Symptom-informed updates increase predicted probabilities of hEDS and POTS in comorbid patientsNew data reveals links between hEDS, POTS, and MCAS conditions

Background. Hypermobile Ehlers-Danlos syndrome (hEDS), postural orthostatic tachycardia syndrome (POTS) and mast cell activation syndrome (MCAS) are reported to co-occur frequently. It is unclear how strongly, whether symptom profiles sharpen prediction of a second diagnosis given a first, and whether the published literature can support such inference at all. Methods. We pooled 22 published cohorts-aggregate prevalence data, no primary human-subjects data-using Bayesian hierarchical random-effects models on the logit scale, and propagated the resulting posteriors through naive and tempered symptom updating. We introduce a feasibility screen derived from the Frechet-Hoeffding bounds that tests whether separately pooled marginals can describe a single population, and we characterise the identifiability of latent class structure under disease-selected sampling. Results. Directed comorbidity is strongly asymmetric: {pi}POTS|hEDS = 46.6% (95% CrI [32.5,61.5]) against {pi}hEDS|POTS = 12.1% ([3.5,38.5]), a near-fourfold gap, with {pi}MCAS|POTS lowest at 3.9% ([0.7,21.6]). Prediction intervals exceed credible intervals throughout, indicating substantial between-cohort heterogeneity. The feasibility screen finds 26 of 102 testable cells (25.5%) incompatible with any joint distribution; critically, 21 of these fail the upper Frechet bound and are invisible to the one-sided screen that is the natural first implementation. Among cells surviving the screen, symptom evidence is informative in four of six directions-P(hEDS | POTS,S) rises from 12.1% to 49.7% on a four-symptom panel under tempered updating, and P(POTS | MCAS,S) from 49.5% to 82.1%-but inert in both hEDS-cohort directions. A pathway-dispersion contrast excludes zero in two of six directions, in opposite signs and by margins of 0.1-0.2 percentage points, consistent with chance at this number of comparisons. We show latent class structure is not identified from disease-selected aggregate data, and that the single cohort reporting trivariate structure (N = 8) yields an exactly balanced table (OR = 1.00, 95% CI [0.063, 15.99]). Conclusions. The pooled directional probabilities are usable as clinical priors, with intervals wide enough to preclude precision. Symptom-conditioned prediction is supported in some directions but not those most often invoked clinically, and every estimate rests on cohorts dominated by self-reported ascertainment. The principal methodological contribution is the two-sided feasibility screen: applied here it shows that a quarter of the testable literature cannot describe one coherent population, and that a one-sided implementation understates this sixfold. Keywords: hypermobile Ehlers-Danlos syndrome; postural orthostatic tachycardia syndrome; mast cell activation syndrome; comorbidity; Bayesian meta-analysis; random-effects model; Frechet bounds; identifiability; latent class analysis; collider bias