The variability of force platform data in normal and cerebral palsy gait

OBJECTIVE

To determine the natural variability and symmetry of force platform parameters in normal and cerebral palsy gait.

DESIGN

Force platform data has been analysed using coefficient of variation, analysis of variances and symmetry index.

BACKGROUND

It is important to establish the natural variability of force platform data so that changes in gait due to natural variability and those due to intervention can be determined.

METHODS

Force platform data and walking speed were recorded in 15 normal volunteers and 11 children with cerebral palsy on 3 consecutive days. Five recordings were made of each leg on each day.

RESULTS

The force platform parameters in both groups showed greater asymmetry in the horizontal plane. In the cerebral palsy group significant asymmetry was found in all 3 directions. The variation of the vertical ground reaction force peaks (Fz1 and Fz3) and of the anterior-posterior negative peak (Fy3) for both groups was low. Fz3 was the most reproducible force parameter. The other force parameters showed high variability and are not considered reliable measures of gait. The temporal parameters and the walking speed were reproducible measures.

CONCLUSIONS

Only selected ground reaction force parameters showed acceptable stability and reproducibility. Therefore caution has to be taken in the selection of these parameters, when assessing the effects of any intervention on cerebral palsy gait.

The reinforcement of mating preferences on an island

We develop a haploid model for the reinforcement of female mating preferences on an island that receives migrants from a continent. We find that preferences will evolve to favor island males under a broad range of conditions: when the average male display trait on the island and continent differ, when the preference acts on that difference, and when there is standing genetic variance for the preference. A difference between the mean display trait on the continent and on the island is sufficient to drive reinforcement of preferences. Additional postzygotic isolation, caused, for example, by either epistatic incompatibility or ecological selection against hybrids, will amplify reinforcement but is not necessary. Under some conditions, the degree of preference reinforcement is a simple function of quantities that can be estimated entirely from phenotypic data. We go on to study how postzygotic isolation caused by epistatic incompatibilities affects reinforcement of the preference. With only one pair of epistatic loci, reinforcement is enhanced by tighter linkage between the preference genes and the genes causing hybrid incompatibility. Reinforcement of the preference is also affected by the number of epistatically interacting genes involved in incompatibility, independent of the overall intensity of selection against hybrids.

The strength of indirect selection on female mating preferences

An important but controversial class of hypotheses concerning the evolution of female preferences for extreme male mating displays involves "indirect selection." Even in the absence of direct fitness effects, preference for males with high overall fitness can spread via a genetic correlation that develops between preference alleles and high fitness genotypes. Here we develop a quantitative expression for the force of indirect selection that (i) applies to any female mating behavior, (ii) is relatively insensitive to the underlying genetics, and (iii) is based on measurable quantities. In conjunction with the limited data now available, it suggests that the evolutionary force generated by indirect selection on preferences is weak in absolute terms. This finding raises the possibility that direct selection on preference genes may often be more important than indirect selection, but more data on the quantities identified by our model and on direct selection are needed to decide the question.

The false negative rate in cervical cytology. Comparison of monolayers to conventional smears

OBJECTIVE

To compare cervical monolayers prepared by a density gradient technique to conventional smears.

STUDY DESIGN

The study evaluated 2,863 patient samples. After conventional smears were prepared, the residual samples (collected in preservative fluid) were processed by the monolayer preparation system, which disaggregates the cells, removes nonclinical debris and layers the diagnostic material onto a microscopic slide. Slides were screened in Germany and the United States in masked studies.

RESULTS

In the U.S. study, the false negative rate for the conventional smears was 9.4% when endocervical components were present and 16.7% when absent. The false negative rate for the monolayer preparation was 4.0% when endocervical components were present and 5.7% when absent. A comparison of the diagnostic results showed that the monolayer made possible the detection of 48 cases of low grade disease or higher that were not found on the conventional smear. The German study was independent but used the same samples. The results, although based upon the Papanicolaou classification, were similar.

CONCLUSION

Cervical samples that are preserved in a liquid suspension and prepared using the density gradient monolayer technology have many advantages over conventional cervical cytologic smear preparations. The quality of cell preservation and presentation is superior. Elimination of obscuring material, homogeneity of the sample and standardized quality work together to produce preparations that can be effectively and reliably screened by cytotechnologists. These preparations should greatly enhance the reliability of machine-based screening in the future.

Is the objective assessment of cerebral palsy gait reproducible?

It is important to determine the reproducibility of objective measures of gait in children with motor disorders in order that the effect of interventions to improve gait can be monitored. Data for walking velocity and vertical ground reaction force expressed as a percentage of body weight were collected from 15 normal children and 11 with spastic cerebral palsy. Five recordings were obtained for each foot of each child on 3 consecutive days, and calculations of variability made for each set of five recordings. Children with cerebral palsy had slower walking velocities and greater ground reaction force values, and both these parameters had low intraindividual variation for both normal children and those with cerebral palsy (coefficients of variation < 12.5%). This good reproducibility applied to intrasubject and intraday variability; no learning effect was seen over 3 days' recordings. There was no significant difference in variability whether the first three recordings or all five were used. These techniques of gait measurement in children have good reproducibility and a potential role in the objective assessment of medical and surgical interventions.

Estimating the covariance structure of traits during growth and ageing, illustrated with lactation in dairy cattle

Quantitative variation in traits that change with age is important to both evolutionary biologists and breeders. We present three new methods for estimating the phenotypic and additive genetic covariance functions of a trait that changes with age, and illustrate them using data on daily lactation records from British Holstein-Friesian dairy cattle. First, a new technique is developed to fit a continuous covariance function to a covariance matrix. Secondly, this technique is used to estimate and correct for a bias that inflates estimates of phenotypic variances. Thirdly, we offer a numerical method for estimating the eigenvalues and eigenfunctions of covariance functions. Although the algorithms are moderately complex, they have been implemented in a software package that is made freely available. Analysis of lactation shows the advantages of the new methods over earlier ones. Results suggest that phenotypic variances are inflated by as much as 39% above the underlying covariance structure by measurement error and short term environmental effects. Analysis of additive genetic variation indicates that about 90% of the additive genetic variation for lactation during the first 10 months is associated with an eigen-function that corresponds to increased (or decreased) production at all ages. Genetic tradeoffs between early and late milk yield are seen in the second eigen-function, but it accounts for less than 8% of the additive variance. This illustrates that selection is expected to increase production throughout lactation.

Spinal cord dysfunction in neonatal meningococcal meningitis

A 3-week-old infant with meningococcal meningitis is described whose clinical course was complicated by paraparesis and urinary retention. This rare complication of meningococcal meningitis has not previously been reported in the neonatal age group.

Preliminary evaluation of Cyto-Rich: an improved automated cytology preparation

A new cervical cytology monolayer preparation system called Cyto-Rich was evaluated. Using samples from 557 patients, Cyto-Rich monolayers were compared to matched conventional smears. After conventional smears were prepared and spray fixed, residual exfoliated cells were transferred to preservative fluid. The cell suspensions were gently disaggregated and the epithelial component enriched with gradient centrifugal sedimentation. The batched samples were then placed on the Cyto-Rich work station where slides are automatically prepared and stained. The results demonstrate that Cyto-Rich prepared monolayers are vastly superior to the conventional smears for cell presentation. While the study showed 99% overall concordance. Cyto-Rich improved the detection of low-grade cervical intraepithelial lesions.

Analysis of the inheritance, selection and evolution of growth trajectories

We present methods for estimating the parameters of inheritance and selection that appear in a quantitative genetic model for the evolution growth trajectories and other "infinite-dimensional" traits that we recently introduced. Two methods for estimating the additive genetic covariance function are developed, a "full" model that fully fits the data and a "reduced" model that generates a smoothed estimate consistent with the sampling errors in the data. By decomposing the covariance function into its eigenvalues and eigenfunctions, it is possible to identify potential evolutionary changes in the population's mean growth trajectory for which there is (and those for which there is not) genetic variation. Algorithms for estimating these quantities, their confidence intervals, and for testing hypotheses about them are developed. These techniques are illustrated by an analysis of early growth in mice. Compatible methods for estimating the selection gradient function acting on growth trajectories in natural or domesticated populations are presented. We show how the estimates for the additive genetic covariance function and the selection gradient function can be used to predict the evolutionary change in a population's mean growth trajectory.

A quantitative genetic model for growth, shape, reaction norms, and other infinite-dimensional characters

Infinite-dimensional characters are those in which the phenotype of an individual is described by a function, rather than by a finite set of measurements. Examples include growth trajectories, morphological shapes, and norms of reaction. Methods are presented here that allow individual phenotypes, population means, and patterns of variance and covariance to be quantified for infinite-dimensional characters. A quantitative-genetic model is developed, and the recursion equation for the evolution of the population mean phenotype of an infinite-dimensional character is derived. The infinite-dimensional method offers three advantages over conventional finite-dimensional methods when applied to this kind of trait: (1) it describes the trait at all points rather than at a finite number of landmarks, (2) it eliminates errors in predicting the evolutionary response to selection made by conventional methods because they neglect the effects of selection on some parts of the trait, and (3) it estimates parameters of interest more efficiently.

Genetic segregation and the maintenance of sexual reproduction

Sexual reproduction confronts evolutionary biology with a paradox: other things being equal, an asexual (all-female) population will have twice the reproductive potential of a competing sexual population and therefore should rapidly drive the sexual population to extinction. Thus, the persistence of sexual reproduction in most life forms implies a compensatory advantage to sexual reproduction. Work on this problem has emphasized the evolutionary advantages produced by the genetic recombination that accompanies sexual reproduction. Here we show that genetic segregation produces an advantage to sexual reproduction even in the absence of an advantage from recombination. Segregation in a diploid sexual population allows selection to carry a single advantageous mutation to a homozygous state, whereas two separate mutations are required in a parthenogenetic population. The complete fixation of advantageous mutations is thus delayed in a heterozygous state in asexual populations. Calculation of the selective load incurred suggests that it may offset the intrinsic twofold reproductive advantage of asexual reproduction and maintain sexual reproduction in diploid populations.

The evolution of growth trajectories and other complex quantitative characters

Growth trajectories differ from many other quantitative characters in that they are characterized by a continuous function rather than by a finite number of discrete measurements. We review here recently developed methods for predicting the evolution of growth trajectories under the influence of natural or artificial selection. Using our method, analysis of genetic data from mice shows that the patterns of genetic variation arising from developmental processes impose constraints on evolution of growth trajectories. These constraints can be quantified to reveal the families of growth trajectories that can be produced by selection and those families that cannot. The data suggest there may be relatively few evolutionary degrees of freedom for growth trajectories despite the presence of abundant additive genetic variation to alter size and (or) growth rate at every age. The description of these constraints may be useful to both biologists who would like to determine the evolutionary options available to natural populations and to breeders who would like to alter growth trajectories to economically improve domesticated species. Our methods and conclusions can be generalized to other kinds of "infinite-dimensional" or complex characters, including morphological shapes and norms of reaction.

Symptoms and signs of progressive hydrocephalus

The clinical features of 107 cases of children with hydrocephalus and measured raised intraventricular pressure were analysed retrospectively. Fifty one children had recently been diagnosed as having hydrocephalus, and the remainder had had shunts injected to direct the cerebrospinal fluid. The most common symptoms in the group were vomiting, behavioural changes, drowsiness, and headaches. The most common clinical signs were inappropriately increasing occipitofrontal head circumferences, tense anterior fontanelles, splayed sutures, and distension of the scalp veins. Half the infantile cases of hydrocephalus were without symptoms, and a quarter of the cases with cerebrospinal fluid shunts and measured raised intraventricular pressure were without signs. There were no fewer than 33 different clinical signs including several unusual ones, such as macular rash and sweating. We believe that the presentation of hydrocephalus with raised intraventricular pressure is sufficiently variable, unusual, or even absent to justify the direct measurement of intracranial pressure.