Humans spend one-third of their lives sleeping. The discovery of the biological clock has deepened our understanding of bodily changes during sleep. Many diseases can occur or worsen during sleep, leading to the development of sleep medicine, which explores sleep disorders. Sleep-related breathing disorders, characterized by abnormal breathing during sleep, rank second among sleep disorders. Sleep-related breathing disorders include obstructive sleep apnea (OSA), central sleep apnea (CSA), sleep-related hypoventilation, sleep-related hypoxemia, and isolated symptoms and normal variants. Obstructive sleep apnea is the most common.
Definition
Common abnormal breathing events during sleep are defined as follows.
Sleep apnea is defined as the cessation of airflow at the nose and mouth (a reduction of ≥90% from baseline) lasting ≥10 seconds during sleep. Sleep apnea events are classified into two types:
Obstructive sleep apnea is characterized by the cessation of airflow at the nose and mouth while thoracic and abdominal breathing efforts persist, often showing paradoxical movement.
Central sleep apnea is characterized by the simultaneous cessation of airflow at the nose and mouth and the absence of thoracic and abdominal breathing efforts, with both diaphragmatic and intercostal muscles ceasing activity.
Hypopnea is defined as a reduction in airflow at the nose and mouth by ≥30% from baseline, accompanied by a ≥4% drop in blood oxygen saturation, lasting ≥10 seconds; or a reduction in airflow by ≥50% from baseline, accompanied by a ≥3% drop in blood oxygen saturation, lasting ≥10 seconds.
Apnea-hypopnea index (AHI) is defined as the count of apnea and hypopnea events per hour of sleep.
Microarousal is defined as the changes in EEG frequency lasting more than 3 seconds during non-rapid eye movement (NREM) sleep, including θ waves and α waves, with frequencies >16 Hz (excluding spindle waves).
Classification
Obstructive sleep apnea
This disorder is characterized mainly by obstructive sleep apnea events and hypopnea, caused by recurrent upper airway obstruction during sleep. It is further divided into adult and pediatric obstructive sleep apnea based on the age of onset.
Central sleep apnea
This disorder is characterized mainly by central sleep apnea events, often resulting from central nervous system diseases or congestive heart failure, which prevent the respiratory center from sending effective signals. It is divided into eight types based on different causes, including central sleep apnea with Cheyne-Stokes breathing, disease-induced central sleep apnea without Cheyne-Stokes breathing, high-altitude periodic breathing-induced central sleep apnea, drug or substance-induced central sleep apnea, primary central sleep apnea, primary central sleep apnea of infancy, primary central sleep apnea of prematurity, and treatment-emergent central sleep apnea.
Figure 1 Classification of sleep apnea and hypopnea
A. Obstructive sleep apnea: Cessation of airflow at the nose and mouth while thoracic and abdominal breathing efforts persist; B. Central sleep apnea: Cessation of both airflow at the nose and mouth and thoracic and abdominal breathing efforts. C. Mixed sleep apnea: Starts with CSA and transitions to OSA during the apnea event. D. Hypopnea: Reduction in airflow amplitude without complete cessation.
Sleep-related hypoventilation
This disorder is characterized by alveolar hypoventilation during sleep as a pathophysiological change. Initially, it may occur only during sleep, but as the disease progresses, it can develop into daytime alveolar hypoventilation. It mainly includes obesity hypoventilation syndrome, congenital central alveolar hypoventilation syndrome, late-onset central hypoventilation with hypothalamic dysfunction, idiopathic central alveolar hypoventilation, drug or toxin-induced sleep-related hypoventilation, and disease-induced sleep-related hypoventilation.
Sleep-related hypoxemia
This refers to hypoxemia during sleep caused by systemic or neurological diseases that cannot be explained by other sleep-related breathing disorders. It is often secondary to airway diseases, pulmonary parenchymal diseases, chest wall diseases, pulmonary vascular diseases, and neuromuscular diseases.
Isolated symptoms and normal variants
These mainly include snoring and sleep-related groaning.
Snoring is the sound produced by the vibration of surrounding soft tissues when airflow passes through a narrowed upper airway, without other abnormal breathing events.
Sleep-related groaning involves prolonged expiration with a monotonous groaning-like sound after deep inspiration, commonly occurring in young adults. If not accompanied by other symptoms or diseases, it generally does not require treatment.
Since sleep-related breathing disorders primarily occur during sleep, their clinical manifestations are often subtle and easily overlooked or misdiagnosed. It is important to increase awareness of these disorders in clinical practice, carefully inquire about medical history, and diagnose and treat them early to improve the outcomes.
To be continued