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Biomedical subjects

A L Evans

Publications and source records attributed to A L Evans.

At least 19 recordsLinked to original sources
PubMed1996

Objective detection of hemifield and quadrantic field defects by visual evoked cortical potentials.

AIMS/BACKGROUND: An objective method for detecting hemifield and quadrantic visual field defects has been developed using steady state visual evoked cortical potentials (VECPs), an adaptive noise canceller (ANC), and Hotelling's t2 statistic. The purpose of this study was to determine the sensitivity and specificity of the technique. METHODS: Nine subjects (mean age 44 years) were investigated with field loss due to a variety of causes including both anterior and posterior visual pathway lesions. Dynamic perimetry was performed by means of a Goldmann or Tübingen perimeter. VECP recordings were made from each visual field quadrant (23 degrees X 23 degrees) by means of a steady state reversing checkerboard (7.7 rev/s). The central 5 degrees of the visual field and the vertical and horizontal meridians were masked during these measurements. Recordings were made from three electrode sites, positioned over the visual cortex, relative to a mid frontal electrode. Each recording lasted 2 minutes, during which time fixation was monitored. The data from each recording were divided into 4 second segments, and the amplitude and phase of the VECP signal measured using the ANC. Hotelling's t2 statistic was applied to determine the probability of signal detection. Receiver operating characteristic curves were used to find the optimum signal detection threshold for identification of the visual field defects. RESULTS: The results of the study confirmed patterns of subjective visual field loss. The technique had a sensitivity and a specificity of 81% and 85%, respectively, for detecting 'non-seeing' areas in the inferior visual field, and 82% and 89%, respectively, for detecting 'non-seeing' areas in the superior visual field. CONCLUSION: These results demonstrate that the technique is of potential clinical value to ophthalmologists and neurologists when subjective perimetry is not possible.

Adolescent↗
PubMed1995

Gait speed and activities of daily living function in geriatric patients.

OBJECTIVE: To establish the relationship between gait speed (GS) and functional independence in elderly people. DESIGN: GS is suggested as being a criterion standard in rehabilitation reflecting muscle strength. This study assessed the relationship between gait speed and functional independence in Activities of Daily Living (ADL). GS was measured by portable accelerometer over 2 meters. The mean of 3 attempts was taken. ADL function was measured by an occupational therapist using the modified Barthel ADL Index. The relationship between these measures was assessed by a statistician. SETTING: A geriatric unit in a hospital in Scotland. PATIENTS: One hundred sixty-one inpatients and outpatients were selected at random from the patients of a geriatric unit over a 3-month period. Patients were eligible if they were mobile with or without a walking aid. INTERVENTIONS: GS was measured by portable ultrasonic accelerometer. Patients were reviewed by an occupational therapist, blinded to their GS, who recorded functional capacity. Case sheet review provided diagnostic details and cognitive function. The type of floor surface was recorded. MAIN OUTCOME MEASURES: GS (m/sec), and Barthel score. RESULTS: Patients with GS of < .25m/sec were more likely to be dependent in one or more ADL function, p < .01. Those with a GS between .35 and .55m/sec were more likely to be independent in all ADL functions, p < .001. Patients whose GS was > .55m/sec did not maintain this independence. There was no relationship between GS and floor surface or cognitive function. CONCLUSIONS: GS is a useful indicator of ADL function in geriatric patients.

Activities of Daily Living↗
PubMed1994

A personal computer-based visual evoked potential stimulus and recording system.

A system for recording electroretinograms and visual evoked cortical potentials has been constructed with the use of a personal computer and a digital signal processing card. The system is based on widely available commercial hardware. It has been designed to be capable of performing routine visual electrophysiology as well as allowing the development of novel visual stimuli and signal detection techniques. The system enables both transient and steady-state stimulation rates. Pattern stimuli can be presented in pattern-reversal, pattern-onset, pattern-offset or motion-onset modes. In addition to conventional signal averaging, the digital signal processing card can also provide on-line Fourier analysis and is facilitating the development of adaptive filtering techniques for the detection of steady-state visual evoked cortical potentials. This versatile system is in regular clinical use for the measurement of electroretinograms and visual evoked cortical potentials.

Electrophysiology↗
PubMed1994

The measurement of steady-state visual evoked cortical potentials using an adaptive noise canceller.

A new software method for measuring steady-state visual evoked cortical potentials (VECPs) has been developed using the principle of adaptive noise cancelling. The steady-state VECP is composed of narrow-band frequency components at harmonics of the stimulus frequency. In clinical recordings, these signal components are masked by wide-band noise, predominantly electroencephalographic activity and muscle noise. The stimulus frequency is exactly known and by using a reference sinusoid at the stimulus frequency (or its harmonics) the adaptive noise canceller (ANC) is able to cancel uncorrelated noise components from the recording. In effect, the ANC functions as an adaptive narrow-band-pass filter at the reference frequency. The performance of the ANC has been evaluated using both simulated and physiological signals. The output of the ANC provides temporal information on the signal amplitude and phase, and can be used to calculate the reliability of signal detection. For this application, the ANC has a number of advantages over the fast Fourier transform: it is a more sensitive detector, it requires fewer calculations, it is less computationally intensive, it requires less memory and it can be implemented in real time.

Algorithms↗
PubMed1994

Isothermal anneal effect on microcrack density around leucite particles in dental porcelain.

Because of the large differential in thermal expansion coefficient between leucite and the surrounding glass matrix, microcracks form around the leucite crystallites during the manufacture of dental porcelain frits. These microcracks decouple leucite from the surrounding glass matrix and affect the bulk thermal expansion of the porcelain frit (Binns, 1983). The purpose of this study was to determine if the microcrack density in a dental porcelain decreased as a result of isothermal heat treatment. Ten specimens of a commercial dental porcelain that had previously exhibited an increase in thermal expansion as a function of isothermal heat treatment were prepared and divided into two groups. The experimental group was heated to 750 degrees C and held for 16 minutes at that temperature. The control group received no anneal. The mean microcrack densities were determined by quantitative stereology to be 575 cm2/cm3 +/- 75 cm2/cm3 (mean +/- SEM) for the control group (no anneal) and 231 cm2/cm3 +/- 25 cm2/cm3 for the experimental group (16-minute anneal at 750 degrees C). The specimens annealed at 750 degrees C had a significantly lower microcrack density (p < 0.001) than those that received no anneal. A model was developed to estimate the effect of microcracking on thermal expansion of the porcelain, and a 6% increase in the coefficient of thermal expansion of the porcelain was predicted from this model as a result of this decrease in microcrack density.

Aluminum Silicates↗
PubMed1993

Malfunction of medical equipment as a result of mains borne interference.

Medical equipment has become more intelligent as the manufacturers have incorporated the latest microprocessor based technology. Equipment malfunction can be caused at any time by inherent errors in the control program but it is particularly important that this is designed to cope with the effects of electrical interference which, in addition, may cause corruption of the software. We have considered interference found in the mains supply in the hospital environment. Using a test protocol with appropriate interference simulators, a wide range of medical equipment was removed temporarily from use and its immunity to electrical mains borne interference tested. Battery operated mains rechargeable devices were unaffected by mains voltage variations including drop-outs and sags whereas mains powered devices were affected to varying degrees of severity. In particular, repetitive drop-outs caused loss of power due to fuse blowing in some life support equipment. Impulses affected 25% and pulse bursts 50% of the equipment tested with some evidence that the more recent designs coped better. The EEC Directive on electro-medical compatibility compliance may cause the design of equipment to be improved but hospitals will have to cope with the above problems in their existing equipment for many years to come.

Electricity↗
PubMed1993

Patterns of central motor reorganization in hemiplegic cerebral palsy.

Central motor reorganization was studied in 33 subjects with hemiplegic cerebral palsy. Corticospinal projections were investigated using focal magnetic stimulation of the motor cortex. Reflex pathways were examined with digital nerve stimulation. Cross-correlation analysis of multi-unit EMG was used to detect activity in branched common stem last order presynaptic inputs to motor neuron pools. The neurophysiological findings were related to the clinical outcome. In 21 of the subjects studied (64%), there was evidence for reorganization of central motor pathways. The clinical and neurophysiological findings revealed two different forms of reorganization. In both forms focal magnetic stimulation demonstrated novel ipsilateral motor pathways from the undamaged motor cortex to the hemiplegic hand. Ipsilateral projections were not demonstrated from the damaged motor cortex. Eleven subjects had intense mirror movements. In these subjects cross-correlation analysis and reflex testing suggested that corticospinal axons had branched abnormally and projected bilaterally to homologous motor neuron pools on both sides of the spinal cord. The remaining 10 subjects did not have intense mirror movements and in these subjects there was no evidence for last order branching of corticospinal axons. It was found that good function of the hemiplegic hand was associated with the presence of EMG responses in that hand following magnetic stimulation of the contralateral motor cortex. When EMG responses were absent, hand function was poor unless the subject had intense mirror movements.

Adolescent↗
PubMed1992

Characterisation and correction of a mammalian cell mutant defective in late step of base excision repair.

An Indian muntjac cell line, SVM, is unusually sensitive to cell killing induced by a range of alkylating agents. Cells transfected with the Escherichia coli ada gene or human genomic DNA have allowed the response of SVM to alkylating agents to be dissociated into two distinct components. Thus, in SVM, which expresses very low levels of alkyltransferase (AT), O6-alkylguanine appears to be the major cytotoxic, clastogenic, and recombinogenic lesion following exposure to agents such as methylnitrosourea (MNU). However, SVM is also very sensitive to agents such as dimethylsulfate (DMS), which produce only very low levels of O6-methylguanine damage. Sensitivity to DMS resides in an inability to complete base excision repair, with the appearance of persistent single-strand DNA breaks (SSBs), and does not appear to involve defects in glycosylase, apurinic/apyrimidinic endonuclease, or DNA ligase activities. Another, possibly related, phenotypic trait in SVM is its limited ability to ligate transfected linear plasmid DNA. Transfectants of SVM, harboring human DNA sequences, show a significant correction of DMS-induced cytotoxicity and clastogenicity and a reduction in the levels of DMS-induced DNA SSBs. The DMS-resistant transfectants have an increased ability to ligate linear plasmid DNA, and also express AT, making these lines resistant to alkylating agents such as MNU. These results suggest that cells possess a mechanism that regulates AT expression, plasmid break-joining ability, and certain aspects of base excision repair. Transfectants of SVM containing human DNA provide a means to isolate genes involved in a coordinate response to alkylation damage.

Alkylating Agents↗
PubMed1991

Cutaneomuscular reflexes recorded from the first dorsal interosseous muscle of children with cerebral palsy.

Cutaneomuscular reflexes were recorded from the first dorsal interosseous muscles of 21 children with spastic cerebral palsy. The reflex responses in children with cerebral palsy were consistent with dysfunction of the pyramidal system, but differed from those seen in adults with central nervous lesions. This provides further evidence that the effect of a central nervous lesion on the reflex control of movement is dependent on whether it occurs in early or adult life.

Adolescent↗
PubMed1991

Effect of cooling rate on leucite volume fraction in dental porcelains.

Prasad et al. (1988) have shown that slow cooling of dental porcelain produces increases in thermal expansion sufficient to make a compatible metal-porcelain system incompatible. The present study was undertaken to determine whether the increase in porcelain thermal expansion might be attributable to crystallization of additional leucite during slow cooling of the porcelain. Eight x-ray diffraction specimens for each of six commercial dental porcelains and for the Component No. 1 frit of the Weinstein and Weinstein (1962) and Weinstein et al. (1962) patents were fabricated and divided into two groups. Specimens in the first group (termed fast-cooled) were cooled in the conventional manner by removing them from the furnace at the maximum firing temperature immediately into room air. Specimens in the second group (termed slow-cooled) were cooled slowly by interrupting power to the furnace muffle and allowing them to cool inside the closed furnace. Quantitative x-ray diffraction was performed on the fast- and slow-cooled porcelain specimens with standards containing leucite volume fractions of 0.111, 0.223, 0.334, and 0.445. Unpaired, one-tailed t tests were performed on the fast- and slow-cool data, and a significant increase (p less than 0.05) in the amount of leucite (as a function of the slow cooling) was found for each of the porcelains. The increases in the leucite volume fractions resulting from the slow cooling ranged from a low of 8.5% to a high of 55.8%, with an average increase of 26.9%.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum Compounds↗
PubMed1991

Color changes in post-cure heat-treated resin composites.

The purpose of this study was to determine the color changes that arise from post-cure heat treatment of selected light-cured resin composites by using colorimetric as well as observational methods. Brands of composite were selected that were solely Bis-GMA-based and Bis-GMA/urethane based. These materials also represented a range of filler content (low, medium, and high). The RGB values of cured resin discs were obtained immediately after light-curing as well as after a 5-minute post-cure heat exposure. The range of post-cure temperature was from 100-300 degrees C in 25 degrees C increments. The mean delta L*, delta a*, delta b* and delta E* were determined from red, green and blue data using triplicate replications for each heating condition. Twenty-one observers were asked to determine if the randomly arranged post-cure heated specimens differed in color from the unheated, cured control specimen for each brand. The results indicate that a delta E* value of approximately 3 resulted in greater than 50% of the observers noting a color difference between the post-cure heated specimen and the unheated control. The degree of color change was influenced more by the amount of resin content in the composite systems rather than by the particular resin composition. Microfill resins showed greater potential for color change than did the other types of materials. If clinicians are considering using conventional light-cured composite materials for inlays, the choice of material as well as the post-cure temperature will influence the ability to match the inlay with the original shade of composite selected.

Analysis of Variance↗