Severe:  Apnea hypopnea index is greater than 30.
Specifiers
Disease severity is measured by a count of  the number of apneas plus hypopneas per hour
of sleep (apnea hypopnea index) using polysomnography or other ov ernight monitoring.
Overall severity is also informed by levels of nocturnal desaturation and sleep fragmen-
tation (measured by brain cortical arousal frequency and sleep stages) and degree of as-
sociated symptoms and daytime impairment. However, the exact number and thresholds
may vary according to the specific measureme nt techniques used, and these numbers may
change over time. Regardless of the apnea hy popnea index (count) pe r se, the disorder is
considered to be more severe when apneas and hypopneas are accompanied by significant
oxygen hemoglobin desaturation (e.g., when more than 10% of the sleep time is spent at
desaturation levels of less than 90%) or wh en sleep is severely fragmented as shown by an