BACKGROUND: Functional neurological disorder (FND) is characterized by neurological symptoms identified through positive clinical signs that are incongruent with recognized structural neurological disease. Postural orthostatic tachycardia syndrome (POTS), in contrast, is defined by excessive orthostatic tachycardia and is associated with a range of multisystem autonomic symptoms. Shared clinical features can blur diagnostic boundaries. As a result, new or evolving symptoms in patients with a prior functional diagnosis may be overlooked, increasing the risk of diagnostic overshadowing.

CASE REPORT: An 18-year-old woman with a prior diagnosis of FND presented with fluctuating lower-extremity weakness, gait disturbance, and recurrent drop attacks. She also reported persistent orthostatic intolerance and symptom improvement with recumbency. These symptoms had previously been attributed to the functional diagnosis. Neurological examination demonstrated positive signs of functional weakness, consistent with the prior FND diagnosis. The Composite Autonomic Symptom Score (COMPASS-31) was markedly elevated. Head-up tilt table testing demonstrated sustained sinus tachycardia, with reproduction of typical orthostatic symptoms; other autonomic responses were largely preserved. Generalized joint hypermobility (Beighton score 4/9 with positive historical hypermobility) met the 2017 criteria for hypermobile Ehlers-Danlos syndrome based on family history and chronic musculoskeletal pain. Over several months, functional status improved from wheelchair dependence to independent standing, with reduced symptoms during multidisciplinary care; these findings are reported as a temporal association rather than causal inference.

CONCLUSIONS: This case demonstrates the coexistence of POTS in a patient with established FND and highlights the importance of systematic autonomic evaluation in individuals with persistent orthostatic symptoms. A prior functional diagnosis should not preclude investigation for coexisting physiological disorders.

Keywords: Orthostatic Intolerance, Conversion Disorder, Psychogenic Nonepileptic Seizures, postural orthostatic tachycardia syndrome, Autonomic Nervous System Diseases