Deconstructing and reconstructing joint hypermobility on an evo-devo perspective.

Joint hypermobility is a common characteristic in humans. Its non-casual association with various musculoskeletal complaints is known and currently defined as "the spectrum". It includes hypermobile Ehlers-Danlos syndrome (hEDS) and hypermobility spectrum disorders (HSD). hEDS is recognized by a set of descriptive criteria, while HSD is the background diagnosis for individuals not fulfilling these criteria. Little is known about the etiopathogenesis of "the spectrum". It may be intended as a complex trait according to the integration model. Particularly, "the spectrum" is common in the general population, affects morphology, presents extreme clinical variability, and is characterized by marked sex bias without a clear Mendelian or hormonal explanation. Joint hypermobility and the other hEDS "systemic" criteria are intended as qualitative derivatives of continuous traits of normal morphological variability. The need for a minimum set of criteria for hEDS diagnosis implies a tendency to co-vary these underlying continuous traits. In evolutionary biology, such a covariation (i.e. integration) is driven by multiple forces, including genetic, developmental, functional and environmental/acquired interactors. The etiopathogenesis of "the spectrum" may be resolved by a deeper understanding of phenotypic variability, which, superimposes with normal morphological variability.

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Authors: Marco Castori