Apnea is defined by the cessation of respiratory airflow. The length of time necessary to be qualified as a true apneic event has changed dramatically over the last few decades: 2 minutes in 1956,  1 minute in 1959,  30 seconds in 1970,  and 20 seconds or shorter if associated with bradycardia or cyanosis in 1978.  The reduction of the duration in the definition of apnea reveals doctors’ desire to intervene early enough to avoid systemic consequences.
The 3 main categories of apnea are central, obstructive, and mixed. Central apnea is a result of inadequate medullary responsiveness and thus no or poor muscle coordination for breathing. Obstructive apnea is when there is an obstruction of the airway passages and therefore poor to no air exchange. Often times with obstructive apnea, there is a vigorous inspiratory effort but it is ineffective. Mixed apnea refers to an episode where combinations of both central and obstructive forces are involved.
Infant apnea is defined by the American Academy of Pediatrics as "an unexplained episode of cessation of breathing for 20 seconds or longer, or a shorter respiratory pause associated with bradycardia, cyanosis, pallor, and/or marked hypotonia."  Apnea is more common in preterm infants. Apnea of prematurity requires a specific assessment and treatment and is not discussed in full in this article. Apnea is rare among full-term healthy infants and, if present, usually indicates an underlying pathology.
The ED physician may not experience many patients with pure apneic events but more likely will have an infant's caregiver come in and report that his or her child appeared to stop breathing, changed color, or became limp. This is an apparent life-threatening event (ALTE).
ALTE was defined by the 1986 National Institutes of Health Consensus Development Conference on Infantile Apnea and Home Monitoring as follows:
"An episode that is frightening to the observer and is characterized by some combination of apnea (central or occasionally obstructive), color change (usually cyanotic or pallid but occasionally erythematous or plethoric), marked change in muscle tone (usually marked limpness), choking or gagging. In some cases, the observer fears that the infant has died. Previously used terminology, such as 'aborted crib death' or 'near miss SIDS' should be abandoned as it implies a, possibly, misleading close association between this type of spell and SIDS."
ALTE is not a diagnosis. It is the description of an event or a presenting complaint. The potential underlying diagnoses run the spectrum from benign to extremely serious. The challenge with the assessment of the patient who experienced an ALTE lies in scrutinizing the patient's history to discern first, if the event was in fact a true episode of apnea, cyanosis, or tone change, and to then use the physical examination findings and various diagnostic studies, if needed, to deduce the reason the event took place.
Apnea is a symptom that has large possibility of etiologies. In this article, some of the major etiologies of apneic events that an ED physician will encounter are discussed, namely, apnea of prematurity, ALTE, obstructive sleep apnea, and miscellaneous forms of apnea that are toxin mediated, secondary to head trauma, or caused by infections.
Apnea refers to a cessation of respiratory airflow and has 3 major types.
Central apnea occurs when there is a lack of respiratory effort due to either a cessation of output from the central respiratory centers or the inability of the efferent peripheral nerves and respiratory muscles required for oxygenation and ventilation to receive or process the signals from the brain. This can be due to immaturity of the system, as seen in certain premature infants, who have a decreased response to hypercapnia (increased carbon dioxide levels). Patients with central apnea have no respiratory effort. This can be seen by a lack of chest wall movement and no breath sounds will be appreciated on auscultation. 
Another cause of central apnea is head trauma, as it may interfere with the afferent and efferent signals of the central respiratory center. Head trauma may be the result of abuse and must always be considered in the apneic pediatric patient without an obvious cause. Toxin-mediated apnea is another form of central apnea, as it may cause central nervous system depression and decrease the respiratory drive.
Obstructive apnea, as the name suggests, results from attempts to breathe through an occluded airway. Obstructive sleep apnea (OSA) is the most common form of obstructive apnea in children. Obstructive sleep apnea is on the sleep-disordered breathing (SDB) spectrum. The sleep-disordered breathing spectrum includes snoring, obstructive hypoventilation and, at its extreme, obstructive sleep apnea. Obstructive sleep apnea results from a number of factors, including the patient having smaller airway patency (may be congenital but also includes adenoid and tonsillar hypertrophy) coupled with a decreased neuromuscular tone of the pharyngeal dilator muscles during sleep.
Additional conditions may put an individual at risk for obstructive sleep apnea and include mucopolysaccharidosis, craniofacial anomalies, and obesity. Other causes of obstructive apnea are an aspirated foreign body and vocal cord paralysis.
Rosen et al ascertain the prevalence of and risk factors for obstructive sleep apnea syndrome in children with sickle cell anemia. The study concluded that the prevalence of obstructive sleep apnea syndrome in children with sickle cell anemia is higher than in the general pediatric population. 
Mixed apnea has characteristics of both central apnea and obstructive apnea. Examples can include a patient with a partial obstructive apnea (due to adenotonsillar hypertrophy) who has undergone sedation (causing central apnea), or a premature infant with central apnea who has an obstruction due to nasal congestion brought on by a viral illness. Gastroesophageal reflux is thought to cause this mixed picture as regurgitated gastric contents may occlude the airway and block laryngeal chemoreceptors to send signals for dilation to the brain.
An inverse relationship is found with apnea of prematurity for both birth weight and gestational age. Because the technology in the area of NICU care is increasing, the number of infants who are surviving ultra-premature births has expanded and therefore the number of children experiencing apnea of prematurity is also growing.
The true incidence of apparent life-threatening event (ALTE) is actually unknown, but it may account for 2.3% of hospitalized children, and between 0.5% and 0.6% of all newborns. 
Obstructive sleep apnea (OSA) has been previously shown to occur in almost 2% of the pediatric population, but that number is rapidly increasing secondary to the explosive incidence of obesity in the United States and is thought to be closer to 5-10%. In older pediatric textbooks, the classic picture of a patient with obstructive sleep apnea was of a patient who was thin and may have even been considered to have failure to thrive. However, currently, the typical patient with obstructive sleep apnea is significantly overweight.
One study has shown that the incidence of apnea can range from 1.2-23.8% in hospitalized infants with respiratory syncytial virus (RSV) bronchiolitis; however, the populations in the studies included premature and neuromuscularly impaired infants. 
The worldwide incidence of ALTE is unknown. One report from Sweden places the incidence of apnea during the first 4 days of life at 0.35 case per 1,000 population. 
As discussed in Pathophysiology, there are different types of apnea, and each has its own unique set of possible causes. The outcome may vary significantly from one cause to another.
Apnea of prematurity frequently persists beyond term gestation in infants delivered at prior to 28 weeks' gestational age. These persistent apnea events may contribute to prolonged hospitalization and mortality. Clearly, if a premature infant with apnea is not taken care of in an appropriate medical setting, the morbidity and mortality can be significant.
The morbidity and mortality rates for the patient who has had an ALTE is difficult to assess due to the multiple and varied etiologies ranging from benign to the severely ill.
Perhaps the most important question regarding patients who have had an ALTE is the degree to which they are associated with sudden infant death syndrome (SIDS).
- Recent studies have shown that 4-13% of SIDS cases had a history of apnea.
However, multiple studies fail to demonstrate a link between ALTE and SIDS.
- The dramatic reduction of SIDS with the advent of the Back to Sleep program initiated by the American Academy of Pediatrics (AAP) in 1994, which has parents putting children to sleep supine, has not been associated with an appreciable reduction in the incidence of ALTE. [11, 12]
- More than 80% of SIDS cases occur between midnight and 6 am,  while more than 80% of ALTEs occur between 8 am and 8 pm. 
- In a recent study with the single largest ALTE cohort with extensive follow up, it was demonstrated that, for patients who present with ALTE and appear well upon presentation, 1 in 9 will subsequently suffer from child abuse, and 1 in 20 will develop an adverse neurological outcome (it is important to note that most of these patients did present again within 1 month of the original incident). 
Among the various outcomes, psychological morbidity certainly occurs. Many children who experience an ALTE are ultimately given a home monitoring device. The period of monitoring may last a year or more. This monitoring can be stressful for the families because a family member must be relied on to respond to monitor alarms at all times.
Untreated obstructive sleep apnea can result in failure to thrive, cor pulmonale, and loss of intellectual quotient.
Apnea from miscellaneous sources, such an overwhelming sepsis, various infectious agents (RSV, influenza, pertussis, human metapneumovirus), toxic agents, or trauma, all carry very significant morbidity and mortality rates.
Most studies do not show a gender difference in the incidence of apnea of prematurity.
For ALTEs, the male-to-female ratio is variable, but, in some studies, it has been as high as 2:1.
Sleep-disordered breathing continuum, of which OSA is on, appears to have a male predilection.
The risk for apnea of prematurity is clearly linked to a younger gestational age at birth as well as lower birth weights. Almost all infants born less than 28 weeks’ gestation suffer from apnea. For infants born at 30-31 weeks, the risk is approximately 50%, and, for those born at 32-33 weeks, the risk is about 14%. The risk for those born at 34-35 weeks is 7%. 
The typical infant presenting after an ALTE is 8-14 weeks. Approximately 7% of these infants were born prematurely.
OSA can occur at any age; however, its incidence is bimodal. It has its first peak at age 2-6 years and then again later in adulthood.
Infections such as bronchiolitis, classically caused by RSV but also influenza and human metapneumovirus, can cause apnea. Acquiring the respiratory illness at a younger age, younger than 3 months, puts the patient at much higher risk for apnea. 
A bimodal distribution exists for apnea caused by ingestions. Accidental ingestions most commonly occur after children are capable of a pincer grasp, approximately 9 months, until early childhood, whereas nonaccidental ingestions or illicit drug behavior occurs during adolescence.
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