Hung-up reflex: the history of a clinical sign and differential diagnosis.

The hung-up reflex (HUR) is a tonic reflex response characterized by delayed muscular relaxation following the elicitation of a tendon reflex. First described by Woltman in patients with myxedema and hypothyroidism, it is best demonstrated in the Achilles tendon. Currently, two primary techniques are used for its measurement: photomotography and the anemometer. The pathogenic mechanism underlying this phenomenon is closely linked to the physiological process of cellular membrane recovery, particularly the reabsorption of calcium by the sarcoplasmic reticulum. This sign has been documented in various pathologies, including Huntington's disease, stiff-person syndrome, stiff-limb syndrome, hypothyroidism, myxedema, and Sydenham's chorea. Over time, it has played a significant role in the clinical examination of movement disorders. The objective of this review is to analyze the historical description, pathophysiological mechanisms, and clinical significance of HUR, aiming to enhance the understanding of its presence in different pathological entities and facilitate differential diagnoses.