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PubMed1995

Mechanism of iris prolapse: a qualitative analysis and implications for surgical technique.

A theoretical explanation of the mechanism of iris prolapse is presented using simple flow mechanics and the Bernouille principle, which states that fluid pressure varies inversely to fluid velocity. Rapid aqueous escape into an anterior chamber perforation thus creates a relative vacuum anterior to the iris. Further analysis shows that the tendency of the iris to prolapse into an anterior chamber wound increases as an inverse function of the fourth power of the radial distance between the iris and the perforation. Anterior wound placement, clear of the iris plane, should help to prevent iris prolapse. Minimizing the irrigation velocity (and hence the fluid outflow velocity) should also help to prevent intraoperative prolapse.

Acute Disease↗
PubMedDate not supplied

Analysis of mitral inertance: a factor critical for early transmitral filling.

The upslope of the transmitral E wave depends on the combined influence of the rate of change of the atrioventricular gradient and the inertial mass of blood within the mitral apparatus (inertance). To use observed transmitral velocity to predict the atrioventricular pressure (delta p) difference requires knowledge of the magnitude of mitral inertance (M, inertial mass divided by effective orifice area), closely related to the length over which blood accelerates and decelerates on passing through the valve. To define the magnitude and determining factors for mitral inertance in typical valvular geometries, we used an in vitro model in which a known atrioventricular gradient (delta p, range 3.8 to 39 mm Hg) was applied instantaneously to orifices (areas of 0.5, 1, 1.5, 2, and 2.5 cm2) and conduits (volume 2.5 to 24 ml). Continuous wave Doppler spectra were recorded and the slope (dv/dt) of the tangent to the upslope was measured manually. From slope and pressure difference, inertance was calculated as delta p/(dv/dt). In 103 combinations of pressure gradients and orifices or conduits, inertance ranged between 1.9 and 12.7 gm/cm2. Linear regression showed that inertance M was highly correlated with orifice diameter D (M = 3.17 D; r = 0.84; p < 0.0001) and, in the conduits, with diameter and length L (M = 4.1 D + 0.7 L-1.8; r = 0.87; p < 0.0001). Inertance was not significantly related to the pressure gradient. In conclusion, inertance depends mainly on the mitral apparatus geometry and most strongly on orifice diameter. Knowledge of mitral inertance should help to extract quantitative data on atrioventricular pressure difference from the upslope of the transmitral E wave.

Binomial Distribution↗
PubMed1998

Estimating the incidence rate ratio in cross-sectional studies using a simple alternative to logistic regression.

PURPOSE: Logistic regression is often used for the analysis of cross-sectional studies, and prevalence odds and odds ratios are obtained. Other methods have been proposed for estimating prevalence ratios. An alternative regression method is also available for estimating rate ratios. Its application to cross-sectional studies is discussed. METHODS: When dealing with chronic conditions, it is possible to model binomial data using the complementary log-log link function log(-log(1-pi)), where pi is the prevalence, an option available on many statistical software packages. In effect, these are models for the disease incidence rate lambda, which is assumed to be constant over the underlying follow-up period t. This approach is based on the well-known relationship 1-pi-exp(-lambda t). The cumulative effect of age on prevalence (effectively "time of follow up") can be accounted for in the model, by specifying it as an offset. RESULTS: The regression coefficients associated with the covariates included in the model estimate rate ratios, rather than odds or prevalence ratios. The method is applied to the analysis of the prevalence of respiratory symptoms in 4395 children aged 7-9 years who are residents of Huddersfield (northern England), surveyed in the framework of the SAVIAH (Small Area Variations of Air Quality and Health) study. CONCLUSIONS: By considering saturated models including only sex as a covariate, direct comparison of crude and fitted parameters (odds, prevalence, and rate ratios) shows that, for short follow-up periods, the complementary log-log model is a valid alternative to logistic regression. More complex models including other covariates are also discussed.

Air Pollution↗
PubMed1995

Oral ciprofloxacin versus intravenous cefuroxime as prophylaxis against postoperative infection in vascular surgery: a randomised double-blind, prospective multicentre study.

OBJECTIVES: To test the hypothesis that oral ciprofloxacin is equally effective as intravenous cefuroxime in preventing postoperative infectious complications in patients undergoing peripheral arterial surgery involving the groins. DESIGN: Prospective, randomised, double-blind multicentre study. MATERIALS: 580 patients undergoing arterial surgery involving the groins were randomised to ciprofloxacin (Ciproxin, Bayer) 750 mg x 2 p.o. or cefuroxime (Zinacef, Glaxo) 1.5 g x 3 i.v. given only on the day of surgery. The primary endpoint was wound/graft infection within 30 days postoperatively. Wound infection was defined as pus. RESULTS: The wound infection rate in the ciprofloxacin group was 9.2% (27 patients) and in the cefuroxime group 9.1% (26 patients) according to intention to treat. For correct treatment the corresponding numbers were 9.5% (23 patients) and 9.7% (22 patients), respectively. There were three graft infections (0.5%). The infection rate was 7.1% (31/433) in the absence and 14.9% (22/147) in the presence of distal ulcers (p < 0.05). S. allreus was the most common bacteria isolated. Forty percent of the wound infections were localised to the groins. By multivariate analysis presence of distal ulcer was the only factor of prognostic significance. CONCLUSIONS: The infection rate was similar in the two groups. Thus, oral administration of ciprofloxacin is an attractive, cost-effective and safe alternative to prophylaxis in vascular patients capable of taking oral medication on the day of surgery.

Administration, Oral↗
PubMed1991

The epidemiology of multiple-clone Plasmodium falciparum infections in Gambian patients.

The occurrence of multiple-clone Plasmodium falciparum haploid blood-stage infections is a pre-requisite for cross-fertilization and genetic exchange at the diploid stage in the mosquito. Using monoclonal antibodies against 3 polymorphic blood-stage antigens, a method of two-colour differential immunofluorescence allowed the resolution of between 1 and 4 clones/isolate. A mean of 2 P. falciparum clones was detected in the blood of malaria patients in The Gambia. The mean number of clones/patient showed no correlation with age, parasitaemia, or disease severity. There was a slight difference in mean number between sample periods, probably reflecting temporal differences in transmission intensity. A statistical analysis of 2-locus genetic diversity of clones within isolates concludes that not all multiple-clone infections result from superinfection, but that some are due to single multiple-clone inoculations.

Alleles↗
PubMed1991

Population genetics of Plasmodium falciparum within a malaria hyperendemic area.

Serotyping with monoclonal antibodies was used to estimate the number and frequencies of allelic variants of two merozoite surface proteins, MSP1 and MSP2, and an exported protein Exp-1, in a sample of 344 clinical isolates of Plasmodium falciparum from an urban region of The Gambia. Represented among the isolates were 36, 8 and 2 alleles of the MSP1, MSP2 and Exp-1 loci respectively. Relative frequencies of these alleles remained stable in the parasite population over the 2 years of the study. A computer program was used to calculate from the frequencies of individual alleles at the three loci, the probable number of different genotypes in samples from the population, assuming random assortment among the loci. No significant difference was found between the expected and the observed genotype diversity. It is concluded that recombination among unlinked loci is a common consequence of sexual reproduction of P. falciparum in The Gambia. Slightly lower genotype diversity was observed in each of two villages, which may be a consequence of smaller population size compared with the urban region.

Alleles↗
PubMed1991

Naturally occurring immunoglobulin M antibodies to Toxoplasma gondii in Japanese populations.

Varying degrees of naturally occurring immunoglobulin (Ig) M antibodies to Toxoplasma gondii were observed in almost all of adult Japanese populations negative for IgG antibodies as determined by an enzyme-linked immunosorbent assay (ELISA). The antibody activity was reduced by absorption of sera with Toxoplasma soluble antigen or by 2-mercaptoethanol treatment of the IgM fraction after sucrose density gradient centrifugation. Umbilical cord sera had almost no detectable IgM antibodies, and the ELISA value in infants increased with age until 1 year. Most paired sera collected from adults at a 1-year interval showed constant IgM antibody levels, suggesting retention of natural IgM antibodies throughout their lives without changing ELISA values of greater than or equal to 0.1/year. Polar staining was observed in immunofluorescent studies for 14 serum samples with natural IgM antibody levels of greater than or equal to 0.5, while samples obtained from a pregnant woman with natural IgM antibody levels of greater than or equal to 0.9 reacted to the whole organism as was observed in positive sera of acute patients. Western blotting analysis revealed that natural IgM antibodies were directed to multiple antigen molecules. The band pattern differed slightly from one serum to another, but almost consistent patterns were observed in sera sequentially obtained from an adult over 4 years.

Absorption↗
PubMed1992

The role of nematode parasites in Soay sheep (Ovis aries L.) mortality during a population crash.

In early 1989, two-thirds of the Soay sheep population on St Kilda died over 12 weeks. Post-mortem examinations revealed emaciated carcasses and considerable nematode burdens, with protein-energy malnutrition as the probable cause of death. Haematological and blood biochemical changes in the sheep, as well as fecundity of gastrointestinal nematodes, suggested the hosts were immunosuppressed. In parallel, laboratory experiments in which Soay sheep on a high plane of nutrition were artificially infected with Ostertagia circumcincta, showed no clinical signs or mortality when supporting worm burdens similar to those recorded in dead sheep on St Kilda. Anthelmintic treatment of a group of animals increased daily survival rates in ewes and male lambs, although treated animals became re-infected as the 'crash' progressed. It is suggested that parasites contribute to mortality in malnourished hosts, exacerbating the effects of food shortage.

Analysis of Variance↗
PubMed2000

Life-history strategies and population dynamics of abomasal nematodes in Svalbard reindeer (Rangifer tarandus platyrhynchus).

The observation that the total abundance of adult nematodes in the abomasum of Svalbard reindeer increases between October and April suggests adaptation to cope with the Arctic winter. Here we investigate the extent to which selection has led to similar life-history strategies in the 3 most numerous trichostrongyle species. The life-histories are found to differ markedly. We use flexible statistical models for the abundance and dispersion of parasites in the host population. One of the taxa, Marshallagia marshalli, was most abundant and had its highest egg output in the winter. In contrast, the abundance of the most common taxa, Ostertagia gruehneri, m. gruehneri was stable or declined from autumn to late winter, and the closely related taxa, O. gruehneri, m. arcticus, showed a similar over winter drop. The faecal egg output of these 2 taxa was highest in summer, as found in temperate trichostrongyle species. Despite the apparent contamination of summer pastures with O. gruehneri, calves showed negligible burdens until their second summer and the abundance of infection reached an asymptote within their third year. In contrast, the abundance of M. marshalli in calves showed a rapid increase over the first summer and by late winter was similar to peak levels found in adults (8000 worms). This increase could not be accounted for by the developing abomasum larvae population and is therefore evidence for transmission over the winter for this taxa. While M. marshalli showed little between-year variation, O. gruehneri showed 2-fold fluctuation in the abundance of infection. O. gruehneri may therefore play a role in the fluctuating population dynamics of the host. Since there was no apparent decline in abundance with host age in any of the 3 taxa there was no evidence of reindeer mounting an immune response.

Abomasum↗
PubMed1998

Effect of management practices on screw-worm among sheep in São Paulo State, Brazil.

The screw-worm Cochliomyia hominivorax (Coquerel), whose larvae develop in wild and domestic animals as well as in man, is one of the major causes of myiasis in Brazil. Sheep raising is expanding in the state of São Paulo, where information about the infestation of screw-worm is necessary for appropriate control measures. The present data were obtained from questionnaires sent to all sheep breeders belonging to Associacão Paulista de Criadores de Ovinos--ASPACO (São Paulo State Association of Sheep Breeders). Response reliability was tested by random visits paid to 10.6% of the respondents. The results show that screw-worm is the most frequent ectoparasite in the flocks and is responsible for the greatest economic losses. Its occurrence is strongly associated with flock size, reaching 100% in flocks consisting of more than 500 animals. Infestation is higher in spring and summer and lowest in the fall. The number of cases increases in the presence of some management practices such as tail amputation. Furthermore, several other associations between the indicators surveyed were also found to be significant. The data obtained will permit the elaboration of strategies for the control of this parasitosis.

Age Factors↗
PubMed2000

[Proof of equivalence as a new issue in confirmatory statistics].

A correct confirmatory analysis of studies aiming at demonstration of equivalence requires the use of a class of testing procedures that is still widely unknown among non-specialists and not even mentioned in traditional textbooks on Applied Statistics. The naive approach to equivalence testing, i.e. performing an ordinary two-sided test and inferring equivalence from the absence of a significant difference, is unacceptable since it entails an uncontrolled increase of the risk of false positive conclusions. A technically simple approach to avoiding such fallacies is based on the principle of confidence interval inclusion. However, the latter suffers from the disadvantage of yielding testing procedures that tend to exhibit comparatively poor power properties. For the sake of keeping the numbers of patients or subjects to be recruited for the respective trials as low as possible, application of optimum testing procedures, also for equivalence assessment, has much to recommend it. The rationale of such an optimum test for equivalence is described in some detail for four standard settings frequently encountered in clinical research.

Binomial Distribution↗
PubMed1999

Stability and change in longitudinal water-level task performance.

Three longitudinal samples of children (N = 481), 8 to 16 years old, were assessed 3 times at yearly intervals on 8 water-level items. The within-child change in task performance over age is viewed as a stochastic process of the child changing or remaining in 1 of 3 latent (strategy) states: (a) bottom-parallel responders, (b) random responders, or (c) accurate responders. A random-effects binomial mixture distribution is used to model performance at each age. Change over age is gauged by a stochastic transition model. Although there was improvement in task performance over age, the more general finding is that strategy stability, not change, is most typical.

Adolescent↗
PubMed1995

Regression analyses of counts and rates: Poisson, overdispersed Poisson, and negative binomial models.

The regression models appropriate for counted data have seen little use in psychology. This article describes problems that occur when ordinary linear regression is used to analyze count data and presents 3 alternative regression models. The simplest, the Poisson regression model, is likely to be misleading unless restrictive assumptions are met because individual counts are usually more variable ("overdispersed") than is implied by the model. This model can be modified in 2 ways to accomodate this problem. In the overdispersed model, a factor can be estimated that corrects the regression model's inferential statistics. In the second alternative, the negative binomial regression model, a random term reflecting unexplained between-subject differences is included in the regression model. The authors compare the advantages of these approaches.

Binomial Distribution↗
PubMed2000

Combining independent p values: extensions of the Stouffer and binomial methods.

The Stouffer z method, and other popular methods for combining p values from independent significance tests, suffer from three problems: vulnerability to criticisms of the individual studies being pooled, difficulty in handling the "file drawer problem," and vague conclusions. These problems can be reduced or eliminated by supplementing a test of combined probability with a variety of new analyses described here. Along with other advantages, these analyses provide a way to address the file drawer problem without making limiting assumptions about the nature of studies not included in the pooled analysis. These analyses can supplement a traditional meta-analysis, yielding conclusions not provided by widely used meta-analytic procedures.

Binomial Distribution↗
PubMed2004

Biases of success rate differences shown in binomial effect size displays.

The intent of a binomial effect size display (BESD) is to show "the [real-world] importance of [an] effect indexed by a correlation [r]" (R. Rosenthal, 1994, p. 242) by reexpressing this correlation as a success rate difference (SRD) (e.g., treatment group success rate-control group success rate). However, SRDs displayed in BESDs generally overestimate real-world SRDs implied by correlations of (a) dichotomous X and Y variables (phi coefficients), (b) dichotomous X and continuous Y variables (point-biserial coefficients [rpbs]). and (c) continuous X and Y variables (rxys). Furthermore, overestimation biases are larger for rxys than for rpbs. Differences in the sizes of biases linked to different correlations suggest that BESD SRDs reported for different correlations are not comparable. The stochastic difference index (N. Cliff, 1993: A. Vargha & H. D. Delaney, 2000) is recommended as an alternative to the BESD.

Binomial Distribution↗
PubMed1990

Enamel defects in the primary dentition of children born weighing less than 2000 g.

The findings for enamel defects in the primary dentition of a group of low birthweight (LBW) children were related to their perinatal medical histories. Examination of 110 LBW children and 93 control children aged 5 years showed that significantly more of the LBW children (71%) had hypoplasia than controls (15%), but there was no difference in opacities alone (LBW 25%, control 26%). In the LBW group, defects were seen more often in children classified as ill during the perinatal period, who received ventilator support or intravenous alimentation and in those children born at less than 32 weeks gestation, compared to the LBW children without these perinatal problems. It is speculated that the high incidence and the cause of enamel defects in sick preterm infants may be due to oxygen deprivation and mineral substitute depletion.

Analysis of Variance↗
PubMed1993

An investigation into sugar-containing and sugar-free over-the-counter medicines stocked and recommended by pharmacists in the north western region of England.

Sugar-containing paediatric medicines can contribute to the aetiology of dental caries, and as many are sold over-the-counter (OTC) as are prescribed. This study evaluated a dental health education initiative among pharmacists in the north-west of England in an effort to increase the sales of sugar-free preparations. It also investigated the stocks they held, their recommendations to their customers and their knowledge of, and attitudes to, sugar in medicine as a cause of dental caries. Following a baseline questionnaire, a randomly selected group of pharmacists were sent a dental health education leaflet informing them of the role of sugar-containing medicines in the aetiology of dental caries and a list of sugar-free alternatives which they were asked to stock and recommend. Twelve months later, a second questionnaire was completed by both the test and control groups of pharmacists. The most commonly stocked and recommended OTC paediatric medicines contained sugar. Of the 14 preparations stocked by over 90% of the pharmacists only two were sugar-free. During the year of the study the mean number of sugar-free medicines stocked by the pharmacists in the test group reduced from 7.70 to 7.12 (P < 0.05) and in the control group from 8.39 to 7.45 (P < 0.05). Seventy-seven per cent of the pharmacists thought that sugar in medicines could contribute to caries, but recommended sugar-containing preparations more frequently than sugar-free alternatives. The dental profession needs to actively promote medicines that are sugar-free if there is to be a change from the use of iatrogenic sugar-containing preparations.

Binomial Distribution↗