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Stridor is a harsh, high-pitched musical sound caused by oscillation of a narrowed airway while breathing, indicating partial airway obstruction. It occurs most commonly during inspiration, but may be biphasic or expiratory based on the anatomic location of the narrowing. Stridor is an important clinical finding that warrants investigation, as the etiologies range from benign, self-limited disease to severe illness leading to a rapidly progressive airway obstruction.

The differential diagnosis of stridor varies widely based on age of presentation. Stridor in neonates is nearly always caused by anatomic abnormalities, while older children and adolescents with stridor generally have underlying infectious, inflammatory, or environmental triggers.

Laryngomalacia is the most common cause of stridor in infants, accounting for approximately 60% of cases. In children with laryngomalacia, stridor begins within the first few weeks of life. It is caused by inward collapse of the supraglottic structures during inspiration; therefore, symptoms are worse with agitation, crying, and supine position. Stridor worsens over the first few months of life and begins to resolve around 6 months of age.

Subglottic stenosis is another common cause of stridor in infants. Stenosis can be congenital, or acquired from airway manipulation, such as endotracheal intubation. Stridor caused by subglottic stenosis is often biphasic and may manifest during the patient's first upper respiratory tract infection.

Vocal cord paralysis accounts for 10% of all congenital laryngeal lesions and is another common cause of stridor in infants. Vocal cord paralysis may be congenital, or acquired via birth trauma or as a complication of neonatal surgery, such as congenital heart disease or tracheoesophageal fistula repair. It is often associated with other abnormalities, such as Arnold–Chiari malformation, myelomeningocele, or hydrocephalus. Children with unilateral vocal cord paralysis may present with a weak cry or aspiration. However, with bilateral vocal cord paralysis, the cry may be normal and patients present with stridor and airway obstruction. Other causes of stridor in infants attributed to laryngeal, tracheal, vascular, or craniofacial abnormalities are listed in Table 14–1.

Table 14–1. Differential Diagnosis of Stridor

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