Update on skull base surgery.

The field of skull base surgery has changed significantly during the past decade. Various surgical approaches have been developed, which when used alone or in combination provide optimal exposure for the resection of a given lesion with minimal morbidity. This recent evolution in the field of skull base surgery has allowed the surgical management of larger lesions in previously inaccessible locations. Furthermore, techniques in the preoperative, intraoperative, and postoperative management of the intrapetrous internal carotid artery have circumvented many prior limitations of cranial base resections. The goals of this article are to provide an overview of the postlateral skull base approaches in use today, discuss the regions of the skull base that are accessible, and describe the state of the art in management of the intrapetrous internal carotid artery.

Feasibility of a visual prosthesis for the blind based on intracortical microstimulation of the visual cortex.

The feasibility of producing a visual prosthesis for the blind using intracortical microstimulation (ICMS) of the visual cortex was studied in a 42-year-old woman who had been totally blind for 22 years secondary to glaucoma. Thirty-eight microelectrodes were implanted in the right visual cortex, near the occipital pole, for a period of 4 months. Percepts reported as small spots of light, called phosphenes, were produced with 34 of the 38 implanted microelectrodes. Threshold currents for phosphene generation with trains of biphasic pulses were as low as 1.9 microA, and most of the microelectrodes had thresholds below 25 microA. Phosphene brightness could be modified with stimulus amplitude, frequency and pulse duration. Repeated stimulation over a period of minutes produced a gradual decrease in phosphene brightness. Phosphenes did not flicker. The apparent size of phosphenes ranged from a "pin-point' to a "nickel' (20 mm diameter coin) held at arm's length. Phosphene size usually decreased as stimulation current was increased but increased slightly as the train length (TL) was increased. At levels of stimulation near threshold, the phosphenes were often reported to have colours. As the stimulation level was increased, the phosphenes generally became white, greyish or yellowish. Individual phosphenes appeared at different distances from the subject. When two phosphenes were simultaneously generated, the apparent distances of the individual phosphenes sometimes changed to make them appear to be at about the same distance. When three or more phosphenes were simultaneously generated, they became coplanar. Except for rare occasions, phosphenes extinguished rapidly at the termination of the stimulation train. When stimulation TLs were increased beyond 1 s, phosphenes usually disappeared before the end of the train. The duration of phosphene perception could be increased by interrupting a long stimulation train with brief pauses in stimulation. Intracortical microelectrodes spaced 500 microns apart generated separate phosphenes, but microelectrodes spaced 250 microns typically did not. This two-point resolution was about five times closer than has typically been achieved with surface stimulation. With some individual microelectrodes, a second closely spaced phosphene was sometimes produced by increasing the stimulation current. Phosphenes moved with eye movements. When up to six phosphenes were simultaneously elicited, they all moved with the same relative orientation during eye movements. All phosphenes were located in the left hemi-field with the majority above the horizontal meridian. There was a clustering of most of the phosphenes within a relatively small area of visual space. The potentially greater microelectrode density and lower power requirements of ICMS compared with surface stimulation appears encouraging for a visual prosthesis. However, further studies with blind subjects are required to optimize stimulation parameters and test complex image recognition before the feasibility of a visual prosthesis based on ICMS can be established.

Chronic invasive monitoring for identifying seizure foci in children.

In both children and adults, it has been well established that the precise localization of a seizure focus allows for the best possible resection and outcome. Long-term invasive monitoring with DE has been the most widely used modality to evaluate patients with intractable seizures of temporal lobe origin. The study of epilepsy in children, however, requires assessment of the cause of the seizures with a resultant decision regarding the optimal modality of study for the particular problem. Complex partial seizures of medial temporal lobe origin without clear concordance or lateralizing information are best studied using DE. With MR imaging anatomic localization, stereotactic insertion of DE can be performed accurately and relatively safely. The information obtained permits lateralization and focus localization if the lesion is within the medial structures. The ability of DE to study epilepsy outside of the medial temporal lobe, however, is suboptimal. Many centers have used surface electrodes to map temporal lobe foci successfully. SE and SGE have not been widely used in children. Subdural electrode arrays, however, are superior to DE in extratemporal epilepsy because the cortical surface contacts provide the best recordings of epileptiform activity from a wide range of extratemporal regions. The limitations of intraoperative ECoG in the awake child are well understood. It is rare that a child can undergo surgery under local anesthesia and cooperate to the extent necessary for definitive mapping of the seizure focus and eloquent areas. Thus, invasive long-term monitoring would seem ideal in the epileptic pediatric patient because it allows for a more relaxed pace in which to record events, and because of the indwelling nature of these electrodes, it is possible for the investigator to map eloquent areas of the brain before resection. This type of evaluation completely and accurately maps interictal and ictal activity, and through cortical stimulation or mapping of somatosensory areas by evoked potentials, eloquent areas of the brain, such as motor and speech regions, can be identified. This method is limited to older children because the young child often has inadequate cortical development to localize these areas definitively. One should note that in young children (under the age of 4 years), the absence of a response to standard cortical stimulation does not indicate nonfunctional cortex. Long-term intractable seizures and significant antiepileptic drug requirements may contribute to developmental and psychosocial deficiencies in the epileptic patient.(ABSTRACT TRUNCATED AT 400 WORDS)

Diurnal variation of cerebrospinal fluid immunoreactive corticotropin-releasing hormone levels in healthy volunteers.

CRH is not only secreted into hypophyseal protal blood where it is believed to regulate the circadian rhythm of pituitary-adrenal activity, but is also measurable in cerebrospinal fluid (CSF). Altered CSF immunoreactive CRH (IR-CRH) levels have been found in patients with a number of neuropsychiatric disorders and have been implicated in some of the symptoms of these disorders. To further study the potential functional relevance of CRH in human CSF, we examined whether a nonuniform temporal pattern of IR-CRH levels existed in CSF using hourly sampling over a 30-h period in six healthy volunteers. CSF was withdrawn continuously at 6 mL/h through a catheter placed in the lumbar subarachnoid space and connected to a miniroller pump and fraction collector. A significant diurnal variation in CSF IR-CRH levels was observed (P < 0.001), with highest levels between 1830-2330 h and lowest levels around 0730 h. This pattern was nearly opposite that of plasma cortisol levels, which showed the expected peak around 0800 h and nadir around 2000-2200 h. In addition, CSF IR-CRH levels in three of the six volunteers showed significant negative correlations with simultaneous plasma cortisol levels. These data suggest that CSF IR-CRH concentrations are negatively modulated by peripheral cortisol secretion, which may be one factor involved in the entrainment of this rhythm. Although the functional significance of this diurnal variation in CSF IR-CRH levels is unknown, the presence of a distinct temporal organization of CRH release into the CSF in humans is compatible with the idea that CSF may play a functional role in or otherwise reflect nonsynaptic information processing in the central nervous system. Diurnal factors should be taken into account in future studies of CRH concentrations in human CSF.