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Longitudinal research may take numerous different forms. They are generally observational, however, may also be experimental.

Some of these are briefly discussed below:. Repeated cross-sectional studies where study participants are largely or entirely different on each sampling occasion;.

Prospective studies where the same participants are followed over a period of time. These may include:. Cohort panels wherein some or all individuals in a defined population with similar exposures or outcomes are considered over time;.

Representative panels where data is regularly collected for a random sample of a population;. Linked panels wherein data collected for other purposes is tapped and linked to form individual-specific datasets. Retrospective studies are designed after at least some participants have already experienced events that are of relevance; with data for potential exposures in the identified cohort being collected and examined retrospectively. Longitudinal cohort studies, particularly when conducted prospectively in their pure form, offer numerous benefits.

These include:. The ability to identify and relate events to particular exposures, and to further define these exposures with regards to presence, timing and chronicity;. Excluding recall bias in participants, by collecting data prospectively and prior to knowledge of a possible subsequent event occurring, and;.

Numerous challenges are implicit in the study design; particularly by virtue of this occurring over protracted time periods. We briefly consider the below:. Incomplete and interrupted follow-up of individuals, and attrition with loss to follow-up over time; with notable threats to the representative nature of the dynamic sample if potentially resulting from a particular exposure or occurrence that is of relevance;. Difficulty in separation of the reciprocal impact of exposure and outcome, in view of the potentiation of one by the other; and particularly wherein the induction period between exposure and occurrence is prolonged;.

The potential for inaccuracy in conclusion if adopting statistical techniques that fail to account for the intra-individual correlation of measures, and;. Conducting longitudinal research is demanding in that it requires an appropriate infrastructure that is sufficiently robust to withstand the test of time, for the actual duration of the study.

It is essential that the methods of data collection and recording are identical across the various study sites, as well as being standardised and consistent over time.

Data must be classified according to the interval of measure, with all information pertaining to particular individuals also being linked by means of unique coding systems. Recording is facilitated, and accuracy increased, by adopting recognised classification systems for individual inputs 2. Numerous variables are to be considered, and adequately controlled, when embarking on such a project. These include factors related the population being studied, and their environment; wherein stability in terms of geographical mobility and distribution, coupled with an ability to continue follow-up remotely in case of displacement, are key.

It is furthermore essential to appropriately weigh the various measures, and classify these accordingly so as to facilitate the allocation effort at the data collection stage, and also guide the use of possibly limited funds 3.

Additionally, the engagement and commitment of organisations contributing to the project is essential; and should be maintained and facilitated by means of regular training, communication and inclusion as possible.

The frequency and degree of sampling should vary according to the specific primary endpoints; and whether these are based primarily on absolute outcome or variation over time. Ethical and consent considerations are also specific to this type of research.

All effort should be made to ensure maximal retention of participants; with exit interviews offering useful insight as to the reason for uncontrolled departures 3. The Critical Appraisal Skills Programme CASP 4 offers a series of tools and checklists that are designed to facilitate the evaluation of scientific quality of given literature. This may be extrapolated to critically assess a proposed study design.

Additional depth of quality assessment is available through the use of various tools developed alongside the Consolidated Standards of Reporting Trials CONSORT guidelines, including a structured point checklist proposed by Tooth et al. Following adequate design, the launch and implementation of longitudinal research projects may itself require a significant amount of time; particularly if being conducted at multiple remote sites.

Time invested in this initial period will improve the accuracy of data eventually received, and contribute to the validity of the results. Regular monitoring of outcome measures, and focused review of any areas of concern is essential 3. These studies are dynamic, and necessitate regular updating of procedures and retraining of contributors, as dictated by events. The statistical testing of longitudinal data necessitates the consideration of numerous factors.

Central amongst these are I the linked nature of the data for an individual, despite separation in time; II the co-existence of fixed and dynamic variables; III potential for differences in time intervals between data instances, and IV the likely presence of missing data 6. Note, in both cases, the assumption of equal interval lengths and normal distribution in all groups; and that only means are compared, sacrificing individual-specific data.

II mixed-effect regression model MRM focuses specifically on individual change over time, whilst accounting for variation in the timing of repeated measures, and for missing or unequal data instances, and III generalised estimating equation GEE models that rely on the independence of individuals within the population to focus primarily on regression data 6.

Introduction Asterixis is a disorder of motor control characterized by an inability to actively maintain a position and consequent irregular myoclonic lapses of posture affecting various parts of the body independently. Pathophysiology The exact mechanism of generation of asterixis remains elusive several decades since its first description.

Clinical Significance Asterixis is an uncommon but significant sign in central nervous system CNS disorders [ Table 1 ]. Bilateral asterixis Unilateral asterixis Metabolic: Liver failure, azotemia, respiratory failure Focal brain lesions at: Drugs: Thalamus Sedatives: Benzodiazepines, barbiturates Corona radiate Anticonvulsants: Phenytoin phenytoin flap , carbamazepine, valproic acid, gabapentin Anterior cerebral artery territory Antipsychotics: Lithium Primary motor cortex Antibiotics: Ceftazidime Parietal lobe Others: Metoclopramide Cerebellum Dyselectrolytemia: Hypomagnesemia, hypokalemia Midbrain Pons Bilateral structural brain lesions.

Open in a separate window. Conclusion Asterixis is an interesting yet poorly understood sign in clinical neurology. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.

References 1. Asterixis: One type of negative myoclonus. Adv Neurol. Gokula RM, Khasnis A. The neurological changes in the more common types of severe liver disease. Trans Am Neurol Assoc. Asterixis associated with anatomic cerebral lesions: A study of 45 cases. Acta Neurol Scand. Asterixis - History and terminology.

Mendizabal M, Silva MO. Images in clinical medicine. N Engl J Med. Yokota T, Tsukagoshi H. Cortical activity-associated negative myoclonus.

J Neurol Sci. Cortical origin of mini-asterixis in hepatic encephalopathy. The Encyclopedia of Movement Disorders. Amsterdam: Elsevier; Unilateral asterixis due to a lesion of the ventrolateral thalamus. J Neurol Neurosurg Psychiatry. Asterixis as a presentation of cerebellar ischemic stroke. Password Forgot login? Discover Featured Music Videos People.

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