Obstructive Sleep Apnea Syndrome can actually range from very mild to very severe. The of Obstructive sleep apnea Syndrome is often established using the apnoea/hypopnoea index (AHI), which is basically the number of apnoeas plus the number of hypopnoeas per hour of sleep - (hypopnoea being reduction in airflow). An AHI of less than 10 is not likely to be associated with clinical problems.