail including an analysis of technical problems (17 in 135=13% of cases) and technical failures (11 of 135=8%). The results of monitoring brainstem pathways contralateral to the lesion are detailed. It is concluded that the technical principles of evoked potential monitoring in posterior fossa surgery are well established. The applications and limits of this technique including its modifications are described.

Clinical and neurophysiological observations after superior petrosal vein obstruction during surgery of the cerebellopontine angle: case report.

IMPORTANCE Preservation of venous drainage during surgery of the cerebellopontine angle has received little attention. CLINICAL PRESENTATION We describe changes in brainstem auditory evoked potentials after temporary obstruction of the superior petrosal vein during surgical resection of a small meningioma at the petrous apex via a standard suboccipital-lateral approach. Temporary clipping of the petrosal vein resulted in deterioration of the brainstem auditory evoked potentials. The tumor was removed with preservation of the superior petrosal vein. CONCLUSION A transient postoperative cochlear nerve deficit emphasizes the importance of venous drainage and its preservation during surgery for small lesions of the cerebellopontine angle that do not distort normal anatomic structures.

Delayed hearing loss after surgery for acoustic neurinomas: clinical and electrophysiological observations.

In a series of 26 patients with medium-sized and large acoustic neurinomas and documented hearing before surgery, 7 patients had preservation of hearing initially after the procedure but then developed delayed hearing loss. The most prominent intraoperative electrophysiological finding in these cases was a gradual deterioration of brain stem auditory evoked potentials (BAEP), especially of Wave V. Four additional patients with a similar gradual intraoperative loss of BAEP and severe postoperative hearing deterioration received vasoactive treatment after surgery (low-molecular weight dextran). In all 4 patients, including 1 patient with documented total deafness after surgery, hearing was preserved. Initial preservation of cochlear nerve function after the removal of an acoustic neurinoma does not guarantee postoperative hearing. Intraoperative BAEP help to identify patients at risk for delayed postoperative hearing loss. The pathophysiological mechanism can be attributed to disturbances of the microcirculation in endoneurial vasa nervorum caused by the mechanical manipulation of the cochlear nerve.

Isolated unilateral internuclear ophthalmoplegia following head injury: Findings in magnetic resonance imaging

Internuclear ophthalmoplegia (INO) is most commonly caused by multiple sclerosis or cerebrovascular diseases. The authors report on two cases with isolated unilateral INO after head trauma with verified anatomical lesion in the brainstem in the region of the medial longitudinal fascicle. In both cases a small contusion could be detected in the MRI. Concomitant brainstem injury was excluded with brainstem auditory evoked potentials (BAEP). A review of the world literature disclosed only one case with isolated posttraumatic INO with magnetic resonance imaging of the lesion. In posttraumic disorders of eye motility emphasis should be placed on clinical diagnosis in combination with neuroradiologic and electrophysiologic examination to achieve accurate topographical identification of the lesion actually being responsible for the symptomatology.

Mapping and monitoring for brainstem lesions

Publisher Summary This chapter discusses that surgical treatment of brainstem lesions is limited by dense concentration of functionally important neural pathways, nuclei, and fibers in ponsvariolii and medulla oblongata, and the lack of reliable visible anatomical landmarks. Since most brainstem lesions are approached via the IVth ventricle, the presence of functionally intact brain tissue between ependyma and lesion already posses a potential risk due to the surgical corridor itself. The chapter explores that in pontine lesions, facial and abducens nerve deficits, as well as conjugated gaze palsies can affect the quality of life. Deficits of lower cranial nerve nuclei are severe life-threatening complications following the surgery of medullary lesions. The chapter reviews that direct electrical stimulation during surgery has emerged as a reliable and safe technique for the identification of superficially located nuclei and fibers, in order to define the safe entry zones into pons and medulla.

Useful Parameters in Intraoperative Monitoring of Evoked Potentials

The basic concept of intraoperative monitoring using evoked potentials is based upon the assumption that neuronal tract systems which can be monitored with electrophysiological methods may be saved from permanent functional damage. After having established this difficult technique and having more precisely delineated the range of normal variations (2, 4, 6), we can now try to describe more precisely some possibly useful parameters and the relationship between postoperative neurological findings and intraoperative changes in potential.