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Biomedical subjects

M A Howard

Publications and source records attributed to M A Howard.

At least 37 records · Page 2Linked to original sources
PubMed1998

Measurement of friction on straight catheters in in vitro brain and phantom material.

As part of our studies on the magnetic stereotaxis system (MSS), a means of delivering therapies to the bulk brain, we have measured the frictional forces on a thin, straight tube used to simulate a catheter. Experiments were done with a spring-loaded, stainless steel tube of 1.9-mm diameter which was passed through 5.5 cm of gelatin phantom or, alternatively, through in vitro calf brain. The dynamic response of the tube to sudden displacement of the outer end of the spring yields estimates of the tube's friction per unit length. Twenty-three runs in the two media were analyzed for the static and dynamic frictional forces exhibited. In these series the static frictional forces were found to be (0.0132 +/- 0.0012) N cm-1 [(1.32 +/- 0.12) g cm-1] of length in the gelatin phantom and (0.0079 +/- 0.0008) N cm-1 [(0.79 +/- 0.08) g cm-1] of length in brain. The kinetic friction coefficient, b, was found to be (8.4 +/- 2.1) N s m-1/cm length of catheter in brain and (16.3 +/- 7.6) N s m-1/cm length of catheter in the phantom material. Based on these figures, the MSS will be capable of moving straight catheters of similar friction that are 20-cm long at rates of displacement of 0.02 to 0.05 cm s-1 in the white and grey matter of the brain. Future studies will evaluate the forces arising from curved paths. Unanswered questions remain as to the mechanical difference between in vivo and in vitro brain, between animal and human brain, and the involvement of sulci in practical paths of motion.

Biomechanical Phenomena↗
PubMed1998

Posterior ventricular catheter burr-hole localizer. Technical note.

Proper ventricular catheter placements are associated with improved shunt performance. When placing ventricular catheters via the posterior approach, the surgeon must determine an optimum trajectory and then pass a catheter along that trajectory. The incidence of optimal posterior catheter placements is increased by using a posterior catheter guide (PCG); however, errors may still occur because of poor selection of a posterior burr-hole site. In this report an easy-to-use posterior burr-hole localizer (Localizer) is described that defines the optimum burr-hole location based on geometric relationships involving the ear and supraorbital rims. The basic design principle of the Localizer was formulated and tested by using neuronavigational imaging tools to examine normal adult ventricular anatomy in relation to surface landmarks and by reviewing imaging studies obtained in 50 adult patients with hydrocephalus. Subsequently, the Localizer was used in 28 consecutive patients scheduled to undergo shunt surgery performed by using the PCG. In all cases the catheter entered the ventricle on the first pass and postoperative imaging studies demonstrated successful placement in the ipsilateral anterior horn. There were no catheter-related complications. These early results indicate that the Localizer and PCG devices may be safe and effective when used in combination for placement of posterior ventricular catheters.

Adult↗
PubMed1997

Chronic microelectrode investigations of normal human brain physiology using a hybrid depth electrode.

Neurosurgeons have unique access to in vivo human brain tissue, and in the course of clinical treatment important scientific advances have been made that further our understanding of normal brain physiology. In the modern era, microelectrode recordings have been used to systematically investigate the cellular properties of lateral temporal cerebral cortex. The current report describes a hybrid depth electrode (HDE) recording technique that was developed to enable neurosurgeons to simultaneously investigate normal cellular physiology during chronic intracranial EEG recordings. The HDE combines microelectrode and EEG recordings sites on a single shaft. Multiple microelectrode recordings are obtained from MRI defined brain sites and single-unit activity is discriminated from these data. To date, over 60 HDEs have been placed in 20 epilepsy surgery patients. Unique physiologic data have been gathered from neurons in numerous brain regions, including amygdala, hippocampus, frontal lobe, insula and Heschl's gyrus. Functional activation studies were carried out without risking patient safety or comfort.

Action Potentials↗
PubMed1996

A chronic microelectrode investigation of the tonotopic organization of human auditory cortex.

We investigated the functional organization of human auditory cortex using a new chronic microelectrode technique. Tonotopic mapping data was obtained at the single unit level for the first time in humans. All sound-driven units were noted to have frequency-dependent response patterns. The majority of units (73%) demonstrated sharply tuned excitatory best-frequency responses. Twenty seven percent of units showed wide receptive fields, representing excitatory responses to almost the entire range of frequencies presented. A tonotopic pattern was observed with best frequencies systematically increasing as more medial-caudal recording sites were sampled.

Acoustic Stimulation↗
PubMed1996

Photodynamic therapy of pigmented choroidal melanomas using a liposomal preparation of benzoporphyrin derivative.

OBJECTIVE: To evaluate the effectiveness of photodynamic therapy of pigmented choroidal melanoma using a liposomal preparation of benzoporphyrin derivative monoacid (BPD), verteporfin. DESIGN: Pigmented choroidal melanomas were established in 25 New Zealand albino rabbit eyes. The animals were treated with daily injections of cyclosporine, and tumor growth was monitored with funduscopic examination and ultrasonography. Fifteen minutes after intravenous injection of BPD (2 mg/kg), the tumors were irradiated at 692 nm through an argon-pumped dye laser with the delivered fluence ranging between 40 and 150 J/cm2. Control animals were treated with light only, photosensitizer only, or observation only. Tumor growth was monitored by indirect ophthalmoscopy, fundus photography, fluorescein angiography, and ultrasonography. Histologic examination was performed. RESULTS: Eighteen tumor-bearing rabbits were treated with light and BPD; 16 were followed up for 1 month, and two were killed immediately for histologic examination. Tumors regressed in all eyes treated with 60 J/cm2 or more. With fluence of 40 J/cm2, tumor regrowth was observed in one animal within 10 days of treatment. In the three control groups, all animals showed continuous tumor growth. Histologic examination of the eyes treated with photosensitizer and light immediately after treatment showed prominent vascular occlusion throughout the full thickness of the tumor. One month after treatment, tumor necrosis and infiltration of mononuclear cells and pigment-laden macrophages were the predominant findings. CONCLUSIONS: Photodynamic therapy with BPD may have a role in the treatment of pigmented choroidal melanomas.

Animals↗
PubMed1996

Magnetic neurosurgery.

Because of the complex shape of many brain structures, the ideal brain probe would be highly flexible and give the neurosurgeon the ability to independently and precisely control movement of the probe tip. A magnetic surgery system has been developed that implements this concept. Flexible brain probes with small permanent magnetic tips are placed on the brain surface through a small burr hole and then magnetically manipulated within the brain. Drive forces are produced by an array of six superconducting magnets suspended within a cryostatic helmet. They produce a maximum force of 3 times the threshold needed to move the probe through the parenchymal tissues at its normal speed of 1 mm/s. Computer-controlled magnetic impulses precisely direct the probe movement in any direction desired with movement accuracy of 0.47 mm in phantom gels. Probe position is monitored 3 times per second with orthogonally oriented microchannel plate X-ray systems, X-ray dose from a 3-hour simulated procedure is comparable to that of a chest X-ray. In vivo and in vitro feasibility studies have been carried out in dog and pig brains. Preclinical trials are planned for clinical applications including implantation of flexible brachytherapy threads.

Equipment Design↗
PubMed1996

A hybrid clinical-research depth electrode for acute and chronic in vivo microelectrode recording of human brain neurons. Technical note.

For several decades, important scientific information has been gained from in vivo microelectrode recordings of individual human cerebral cortical neurons in patients with epilepsy. The experimental methods used, however, are technically complex and require a highly skilled intraoperative team. There are also significant experimental time limitations, as well as constraints on the type of behavioral tests conducted, and the brain regions that may be safely studied. In this report, a new method is described for obtaining in vivo microelectrode recordings using a hybrid depth electrode (HDE). High-impedance research recording contacts are interspersed between low-impedance clinical electroencephalographic (EEG) contacts along the HDE shaft. The HDE has the same external physical properties as a standard clinical depth electrode (DE). Following preclinical laboratory testing, 15 HDEs were used in the evaluation of six patients with medically refractory epilepsy. High-quality EEG recordings were obtained in all cases (two acute intraoperative, four from the chronic epilepsy monitoring unit). Action potentials from individual neurons were successfully recorded during all experimental sessions; however, the chronic preparations were clearly superior. Chronic HDEs are placed using a standard stereotactic system, and the locations of recording contacts are documented on a postimplantation imaging study. The quality of the chronic research recordings was excellent over study periods ranging from 5 to 14 days. The patients rested comfortably on the ward and were able to cooperate with complex experimental instructions. Basic neuroscientists participated fully in all aspects of the chronic investigations. The use of an HDE in place of a standard clinical DE may now allow detailed physiological investigations of any brain region targeted for clinical DE implantation.

Brain↗
PubMed1995

Review of magnetic neurosurgery research.

Image guided surgical techniques are often used to provide useful localizing information during the course of otherwise "standard" surgical procedures. In this review, a magnetic surgery system (MSS) is described that couples image guidance methods with a novel magnetic treatment delivery system. The MSS was designed with the unique capacity to manipulate an implant remotely, to follow complex curvilinear paths, and to implement serial movements over time without reoperation. Magnetically tipped intracerebral implants are placed on the brain surface and then directed along a target trajectory by means of externally generated magnetic gradients. The location of the implant is calculated by comparing preoperative magnetic resonance imaging (MRI) imaging data with intraoperative biplanar fluoroscopic images. A command computer presents the surgeon with relevant MRI-derived images and implements movement commands by sending currents of various strength to the six superconducting magnets suspended about the subject's head. The results of multidisciplinary preclinical research and development work are reviewed.

Brain↗
PubMed1995

Treatment of retinal breaks with autologous serum in an experimental model.

PURPOSE: The standard treatment for retinal breaks is thermal adhesion. Breaks in the posterior pole (i.e., macular holes) recently have been treated using vitrectomy and the recombinant cytokine transforming growth factor-beta. This has been shown to achieve closure of the retinal breaks by stimulating localized fibrocellular proliferation. Serum has been shown to contain chemoattractants and mitogens for many types of cells. The authors studied the clinical and histologic effect of autologous serum application to retinal breaks in an experimental model. METHOD: Twenty-four rabbits underwent pars plana lensectomy, vitrectomy, retinectomy, fluid-air exchange, application of test solution (12 with Hank's buffered salt solution and 12 with autologous serum), and air-gas exchange. Clinical examination with indirect ophthalmoscopy was performed, and animals were killed 5, 14, and 28 days after treatment. Tissue sections through the retinectomy were studied by light microscopy, electron microscopy, and immunocytochemistry. RESULTS: None of the serum-treated eyes showed retinal detachment at the site of the retinectomy by evaluation with indirect ophthalmoscopy at each of the time points. In contrast, in control eyes retinal detachment developed at the retinectomy site from 0% at day 5 to 50% at day 14 and 75% at day 28. By light microscopy, serum-treated eyes contained multilayers of fibroblast-like cells adhering the retinectomy edges to the underlying retinal pigment epithelium and choroid. The control eyes had nonadherent retinal edges at the retinectomy site with little sign of fibrocellular response. Results were confirmed by electron microscopy. The fibroblast-like cells by immunocytochemistry contained vimentin, cytokeratin 18, and/or glial fibrillary acidic protein. CONCLUSION: This study suggests that serum induces a localized fibrocellular response at the retinectomy edges compared with control eyes. This response, characterized by light microscopy, electron microscopy, and immunocytochemistry, appears to involve a mixed population of glial, retinal pigment epithelial, and/or fibroblastic cells. These cells seem to enhance adhesion and subsequent reattachment of the edges of the retinectomies at the time points studied when compared with controls.

Animals↗
PubMed1995

A technique revisited: hemodynamic comparison of closed- and open-chest cardiac massage during human cardiopulmonary resuscitation.

OBJECTIVE: To compare the hemodynamics of closed-chest cardiac massage vs. open-chest cardiac massage in patients resuscitated from cardiac arrest that occurred outside of the hospital. DESIGN: Prospective, non-outcome, case series. SETTING: Large urban emergency department. PATIENTS: Ten adult, normothermic, nontraumatic, out-of-hospital, cardiac arrest patients who failed advanced cardiac life support (ACLS) therapy. INTERVENTIONS: Patients presenting to the hospital in cardiac arrest were managed according to the ACLS protocol at the clinician's discretion. Proximal aortic and central venous pressure catheters were placed to measure arteriovenous compression- and relaxation-phase pressure gradients. After 5 mins of baseline measurements during closed-chest cardiac massage, patients underwent a left lateral thoracotomy, and open-chest cardiac massage was performed for 5 mins. MEASUREMENTS AND MAIN RESULTS: The mean coronary perfusion pressure and compression-phase pressure gradients were 7.3 +/- 5.7 and 6.2 +/- 5.4 mm Hg, respectively, during closed-chest cardiac massage, while increasing to 32.6 +/- 17.8 and 32.6 +/- 29.9 mm Hg, respectively, during open-chest cardiac massage. The differences between both measurements were statistically significant (p < .05). CONCLUSIONS: Open-chest cardiac massage is superior to closed-chest cardiac massage in providing relaxation-phase and compression-phase pressure gradients during cardiac arrest in patients failing current ACLS protocols. During open-chest cardiac massage, all patients exceeded the minimum coronary perfusion pressure of 15 mm Hg, which is recommended to obtain a return of spontaneous circulation. Further outcome studies are needed to determine the timeliness and appropriate indications for open-chest cardiac massage.

Aged↗
PubMed1995

Functional design features and initial performance characteristics of a magnetic-implant guidance system for stereotactic neurosurgery.

A helmet with a roughly cubic array of six super-conducting coils is used to apply force on a small permanent magnet pellet in brain or in brain phantom material. This apparatus, called the Magnetic Stereotaxis System, will be used to deliver drugs and other therapies directly into deep brain tissues, under control of a computer and fluoroscopic imaging system. This paper considers only the force application aspects of the instrument. The primary design features of the helmet and power supply controls are presented, along with field plot data and single-axis motion results. The field plot data show that agreement with the finite-element iron-free field calculations is sufficiently high (> 1%) for the instrument. These preliminary motion data indicate accuracy better than 2 mm for the impulsive pellet motion, even though the visual position observations had significantly greater error than the completed imaging system will have. The companion paper will take up analysis of the control aspects of the motion, and our recent solutions to difficulties found in the experimental work described here.

Brain↗
PubMed1995

Characteristics of an improved magnetic-implant guidance system.

The previous companion paper described the motivation, design, and early experiments of a Magnetic Stereotaxis System. The part of the system considered in these papers is a helmet with a roughly cubic array of six superconducting coils used to apply force on small permanent magnet pellets in brain and in brain phantom material. This apparatus will be used to deliver drugs and other therapies directly into deep brain tissues, under control of a computer and fluoroscopic imaging system. Here, we analyze the general stability problems of controlling the currents in the coils for impulsive stepwise motion of the pellet, subject to quench avoidance in the superconducting coils, and in the face of Earnshaw's theorem governing stability in static magnetic fields. We also describe solutions that have been found to the primary difficulties limiting controlled pellet motion in the studies presented in the companion paper.

Brain↗
PubMed1995

A guide to placement of parietooccipital ventricular catheters. Technical note.

Accurate placement of parietooccipital ventricular catheters can be difficult and frustrating. To minimize the morbidity of the procedure and lengthen the duration of shunt function, the catheter tip should lie in the ipsilateral frontal horn. The authors describe a posterior ventricular guide (PVG) for placement of parietooccipital catheters that operates by mechanically coupling the posterior burr hole to the anterior target point. In a series of 38 patients who underwent ventriculoperitoneal shunting with the assistance of the guide, postoperative computerized tomography (CT) scanning revealed that 35 (92.0%) had accurate catheter placement. In comparison, a retrospective review of free-hand posterior catheter placement revealed good catheter position in only 22 of 43 patients (51.1%). The use of the guide added less than 5 minutes to the entire procedure, and there were no complications related to its use. The PVG is a simple and useful tool that aids in the placement of parietooccipital ventricular catheters.

Adolescent↗
PubMed1995

A noninvasive, reattachable skull fiducial marker system. Technical note.

As computer-interactive technologies become more widely used in neurosurgery, radiology, and radiation therapy, the need for an optimum skull fiducial marker system increases. In the past, intracranial localization methods required precisely machined metal frames and rigid pin fixation to the skull. Recently, this function has been performed using "frameless" computer-based systems that calculate brain position relative to a series of external reference points, the most accurate of which are screwed directly into the skull. A penetrating fiducial marker system, however, is not well suited for applications requiring multiple volume registrations over an extended time period. We describe a new skull fiducial marker system that attaches to the maxillary teeth and can be used repeatedly on different occasions. A curved bar, known as a Banana Bar (BB) extends backward from a custom mouthpiece around the side of the patient's head; the bar contains sites of attachment for screw-in radiographic fiducial markers. Repositioning accuracy was quantitated using a photographic technique. A BB prototype was constructed and tested in three subjects. The BB weighs less than 100 g and can be comfortably held in position for up to 30 minutes. It takes less than 1 minute to screw in the mouthpiece and only seconds to secure the BB to the teeth. One hundred twenty photographic measurements were analyzed from 60 repositionings over a minimum 3-week period. Standard deviations for the measurement series ranged from 0.29 to 0.86 mm. Results suggest that the BB may be an inexpensive, efficient, and accurate method for providing the external reference points needed for a wide range of emerging computer-interactive applications.

Brain↗
PubMed1994

Physiologic effects of nucleus basalis magnocellularis stimulation on rat barrel cortex neurons.

Cholinergic neurons in the nucleus basalis magnocellularis (NBM) project to the cerebral cortex and are thought to play an important role in learning and memory, and other cognitive functions. In the present study, we examined the effects of NBM stimulation on the response properties of individual cortical neurons in layer V of the rat somatosensory cortex. Seventy-three neurons were studied before and after a brief electrical stimulation of NBM. Transient changes in spontaneous activity were observed in 60% of the cells, and in most cases this background activity decreased. Recordings lasting more than 1 h stimulation were obtained from 56 cells. Because some NBM stimulation-induced effects lasted several hours, neurons were evaluated in two groups, NBM1 and NBM2. NBM1 neurons were those exposed to either the first NBM stimulation of the day or an NBM restimulation following a more than 5 h stimulation-free period. Neurons exposed to NBM restimulation following a stimulation free interval of less than 5 h were classified as NBM2. Sixty-nine percent of the 32 NBM1 neurons displayed marked decreases in spontaneous activity and/or increases in the response evoked by deflecting a contralateral facial vibrissa. NBM1 stimulation caused some units to respond to previously minimally effective whisker stimuli. Stimulation effects often lasted several hours. By contrast, long-lasting changes were observed in only 25% of the 24 NBM2 neurons, and the only consistent effect was on spontaneous, not stimulus-evoked, activity. Systemic injection of atropine blocked NBM stimulation-induced changes in spontaneous and stimulus-evoked activities. Control neurons, studied without NBM stimulation, failed to display consistent alterations in their response properties during the course of 1 h or more. Results demonstrate that NBM activation produces long-lasting, cholinergically mediated alterations in the response properties of somatosensory cortical neurons. Effects were complex, being influenced by factors such as the time interval between successive stimulations during an experiment. The complexity of these NBM mediated effects should be considered when designing therapies for neurodegenerative disorders characterized by loss of NBM neurons.

Animals↗
PubMed1994

Comparing the extent of hippocampal removal to the outcome in terms of seizure control.

The relationship between the outcome of temporal lobectomy for epilepsy and the size of the hippocampectomy tailored to intraoperative electrocorticographic findings was evaluated in 52 patients, with at least 1 year of follow-up. In 22 patients, < 2.0 cm of hippocampus was removed. Postoperatively, 16 (72.7%) were class I (seizure-free), 3 (13.6%) were class II (rare seizures) and 3 (13.6%) were class IV (no worthwhile improvement). In 30 patients, > or = 2.0 cm of hippocampus was removed, and the results were as follows: 21 (70%) were class I, 4 (13.3%) were class II, 1 (3.3%) was class III (worthwhile improvement) and 4 (13.3%) were class IV. Statistically significant differences were not present.

Electroencephalography↗
PubMed1993

The pathophysiology of infant subdural haematomas.

The widely accepted theories concerning the pathophysiology of infant subdural haematoma (SDH) were formulated in the pre-computed tomographic (CT) scan era. Violent shaking is considered to be a crucial cause of SDH in non-accidentally injured infants. This theory has been re-examined in a clinical and CT scan review, and our findings have been correlated with results of recent head injury research. A retrospective review was conducted of all head injured infants (up to 18 months old) treated at Atkinson Morley's Hospital over a recent 20-year period (n = 100). Twenty-eight infants with a SDH were identified. CT scans were reviewed and each SDH greater than 0.5 cm thick was morphometrically analysed. Seventeen infants were Caucasian, 10 were non-Caucasian and one was of mixed race. A race-dependent pattern of SDH pathophysiology was noted, with non-Caucasian infants with a head injury more likely to have a SDH than Caucasian infants (67 v 21%, p < 0.01). All had a history of, or radiographic and clinical findings most consistent with, an impact injury, and non-Caucasian infants were significantly more likely to have a normal scalp examination despite the impact injury (p < 0.05) and to have developed the SDH after a relatively trivial fall (p < 0.01). They were also more likely to have a large (> 0.5 cm thick) SDH (p < 0.05), and to suffer post-traumatic seizures (p < 0.05). Our findings do not support shaking as the only cause of infant SDH formation and also suggest that non-accidental injury is a less common cause of SDH than it is believed to be.

Child Abuse↗