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BACKGROUND: Poor eye contact and joint attention are early signs of autism spectrum disorder (ASD) and important prerequisites for developing other socio-communicative skills. Teaching parents evidence-based techniques to improve these skills can impact the overall functioning of children with ASD. We aimed to analyse the impact of conducting a group parent-training intervention with video modelling to improve the intelligent quotient (IQ), social and communication functioning and to minimise symptoms in children with ASD and intellectual disability (ID). METHODS: Study design: A multicentre, single-blinded, randomised clinical pilot trial of parent training using video modelling was conducted. SAMPLE: Sixty-seven parents of children with ASD, aged between 3 and 6 years and with IQs between 50 and 70, were randomised: 34 to the intervention group and 33 to the control group. Intervention program: The intervention group received parent training over 22 sessions, and the control group received the standard community treatment. INSTRUMENTS: Pre-evaluation and post-evaluation (week 28), the following were used: Autism Diagnostic Interview, Vineland Adaptive Behaviour Scale I, Snijders-Oomen Nonverbal Intelligence Test, Autism Behaviour Checklist and Hamilton Depression Rating Scale. DATA ANALYSIS: Intention to treat and complier-average causal effect (CACE) were used to estimate the effects of the intervention. RESULTS: There was a statistically significant improvement in the Vineland standardized communication scores in CACE (Cohen's d = 0.260). There was a non-statistically significant decrease in autism symptomatology (Autism Behaviour Checklist total scores) and a significant increase in the non-verbal IQ in the intervention group. After the false discovery rate correction was applied, IQ remained statistically significant under both paradigms. The effect size for this adjusted outcome under the intention-to-treat paradigm was close to 0.4, and when considering adherence (CACE), the effect sizes were more robust (IQ's Cohen's d = 0.433). CONCLUSIONS: Parent training delivered by video modelling can be a useful technique for improving the care given to children with ASD and ID, particularly in countries that lack specialists.

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OBJETIVO: Determinar predictores de buena respuesta con el uso de prótesis en la estenosis traqueobronquial. MATERIALES Y MÉTODO: Estudio retrospectivo descriptivo de una serie de casos entre junio de 2014 y junio de 2016. Se revisaron registros clínicos, protocolos operatorios, estudios histopatológicos y de imagen. Se consignaron antecedentes demográficos y clínicos al ingreso y luego del procedimiento, etiología de la estenosis traqueal, necesidad de oxígeno en litros, estadía hospitalaria, morbilidad asociada al procedimiento, indicación de terapia complementaria posterior y sobrevida. Se analizaron los datos con estadística descriptiva y analítica. RESULTADOS: Se realizaron 68 procedimientos, en 44 pacientes, 24 mujeres, el diagnóstico principal fue estenosis traqueal con 40 casos. La etiología neoplásica fue el 88% de los casos. El Performance status (PS) de ingreso fue mayor o igual a 2 en el 68% y posterior al procedimiento disminuyó a 22% p < 0,05. En 36 casos los pacientes requerían al menos 1 L de oxígeno lo que disminuyó en 13 casos posterior a la intervención p < 0,05. El tiempo de hospitalización promedio fue 6,2 días (1-60). En 13 pacientes con patología neoplásica se indicó terapia paliativa complementaria. La morbilidad asociada al procedimiento fue de 2,9% dado por prótesis desplazada y lesión iatrogénica. La sobrevida fue de 27% a un año. CONCLUSIONES: La etiología benigna, el PS previo a la intervención menor o igual a 3, requerimiento de oxígeno de 1 litro y obstrucción tumoral menor al 70% del lumen fueron elementos de buena respuesta.

AIM: Determine good response predictors in use of stent in benign and malignant tracheobronchial stenosis. MATERIAL AND METHODS: We retrospectively reviewed medical records of patients submitted to the procedure in the period 2014 to 2016. Clinical records, operative protocols, and histopathological and imaging studies were reviewed. Demographic and clinical data, performance status (PS) at admission and after the procedure, etiology of tracheal stenosis, need for oxygen in liters (L) Post-intervention, hospital stay, procedure-related morbidity, indication of therapy (Chemotherapy and/or Radiotherapy) and survival. Data were analyzed with descriptive and analytics statistics. RESULTS: A total of 68 procedures were performed in 44 patients, 24 women. Tracheal stenosis 40 cases were diagnosed. The malignant etiology was 88%. The admission PS was greater than or equal to 2 in 68% and 22% after the procedure. The oxygen requirements prior to the procedure were at least 1 L in 36 cases and decreased in 13 cases after the procedure. The average hospitalization period was 6.2 days (1-60). Complementary therapy was indicated in 13 patients; the morbidity associated with the procedure was 2.9%, displaced installation and iatrogenic injury. The one year survival was 27%. CONCLUSIONS: Benign etiology, el PS minor than or equal to 3, oxygen requirements prior to the procedure of 1 L and tumoral obstruction less than 75% were good response predictors in our study.

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La Gerencia de Producción y Servicios Básicos del INHRR responde a las necesidades de las áreas técnicas del Instituto a través de la producción de insumos requeridos para el control de productos de uso y consumo humano, diagnóstico de enfermedades transmisibles, desarrollo de nuevas técnicas y líneas de investigación, orientadas a garantizar la salud integral de la población venezolana; tales como: animales de laboratorio, hemoderivados de uso diagnóstico, cultivos celulares, medios de cultivo, reactivos, colorantes, agua purificada, agua calidad inyectable y estuches biológicos. Además brinda servicios auxiliares de descontaminación, lavado, embalaje, esterilización de materiales y uniformes de laboratorio. Presta servicio de secuenciación de ácidos nucleicos a los laboratorios del Instituto y usuarios externos, así como también asesoría técnica y científica a entes internos, nacionales e internacionales en actividades de su competencia

The Production and Basic Services Management of INHRR responds to the needs of the technical areas of the Institute through the production of inputs required for the control of products of human use and consumption, diagnosis of transmissible diseases, development of new techniques and lines of research aimed at guaranteeing the integral health of the Venezuelan population; such as: laboratory animals, blood products for diagnostic use, cell cultures, culture media, reagents, dyes, purified water, water for injection and biological kits. It also provides auxiliary services of decontamination, washing, packaging and sterilization of materials and laboratory uniforms. Also it provides nucleic acid sequencing service to the Institute´s laboratories and external users, as well as technical and scientific adviser to internal, national and international entities in activities of their competence

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Un sistema de gestión de la calidad (SGC) según norma ISO 9001:2015, requiere un soporte documental de sus procesos, que permita a la organización demostrar la eficacia de planificación, operación, control, implantación y mejora continúa de su SGC. La producción de animales de laboratorio (AL) requiere además otros documentos que garanticen las buenas prácticas de producción (BPP). Objetivo: Describir y analizar los procesos del Bioterio definiendo su documentación según la ISO 9001:2015, para apoyar la operatividad, lograr mejoras y recomendar acciones que garanticen la trazabilidad y la satisfacción del cliente. Materiales y métodos: Se hace la descripción del SGC y de BPP diseñado e implantado en la producción de AL bajo condiciones convencionales con barreras. Norma ISO 9001:2015; Mapas y diagramas de procesos, interacciones y documentación nueva y existente. Resultados: Se toma como soporte el enfoque de procesos de la ISO 9001:2015; Levantamiento e Identificación de los procesos claves, clientes y partes interesadas; expectativas y requisitos; Definición de las especificaciones del AL a producir; Descripción de diagramas de flujo asociados en recepción, producción, control y entrega; Mantenimiento de la documentación asociada a la gestión de procesos; planes de control, procedimientos, instructivos y registros. Conclusiones: Producción de AL con la documentación especializada que agrega valor al proceso y al SGC. .La documentación coadyuva la valoración, eficacia y adecuación del SGC. Identifican y controla los procesos claves garantizando su función a satisfacción. Estimula la actitud de mejora en la Institución necesaria para el cambio en la cultura de trabajo

A quality management system (QMS) according to ISO 9001: 2015 standard requires documentary support of this processes, which allows the organization to demonstrate the efficiency of planning, operation, control, implementation and continuous improvement of its QMS. The production of Laboratory Animals (AL) also requires other documents that guarantee good production practices (BPP). Objective: the aim of this study was to describe and analyze the processes of the biotery defining its documentation according to ISO 9001: 2015, to support the operation, achieve improvements and recommend actions that guarantee traceability and customer satisfaction. Materials and methods: The description of the SGC and BPP is designed and implemented in the production of AL under conventional conditions with barriers. The ISO 9001: 2015 standard; Maps and diagrams of processes, interactions and new and existing documentation. Results: The process approach of ISO 9001: 2015 is taken as support; survey and identification of key processes, clients and interested parties; expectations and requirements; definition of the LA specifications to be produced; description of associated flow diagrams in reception, production, control and delivery; maintenance of documentation associated with process management; control plans, procedures, instructions and records. Conclusions: The production of AL with specialized documentation that adds value to the process and the QMS. The documentation contributes to the assessment, effectiveness and adequacy of the QMS. They identify and control the key processes guaranteeing their function to satisfaction and stimulate the attitude of improvement in the Institution necessary for the change in the work culture.

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INTRODUCTION: In Argentina respiratory disease is the third leading cause of death in children under 5 years. In 2011 influenza vaccination was included in the National Calendar for children between 6 and 24 months (two doses schedule). Influenza vaccine coverage for second dose was 46.1% in 2013. The aim was to determine the proportion of delayed schedules and missed opportunities, to assess the characteristics of missed opportunities for vaccination and to explore the perception of influenza disease and vaccination from the parents of children between 6 and 24 months in different regions of Argentina in 2013. METHODS: Analytical observational multicenter cross-sectional study. Structured surveys were carried out to the children's parents who were between 6 and 24 months of age during the influenza virus vaccination season (April-October 2013). Chi-Square test was used to assess association and differences between proportions and categorical variables. A logistic regression model was built to identify delay predictor variables in the vaccination schedules. Missed opportunities for vaccination were characterized through the estimation of proportions. RESULTS: 1350 surveys were conducted in the three centers. We detected 65% (884/1340) of delayed influenza vaccination schedules, 97% of them associated with missed opportunities of vaccination. The independent protective factors associated with a decreases risk of delayed schedules were: (a) perception of the importance of influenza vaccination (OR=0.42(0.18-0.94); p=0.035), (b) having less than one year of age (OR=0.75(0.59-0.96); p=0.022), (c) to have received information in pediatric visits or in any media (OR=0.71(0.56-0.90); p=0.004). There was 38% of MOIV in 1st dose and 63.4% in 2nd dose. The main cause of MOIV in 1st dose was not detecting the need for vaccination (39%) and in 2nd dose the unknowledge of the vaccination schedule (35.3%). No cultural reasons were detected. CONCLUSIONS: High frequency of delayed vaccination schedules and missed opportunities were detected. Parents had little concern about the safety of influenza vaccine.

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BACKGROUND: Leukotrienes are among the most important mediators associated with inflammatory responses in patients with exercise induced asthma (EIA). The aim of this study was to investigate the impact of exercise on the urinary leukotriene profile. Hence, we compared post exercise changes of urinary leukotriene E4 (LTE4) concentration between children with EIA and healthy controls. METHODS: Ten children with EIA and 15 controls were enrolled. Both groups underwent a standardised exercise challenge test (ECT). LTE4 concentration was measured in urine samples obtained pre and post ECT, using enzyme immunoassay and adjusted by urinary creatinine concentrations. RESULTS: Median (minimum-maximum) pre ECT concentration of LTE4 was 17.82 (7.58-90.23 pg/ml) in EIA and 17.24 (4.64-64.02 pg/ml) in controls, p=0.86. LTE4 concentration post ECT were 23.37 (4.02-93.00 pg/ml) in EIA and 11.74 (0.13-25.09 pg/ml) in controls, p=0.02. Changes of LTE4 concentration post ECT were 2.54 (-31.98 to 43.31 pg/ml) in cases and -13.53 (-46.00 to 11.02 pg/ml) in controls, p=0.03. There was no significant correlation between basal predicted FEV(1) [%] and changes in LTE4 concentration in cases (i.e., r(s)=0.14) nor controls (i.e., r(s)=0.12). There was a tendency towards more pronounced changes in LTE4 concentration post ECT in children with moderate/mild persistent asthma compared to those with mild but intermittent asthma. CONCLUSIONS: Children with EIA had significantly higher changes of urinary LTE4 concentrations post ECT compared to healthy controls. Urinary measurement of LTE4 may be an interesting and non-invasive option to assess control of EIA in children.

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Elasmobranchs are important, not only as an economic resource but also for their phylogenetic position. Their slow growth and long reproductive cycles, together with the high fisheries pressure exerted, attempt to the knowledge of all biological parameters of species. In this work we analyze the histology of the pregnant uterus of the gatuzo, a viviparous aplacental shark. The mucosa depicts modifications similar to those of placental species and others proper of histotrophism. Likewise, a network-like muscular layer in the mucosa was described for first time in the genus. The presence of vitelline plates in the gut of at term embryos suggests that the maternal supplies are not enough for the big fetus development. The relationships between maternal support and embryo growth are discussed into the matrotrophic and phylogenetic contexts.

Los peces cartilaginosos son importantes tanto desde el punto de vista evolutivo como desde el comercial, ya que integran una parte importante de las pesquerías argentinas. El gatuzo es explotado intensamente, pero se desconocen todos los parámetros biológicos necesarios para establecer medidas adecuadas de manejo sustentable. En este trabajo se estudia la arquitectura del útero gestante de esta especie vivípara aplacentaria. Se observaron cambios estructurales en el epitelio uterino así como secreción mucosa y descamación del mismo. Asimismo se describe una túnica muscular reticuliforme en la mucosa, que no había sido referida para ningún otro elasmobranquio. Se aportan evidencias de un histotrofismo moderado. Se observó la presencia de vitelo en la válvula espiral de los embriones gestantes, lo que junto con la secreción uterina, contribuye al desarrollo de los fetos. Se discute la importancia del aporte materno en el crecimiento embrionario, desde el punto de vista de los diferentes tipos de matrotrofismo.

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Las micosis superficiales son afecciones causadas por hongos que viven sobre la piel y/o en sus anexos. Se realizó un estudio en 214 conejos Nueva Zelanda blancos (Oryctolagus cuniculus) de entre 19 días y 5 años de edad, reproducidos por más de 16 años en el Bioterio del Instituto Nacional de Higiene "Rafael Rangel", con la finalidad de determinar la incidencia y prevalencia de las micosis superficiales y los factores epidemiológicos asociados. El estudio contempló la clínica, el laboratorio y su epidemiología. El tamaño de la muestra se estimó por el programa Epi-Info versión 5,01, estableciéndose 15% de frecuencia esperada, 5% de error y un nivel de 95% de confianza. Se evaluó el estado general de los animales, así como la presencia de alopecias y lesiones descamativas. La toma de muestras de pelos y escamas epidérmicas fue realizada por tracción mecánica y raspado. Se realizó examen directo de las muestras con KOH al 10% más tinta Parker y se sembraron en tubos de agar Mycosel y Lactritmel. Para establecer el patrón de ocurrencia de las micosis se determinó incidencia acumulada, incidencia y prevalencia en tasas y proporciones. Los principales hongos hallados causantes de micosis superficiales fueron: T. mentagrophytes (dermatofito) y Scopulariopsis sp. (no dermatofito). Se obtuvo una incidencia acumulada de 45% en doce meses de observación y 8% de casos sospechosos por animal-mes. La prevalencia osciló entre 3,6% y 4,8% en casos confirmados. Los resultados aportan información para la aplicación de buenas prácticas de laboratorio y contribuyen con el programa de control de calidad del biomodelo.

The superficial mycoses are diseases caused by fungi that live on the skin and/or in their annexes. It was carried out a study in 214 white New Zealand rabbits (Oryctolagus cuniculus) between 19 days and 5 years of age, reproduced for more than 16 years, in the animal facilities of the National Institute of Hygiene «Rafael Rangel¼, to determine the incidence and prevalence of the superficial mycoses and the associated epidemic factors. The study contemplated the clinic, laboratory and epidemiology. The size of the sample was estimated by Epi- Info version 5,01 program settling down 15% of prospective frequency, 5% error and 95% confidence level. Alopecia and descamative lesions, as well as the general state of the animals, was evaluated. Hair samples and epidermal flakes was taking out by mechanical traction and rasped. Direct exam with 10% KOH plus Parker ink was carried out and the samples were seeded in Mycosel and Lactritmel agar tubes. It was determined: accumulated incidence, and prevalence in rates and proportions to establish the occurrence pattern of the mycoses. The main fungi causing superficial mycoses were: T. mentagrophytes (dermatophyte) and Scopulariopsis sp. (nondermatophyte). An accumulated incidence of 45% was obtained in twelve months of observation and 8‰ of suspicious cases per animal-month. The prevalence oscillated between 3,6% and 4,8% in confirmed cases. This result give information for good laboratory practices application and contributes with the quality control program of the biological models.

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La gran cantidad de evidencia acumulada sobre el papel de la homocisteína como factor de riesgo cardiovascular es cada día más fuerte. Una concentración de homocisteína plasmática elevada induce cambios patológicos en la pared arterial asociándose a un incremento en el riesgo de desarrollar aterosclerosis, la cual se manifiesta comúnmente como accidentes cerebrovasculares, cardiovasculares y vasculares periféricos. Diferentes estudios realizados han sido dirigidos a determinar si la disminución de los niveles de homocisteína plasmáticos pueden ser de utilidad para prevenir eventos oclusivos. En la actualidad la realización de exámenes de laboratorio para cuantificar los niveles de homocisteína pueden ser considerados en pacientes con ateroesclerosis prematuras con una historia familiar de arteroesclerosis, donde la hiperhomocisteinemia es un factor de riesgo común en estos pacientes. El tratamiento de la hiperhomocisteinemia es simple y con riesgo terapéutico mínimo. Este desorden es usualmente corregido con suplementos vitamínicos que contienen ácido fólico.

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This study investigated what are the main reasons that led parents to enroll children in a clinic for infants. This was studied by consulting 1368 records during the period from July 1996 to August 2001. The predominant reason for enrolling was orientation/prevention followed by "other" and treatment. This study demonstrated that a program for children from the first year of life encourages parents to have a new vision of dentistry.

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Onions are an important crop for New Mexico with 7,700 acres (3,116 ha) harvested in the state in 2003 (3). In 2002, onions of several cultivars were first noticed with diamond-shaped chlorotic or bleached lesions on seed stalks or leaves, typical of those reported for Iris yellow spot virus (IYSV). A more widespread survey of breeding stocks and commercial onion fields revealed similar symptoms on thrips-infested onions in Dona Ana and Rio Arriba counties. Incidence of disease symptoms ranged from <0.5 to nearly 30%. Symptomatic leaves were assayed for the presence of IYSV using enzyme-linked immunosorbent assay (ELISA; Agdia, Elkhart, IN) and antisera acquired from Agdia. Symptomatic leaves from breeding and commercial fields tested positive for IYSV. The virus was transmitted by Thrips tabaci from symptomatic onions to three onion cvs. New Mex Mesa, New Mex Vado, and New Mex Cryspy in growth chamber tests. All three cultivars showed symptoms of IYSV and tested positive for the disease using ELISA. However, New Mex Vado and New Mex Cryspy cultivars each showed 24% infection (4 infected plants of 17 tested) compared with 59% infection (10 infected plants of 17 tested) for New Mex Mesa, suggesting that not all cultivars are equally susceptible to the virus. To our knowledge, this is the first report of IYSV in onions in New Mexico, which has also been reported in the western United States in Idaho, Oregon, Colorado, and Washington (1,2,4). References: (1) L. J. du Toit et al. Plant Dis. 88:222, 2004. (2) J. M. Hall et al. Plant Dis. 77:952, 1993. (3) National Agricultural Statistics Service, On-line publication. USDA, 2004. (4) H. F. Schwartz et al. Plant Dis. 86:560, 2002.

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The increasing demographic and life expectancy rates, together with the vertiginous technological development during the last two decades, have raised the number of cardiac patients requiring surgical treatment. Therefore, several institutions have been forced to give priority to advanced or more serious cases and to postpone those that do not demand an urgent surgery. This analysis was made from June 15, 1999 to June 15, 2000 and demonstrates the results obtained from maintaining a fast track at the National Institute of Cardiology "Ignacio Chávez". This has favored the practice of surgeries in cases with noncomplex pathologies, null mortality, and low morbidity rates. The analysis also compares results from patients who had been subjected to surgery for the same type of pathologies but following the usual course of admittance and surgery scheduling, proving to be cost-efficient. It also demonstrates the high incidence of inter-auricular communication and the Von Willebrand disease that exist in our environment. Based on the results, we propose to maintain and increase this type of surgeries, addressing their limitations.

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The chicken eggshell is a composite bioceramic containing organic and inorganic phases. The organic phase contains, among other constituents, type X collagen and proteoglycans (mammillan, a keratan sulfate proteoglycan, and ovoglycan, a dermatan sulfate proteoglycan), whose localization depends on a topographically defined and temporally regulated deposition. Although the distribution of these macromolecules in the eggshell has been well established, little is known about their precise localization within eggshell substructures and oviduct cells or their pattern of production and function during eggshell formation. By using immunofluorescent and immuno-ultrastructural analyses, we examined the distribution of these macromolecules in oviduct cells at different post-oviposition times. To understand the role of proteoglycan sulfation on eggshell formation, we studied the effects of inhibition of proteoglycan sulfation by treatment with sodium chlorate. We showed that these macromolecules are produced by particular oviduct cell populations and at precise post-oviposition times. Based on the precise ultrastructural localization of these macromolecules in eggshell substructures, the timing of the secretion of these macromolecules by oviduct cells and the effects on eggshell formation caused by the inhibition of proteoglycan sulfation, the putative role of mammillan is in the nucleation of the first calcite crystals, while that of ovoglycan is to regulate the growth and orientation of the later forming crystals of the chicken eggshell.

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Both original and colonizer populations of Drosophila buzzatii have been analyzed for mtDNA restriction polymorphisms. Most of the mtDNA nucleotide variation in original populations of NW Argentina can be explained by intrapopulation diversity and only a small fraction can be accounted for by between-population diversity. Similar results are obtained using either the estimated number of nucleotide substitutions per site or considering each restriction site as a locus. Colonizer populations of the Iberian Peninsula are monomorphic and show only the most common haplotype from the original populations. Under the infinite island model and assuming that populations are in equilibrium, fixation indices indicate enough gene flow to explain why the populations are not structured. Yet, the possibility exists that populations have not reached an equilibrium after a founder event at the end of the last Pleistocene glaciation. Tajima's test suggests that directional selection and/or a recent bottleneck could explain the present mtDNA differentiation. Considering the significant population structure found for the chromosomal and some allozyme polymorphisms, the among-population uniformity for mtDNA variability argues in favor of the chromosomal and some allozyme polymorphisms being adaptive.

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Se analiza la importancia de un diagnóstico preciso en pacientes que presentan un cuadro de inestabilidad glenohumeral. Catorce pacientes con diagnóstico de inestabilidad multidireccional anteroinferior involuntaria fueron tratados con un plan específico de ejercicios de fortalecimiento muscular. Once de ellos presentaron un resultado bueno o excelente. Cuando la cirugía es necesaria, el procedimiento técnico debe adecuarse al sustrato anatomopatológico del cuadro

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Se analiza la importancia de un diagnóstico preciso en pacientes que presentan un cuadro de inestabilidad glenohumeral. Catorce pacientes con diagnóstico de inestabilidad multidireccional anteroinferior involuntaria fueron tratados con un plan específico de ejercicios de fortalecimiento muscular. Once de ellos presentaron un resultado bueno o excelente. Cuando la cirugía es necesaria, el procedimiento técnico debe adecuarse al sustrato anatomopatológico del cuadro

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Amiodarone has been used for the last 20 years, initially as antianginal and then as antiarrhythmic agent. To the collateral effects described, the optic neuropathy must be added as a complication, not so frequent, caused apparently by the prolonged ingestion of it. Two patients treated with MDD of 250 mg of amiodarone, for a mean of 17 months, are reported. They presented a decrease of the visual acuity. After proposing the possible differential diagnosis, the relation between the use of amiodarone and the optic alteration observed is briefly exposed, relating it with the demyelination neuropathy induced by this drug. This presumption is corroborated by the ultrastructural alterations registered in the optic nerve of animals in which amiodarone was used, similar to those of the peripheral nerves, as an expression of a disorder of the lipidic metabolism, which allow it to be integrated to the so called iatrogenic lipidoses.

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We have studied 15 gene loci coding for enzymes in 121 Trypanosoma cruzi stocks from a wide geographic range--from the United States and Mexico to Chile and southern Brazil. T. cruzi is diploid but reproduction is basically clonal, with very little if any sexuality remaining at present. We have identified 43 different clones by their genetic composition; the same genetic clone is often found in very distant places and in diverse hosts. There is much genetic heterogeneity among the different clones, and they cannot be readily classified into a few discrete groups that might represent natural taxa. These findings imply that the biological and medical characteristics need to be ascertained separately for each natural clone. The evidence indicates that clonal evolution is very ancient in T. cruzi. We propose two alternative hypotheses concerning the relationship between the biochemical diversity and the heterogeneity in other biological and medical characteristics of T. cruzi. One hypothesis is that the degree of diversity between strains simply reflects the time elapsed since their last common ancestor. The second hypothesis is that biological and medical heterogeneity is recent and reflects adaptation to different transmission cycles. A decision between the two hypotheses can be reached with appropriate studies, with important medical consequences.

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