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Biomedical subjects

J Rivest

Publications and source records attributed to J Rivest.

At least 19 recordsLinked to original sources
PubMed2000

A dynamic model of protein digestion in the small intestine of pigs.

A dynamic mathematical model of the digestion of proteins in the small intestine of pigs was developed. The model integrates current knowledge on the transit of digesta along the small intestine, endogenous secretions, digestion of proteins, and absorption of amino acids into a mechanistic representation of digestion. The main characteristics of the model are the following: the small intestine is divided into several segments of variable length but with equal digesta retention time; the rate of transfer of digesta between segments is based on the progression of myoelectric migration complexes; pancreatic and biliary secretions are poured into the first segment, whereas intestinal secretions enter all intestinal segments; protein hydrolysis is described by first-order equations; and an intestinal absorption capacity is used to estimate absorption of hydrolyzed protein. Simulation results are consistent with observed data, although more information is needed to represent reality more closely. The sensitivity analysis shows that parameters for protein hydrolysis largely determine protein digestibility. The absorption capacity of the small intestine limits the absorption of amino acids at the beginning of a meal and modulates the appearance of amino nitrogen in the portal vein. It also shows that amino acid absorption can be limiting to protein digestibility when large amounts of protein are eaten in a single daily meal. The model is useful in evaluating the dynamics of protein digestion and absorption of feedstuffs. The model can be used in evaluating protein digestion of different feedstuffs and feeding strategies.

Animals↗
PubMed1999

Differential diagnosis of parkinsonism.

The diagnosis of Parkinson's disease is predominantly clinical, based on a combination of the cardinal features of tremor, bradykinesia and rigidity. The differential essentially lies between other conditions resulting in tremor, of which essential tremor is the commonest, and other akinetic-rigid syndromes. These include progressive supranuclear palsy, multiple system atrophy, toxins and other degenerative disorders, including diffuse Lewy body disease and corticobasal degeneration. The key clinical features of these disorders and a practical diagnostic approach are briefly reviewed in this article.

Diagnosis, Differential↗
PubMed1999

COMT inhibitors in Parkinson's disease.

The COMT inhibitors, tolcapone and entacapone, are a new class of Parkinson's medications. By inhibiting the enzyme catechol-o-methyl-transferase (COMT), they prevent peripheral degradation of levodopa, allowing a higher concentration to cross the blood-brain barrier. Pharmacokinetic studies have shown that both tolcapone and entacapone significantly prolong the elimination half life, and increase the area under the curve of levodopa without increasing C max. Clinical studies with COMT inhibitors have shown benefit in both stable and fluctuating PD patients with improvement in motor function with lower levodopa doses. Fluctuating patients also had increased "on" time and reduced "wearing off". Side effects were most commonly related to increased dopaminergic stimulation. Specific side effects included diarrhea and elevated liver enzymes. The recent reports of three cases of fulminant hepatitis with the use of tolcapone has led many countries to remove this compound from their market. Concerns about a possible class effect should impose close monitoring of liver function tests with the use of any of the nitrocatechols.

Antiparkinson Agents↗
PubMed1998

Prediction of methane production from dairy cows using existing mechanistic models and regression equations.

Ruminants may contribute to global warming through the release of methane gas by enteric fermentation. Until now, methane emissions from ruminants were estimated using simple regression equations. The objective of this study was to compare the capacity of dynamic and mechanistic models to that of regression equations to predict methane production from dairy cows. The updated version of the model of Baldwin et al. and a modified version of the model of Dijkstra et al. and the regression equations of Blaxter and Clapperton and Moe and Tyrrell were challenged with 32 experimental diets selected from 13 publications. The predictive capacity of mechanistic models and regression equations was evaluated by comparing predicted and observed methane production using regression analysis. Results of regression showed better prediction of methane production with mechanistic models than with regression equations. The modified model of Dijkstra et al. predicted methane production with the higher R2 (.71) and the smaller error of prediction (19.87% of the observed mean). The model of Baldwin et al. predicted methane production with a similar R2 (.70) but a higher error of prediction (36.93%). However, a large proportion of this error can be eliminated by a correction factor. Predictions using the equations of Moe and Tyrrell and Blaxter and Clapperton were poor (R2 = .42 and .57; error of prediction = 33.72% and 22.93%, respectively). This study demonstrated that from a large variation in diet composition, mechanistic models allow the prediction of methane production more accurately than simple regression equations.

Animal Feed↗
PubMed1997

Perceptual learning of orientation discrimination by more than one attribute.

Using a perceptual learning paradigm, we evaluated whether information from the attributes: color, luminance and motion is combined to provide orientation coding. Four observers were trained to discriminate the orientation between color-defined bars, four between luminance-defined bars, and four between motion-defined bars. Before and after training, they were tested with each of the three attributes separately and all superimposed, at the same and at a different location as the one seen during training. A similar improvement was found whether the bars seen after training were defined by the same, or by a different attribute as the one seen during training, or by the three attributes superimposed. This improvement was significantly more substantial at the location where the bars were presented during training. Moreover, orientation discrimination was always better when the bars were defined by three attributes than by any one alone. Because the improvement was retinotopic and not restricted to the attribute seen during training, we suggest that training changed the sensitivity of orientation-selective cells responsive to color, luminance and motion. Moreover, the overall better performance with additional attributes supported an integration of information from color, luminance, and motion at a common site for orientation coding.

Analysis of Variance↗
PubMed1996

Localizing contours defined by more than one attribute.

Two experiments were run in order to test how information from different attributes is combined to localize contours. In Expt 1 the apparent position of a contour defined by one attribute was measured while a contour defined by another attribute was presented beside it. Interactions were found between all pairings of luminance, color, motion and texture. These results suggested that the information associated with each contour is integrated at a common site. In Expt 2 the precision of localization was measured for contours defined by one, two or three attributes (combinations of luminance, color and texture). The improvement in precision with additional attributes again supported an integration of contour information at a common site prior to a decision of localization.

Color Perception↗
PubMed1996

Botulinum toxin in the treatment of writer's cramp.

Forty-four patients with disabling writer's cramp (WC) and one with a musician's cramp were treated with botulinum toxin (BT) injections for a mean period of 12 (range, 3-48) months. The forearm muscles causing the dystonic position were identified by inspection while writing; BT was then administered under electromyographic (EMG) guidance. The degree of improvement in writing and amelioration of pain were rated with self-assessment scales. Patients reported significant improvement in writing after 56% treatment sessions (TS) and in pain after 62% TS. Mild weakness occurred after 32% TS. Twenty-nine patients discontinued treatment, generally after the initial BT injection. In 16 patients who remained on treatment with a mean follow-up of 21 (range, 3-48) months, the improvement in writing and pain was present after 76 and 79% of the TS, respectively. We conclude that BT injections offered a worthwhile and sustained functional improvement to 36% of our patients with WC.

Adult↗
PubMed1995

The contribution of color to depth perceived from motion parallax.

Perceived depth was measured in a colored stimulus while stimulus movement yoked to head displacement simulated a depth of 1 cm. Velocity judgments were also made for similar stimuli moving at the same average speed but without head movement. Both measures decreased to a minimum of about 30-40% of the veridical values when the stimuli were equiluminous. Perceived depth and speed also decreased for a monochromatic stimulus as a function of luminance contrast but much more abruptly than for the chromatic stimuli. The results indicate that equiluminous color stimuli contribute to the perception of depth from motion parallax and that the contribution is not mediated by residual luminance.

Color Perception↗
PubMed1994

Interhemispheric depth judgement.

Interhemispheric depth comparisons were studied by requiring subjects to align in depth two textured plates, one presented to the left hemifield and the other to the right. Callosal agenesis subjects and neurologically-normal control subjects adjusted the plates so that they appeared to be at the same distance. Subjects viewed the plates monocularly or binocularly while keeping their head still, moving it side-to-side or moving it up and down. Subjects fixated a target located between the two plates while performing the task. For all subjects, the results showed that the deviations from veridical settings were significantly smaller for the binocular than for the monocular viewing conditions. Moreover, there were no significant differences among the three binocular viewing conditions (horizontal, vertical or no head movement), indicating that neither vertical nor horizontal motion parallax improves the precision of depth judgement when binocular disparity is available. These results further suggest that the precision of interhemispheric comparison for binocular depth is not affected by the absence of the corpus callosum. Looking at the plates monocularly, the control subjects judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that motion parallax is a useful depth cue when relative motion is extracted from different hemifields. Unlike the control subjects, the callosal agenesis subjects did not judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that interhemispheric comparison of depth using relative motion is not possible without the corpus callosum.

Adult↗
PubMed1992

Treatment of cervical dystonia hand spasms and laryngeal dystonia with botulinum toxin.

One hundred and twenty-six patients with different forms of focal dystonia (89 with cervical dystonia, 12 with hand cramps and 25 with laryngeal dystonia) were treated with localised injections of botulinum toxin. Mean doses per muscle were 200 mouse units (m.u.) for treating cervical dystonia, 40-120 m.u. for forearm muscles in writers' cramp and 3.7 m.u. for the thyroarytenoid muscle in laryngeal dystonia. Responder rates have been above 80% in all patient groups and beneficial effects could be reproduced over follow-up periods of up to 4 years. The commonest side-effects were dysphagia after treatment of spasmodic torticollis, weakness of neighbouring muscles after injections for hand cramps and breathiness and hypophonia following laryngeal injections. All these were transient and generally well tolerated. It is concluded that botulinum toxin injections are a safe and effective treatment in all three types of focal dystonia.

Botulinum Toxins↗
PubMed1992

Botulinum toxin treatment of spasmodic torticollis.

We reviewed the efficacy and adverse effects of repeated botulinum toxin injections into hyperactive neck muscles of 107 successive patients with spasmodic torticollis. They received 510 injection treatments over a median period of 15 months (range 3-42 months). One patient failed to benefit at all, but 101 (95%) patients reported considerable (moderate or excellent) benefit from at least one treatment. On a global subjective response rating, 93% of 429 treatments resulted in some improvement and 76% in moderate or excellent improvement. Pain reduction followed 89% of 190 treatments with moderate or excellent reduction after 66%. Median duration of benefit was 9 weeks. All torticollis types responded equally well and injections into two (or more) involved neck muscles were more effective than injection into a single muscle. The most frequent adverse effect was dysphagia, occurring after 44% of all treatments, but this was severe after only 2%. Antibodies to botulinum toxin were detected in the serum of three out of the five patients in whom loss of treatment efficacy occurred. We conclude that botulinum toxin treatment is the most effective available therapy for spasmodic torticollis and practical advice is provided for anyone wishing to set up the technique.

Adult↗
PubMed1991

Writer's cramp: treatment with botulinum toxin injections.

Twelve patients with writer's cramp were treated with injections of botulinum toxin. The overactive muscles were identified by clinical observation of the subjects while they were writing. Repeated injections were given at 2-week intervals until the optimal response was obtained. Eleven patients reported some benefit, which was considered by seven to be significant. Eight of the 10 patients who had pain reported moderate to significant relief. Five patients had local complications, consisting of disabling weakness of target or neighbouring muscles. These preliminary results suggest that this treatment can be successfully applied to many patients with writer's cramp without performing complex electromyographic recordings while the patients are writing.

Adult↗
PubMed1991

Periodic lateralized epileptiform discharges with transitional rhythmic discharges: association with seizures.

Low amplitude rhythmic discharges (RDs) closely associated in time and in spatial distribution to inter-ictal epileptiform discharges are not seen in scalp EEGs of patients with non-periodic focal epileptiform discharges (NPEDs) but they are unexpectedly common in patients with periodic lateralized epileptiform discharges (PLEDs). A classification of PLEDs into PLEDs Proper (PLEDs without RDs), and PLEDs Plus (PLEDs with RDs) is proposed. Such a classification is useful for an easier appreciation of PLEDs' pleomorphism. It underscores the changing periodicity of PLEDs and allows for a more inclusive definition of the phenomenon than the one reported by the literature. It also emphasizes the importance of recognizing RDs in scalp EEGs as transitional anomalies intercalated between inter-ictal PLEDs and ictal seizure discharges, analogous to those observed by Ralston in animal models. Indeed, the occurrence of recorded seizures is higher in patients with PLEDs Plus than in those with PLEDs Proper or NPEDs.

Electroencephalography↗
PubMed1991

Pharmacological modulation by cetirizine and ebastine of the cutaneous reactivity to histamine.

The peripheral H1-inhibiting effects of cetirizine 10 mg and ebastine 10 mg were compared at the skin level after single oral administration. The study was performed in 9 healthy subjects under double-blind randomized crossover conditions. Both drugs were significantly effective up to 24 h. The suppressive effect of cetirizine was significantly more rapid and more marked.

Administration, Cutaneous↗
PubMed1990

Trunk and head tremor as isolated manifestations of dystonia.

Five patients presenting with isolated tremors of the trunk or neck are described. Their clinical features were similar to seven other patients who presented with head tremor, or arm and head tremor, but then eventually developed obvious torticollis, sometimes with arm dystonia. We conclude that isolated tremor of the trunk or head, especially of slow frequency (2-5 Hz), and in the case of the head in a "no-no" direction, may be the initial manifestation of focal dystonia.

Adult↗
PubMed1990

Dystonia in Parkinson's disease, multiple system atrophy, and progressive supranuclear palsy.

Adult-onset dystonia-parkinsonism is a syndrome in search of a pathology. We therefore reviewed the literature on dystonic manifestations in autopsy-proven cases of multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and idiopathic Parkinson's disease (PD). Only 6 of 140 autopsy reports of MSA remarked on the presence of dystonia in life, but personal observations suggest prominent antecollis may develop at some stage in up to 1/2 of sufferers. Similarly, very few (15/118) clinicopathologic observations on PSP included convincing dystonic manifestations, in contrast to some clinical reports where blepharospasm and early limb dystonia were prominent. Virtually any form of focal and segmental dystonia may sometimes occur with clinically diagnosed PD, with occasional descriptions of hemidystonia-hemiparkinsonism. However, there is pathologic confirmation of this diagnosis in only 1 case. With many patients thought clinically to have PD proving pathologically to have another cause for their parkinsonism, the true frequency and the range of dystonic manifestations acceptable in PD remain unknown.

Brain Diseases↗