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What is Apnea?
Apnea is the complete cessation of breathing for greater than 10 seconds. Apnea can be obstructive or central. In obstructive apnea there is absence of airflow but the respiratory muscles continue to exert to inhale air but in the central apnea there is no respiratory muscle effort despite the absence of airflow. Majority of the patients have both elements of obstructive and central sleep apneas.

What is Hypopnea?
A hypopnea is defined as a decrement in airflow of 50 percent or more associated with a 4 percent fall in oxygen saturation and/or electroencephalographic (EEG) arousal.

What is RERA?
Respiratory effort related arousal are arousals that do not satisfy the definitions for apnea or hypopnea but are characterised by repetitive respiratory efforts to overcome the cessation of breathing thereby leading to the arousal of the individual from sleep.

What are the types of sleep apneas?
Obstructive Sleep Apnea
Central Sleep Apnea
Obesity Hypoventilation Syndrome
Upper Airway Resistance Syndrome

Of these Obstructive sleep apnea is the most commonest. In some individuals a combination of sleep disorders may be present resulting in diagnostic and therapeutic difficulties.

What is Obstructive Sleep Apnea?
Obstructive sleep apnea is characterized by cessation of breathing due to obstruction in the upper airway caused by reduction in the tone of the pharyngeal and other upper airway supportive muscles. This leads to sleep related reduction in the blood oxygen content. Individuals suffering from OSA generally have loud snoring, fragmented sleep, daytime sleepiness. In addtion they are at increased risk for hypertension, diabetes, coronory artery disease, stroke.  

What is the criteria for the diagnosis of Obstructive sleep apnea syndrome?
In order to diagnose an individual with obstructive sleep apnea, the following criteria should be satisfied.

Criterion A or B, plus criterion C to be diagnosed with OSAS:

A. Excessive daytime sleepiness that is not explained by
other factors
B. Two or more of the following that are not explained
by other factors:
 Choking or gasping during sleep
 Recurrent awakenings from sleep
 Unrefreshing sleep
 Daytime fatigue
 Impaired concentration
C. Overnight monitoring demonstrates 5 to 10 ormore
obstructed breathing events per hour during sleep or
greater than 30 events per 6 hours of sleep. These
events may include any combination of obstructive
apnea, hypopnea, or respiratory effort–related
arousals.

How is the severity of the obstructive sleep apnea measured?
The standard measurement for the severity of the OSA is based on the AHI or Apnea-Hypopnea Index.It is a measure of the number of apnea and hypopneas per hour of sleep. Generally an AHI greater than 5 to 10 is suggestive of obstructive sleep apnea. As the AHI index increases, the severity of the sleep apnea also increases. While an AHI of 5-15 indicates mild sleep apnea, 15-30 indicates moderate sleep apnea and greater than 30 indicates severe sleep apnea.

What is central sleep apnea?
Central sleep apnea is caused by repetitive cessation of breathing due to the absence of effort by the respiratory muscles. These are due to central causes in the brain and the nervous system. The common causes of central sleep apnea is stroke and heart failure. During sleep study the central sleep apnea can be diagnosed by the absence of thoracic muscle movement and along with the cessation of airflow.

What is the criteria for the diagnosis of central sleep apnea?
In order to diagnose central sleep apnea, the following criteria must be satisfied.
A. At least one of the following symptoms that is not
explained by other factors:
 Excessive daytime sleepiness
 Frequent nocturnal arousals/awakenings
B. Overnight monitoring that demonstrates 5 to 10 or
more central apneic events plus hypopneic events
per hour of sleep.
C. Normocarbia while awake (PaCO2 less than 45 mmHg).

What is Pickwickian Syndrome?
Pickwickian syndrome is a nickname for Obesity Hypoventilation syndrome. It is based on a character in charles dickens Posthumous papers of the pickwickian club. The character is a obese boy with loud snoring and excessive sleepiness.For the clinical diagnosis of Obesity hypoventilation syndrome, the individual should be morbidly obese with BMI greater than 40kg/m2 and hypercapnia of greater than 45mmHg during wakefulness. To differentiate OHS with COPD, pulmonary function testing is done. In OHS restrictive pattern is seen while in COPD obstructive pattern is seen. People with OSA do not have hypercapnia during wakefullness and this can be used to differentiate with OHS (Hypercapnia - excessive blood carbondioxide)

What is Upper Airway Resistance Syndrome?
In UARS, there is increased negative intrathoracic pressure that prevents the easy airflow in the upper airway. This lead to arousal from sleep and hence disturbed fragmented sleep. The sleep study would not reaveal presence of apnea or hypopnea. The eosophageal manometry usually reveals the increased pressure in the upper airways. Some researchers believe that URAS is a milder form of sleep apnea syndrome and in some individuals may progress to obstructive sleep apnea syndrome.