EPIDEMIOLOGY Sleep-related breathing disorder (SRBD) encompasses a broad clinical spectrum of recurring partial or complete obstruction of the upper airway. This ranges from simple or primary snoring to severe obstructive sleep apnoea (OSA). Patients who have symptoms suggestive of OSA but do not have objective parameters in a sleep study confirming the condition can be classified as having upper airway resistance syndrome (UARS). These patients may have sleep fragmentation but do not have significant oxygen desaturation events or many obstructive events. A definition of apnoea, hypopnea and the apnoea-hypopnea index (AHI) is covered in Chapter 74, Obstructive sleep apnoea: medical management.