Chapter 14. Stridor

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.

Stridor in older infants and children is often caused by acquired infectious or inflammatory conditions. These conditions can cause airway edema, which causes airway narrowing and an exponential increase in airway resistance. Viral croup, or laryngotracheobronchitis, is the most common cause of acute airway obstruction in older infants and children. It can be caused by a number of viral infections, the most common being parainfluenza, and occurs most commonly in patients aged 3 months to 5 years. Patients begin with upper respiratory tract symptoms, such as rhinorrhea, pharyngitis, and a low-grade fever 1–3 days prior to showing signs of airway involvement. They then develop a hoarse voice, “barking” cough, and inspiratory stridor. Symptoms resolve within 1 week. Older children are generally not as severely affected as younger children.

Though increasingly uncommon in the post-Haemophilus influenzae type B (Hib) vaccine era, epiglottitis is a severe, life-threatening cause of stridor and should remain on the differential for patients presenting with acute airway obstruction. The incidence of epiglottitis has fallen dramatically since the advent of the Hib vaccine; however, cases of epiglottitis are still seen in unimmunized patients. Causes of epiglottitis in immunized patients include Staphylococcus aureus, Streptococcus pyogenes, and Streptococcus pneumoniae. The course often begins with pharyngitis, fever, and rhinorrhea, but rapidly progresses within hours to severe respiratory distress. Patients are toxic-appearing and may lean forward in a “tripod” position with their necks extended as a position of maximal comfort. Drooling can be seen because of inability to swallow. Stridor is a late finding and indicates severe airway obstruction.

Foreign body aspiration is important to consider in young children with acquired airway obstruction. Acute onset stridor occurring without other associated symptoms should raise concern for either esophageal or tracheal foreign body aspiration. Children aged 6 months to 3 years account for most cases of aspiration, though older children with developmental delay or behavior disorders are also at risk. A history of coughing, choking, or gagging should always be taken seriously even if the child does not present with stridor or respiratory distress, as some patients may have an asymptomatic interval before developing complications, such as mucosal erosion, secondary infection, or airway obstruction.1

A careful history and physical examination can help narrow the differential diagnosis of stridor. Key components of the history include the age and acuity of stridor onset, as well as the severity of symptoms. It is important to identify whether stridor is inspiratory, expiratory, or biphasic. Inspiratory stridor is caused by narrowed airway diameter during the negative pressure of inspiration and indicates a lesion above the vocal cords, while expiratory stridor suggests pathology of the intrathoracic airways. Biphasic stridor often indicates a fixed lesion that is unchanged by alterations in airway pressure.

Onset of stridor shortly after birth suggests vocal cord paralysis. In contrast, laryngomalacia is typically noted after 4 weeks of life. Previous airway manipulation, such as prolonged endotracheal intubation, predisposes to subglottic stenosis. Associated feeding difficulties, coughing during or shortly after feeds, and poor weight gain may indicate gastroesophageal reflux, poor pharyngeal tone (e.g., developmental delay, neuromuscular disorder), laryngeal cleft, tracheoesophageal fistula, or vascular ring. A hoarse cry with or without mild stridor may occur with unilateral vocal cord paralysis; bilateral vocal cord paralysis often results in severe obstruction. In older children, fever, cough, respiratory distress, drooling, and symptoms of upper respiratory tract infection, such as rhinorrhea or sore throat suggest croup, epiglottitis, or parapharyngeal abscess. Pain with neck extension also suggests parapharyngeal abscess.

The physical examination begins by observing the patient's general appearance at rest with attention to signs of respiratory distress and position of greatest comfort. Children with severe laryngeal obstruction usually hyperextend their neck as they attempt to straighten the upper airway and maximize its patency. In a neonate, certain physical examination maneuvers provide accurate assessment of the functional anatomy of the airway. Relief of stridor with placing the infant in prone position suggests supraglottic (or higher) obstruction as may occur in children with laryngomalacia, Pierre–Robin sequence (from micrognathia), or Down syndrome (from macroglossia). The latter two conditions are also relieved by pulling the infant's mandible and tongue forward. Nasal catheters should be passed to determine the patency of the nasopharyngeal airway (i.e., choanal atresia).

Any cause of neck swelling (especially diphtheria) or cervical lymphadenopathy (particularly with tuberculosis and Epstein–Barr virus infection) may cause stridor as a result of extrinsic airway compression. Cutaneous findings may be helpful, as café au lait macules raise suspicion for neurofibromatosis and associated airway neurofibromas. Cutaneous hemangiomas located in the beard distribution (any point along the preauricular, mandibular, perioral and submental areas) are also associated with a higher risk of subglottic hemangiomas that can cause stridor.

For many common causes of stridor, the diagnosis can be made clinically, without need for laboratory or radiographic studies.

Radiography

In most instances, the use of radiographic studies is not necessary in children with suspected croup. However, if the diagnosis is in question, anteroposterior and lateral neck radiographs should be obtained. In the anteroposterior view the “steeple sign,” the classic finding in croup, reveals narrowing of the subglottic air column. Widening of the hypopharynx is observed in the lateral view. This finding is not present in all children with croup and can be found in some healthy children depending on the phase of inspiration. Lateral views can also reveal widening of the prevertebral soft tissues, which suggests retropharyngeal abscess (Figure 14–1) or cellulitis. Though epiglottitis can be seen on a lateral neck radiograph as a “thumb sign,” this should not be used for diagnostic purposes (Figure 14–2). When epiglottitis is suspected, the child should be immediately transported to the operative suite for endotracheal intubation.

Chest radiographs can be useful in suspected foreign body aspiration. Radio-opaque objects, such as coins, can be seen both in the esophagus, and in the airway. Radiolucent objects will not be directly visualized; however, ipsilateral hyperinflation, particularly that which persists on a lateral decubitus view (with the affected side downward), suggests a foreign body in the mainstem bronchus. Additionally, chest radiographs may reveal evidence of airway bowing, as seen with either a double or a right-sided aortic arch.

Airway Fluoroscopy

Dynamic study of the airway with fluoroscopy can be helpful in localizing obstruction in patients with stridor. Fixed lesions (e.g., subglottic webs) are visualized throughout the respiratory cycle while functional lesions (e.g., laryngomalacia) change. Fluoroscopy (1) allows evaluation of hemidiaphragm movement; (2) allows assessment for air trapping; and (3) requires less patient cooperation than plain radiographs. However, when an airway foreign body is likely, airway fluoroscopy is less useful since endoscopy will be required regardless of fluoroscopy results.2

Flexible Fiberoptic Laryngoscopy

Flexible fiberoptic laryngoscopy provides visualization from the nasopharynx down to the vocal folds but cannot accurately evaluate the subglottis or trachea. This test is useful in diagnosing laryngomalacia as well as assessing for nasopharyngeal inflammation consistent with severe gastroesophageal reflux. For more direct assessment of the subglottic region, direct laryngoscopy and bronchoscopy with general anesthesia is required.

Barium Swallow

Barium swallow has been used for diagnosis of vascular rings in the past; however, with the advent of advanced CT angiography and MRI techniques, these newer studies are preferred when vascular rings are suspected.

Other Studies

Laboratory studies are rarely indicated in the evaluation of stridor. For patients in whom viral laryngotracheobronchitis is suspected, a viral respiratory PCR panel can be useful in identifying a viral etiology. Additionally, for patients with concern for bacterial tracheiits, culture of the exudate can be helpful in determining the bacterial pathogen.

The gold standard for evaluation of stridor is direct visualization of the airway by an otorhinolaryngologist; however, not all children warrant subspecialty referral. In order to determine the children who may benefit the most from imaging or an ENT evaluation, a diagnostic approach is discussed below and an algorithm is presented in Table 14–2.3–8

While many of the common causes of stridor can be diagnosed clinically and managed as an outpatient by the primary care provider, there are several indications for subspecialty or emergency department referral.

Neonates presenting with stridor immediately after birth warrant evaluation by an otorhinolaryngologist to rule out significant laryngeal or tracheal abnormalities.3,4 Pediatricians can monitor infants with stridor whose history and physical examination is consistent with tracheomalacia, unless they demonstrate poor weight gain, cyanosis, or respiratory distress. These findings may indicate severe malacia requiring intervention or may portend a more significant diagnosis.5

Children presenting with symptoms of a viral upper respiratory tract infection and stridor consistent with viral laryngotracheobronchitis can be managed as an outpatient as long as they have stridor only with agitation and no evidence of respiratory distress. Patients with stridor at rest, moderate subcostal retractions, decreased air entry, cyanosis, or depressed mental status require hospitalization.

Any child with suspected esophageal or tracheal foreign body aspiration, epiglottitis, or evidence of severe respiratory distress warrants immediate emergency department referral. Additionally, older children and adolescents with stridor require evaluation in the emergency department if the stridor is acute or by an otorhinolaryngologist if symptoms are insidious.