SLEEP DISORDERED BREATHING

SLEEP DISORDERED BREATHING

The Obstructive Sleep Apnea Spectrum

In normal breathing, the air flows smoothly past the soft tissues at the back of the throat (such as the soft palate, uvula and tongue). Although the muscles are more relaxed during sleep, the airway remains open.

Snoring is due to the vibration of the pharyngeal soft tissue as air passes through a narrowed airway, which results in turbulent flow. Primary (or benign) snoring is simply turbulent breathing that does not lead to arousals, airflow interruptions or oxygen desaturations.

In contrast, Upper Airway Resistance Syndrome (UARS) occurs when the pharynx partially collapses due to negative pressure during inspiration, reducing the amount of air flow, which may happen with or without the presence of snoring. This is not associated with apneas, hypopneas or significant oxygen desaturations, but it triggers micro arousals due to the increased respiratory effort. Even when they do not cause a full, conscious awakening, these micro arousals interfere with the normal sleep cycle architecture, which can have a negative impact on systemic health, daytime symptoms and performance. The night-long struggle to breathe maintains the body in a "fight or flight" state, which keeps the blood pressure and heart rate elevated and does not allow normal restorative sleep processes to take place. As a result, patients do not feel rested in the morning and manifest many of the same symptoms as patients with OSA. The upper airway collapsibility of patients with UARS is intermediate between that of OSA and normal patients.

Obstructive Sleep Apnea (OSA) represents the most severe end of the pathology spectrum: it consists of repeated, transient episodes of apnea (absence of breathing for 10 seconds or more) or hypopnea (a 30% or more reduction in airflow for 10 or more seconds combined with a 4% or greater oxygen desaturation), due to negative inspiratory pressure as well as the partial blockage of the airway by the tongue and other soft tissues at the back of the throat. The resultant hypoxia leads to an increase in respiratory effort, which in turn triggers an arousal, restoring pharyngeal muscle activity, opening the airway and allowing normal breathing to resume, until the onset of sleep initiates the cycle again. This can occur hundreds of times a night, with some episodes lasting over one minute. If these events occur an average of more than 5 times per hour (AHI >5), the condition is classified as pathological.

The AHI index is the number of apneas or hypopneas that the patient experiences per hour. This number determines the severity of the condition and is a key determinant in deciding which treatment course is most appropriate.

Severity Classification of OSA:

It is estimated that 24% of men and 9% of women have some form of obstructive apnea.

Untreated snoring and UARS can progress to obstructive apnea, as the continual mechanical trauma caused by the vibration of the soft tissues at the back of the throat leads to inflammation, which results in a gradual enlargement of the tissues and progressive narrowing of the airway.

Central and Mixed Sleep Apnea

Not all cases of sleep apnea are caused by airway obstruction.

Central sleep apnea is a condition in which an instability in the breathing control mechanisms results in a reduced ventilatory drive. Common causes include heart failure, neurologic abnormalities (especially those involving the brainstem), alveolar hypoventilation disorders and idiopathic central apnea syndrome. Patients with idiopathic CSA often complain of insomnia and awakening during the night. Cheyne-Stokes respiration, or periodic breathing, occurs in a high percentage of patients with heart failure and is an independent risk factor for death. Treatment options for CSA include supplementary oxygen, CPAP and medical therapy (such as acetazolamide).

In about 15% of sleep apnea patients, both obstructive and central apnea are present (Complex/Mixed Sleep Apnea). Typically these are patients presenting with OSA in whom CPAP therapy appears to trigger central apneas while correcting the obstructive form of the disease, which tends to cause significantly disrupted breathing and sleep and more CPAP interface problems than normal. Although it is not possible to predict such treatment-emergent apneas at the diagnostic stage, many of these tend to spontaneously resolve within a few months.

Obesity Hypoventilation Syndrome

Obesity Hypoventilation Syndrome is characterized by decreased alveolar ventilation resulting in poor blood oxygen saturation, despite the absence of obstructive episodes. Typically this syndrome is found in males over 50 presenting with exercise-induced shortness of breath. Commonly associated conditions include diabetes, hypertension and heart disease. Correction of co-existing obstructive apnea (if any) does not eliminate all subjective symptoms in these cases and the patient may present with residual daytime sleepiness - therefore it is important to return for evaluation with the sleep physician in all cases where such symptoms persist. Treatment of OHS usually involves nasal intermittent positive pressure ventilation (NIPPV) .