There are three main types of sleep apnea: obstructive sleep apnea (OSA), central sleep apnea (CSA), and complex sleep apnea syndrome, also known as treatment-emergent central sleep apnea. OSA is the most common form, characterized by physical blockage of the upper airway. CSA occurs when the brain fails to send proper signals to muscles that control breathing. Complex sleep apnea syndrome is a combination of both OSA and CSA.