Classification and definition of respiratory disorders during sleep.

Growing interest in sleep disorders has led to increased research in this direction. Increasingly sophisticated instrumental tests have disclosed new breathing patterns and complex syndromes. The initial identification of obstructive apneas was followed by studies characterizing snoring, hypopneas, respiratory effort-related arousal and flow limitation events. Since Pickwickian syndrome, an historical term currently deemed obsolete and confusing, sleep investigations have differentiated secondary hypoventilation, central hypoventilation and syndromes resulting from narrowing of the upper airways (snoring, upper airway resistance syndrome, obstructive sleep apnea-hypopnea syndrome). Inevitably, this has given rise to some confusion in the classification of events and syndromes which recent studies have attempted to clarify.