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Apparent life-threatening event (ALTE) and Sudden infant death syndrome (SIDS)

Kiran Maski, MD, MPH

Kiran Maski, MD, MPH is an Assistant Professor at Harvard Medical School and child neurologist and sleep medicine specialist at Boston Children’s Hospital. Dr. Maski obtained her undergraduate and medical degrees from the University of Wisconsin. She completed her residency training in pediatrics at Tufts-New England Medical Center and in child neurology at Boston Children’s Hospital. Dr. Maski completed her sleep fellowship at Boston Children’s Hospital. Her clinical and research interests are in sleep disorders in children with neurologic diseases and primary neurologic sleep disorders such as narcolepsy.


An Apparent life-threatening event (ALTE) is an acute, unexpected episode in an infant. ALTE is witnessed by and is frightening to a caregiver due to some combination of symptoms such as:

  • apnea (episode of no breathing)
  • color change (pale or blue)
  • change in muscle tone (stiff or floppy)
  • choking
  • gagging

The term ALTE is non-specific and has a broad range of causes.

Sudden infant death syndrome (SIDS) is defined as an unexplained sudden death of an infant under 1 year of age. Research has not shown that an ALTE is a precursor to SIDS. However, in many infants ALTEs can represent a sign of serious underlying disease.


The accepted definition of ALTE was established at a consensus conference of the National Institutes of Health in 1986 and is “an episode that is frightening to the observer and that 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”. The term is applied only to children less than one year of age. In many cases, such episodes are stopped with stimulation or resuscitation. ALTE is not considered a precursor to SIDS as only 5% of SIDS cases had reported prior ALTEs.

A careful history and physical examination are needed. Additional laboratory testing and other diagnostic tests may be recommended to determine the cause of an ALTE. In over half the cases of ALTEs, no cause is ever found.

Symptoms/Physical Abnormalities

Patients with ALTEs may present with:

  1. color change (pale or blue)
  2. breathing changes (slowed or absent breathing or irregular breathing patterns)
  3. abnormal muscle tone (stiff or floppy)
  4. a change in alertness (less responsive or unresponsive)


The cause of an apparent life-threatening event is broad and ranges from normal phenomena in infants, such as periodic breathing or brief choking and gagging during a feeding to true medical or surgical emergencies. Often the infant appears healthy by the time he or she presents for medical attention;  in as many as half the ALTE cases, no underlying cause of the event is ever identified despite appropriate evaluation. Some infants receive more than one diagnosis.

In cases for which a cause is found, the three most common underlying diagnoses are:

  1. gastroesophageal reflux (GER)
  2. seizure
  3. lower respiratory tract infection

Of these, gastroesophageal reflux is the most common diagnosis, accounting for 30% of diagnoses in case series. However, whether GER is the true etiology of the event is highly uncertain because GER is common in healthy infants. A causal relationship between GER and ALTE is often difficult to establish. GER may be the true cause of the ALTE if the event was characterized by choking or gagging during or immediately after a feeding — meaning, if the episode was characterized by obstructive apnea due to laryngospasm rather than a lack of respiratory effort, or if gastric contents were found in the infant’s mouth or nose during the episode. Furthermore, the clinician will try to distinguish between GER symptoms and feeding difficulties (poor suck, poor swallowing coordination) by history, physical examination and, if necessary, a diagnostic swallow study.

The second most common diagnosis assigned after evaluation for an apparent life-threatening event is seizures, accounting for 15-30% of all cases when febrile seizures are included. Patient with an ALTE attributed to a seizure typically present with a change in muscle tone and no history of choking or gagging during the event. The electroencephalogram (EEG) can be normal immediately after the ALTE. In one study of infants with ALTEs who were diagnosed with seizures, less than half were diagnosed within 1 week of their ALTE presentation and 71% were diagnosed within 1 month.  Abnormal brain magnetic resonance imaging (MRI) and developmental delay were only found in those patients who went on to develop epilepsy.

Lower respiratory tract infections are the third most common diagnosis for ALTE, accounting for approximately 10-20% of all patients. Pertussis (whooping cough) was diagnosed in 6% to 9% of patients presenting to the emergency department with ALTE and Respiratory Syncytial Virus (RSV) infection in up to 15% in some study series.

Apparent life-threatening events can also be the initial presentation of rare metabolic disorders including inborn errors of metabolism. Specific inborn errors of metabolism may present with acute life threatening illness, such as:

  • organic acidurias
  • urea cycle disorders
  • maple syrup urine disease
  • fatty acid oxidation disorders

Non-accidental trauma has been shown to be a missed cause of ALTE so clinicians will evaluate for signs of bruising and trauma and may ask questions regarding concerns of child maltreatment.

Additional Concerns and Complications

Infants who appear with the following symptoms and/or history have a higher chance of having a more serious medical condition that caused the ALTE.:

  • lethargy
  • breathing problems
  • frequent vomiting
  • cannot take oral intake
  • signs of trauma
  • more than one ALTE in a 24-hour period

Diagnostic Investigations

Physical Examination

The physical examination usually begins with a general assessment of appearance and complete vital signs, including blood pressure and blood oxygenation measurements. Height, weight, and head circumference should also be measured and compared to norms. Attention will be paid to the neurologic, cardiac, and respiratory systems as well as possible signs of abnormal features of the child (dysmorphic findings suggesting a genetic disease) or signs of abuse such as bruising or a bulging soft spot on the head.

Diagnostic Testing

There is no widely accepted consensus for diagnostic testing in infants presenting with ALTE; instead testing undertaken is dependent on the individual patient’s history and physical exam. Some routine lab testing may include:

  • evaluation of electrolytes
  • blood sugar
  • blood counts to evaluate for infection or anemia
  • blood venous gas to measure concerns of breathing problems
  • urine analysis to look for a urinary tract infection

Other metabolic labs may be requested to evaluate for more rare conditions such as inborn errors of metabolism. Additional tests, including an electrocardiogram (ECG) for cardiac arrhythmias, electroencephalogram (EEG) for seizure activity, and chest x-ray (for respiratory disorders), may be performed. Depending on the history and examination, the clinicians may request imaging of the brain.

Therapeutic Intervention/Home Monitoring

Treatment is based on the underlying cause of the apparent life-threatening event. The use of home cardiorespiratory monitors which alert caregivers to bradycardia or absence of chest wall movement (and in some cases to blood oxygen desaturation) is controversial. There is no evidence that home monitors prevent SIDS or provide a therapeutic benefit after an ALTE. However, some experts agree that on a case-by-case basis such monitors may be appropriate for infants with a high risk of apnea, bradycardia, or desaturations due to prematurity or for infants with unstable airways, dysregulated breathing, or chronic lung disease.


The American Academy of Pediatrics (AAP) Task Force on sudden infant death syndrome has stated that there is no evidence that ALTE is a precursor to SIDS. In fact, the risk factors for SIDS and ALTE appear to be largely distinct:

  1. ALTEs tend to occur during the day and SIDS at night
  2. ALTEs peak in the first two months of life compared to SIDS between two and four months of age
  3. SIDS is more common in boys; ALTE occurs equally among genders

While SIDS has several modifiable risk factors, particularly not placing an infant in the prone (on stomach) sleeping position, the same has not been found for ALTEs. In fact, while the 1994 recommendation from the AAP that infants be placed supine (on back) for sleep led to a dramatic reduction in SIDS rates, no reduction in ALTE rates was noted.

Risk of death

The risk of death in infants presenting with ALTE ranges from 0.5% to 1.1% in 12 month to 5 year follow-up periods in studies which tracked the outcomes of patients presenting with a first ALTE.  Causes of death included:

  • pneumonia
  • an underlying neurologic disorder
  • child abuse
  • SIDS

Risk factors for a serious underlying diagnosis include a history of multiple ALTEs and suspected child maltreatment. Infants with a history of prematurity are at increased risk for subsequent severe events likely due to immature respiratory control. These infants  are also possibly at increased risk of death, although in some studies this risk appears to return to baseline once the infant reaches between 43 and 48 weeks post-conceptional age.


The risk of sudden infant death syndrome is minimized with

  • supine (on back) sleep position
  • the use of a firm sleep surface without loose bedding
  • avoidance of overheating
  • avoidance of tobacco product exposure in the infant’s environment

Several studies have concluded that breastfeeding has a protective effect against SIDS. Pacifier use also appears to have a protective effect against SIDS, reducing risk by two-fold or even greater in several studies. Pacifiers may be protective due to alterations in jaw and upper airway positioning during sleep.

As mentioned earlier, there is no evidence that home cardiorespiratory monitors are protective against SIDS or recurrent ALTEs.

Caretaker Augmentation

Helpful websites for caretakers include:


  • Acute life-threatening event (ALTE): Not a specific diagnosis but a description of a sudden occurrence of certain alarming symptoms such as prolonged periods of no breathing (apnea), change in color or muscle tone, coughing, and gagging in children under 1 year of age
  • Apnea: Episode of no breathing
  • Bradycardia: slow heart rate
  • Cyanosis/cyanotic: Color change in which a child turns blue or turns blue around the mouth (the latter is called perioral cyanosis)
  • Echocardiogram: An ultrasound of the heart to look at heart structure
  • Electrocardiogram (EKG): A cardiac study that measures the electrical activity of the heart
  • Electroencephalogram (EEG): A brain wave study performed to evaluate for seizure activity; can vary in time depending on how ordered <1 hour to 72 hours)
  • Erythematous: Red-appearing
  • Gastro-esophageal reflux disease (GERD): Occurs when stomach acid or contents flows back into the food pipe (esophagus); high acid content of this backflow (reflux) can irritate the lining of the esophagus and even the upper airway
  • Hypotonia or low tone: Floppy appearing
  • Pallor: Pale/whitish in color
  • Plethoric: Swollen-appearing
  • Sudden infant death syndrome (SIDS): An unexplained death, usually during sleep, of a seemingly healthy baby less than a year old.


  1. Fu LY, Moon RY. Apparent life-threatening events: an update. Pediatrics in review 2012;33(8):361-368;quiz 368-369.
  2. McGovern MC, Smith MB. Causes of apparent life threatening events in infants: a systematic review. Archives of disease in childhood 2004;89(11):1043-1048.
  3. Moon RY. SIDS and other sleep-related infant deaths: expansion of recommendations for a safe infant sleeping environment. Pediatrics 2011;128(5):e1341-1367.

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