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return to: Pediatric Airway

see also: Laryngomalacia

the sound of stridor - click below:

General

1. Stridor is derived from the latin word stridulus, meaning a harsh, shrill sound.
   1. Stridor is described as a high-pitched, monophonic sound derived from the upper airways (compared to the polyphonic sound heard in the lower airway in wheezing).
2. Stridor is a common presentation in pediatric patients and requires prompt evaluation. The first priority in the evaluation of a child with stridor is to determine if there is respiratory compromise.
3. The age of onset, patient history, tonal quality, and relation to the timing of the respiratory cycle is critical to the workup of a patient with stridor.
4. Stridor can be inspiratory, expiratory, or biphasic; this may aid in determining the anatomic location of the airway obstruction.
   1. Inspiratory stridor is more likely to be caused by extrathorasic obstruction to air flow while expiratory stridor is more likely to occur with intrathorasic pathology.

Pathophysiology:

1. Stridor is caused by restriction of airflow through the upper airways. As the radius of the airway deceases by a factor of 1, the area of the airway decreases by a power of 4.
2. The decrease in the area of the airway leads to a proportional increase in velocity due to the Venturi Effect (this same effect can be seen when a thumb is placed over the end of a garden hose). The increase in velocity creates a low-pressure vacuum, exacerbating airway collapse (Bernoulli Principle).
3. This ultimately leads to increased airway resistance, increase effort of breathing, and the clinical finding of stridor.

• ​R- Resistance, r- radius 

4. Due to the smaller diameter of the pediatric airway compared to that of adults, even minor changes can lead to a marked reduction in overall airway calibre.
5. As demonstrated in the diagram below, 1mm of edema in the average adult airway leaves 81% of the cross-sectional area patent, while the same 1mm of edema in a pediatric patient results in only 44% patency.  

Effect of Reductions in Airway Radius in Pediatric Patients vs. Adults 

Etiology

1. In a study of 219 patients with the primary presenting symptom of stridor, the principle diagnosis was due to: (Holinger et al. 1980)
   1. Congenital anomalies: 87%
      1. Laryngeal: 75%
      2. Tracheal: 16
      3. Bronchial: 5%
   2. Traumatic: 5.5%
   3. Infectious: 5.5% 

Anatomy

1. The upper airway can be divided into two anatomic regions: the intrathorasic and extrathorasic airway.
   1. The extrathorasic airway is defined as the region above the superior thoracic aperture including:
      1. The nasopharynx, epiglottis, larynx, vocal folds, and upper segment of trachea.
      2. The narrowest portion of the extrathorasic airway in pediatric patients is the cricoid cartilage.
   2. The intrathroasic airway is defined as the region below the superior thoracic aperture including:
      1. The lower portion of the trachea and the mainstem bronchi.
2. Pathology of the extrathorasic airway is the most common cause of stridor in the pediatric population.
3. Generally, Inspiratory stridor is more likely to be caused by extrathorasic obstruction to air flow while expiratory stridor is more likely to occur with intrathorasic pathology.
4. The pediatric airway differs from that of an adult in four ways: 
   1. The position of the larynx is more anterior (often making direct visualization more difficult).
   2. The vocal cords are shorter and more concave. 
   3. The epiglottis is more 'U-shaped' allowing it to protrude further into the pharynx. 
   4. The cricoid cartilage is the narrowest portion of the airway in patients under 8 years of age.  

Age of Onset

1. Age of onset is a key factor in developing a differential diagnosis for stridor in pediatric patients. Congenital abnormalities of the upper airway typically present in the first few weeks to months of life and are the most common causes of stridor (87%).
   1. Common causes of stridor at birth include: 
      1. vocal cord paralysis, choanal atresia, laryngeal web, or vascular ring.
   2. Common causes of stridor during the first few weeks of life include: 
      1. laryngomalacia, tracheomalacia, and subglottic stenosis
   3. Common causes of stridor from 1-4 years of age
      1. croup, epiglottitis, foreign body aspiration
   4. Stridor occurring in toddlers is most likely due to foreign body aspiration.
   5. Infectious causes can occur in children of all ages.

Acuity of Onset

1. Acute onset of stridor in toddlers should raise the suspicion for a foreign body aspiration.
   1. In some children, stridor will not appear for several due to reactive inflammation of the airway, so a remote history of aspiration should be evaluated in the patient's history.
2. The onset of stridor along with fever, chills, and toxic appearance should allude to an infectious cause of epiglottitis or tracheitis.
3. Chronic stridor may represent an indolent structural process such as laryngomalacia, laryngeal web, or laryngotracheal stenosis.

Past Medical History

1. Evaluation should include aspects of a patient's prenatal and perinatal history and should include:
   1. Prematurity, birth weight, mode of delivery, intubation/ventilation, infections, allergies, congenital syndromes.
      1. A history of intubation should focus on:
         1. Number of intubations, duration of intubation, size of ETT
            1. This will give an idea about potential trauma to the airway. 
            2. A history of intubation is associated with laryngotracheal stenosis or vocal cord paralysis.
   2. A history of atopy may indicate angioedema
   3. A viral prodrome with fevers suggests an infectious etiology.

Physical Exam

1. Vital signs are crucial in identifying patients with impending respiratory collapse.
   1.  Continuous pulse oximetry and close observation is paramount in a patient presenting with acute stridor.
2. Observation of respiratory effort by assessing for: retractions (intercostal, supraclavicular, and subxyphoid), nasal flaring, drooling, cyanosis, agitation.
3. Auscultation helps to determine the possible location of the airway obstruction. Timing of stridor with a respiratory cycle, as well as its tonal quality, is helpful in elucidating the location of the airway narrowing.
   1. Classically, inspiratory stridor indicates epiglottic pathology while expiratory stridor alludes to intrathorasic obstruction.
   2. Biphasic stridor is indicative of a "fixed" obstruction, typically located in the larynx or cricoid cartilage.
   3. In addition to auscultating the chest, the stethoscope should be placed over the patient's nose and mouth in attempts to listen for nasopharyngeal or oropharyngeal obstruction.
4. A weak cry or hoarseness can allude to glottic etiologies

Precipitating Factors: Parents are often astute in noticing what factors aggravate or decrease their child's stridor.  

1. Worsening with agitation: Laryngomalacia or subglottic hemangioma.
2. Worsening while supine: laryngomalacia, tracheomalacia, macroglossia, or micrognathia.
3. Worsening with feeding: laryngomalacia, TE fistula, vascular malformation.

Diagnostic Testing

1. Imaging
   1. AP and lateral radiographs of the neck may be useful in assessing for size of the epiglottis, retropharyngeal profile, and defining general tracheal anatomy.
   2. AP and lateral radiographs of the chest may identify radio-opaque foreign bodies.
   3. Inspiratory and expiratory films may be useful in demonstrating air-trapping due to airway obstruction.
      1. A positive exam will reveal hyperlucency of the obstructed lobe during expiration compared to inhalation due to air-trapping.
      2. May also see a shift of the mediastinum to the side opposite the obstruction.
2. Flexible Fiberoptic Laryngoscopy  
   1. Often, direct visualization of the airway is necessary to confirm a suspected diagnosis.
   2. Flexible laryngoscopy and flexible bronchoscopy is helpful in identifying the exact anatomic location of the obstruction.
3. Direct bronchoscopy under general anesthesia allows for intervention such as removal of foreign bodies or tissue biopsy.
4. Labs
   1. A CBC is helpful if a patient presents with an infectious picture.
   2. Arterial blood gasses may be indicated if a patient present with significant respiratory distress. This allows for an assessment of potential hypoxia and hypercarbia.

References

Leung, A., et al. Diagnosis of Stridor in Children. American Family Physician. November 1999.

Olney, D., Smith, R., et al. Laryngomalacia and its Treatment. The Laryngoscope. 1999. 

Holinger, LD. Etiology of stridor in the neonate, infant and child. Ann Otol Rhinol Laryngol. 1980