Stidor is a harsh, vibratory sound, caused by turbulent airflow through an obstructed upper airway.

• Stridor is best heard over the upper airways
• It usually occurs on inspiration → inward pressure exerted on upper airway to collapse, further diminishing the diameter of airways outside the chest. It doesn't usually occur on expiration, because the column of air pushing up from below opens the upper airway, diminishing obstruction to flow
• Stridor however, can occur on inspiration → if obstruction in the upper airways is so severe severe, or it starts involving the chest cavity (e.g. the portion of the trachea down there), even the column of air pushing up in expiration may not be able to assist

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Patient information

Wait ... I understand how with wheeze, you hear it on breathing out. Because when you breath out, you push air out of your body so your chest becomes smaller. But on inspiration, why does this cause collapse? Don't you expand?
You do. But you expand the LOWER airways . The upper airways however, collapse .

I see. And because stridor is generated by airways OUTSIDE the chest, it is on inspiration that you hear it - because this is when this part of the airway collapses?
Exactly !

• It is caused by obstructions of portions of the airway outside the chest cavity
• Airflow velocity within the respiratory tree is higher than normal, causing a vibratory sound

• Infectious/inflammation:
  • Epiglottitis, which was historically caused by Haemophilus influenzae, now uncommon due to the Hib vaccine
  • URTI (croup), croup is the most common cause of acute stridor in kids 6mo-2yo
  • Bacterial tracheitis, which can be secondary to a viral infection in the trachea (e.g. croup)
  • Allergies
  • Laryngitis
  • Abscess on the tonsils
  • Swollen tonsils or adenoids (e.g. with tonsilitis)
  • Secretions, e.g. phlegm/sputum
• Anatomic:
  • Laryngomalacia, the most common cause of stridor
  • Foreign body inhalation, most common cause of acute stridor in kids 1-2yo
  • Vocal cord dysfunction, the next most common cause of infant stridor
  • Tracheomalacia, the most common cause of expiratory stridor
  • Tumor (e.g. vocal cord cancer)
  • Airway injury, including due to Dx tests (bronchoscopy, laryngoscopy), neck surgery, use of a breathing tube
  • Swelling of the neck/face

• Stridor occurs on inspiration → obstruction in upper airways
• Stridor occurs on expiration → severe obstruction in upper airways
• "Tripoding", positioning their elbows/hands on their knees, leaning forward, with their neck extended, to give accessory muscles of respiration their most advantageous position, and to open up the upper airways → upper airway obstruction
• Barking cough → croup
• Worsening at night → croup
• Previous URTI → croup, bacterial tracheitis
• Coryzal → infection
• Fever → infection (including ruling out tracheitis, a medical emergency)
• Acute onset → foreign body aspiration, infections e.g. croup, epiglottitis
• Long duration → anatomical (laryngomalacia, laryngotracheal stenosis)
• Age of onset:
• • Birth → congenital anatomical (down syndrome)
  • 4-6 weeks → laryngomalacia
  • 1-4yo → croup, epiglottitis, foreign body aspiration
• Previous stridor → if no, rules out anatomical (e.g. laryngomalacia)
• Difficulty swallowing, causing drooling → upper airway obstruction, especially epiglottitis, foreign body in the esophagus
• Hoarseness of void → croup, vocal cord paralysis
• Recently playing with a small object prior? → foreign body
• Congenital abnormalities, or neck surgeries → anatomic
• Haemophilus influenza vaccine → acute epiglottitis is unlikely

• CXR of chest and/or neck → anatomic causes? foreign bodies?
• Flexible laryngoscopy → used to view down to the larynx
• Bronchoscopy → visualize below the larynx

• ABC's, including oxygenation

• Breath sounds
• Wheeze (due to narrowing of airways INSIDE the chest cavity, cf. stridor which is narrowing of airways OUTSIDE the chest cavity)