Introduction: The description of Upper Airway Resistance Syndrome (UARS) let us to recognize the importance of the pair 'respiratory effort-arousal' on sleep-disordered breathing pathophysiology.
Development: First part of this paper reviews knowledge about respiratory arousal pathophysiology. Arousal response is normally needed to end obstructive respiratory episodes, but it is also the cause of sleep fragmentation. Among respiratory stimuli able to provoke arousal (respiratory effort, hypoxemia and hypercapnia), respiratory effort is the most constant. Neurophysiological mechanisms involved in arousal, sleep and vegetative consequences, and the possible role of non visible arousals, are also discussed. In UARS, because of the absence of apnea/hypopnea and significative O2 desaturations, arousals are induced by the increased respiratory effort. Diagnosis needs the simultaneous recording of polysomnography and esophageal pressure. Some symptoms and signs of UARS are similar to those of Obstructive Sleep Apnea Syndrome. However, UARS shows any differences: a lower Body Mass Index, less constant snoring, males and females are similarly affected or higher frequency of craniofacial abnormalities. Diagnostic difficulties may be due to confusion between hypopneas and episodes of increased resistance of upper airway, or to the lack of definitive diagnostic criteria. Finally, differential diagnosis needs a broad knowledge of disorders of excessive daytime sleepiness.