Introduction

The term "upper airway resistance syndrome" (UARS) was first coined in 1992 (1) and 1993 (2). Prior to these reports, in the late 1980s, we conducted a series of studies at Stanford University aimed at the pathophysiologic mechanisms of subtle flow limitation associated with snoring during sleep. Using a pneumotachometer and esophageal pressure monitoring it became apparent that there existed a number of individuals with impaired respiration during sleep, who did not fit the typical diagnostic criteria of obstructive sleep apnea (OSA). This is to say that they presented with daytime sleepiness and snoring but without clear polysomnographic diagnostic criteria of apnea, hypopnea or oxygen desaturation. Despite the absence of these diagnostic criteria for OSA they still showed signs of abnormal breathing during sleep when investigated with pneumotachometers and esophageal pressure monitoring instead of oronasal thermistors. This led us to pose the question: "Obstructive sleep apnea or abnormal upper airway resistance during sleep?" (3). In 1991, we published the pathophysiologic phenomena of increased upper airway resistance leading to sleep fragmentation in the absence of apnea, hypopnea, and hypoxemia (4). For further reference into the historic development of UARS please refer to the article by Exar and Collop (5) for a more comprehensive review.

Fifteen years later, the majority of patients with UARS remain undiagnosed, although, a fair number of patients with OSA are being diagnosed with UARS. Others are diagnosed with "habitual" or primary snoring (PS). There are three reasons for this profound misunderstanding. First is the widespread use of the term "sleep-disordered breathing" (SDB) or "sleep-related breathing disorder" (SRBD). The fundamental problem in using these terms lies with the fact that they include any breathing abnormality during sleep, independent of type, pathogenetic background or clinical manifestation. The second reason is that many clinicians still do not have a clear understanding of the distinct diagnostic criteria and the differences in the clinical presentation of UARS and obstructive sleep apnea-hypopnea (OSAH) without daytime sleepiness and obstructive sleep apnea-hypopnea syndrome (OSAHS) (with daytime sleepiness). To make the issue even more complicated, in the second edition of The International Classification of Sleep Disorders (ICSD-2), the term "obstructive sleep apnea" (OSA) now incorporates the majority of these terms. UARS is also subsumed under this diagnosis because it was felt by the ICSD-2 SRBDs task force that pathophysiology does not significantly differ from that of OSA (6). And finally, many laboratories still do not use adequately sensitive methods to detect subtle flow limitation during sleep: they still use oronasal thermocouples.

The goal of this chapter is to review the current knowledge about snoring and UARS and to describe its typical clinical and diagnostic features.

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