Erythromelaigia is a poorly understood clinical syndrome characterized by painful, hot, red extremities. We assessed the frequency and types of abnormalities observed during tests of vascular, peripheral neurophysiologic, and autonomic function in patients with erythromelalgia.Methods Of" 163 charts of patients fulfilling the clinical diagnosis of erythromelalgia. 93 patients underwent vascular studies Five of them had detailed vascular studies in 10 affected lower extremities performed before and during symptoms, fifty-four patients underwent neurophysiologic testing, 27 had autonomic reflex screening (ARS). and two had recordings of peripheral autonomic surface potentials (PASP).Results. Measurements in the toes during symptoms revealed a mean temperature increase of 11.6 C (P = 0,00011 along with a laser flow increase from a mean of 6.8 mL/min per 100 g tissue to 76.5 mL/min per 100 g tissue (P<.0.0001). Baseline TcPO; in the feet decreased by 6.7 mmHg (P = 0.032) during symptoms. Twenty-one of 54 electromyographic recordings were abnormal: all fulfilled the criteria for axonal neuropathy. Seventeen of 27 ARSs and one PASP showed severe postganglionic sudomotor impairment; five of 17 additionally had peripheral adrenergic dysfunction.Conclusions During symptoms, an increase in flow and temperature is accompanied paradoxically by a decrease in oxygenation of the affected area; a high proportion of patients have a distal small fiber neuropathy with selective involvement of cutaneous sympathetic fibers; in addition, large fiber neuropathy is often present.