The hydromechanics of hydrocephalus: steady-state solutions for cylindrical geometry.

Hydrocephalus is a state in which the circulation of cerebrospinal fluid is disturbed. This fluid, produced within the brain at a constant rate, moves through internal cavities in it (ventricles), then exits through passages so that it may be absorbed by the surrounding membranes (meninges). Failure of fluid to move properly through these passages results in the distention of the passages and the ventricles. Ultimately, this distention causes large displacements and distortion of brain tissue as well as an increase of fluid in the extracellular space of the brain (edema). We use a two-phase model of fluid-saturated material to simulate the steady state of the hydrocephalic brain. Analytic solutions for the displacement of brain tissue and the distribution of edema for the annular regions of an idealized cylindrical geometry and small-strain theory are found. The solutions are used for a large-deformation analysis by superposition of the responses obtained for incrementally increasing loading. The effects of structural and hydraulic differences of white and gray brain matter, and the ependymal lining surrounding the ventricles, are examined. The results reproduce the characteristic steady-state distribution of edema seen in hydrocephalus, and are compared with experiment.

Salamander olfactory bulb neuronal activity observed by video rate, voltage-sensitive dye imaging. I. Characterization of the recording system.

1. In this paper we describe properties of a video imaging system used to acquire voltage-sensitive dye fluorescence signals from the salamander olfactory bulb. Sources of noise in these signals were evaluated in preparations stained with the potentiometric probe RH-414. These were compared with noise levels in signals obtained from a light-emitting diode array designed to stimulate the experimental conditions with light levels similar to those seen in the salamander bulb recordings. These experiments define a number of determinants of video image quality to standardize optical voltage measurements in the salamander olfactory bulb. 2. Images were acquired at video rates using a Newvicon camera in a standard upright microscope and digitized with an eight-bit video frame grabber. 3. Sources of noise related to camera sensitivity, stability of illumination, and mechanical vibration were characterized. Camera dark noise was less than the pixel variability due to photon noise at the camera faceplate. This pixel noise was the limiting factor for discriminating the spatial and temporal properties of the optical responses. 4. No significant noise was found to be related to image digitization, transmission, or readout by the eight-bit frame grabber. Mechanical vibration, light stability, and other sources of noise could be controlled in vitro. In this condition, voltage-sensitive dye signal noise was similar to that in stimulated experiments using the light-emitting diode array. Higher levels of noise were found in vivo; some of this was reduced by sychronizing frame acquisition to the heartbeat. 5. On the basis of photodiode and video measurements, voltage-sensitive dye responses in the salamander olfactory bulb typically fell between 0.75% and 2.5% fractional change of background fluorescence. By appropriately adjusting the video signals before analog-to-digital conversion, we could detect fractional changes of < 0.5%. 6. Both response averaging and low-bandpass spatial filtering improved the signal-to-noise ratios of the images. For small numbers of averaged runs, the best improvement was obtained by low-bandpass spatial filtering. 7. Acquisition of high-spatial resolution video images permitted the use of low-bandpass spatial filters to suppress pixel noise. The degree of spatial enhancement depended on the relationship between the size of the structures of interest, pixel density, and the properties of the convolution filter kernel. This method avoided exposure of the preparation to prolonged illumination and the necessity of applying the large numbers of repeated stimuli required for averaging.

Emergent properties of odor information coding in a representational model of the salamander olfactory bulb.

In the salamander olfactory bulb, mitral output cells exhibit a variety of responses to electrical and odor stimulation, but the cellular interactions within the bulb that give rise to these responses are not completely understood. We have developed a computer model to investigate whether available data are sufficient for formulating a simulated bulb circuit that can generate realistic mitral cell output. A set of coupled difference equations incorporating mathematical descriptions of anatomical and physiological data was used to calculate changes in membrane potentials of olfactory bulb neurons over time. Model mitral cells showed responses to simple orthodromic and antidromic electrical stimuli that were similar to salamander intracellular responses. Without changing the parameters of the equations, simulated odor stimuli were applied that elicited complex patterns of mitral depolarization, spike activation, and hyperpolarization that emerged from the interactions among the numerous elements in the model. As with the electrical stimuli, model mitral responses to odor were also strikingly similar to those of real mitral cells. As an initial test of how different circuit components contribute to the responses, the lateral interactions between mitral cells and bulbar interneurons were manipulated. Tests with reduced lateral interactions and other tests with no inhibitory synaptic connections both produced mitral cell outputs that were uncharacteristic of salamander recordings. The similarity of the model's output to the complex properties of salamander single-cell recordings suggests that several critical features of the bulb circuit responsible for shaping mitral cell responsivity have been captured.(ABSTRACT TRUNCATED AT 250 WORDS)

Lumbar disc surgery: results of the Prospective Lumbar Discectomy Study of the Joint Section on Disorders of the Spine and Peripheral Nerves of the American Association of Neurological Surgeons and the Congress of Neurological Surgeons.

The Prospective Lumbar Discectomy Study enrolled 740 patients in a multiphysician, multicenter, consecutive patient protocol to evaluate the indications and efficacy of lumbar discectomy. Five hundred and thirteen patients could be evaluated at 3 months after surgery. Stepwise logistic regression showed that the factors of fraction of pain referred to the back, work-related injury, absence of back pain on straight leg-raise examination, correspondence of leg pain to typical radicular patterns, leg pain on straight leg-raise examination, and reflex asymmetry were independently predictive of good outcome from surgery. Univariate analysis of the case with different numbers of predictive factors present showed that use of the operating microscope, sensory deficit, central disc bulge, and free disc fragment were correlated with outcome only in subgroups. An analysis of unsatisfactory outcomes showed two patterns: one of failure as a result of mechanical back pain and one of failure as a result of radiculopathy. Factors predictive of outcome did not influence the type of failure. In a stepwise logistic regression analysis, facetectomy and preoperative sensory deficit were associated with increased likelihood of mechanical back pain failure, while preoperative motor deficit was associated with an increased likelihood of radicular failure. The results support several intuitively derived and commonly believed principles of lumbar disc surgery.

Altered skin flap survival and fluorescein kinetics with hemodilution.

Isovolemic hemodilution is known to reduce blood viscosity. Reduction of either the blood cell concentration or the serum total protein has been found to improve blood flow and ultimate survival of skin flaps. Nutritive blood flow and tissue survival were examined in dorsal pedicle skin flaps in three groups of rats at three hematocrit levels--44% (controls), 31%, and 19%. After hemodilution by phlebotomy and crystalloid replacement, the dye kinetics of intravenously administered fluorescein were measured with the fiberoptic perfusion fluorometer. Skin flap survival at the different hematocrits were also examined. Hematocrits and serum total protein were measured before and after hemodilution and the elevation of the flap. The rate of fluorescein elimination and ultimate flap survival were significantly augmented in the two hemodiluted groups. The maximum benefit was seen in the group of animals hemodiluted to 19%. However, significant improvement in blood flow and skin flap survival was noted in the group of animals that were hemodiluted to a more clinically acceptable level (31%). The decreases in serum total protein following flap elevation did differ among the groups. This study supports the hypothesis that isovolemic hemodilution may be a valuable technique for salvaging marginally ischemic tissues.