Sleep Apnea Types and Symptoms: OSA, CSA, and Complex Sleep Apnea Explained

The term "sleep apnea" is commonly used as though it describes a single condition. Clinically, it encompasses three distinct disorders with different mechanisms, different symptom profiles, and — critically — different treatment approaches. A patient with obstructive sleep apnea who is prescribed CPAP and a patient with central sleep apnea who is prescribed CPAP may have very different outcomes; in some forms of central apnea, standard CPAP can actually worsen the condition. Understanding which type of sleep apnea a patient has is not a matter of academic precision — it directly determines what works and what doesn't.

Obstructive Sleep Apnea: The Anatomy of Collapse

Obstructive sleep apnea is the most prevalent form of the disorder, accounting for an estimated 85 to 90 percent of diagnosed cases. Its defining mechanism is physical: during sleep, the muscles of the upper airway — including the tongue, soft palate, and pharyngeal walls — relax to a degree that allows the airway to narrow or collapse entirely. When this occurs, airflow is partially or completely obstructed. The brain eventually registers the resulting oxygen desaturation and triggers a brief arousal that restores airway tone and breathing. The sleeper typically resumes breathing with a gasp or snort, then falls back asleep — often without any conscious memory of the episode.

Several anatomical features increase OSA susceptibility. A large tongue or tonsils, elongated soft palate, retrognathia (recessed jaw), and increased fat deposition around the pharynx all reduce the effective diameter of the upper airway. "OSA is fundamentally a structural problem," says Dr. Rachel Simone, a sleep researcher at the Stanford Center for Sleep Sciences and Medicine. "The question is how much structural compromise someone has, and whether the degree of muscle relaxation during sleep is enough to cross the threshold into obstruction." Obesity is the single strongest modifiable risk factor: each unit increase in BMI is associated with a roughly 14 percent increase in OSA odds in men and a 6 percent increase in women. But OSA is common in non-obese individuals as well — particularly in those with craniofacial anatomy predisposing to a narrow airway.

Symptoms of OSA

The classic symptom triad of OSA is loud snoring, witnessed apneas (a bed partner observing the patient stop breathing), and excessive daytime sleepiness. However, OSA's symptom profile extends considerably further: morning headaches caused by overnight CO2 retention; dry mouth and sore throat from breathing through an open mouth all night; nocturia (frequent nighttime urination, driven by apnea-triggered atrial natriuretic peptide release); cognitive symptoms including memory problems, difficulty concentrating, and slowed processing speed; mood disturbances including irritability and depression; and reduced libido. In severe OSA, patients may fall asleep unintentionally during conversation, meals, or driving.

Central Sleep Apnea: A Failure of Signal

Central sleep apnea accounts for roughly 5 to 10 percent of sleep apnea diagnoses, and its mechanism is fundamentally different from OSA. The airway is anatomically patent — there is nothing blocking airflow. Instead, the brain temporarily fails to send the signal to the respiratory muscles to initiate a breath. The respiratory drive itself is absent during these events, rather than obstructed.

CSA most commonly occurs in specific clinical contexts. Heart failure is the strongest association: estimates suggest CSA occurs in 30 to 40 percent of patients with systolic heart failure, often in the distinctive Cheyne-Stokes breathing pattern — a cyclical crescendo-decrescendo respiratory pattern followed by a central apnea. Opioid use is the second major cause; opioids suppress the brainstem respiratory centers in a dose-dependent fashion, producing irregular breathing and central apneas. High-altitude exposure can produce a transient form of CSA in healthy individuals through a mechanism involving hyperventilation-induced hypocapnia (reduced CO2, a key respiratory drive stimulus). Idiopathic CSA — without an identifiable cause — occurs but is less common.

"Central sleep apnea patients present differently in the clinic," says Dr. James Whitfield, a pulmonologist and sleep medicine specialist at Cleveland Clinic. "They're less likely to complain of snoring — often their complaint is insomnia, difficulty staying asleep, or waking with a sense of breathlessness. The classic OSA presentation of a snoring man with a thick neck is not what we see."

Symptoms of CSA

CSA symptoms overlap with OSA in broad terms — fatigue, non-restorative sleep, and daytime sleepiness are common to both. But the clinical presentation differs in key ways. Snoring is typically less pronounced or absent in CSA (since there is no airway obstruction to generate the sound). Patients more often describe insomnia — specifically, difficulty staying asleep and recurrent nocturnal awakenings — than the heavy snoring and gasping typical of OSA. Waking with a sense of shortness of breath or dyspnea is more characteristic of CSA, particularly in heart failure patients with Cheyne-Stokes breathing.

Complex (Treatment-Emergent) Sleep Apnea

Treatment-emergent central sleep apnea — sometimes called complex sleep apnea — is a phenomenon observed when patients being treated for OSA with continuous positive airway pressure (CPAP) continue to show significant sleep-disordered breathing events, now of central rather than obstructive character. In other words, CPAP successfully eliminates the obstructive events, but central apneas emerge or persist once obstruction is removed. The prevalence is estimated at 5 to 15 percent of patients initiating CPAP therapy.

The mechanisms underlying treatment-emergent CSA are not fully resolved, but current evidence implicates CO2 instability (CPAP may suppress CO2 levels below the apneic threshold in susceptible individuals) and loop gain — the sensitivity of the respiratory control system. Management typically involves adaptive servo-ventilation (ASV), a device that continuously adjusts pressure support based on real-time breath-by-breath monitoring. Notably, ASV is contraindicated in patients with heart failure with reduced ejection fraction, underscoring why accurate diagnosis matters.

AHI Severity Thresholds and Diagnosis

Regardless of type, sleep apnea severity is quantified using the apnea-hypopnea index (AHI) — the average number of apnea (complete cessation) and hypopnea (partial obstruction with arousal or desaturation) events per hour of sleep. Standard clinical thresholds are: fewer than 5 events per hour is normal in adults; 5 to 14 is mild sleep apnea; 15 to 29 is moderate; and 30 or more is severe. In children, the threshold for abnormal is lower — more than 1 to 2 events per hour is considered abnormal, reflecting different normative respiratory patterns during development.

The diagnosis pathway begins with clinical evaluation — history, physical exam, and a validated screening tool such as the STOP-BANG questionnaire. Confirmation requires sleep testing: either an in-lab polysomnography (PSG), which measures brain waves, eye movements, muscle activity, airflow, respiratory effort, oxygen saturation, and cardiac rhythm, or an at-home sleep test (HST), which measures a more limited parameter set adequate for uncomplicated OSA. PSG is preferred for suspected CSA, complex apnea, or cases where a non-OSA sleep disorder is suspected. The type of apnea — obstructive vs. central — is determined by whether respiratory effort is present during the apnea event; PSG measures chest and abdominal movement belts to make this distinction. For more on what the health consequences of untreated sleep apnea can be, see our article on how sleep apnea affects your health beyond sleep.

Pediatric vs. Adult Presentation

Sleep apnea in children presents differently than in adults, and the distinction matters for recognition. Adult OSA is predominantly driven by obesity and aging-related tissue laxity; pediatric OSA most commonly results from adenotonsillar hypertrophy — enlarged adenoids and tonsils that physically narrow the pediatric airway. The behavioral and cognitive consequences in children are often more prominent than the sleepiness that typifies adult presentation. Children with OSA frequently exhibit hyperactivity, inattention, poor school performance, and behavioral problems — symptoms more often attributed to ADHD than to a sleep disorder. Mouth breathing, restless sleep, and bedwetting are also more prominent in pediatric OSA. Treatment in children frequently involves adenotonsillectomy rather than CPAP, with surgical cure rates of 70 to 85 percent in otherwise healthy children.

Frequently Asked Questions

What is the most common type of sleep apnea?

Obstructive sleep apnea (OSA) is by far the most common type, accounting for 85 to 90 percent of all sleep apnea diagnoses. It is caused by physical collapse of the upper airway during sleep rather than any failure of the brain's respiratory drive. Central sleep apnea accounts for approximately 5 to 10 percent of cases, and treatment-emergent (complex) sleep apnea for roughly 5 to 15 percent of patients initiating CPAP therapy.

How is central sleep apnea different from obstructive sleep apnea?

In obstructive sleep apnea, the airway physically collapses during sleep, blocking airflow — respiratory effort continues (the chest tries to breathe) but air cannot pass. In central sleep apnea, the airway is open, but the brain temporarily fails to send the signal to the respiratory muscles to initiate a breath. This distinction is clinically important because it determines appropriate treatment: CPAP works well for OSA but may not address CSA and can occasionally worsen it.

What is an AHI of 15 mean?

An AHI (apnea-hypopnea index) of 15 falls at the lower boundary of moderate sleep apnea. Standard clinical thresholds classify AHI below 5 as normal, 5–14 as mild, 15–29 as moderate, and 30 or above as severe. At an AHI of 15, treatment is generally recommended, as moderate sleep apnea carries meaningful cardiovascular and metabolic risks over time.

Can someone have both obstructive and central sleep apnea?

Yes. Treatment-emergent (complex) sleep apnea is a recognized syndrome in which a patient diagnosed with OSA is found to have central apnea events persisting or emerging after CPAP eliminates the obstructive events. It occurs in an estimated 5 to 15 percent of patients starting CPAP. Management typically involves adaptive servo-ventilation (ASV), though ASV is contraindicated in certain cardiac conditions, making accurate characterization essential.

Do children with sleep apnea snore loudly like adults?

Children with OSA may snore, but the presentation differs from adults. In children, behavioral symptoms — hyperactivity, inattention, poor school performance, and mood changes — are often more prominent than snoring or daytime sleepiness. Mouth breathing, restless sleep, and bedwetting are common pediatric signs. Pediatric OSA most commonly results from enlarged adenoids and tonsils, and treatment is frequently surgical (adenotonsillectomy) rather than CPAP-based.