Intra-operative localization of sensorimotor cortex by cortical somatosensory evoked potentials: from analysis of waveforms to dipole source modeling.

Intra-operative localization of sensorimotor cortex is of increasing importance as neurosurgical techniques allow safe and accurate removal of lesions around the central sulcus. Although direct cortical recordings of somatosensory evoked potentials (SEPs) are known to be helpful for cortical localization, source localization models can provide more precise estimates than subjective visual analysis. In addition to intra-operative analysis of waveforms and amplitudes of SEPs to median nerve stimulation in 20 neurosurgical patients, we used a spatiotemporal dipole model to determine the location of the equivalent dipoles consistent with the cortical distribution of the SEPs. The early cortical SEPs were modeled by 2 equivalent dipoles located in the postcentral gyrus. The first dipole was primarily tangentially oriented and explained N20 and P20 peaks. The second dipole was primarily radially oriented and explained P25 activity. We found consistent localization of the first dipole in the postcentral gyrus, which was always located within 8 mm of the central sulcus, with an average distance of 3 mm. This finding provides an objective basis for using the SEP phase reversal method for cortical localization. We conclude that dipole source modeling of the cortical SEPs can be considered as an objective way of localizing the cortical hand sensory area.

Three-dimensional magnetic resonance images of the brain: application to neurosurgical planning.

Data from single 10-minute magnetic resonance scans were used to create three-dimensional (3-D) views of the surfaces of the brain and skin of 12 patients. In each case, these views were used to make a preoperative assessment of the relationship of lesions to brain surface structures associated with movement, sensation, hearing, and speech. Interactive software was written so that the user could "slice" through the 3-D computer model and inspect cross-sectional images at any level. A surgery simulation program was written so that surgeons were able to "rehearse" craniotomies on 3-D computer models before performing the actual operations. In each case, the qualitative accuracy of the 3-D views was confirmed by intraoperative inspection of the brain surface and by intraoperative electrophysiological mapping, when available.

A 10-year follow-up review of percutaneous microcompression of the trigeminal ganglion.

Percutaneous microcompression of the trigeminal ganglion for the relief of trigeminal neuralgia is a technically simple, nonpainful procedure, carried out under brief general anesthesia. One hundred patients treated by this method have been followed for 1 to 10 years; treatment has been technically successful in 97% of cases. Relief persisted at five years in 80%, and it is estimated that at 10 years the figure will be 70%. There were no deaths, no cerebral damage, no keratitis, and no analgesia dolorosa; 4% of the patients reported dysesthesia.

The brain: integrated three-dimensional display of MR and PET images.

Three patients with intractable epilepsy, two with brain tumors, and one with encephalitis were imaged with magnetic resonance (MR) and positron emission tomography (PET). MR data were used to construct a three-dimensional (3D) computer model of the brain surface depicting the precentral (movement), postcentral (sensation), left inferior frontal (speech), and left superior temporal (hearing) gyri. PET-derived measurements of average surface metabolism were encoded as colors and mapped onto the 3D model by means of a retrospective technique for registering the two scans. The integrated 3D model depicted the location of PET-detected metabolic abnormalities with respect to the gyral anatomy visualized with MR. In each case, the predicted relationships were confirmed intraoperatively by means of inspection of the brain and electroencephalography. Multimodality 3D displays are likely to be particularly valuable for interpreting PET studies of epileptic patients and others with normal MR anatomy.

Effects of dimethyl sulfoxide on systemic and cerebral hemodynamic variables in the ischemic canine myocardium.

The effects of dimethyl sulfoxide (DMSO) infusion on cerebral blood flow (CBF) and systemic hemodynamics were studied in a canine model of myocardial ischemia. Immediately after ligation, cardiac output dropped from 2.09 +/- 0.11 (SEM) in the control group and 1.79 +/- 0.13 in the DMSO group to 1.07 +/- 0.21 and 1.0 +/- 0.08 L/min, respectively; there were no significant differences for 2 h. By the third hour and thereafter, the DMSO group had a significantly (p less than .05) higher cardiac output (1.65 +/- 0.08 L/min) than the control group (1.15 +/- 0.10 L/min). The cardiac output increase at 3 h was accompanied by significantly (p less than .05) lower systemic vascular resistance (SVR) in the DMSO group (5315 +/- 248 dyne X sec/cm5) as compared to the control group (7892 +/- 442 dyne X sec/cm5). There were no significant differences in heart rate, mean arterial pressure, pulmonary artery wedge pressure, or cerebral or pulmonary resistances in the control as compared to the DMSO group. Higher CBF values were noted at one hour and thereafter in the DMSO as compared to the control group (p less than .05). Low-dose DMSO given as an iv bolus improved cardiac output and CBF and lowered SVR in this canine model of experimental myocardial ischemia.

Hydrocephalus as a cause of dementia: evaluation by computerised tomography and intracranial pressure monitoring.

To identity those patients with hydrocephalus as a basis for their presenile dementia it is suggested that such patients be screened non-invasively by CT scan. If there is ventricular dilatation and small cortical sulci, intracranial pressure should be monitored for 48 hours to detect `B' waves. Patients with significant `B' waves (10 mmHg) for at least two hours in the day might benefit from a shunt.

Somatosensory evoked potentials, cerebral blood flow and metabolism following cerebral missile trauma in monkeys.

Somato sensory evoked potentials (SEP), cerebral blood flow and cerebral metabolism were studied in seven rhesus monkeys before and after a right occipito-frontal missile injury with an air rifle. The sensory evoked potential was present shortly after injury though markedly altered in shape. There was a very close correlation (r2 equal to 0.83) between SEP and cerebral blood flow on the uninjured side five minutes after injury. On the injured side, this was also noted but the amplitude of the SEP was much smaller, perhaps due to direct injury. If the flow in either hemisphere fell below 15-20 mls/100 gm/min, the evoked response disappeared, but in several animals a subsequent increase in flow was associated with a return of electrical activity. There was no correlation with cerebral perfusion pressure or cerebral metabolic rates for oxygen or lactate production, though it is likely that this is not due to physiological reasons but rather methodological. It might be inferred from these results that adequate flow is vital for the preservation and return of electrical activity following brain injury.

Effects of trimethaphan and sodium nitroprusside on cerebral blood flow in rhesus monkeys.

In tranquilised spontaneously breathing rhesus monkeys we have found that trimethaphan can reduce systemic blood pressure by 20% with little or no change in CBF. This was in marked contrast to those given nitroprusside which showed signs of loss of autoregulation at a 5% MBP reduction. It is our opinion that the divergence of these results from other work might be explained in part by differing anaesthetic techniques and species variations. On this experience we would be hesitant to use sodium nitroprusside or a hypotensive agent in any patient where cerebral blood flow might be already compromised.