Assessment of stridor in children

INTRODUCTION — Stridor is an abnormal, high-pitched monophonic sound, produced by turbulent airflow caused by the oscillation of a narrowed airway. Its presence suggests significant obstruction of the large airways at the level of the supraglottis, glottis, subglottis, or trachea. The term is derived from the Latin verb stridere, meaning to make a harsh noise or shrill sound, as to creak.

The acoustics of stridor may be explained by Bernoulli's principle, which states that as the speed of a moving fluid increases, the pressure within the fluid decreases. When airflow is forced through a narrowed tube, a local area of low pressure creates a vacuum effect distal to the narrowing. The focal area of low pressure distal to a narrowed airway causes the airway walls to collapse and vibrate, generating the high-pitched squeaking sound that is characteristic of stridor [1]. Stridor is a symptom that prompts evaluation to determine the cause and occasionally requires emergency intervention. The pathophysiology, differential diagnosis, and clinical management of stridor will be reviewed here. Related material is found in the following topics:

●(See "Emergency evaluation of acute upper airway obstruction in children".)

●(See "Evaluation of wheezing in infants and children".)

●(See "Approach to chronic cough in children".)

PATHOPHYSIOLOGY

Anatomy — Anatomically, the large airways can be divided into the extrathoracic and intrathoracic regions:

Extrathoracic — The extrathoracic region includes airways above the thoracic inlet, which, in turn, can be subdivided into two anatomic areas (figure 1B):

●Supraglottic area – This area includes the nasopharynx, epiglottis, larynx, aryepiglottic folds, and false vocal cords. The walls supporting this region are made of soft tissue and muscles and lack cartilaginous support. Therefore, airway collapse and obstruction can occur easily and progress rapidly in this area.

●Glottic and subglottic area – This portion of the airway extends from the vocal cords to the extrathoracic segment of the trachea, just before it enters the thoracic cavity. The glottic area has some cartilaginous support (ie, cricoid cartilage and incomplete tracheal rings), which makes it less vulnerable to collapse than the supraglottic area. The subglottis, surrounded by the cricoid cartilage, is the narrowest part of the trachea. The diameter of the subglottis is 5 to 7 mm at birth and steadily increases to an average diameter on 20 mm in adults. In the narrow airway of an infant, a small decrease in diameter dramatically increases the airway resistance (because resistance is inversely proportional to the radius to the fourth power [r⁴]). As an example, a narrowing by 1 mm in the subglottic area in an infant will decrease the cross-sectional area by approximately 50 percent [2] and increase airway resistance 16-fold [3].

Classic diseases that cause obstruction at the extrathoracic level include laryngomalacia, laryngotracheitis (croup), retropharyngeal abscess, epiglottitis, and craniofacial malformations. (See 'Causes of stridor' below.)

Intrathoracic — The intrathoracic region includes the portion of the trachea that lies within the thoracic cavity, as well as the mainstem bronchi. Compression of the proximal trachea can result in expiratory stridor, whereas compression of the more distal intrathoracic airways tends to cause typical polyphonic wheezing rather than stridor. Congenital disorders are a prominent cause of obstruction at this level (eg, vascular rings and bronchial webs) [4]. Foreign bodies and compression by enlarged lymph nodes or tumors can also occur at this level [5]. (See 'Chronic' below.)

Auscultatory clues — Stridor is a high-pitched monophonic sound made during breathing. It is usually inspiratory and best heard over the anterior neck (movie 1). These acoustic characteristics of stridor distinguish it from wheezing due to asthma or bronchiolitis, which is usually expiratory and tends to consist of multiple sounds that start and stop at different times and is caused by diffuse airflow limitation of the distal intrathoracic airways.

In normal individuals, during inspiration, the extrathoracic pressure is equal to the atmospheric pressure and exceeds the intratracheal pressure. This produces some collapse of the airway, which is not clinically relevant. By contrast, in patients with a non-rigid airway obstruction, the increased Bernoulli effect results in a substantial drop in the airway pressure beyond the obstruction, leading to the collapse of the airway walls and worsening obstruction. This in turn limits the airflow and causes turbulence and vibration of the airway walls, thus generating the monophonic squeak characteristic of stridor [1]. In patients with minor degrees of airway narrowing, the stridor may occur only during activity (eg, crying or exercise).

●Inspiratory stridor – In general, stridor originating from obstruction in the extrathoracic region is more pronounced during inspiration, when the pressure inside the airway falls below atmospheric pressure, causing airway collapse (figure 1A-B).

●Expiratory stridor – In contrast, stridor originating from obstruction in the intrathoracic region is more pronounced on exhalation since intrathoracic pressure rises on expiration and causes airway collapse (figure 2). The sound produced by intrathoracic obstruction can be difficult to distinguish from wheezing; it has a low-pitch, monophonic tone and can have a harsh or honking character.

●Biphasic (inspiratory and expiratory) stridor – A fixed (rather than dynamic) central airway obstruction usually produces noise during inspiration and expiration. The stridor may be more evident in either phase of breathing, depending on the effort the child makes on inspiration compared with expiration. Fixed central and upper airway obstructions may sound similar regardless of location in the intra- and extrathoracic locations. Conditions causing fixed airway obstruction include external compression, intraluminal airway masses (eg, hemangioma or foreign body), and mural changes in the airway, such as subglottic stenosis.

Stridor caused by obstruction at the glottis (vocal cords) may occur during inspiration only, or during both inspiration and expiration, since the shape and size of this part of the airway change little during the process of respiration.

●Stertor or snoring – If airway narrowing originates from the nasal, nasopharyngeal, or oropharyngeal areas (ie, adenoid hypertrophy, micrognathia, macroglossia, and tonsil hypertrophy), the noise generated is typically low pitched. It may be referred to as snoring if it occurs when the patient is sleeping and stertor if it is present when the patient is awake.

CAUSES OF STRIDOR — Stridor may result from a variety of conditions that can be either congenital or acquired (table 1A-B). The age of the patient and acuity of onset helps to narrow the differential diagnosis.

Acute or subacute onset — A variety of causes of acute upper respiratory obstruction can manifest as stridor. Symptoms develop within minutes or over hours or days and may progress rapidly. The main considerations are listed here, and a general approach to management is discussed separately. (See "Emergency evaluation of acute upper airway obstruction in children".)

●Croup – The term "croup" is used to describe acute-onset stridor caused by a range of upper respiratory conditions in children and can include:

•Viral croup (laryngotracheitis) – Laryngotracheitis accounts for more than 90 percent of all cases of stridor in children [6]. It most commonly occurs in children 6 to 36 months of age. It is also seen in younger infants (as young as three months) and in preschool children but is rare beyond age six years. The onset of symptoms is usually gradual, beginning with nasal irritation, congestion, and coryza and progressing to inspiratory and sometimes expiratory stridor. Symptoms generally progress over 12 to 48 hours to include fever, hoarseness, barking cough, and stridor. Croup is usually a mild and self-limited disease but occasionally causes significant respiratory distress and can be life-threatening.

•Spasmodic croup – Spasmodic croup is characterized by brief, recurrent episodes of stridor, typically occurring at night. The onset and cessation of symptoms are abrupt, and the duration of symptoms is short, often with symptoms subsiding by the time of presentation for medical attention. Fever is typically absent, but mild upper respiratory tract symptoms (eg, coryza) may be present. Episodes can recur within the same night and for two to four successive evenings. A striking feature of spasmodic croup is its recurrent nature, hence the alternate descriptive term "frequently recurrent croup." There may be a familial predisposition to spasmodic croup, and it may be more common in children with a family history of allergies. Because there is some clinical overlap with atopic diseases, it is sometimes referred to as "allergic croup." (See "Croup: Clinical features, evaluation, and diagnosis".)

●Bacterial tracheitis – Bacterial tracheitis (or acute bacterial laryngotracheobronchitis) generally occurs during the first six years of life but may occur at any age. Most patients have prodromal symptoms and signs suggestive of viral respiratory tract infection for one to three days before more severe signs of illness develop, including stridor and respiratory distress, often with cough and a preference to lie flat. Although uncommon, it is among the most frequent pediatric airway emergencies requiring admission to an intensive care unit [7]. (See "Bacterial tracheitis in children: Clinical features and diagnosis", section on 'Presentation'.)

●Epiglottitis – Epiglottitis is inflammation of the epiglottis and adjacent supraglottic structures. Without treatment, it can progress rapidly to life-threatening airway obstruction. Young children classically present with respiratory distress, anxiety, and fever, often with dysphagia, drooling, muffled speech, and reluctance to lie flat. In the past, epiglottitis commonly occurred between two and seven years of age. Since the introduction of the Haemophilus influenzae type B (Hib) vaccine, the incidence of epiglottitis has dropped dramatically; however, there are still case reports of HIB epiglottitis occurring despite immunization [8]. Epiglottitis is now most common after seven years of age, but it can occur at any age [9]. (See "Epiglottitis (supraglottitis): Clinical features and diagnosis".)

●Retropharyngeal abscess – Most retropharyngeal abscesses occur in children between two and four years of age. Early in the disease process, the findings may be indistinguishable from those of uncomplicated pharyngitis. As the disease progresses, symptoms include fever, dysphagia and drooling, unwillingness to move neck, muffled or "hot potato" voice, and inspiratory stridor. (See "Retropharyngeal infections in children".)

●Peritonsillar abscess – Peritonsillar abscess generally arises as a complication of tonsillitis or pharyngitis. Its incidence peaks in children older than 10 years of age, when streptococcal pharyngitis becomes more common. The typical clinical presentation is a severe sore throat (usually unilateral), fever, and muffled or "hot potato" voice, sometimes with inspiratory stridor, drooling, trismus, or neck swelling. (See "Peritonsillar cellulitis and abscess".)

●Inducible laryngeal obstruction (vocal cord dysfunction or paradoxical vocal fold motion) – Vocal cord dysfunction typically presents with recurrent acute episodes of dyspnea and stridor; it may be misdiagnosed as asthma or occur concurrently with asthma [10]. The stridor is more pronounced with exercise and resolves during sleep. Patients may complain often with throat tightness, a choking sensation, dysphonia, and cough. The disorder may begin at any age but most commonly presents during adolescence and is sometimes but not always associated with psychosocial disorders and stress [11,12]. (See "Inducible laryngeal obstruction (paradoxical vocal fold motion)".)

●Foreign body aspiration – Foreign body aspiration should always be suspected in an infant or toddler who presents with sudden or intermittent onset of stridor, with or without respiratory distress, and without other physical findings [13]. Its incidence peaks around two to three years of age [14]. If an infant is in an environment with older children, foreign body aspiration can occur at an earlier age. A history of a witnessed choking episode is common but may not be recalled by the caregiver if the symptom appeared to resolve. (See "Airway foreign bodies in children".)

●Anaphylaxis – Anaphylaxis (most commonly caused by food or medications) is a common cause of acute-onset stridor and may be severe and life-threatening, particularly when edema involves the retropharynx and/or larynx. Common manifestations include respiratory symptoms (change in voice quality, choking, stridor, wheezing, drooling, or cough), skin symptoms (hives, itching, and mouth swelling), and/or gastrointestinal symptoms (nausea, vomiting, diarrhea, and abdominal pain) (table 2). Presenting symptoms in infants are similar but can be more difficult to recognize (table 3). (See "Anaphylaxis: Emergency treatment" and "Anaphylaxis in infants".)

●Airway burns

•Thermal epiglottitis and upper airways burns – Microwave-heated liquids are the most common cause of thermal epiglottitis and edematous arytenoidal tissue in infants and young children. The clinical presentation of thermal epiglottitis injuries in children may be similar to that of acute infectious epiglottitis and should be managed as an emergency airway obstruction [15]. Airway burns also should always be considered in any child with body scald burns who presents with stridor or respiratory distress [16]. Smoke or steam inhalation can cause direct thermal injury to the upper respiratory tract. (See "Emergency evaluation of acute upper airway obstruction in children", section on 'Upper airway burns (thermal and chemical)' and "Moderate and severe thermal burns in children: Emergency management".)

•Caustic burns – Ingestion or inhalation of caustic materials (such as powdered detergents) can injure the upper airway and epiglottis, causing stridor. Accompanying symptoms of vomiting, difficulty swallowing, and oropharyngeal swelling may resemble anaphylaxis. (See "Caustic esophageal injury in children" and "Differential diagnosis of anaphylaxis in adults and children", section on 'Caustic ingestion (young children)'.)

●Iatrogenic causes

•Postextubation – Stridor is a well-known clinical complication of endotracheal intubation. In a case series of children who required intubation for more than 24 hours, almost 20 percent had stridor after extubation, although major complications were rare [17]. The main risk factors included intubation on the scene by emergency medical personnel, use of cuffed endotracheal tubes, and young age.

•Therapeutic hypothermia – Therapeutic hypothermia is sometimes used in the management of moderate to severe hypoxic ischemic encephalopathy in infants. Up to 10 percent of patients undergoing this treatment develop stridor [18]. The mechanism is thought to be due to airway trauma during intubation, manifestation of brainstem injury, muscle weakness due to potassium and calcium shifts caused by hypothermia, or laryngospasm/airway edema due to temperature changes.

Chronic — Chronic stridor (persisting for weeks or more) is typically caused by a structural abnormality that may be congenital or acquired, static, or slowly progressive, leading to intrinsic or extrinsic obstruction of the upper airway.

Congenital — Most of the congenital lesions present during the first few weeks of life, but symptoms may be intermittent, and some anomalies such as bronchogenic cysts and laryngeal clefts may present later in infancy or childhood (table 1A).

●Laryngomalacia – Laryngomalacia is the most common cause of chronic extrathoracic airway obstruction in infants [3]. It typically presents during the neonatal period and progresses during infancy but resolves by 12 to 18 months. The stridor is inspiratory and tends to be worse in the supine position and during feeding and sleeping. (See "Congenital anomalies of the larynx", section on 'Laryngomalacia'.)

●Tracheomalacia – Tracheomalacia usually is caused by an intrinsic defect in the cartilaginous portion of the trachea, leading to an increased proportion of membranous trachea (type 1). Most lesions are intrathoracic, causing airway collapse and stridor during expiration, often with a croup-like cough. Most affected infants improve spontaneously by 6 to 12 months of age as airway caliber increases and cartilage develops. (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Tracheomalacia'.)

●Vocal cord paresis/paralysis – Vocal cord paralysis may be associated with neurologic abnormalities, be secondary to recurrent laryngeal nerve injury during vaginal birth or chest surgery (mostly cardiovascular surgeries), or may be idiopathic. Infants with bilateral paralysis typically present with stridor and respiratory insufficiency and may have normal cry. Those with unilateral paralysis tend to present with hoarseness and are at risk for aspiration.

●Vascular ring – Vascular rings (including a pulmonary artery "sling") cause stridor due to external compression of the trachea. The presentation varies from marked respiratory distress in neonates to intermittent stridor in infants and young children. Because the compression is intrathoracic, the stridor usually is loudest in the expiratory phase. In some cases, the esophagus is also compressed, leading to dysphagia, hyperextension of the head during feeding, or other feeding difficulties [19]. Associated anomalies are common, especially cardiac defects. (See "Vascular rings and slings".)

●Bronchogenic cyst – Bronchogenic cysts are a congenital anomaly but typically present during the second decade of life with recurrent coughing, wheezing (which may simulate asthma), and pneumonia [20,21]. Newborns with rapidly enlarging central bronchogenic cysts can develop respiratory distress, cyanosis, and feeding difficulty. (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Bronchogenic cyst'.)

●Laryngeal malformations – Congenital malformations of the larynx include cysts, stenoses, clefts, and webs. They may present in infancy or early childhood with feeding difficulty, failure to thrive, wheezing, stridor, noisy breathing, aspiration, respiratory distress, recurrent pulmonary infections, and/or hoarseness [22,23]. Clefts and webs are often associated with other congenital anomalies. (See "Congenital anomalies of the larynx", section on 'Malformations' and "Common causes of hoarseness in children", section on 'Congenital anomalies'.)

●Infantile hemangiomas – Infantile hemangiomas occasionally affect the larynx or trachea and cause intermittent stridor or respiratory distress, usually increasing during the first few months of life (picture 1A-B). Hemangiomas of the skin are present in approximately one-half of patients with subglottic hemangiomas. The risk of airway hemangioma is higher with segmental hemangiomas located in a cervicofacial, mandibular, or "beard" distribution. (See "Infantile hemangiomas: Epidemiology, pathogenesis, clinical features, and complications".)

●Subglottic stenosis – Subglottic stenosis can be either a congenital anomaly or acquired. (See "Congenital anomalies of the larynx", section on 'Laryngeal atresia' and "Complications and long-term pulmonary outcomes of bronchopulmonary dysplasia", section on 'Glottic and subglottic damage'.)

Acquired

●Vocal cord paralysis – Acquired vocal cord paralysis may be idiopathic, iatrogenic (eg, after cardiothoracic or thyroid surgery or endotracheal intubation), or caused by neurologic abnormalities or injury to the recurrent laryngeal nerve. The pattern of onset may be acute, subacute, or intermittent, depending on the cause. Children with bilateral paralysis typically present with stridor and respiratory insufficiency and may have normal voice/cry. Those with unilateral paralysis tend to present with hoarseness and are at risk for aspiration. (See "Common causes of hoarseness in children", section on 'Vocal fold paralysis'.)

●Subglottic stenosis – Subglottic stenosis can be either acquired due to injury to the glottis after traumatic or prolonged endotracheal intubation in infants or can be a congenital anomaly. (See "Complications and long-term pulmonary outcomes of bronchopulmonary dysplasia", section on 'Glottic and subglottic damage' and "Congenital anomalies of the larynx", section on 'Laryngeal atresia'.)

●Tumor – Any tumor that compresses the airway may present with stridor. Most are intrathoracic (eg, mediastinal) and extrinsic to the airway; this typically results in expiratory stridor. (See "Overview of common presenting signs and symptoms of childhood cancer", section on 'Mediastinal masses'.)

●Respiratory papillomatosis (RRP) – RRP may be caused by human papilloma virus (HPV) type 6 or type 11. It is characterized by papillomatous growth of the epithelium of the airways (larynx and trachea). RRP has a bimodal age distribution and manifests most commonly in children younger than five years (juvenile-onset) or in persons in the fourth decade of life (adult-onset). The larynx is frequently affected. Patients can present with chronic or progressive stridor, hoarseness of voice or weak cry, choking episodes, cough, and wheezing if trachea is also involved. In some cases, upper airway obstruction may be life-threatening and may be the presenting symptom [24].

●Hypocalcemic laryngeal spasm – Laryngeal spasm due to hypocalcemia has been reported as a cause of stridor in pediatric populations. Most reports are in infants with vitamin D deficiency and rickets secondary to dietary insufficiency, but some cases have occurred in the setting of other metabolic and endocrine disorders that lead to severe hypocalcemia, such as renal failure and hypoparathyroidism. The associated stridor can be chronic or intermittent but also may present with acute severe onset [25,26].

CLINICAL ASSESSMENT

Initial rapid assessment — The initial evaluation of children with stridor must begin with a rapid assessment to identify patients who need immediate intervention. Key steps are to evaluate upper airway patency, the degree of respiratory effort, and evidence of hypoxemia and fatigue and to monitor closely for the possibility of rapid deterioration. Diagnostic testing may need to be delayed to provide appropriate supportive care. In an acutely ill patient, diagnostic testing should only be performed in a controlled setting and with close supervision by a clinician who can intervene if the patient's respiratory function deteriorates. The rapid assessment and stabilization of such patient is detailed in a separate topic. (See "Emergency evaluation of acute upper airway obstruction in children", section on 'Evaluation'.)

For children with stable respiratory status, the next step is a detailed history and physical examination to identify the cause of the stridor. In some cases, additional diagnostic testing, including radiography, pulmonary function testing, or airway visualization, may be required.

History — Key elements of the history serve to focus the likely causes of stridor, as outlined in the tables (table 1A-B):

Age — Age of onset is an important determinant that can help identify the cause of stridor.

●Neonates and infants – Congenital disorders (eg, laryngomalacia, subglottic stenosis, and tracheomalacia) generally present in the first few weeks of life. However, some congenital defects such as bronchogenic cysts and laryngeal clefts may also manifest later in infancy or toddler years and should be considered in the differential diagnosis [7-9]. (See 'Chronic' above.)

●Infants and toddlers – Croup is the most common cause of stridor in this age group. Foreign body aspiration is also an important consideration. Epiglottitis is uncommon but potentially life-threatening and also occurs in older children.

●School-aged children and adolescents – Children in this age group are prone to peritonsillar abscess, as well as vocal cord dysfunction, which typically presents with recurrent episodes of stridor and dyspnea.

●All ages – Anaphylaxis and bacterial tracheitis are important considerations in any age group.

Acuity of onset — The acuity of onset and the severity of the symptom further narrows the cause of stridor. Key considerations are outlined here and detailed above (table 1A-B). (See 'Causes of stridor' above.)

●Acute or subacute onset (see 'Acute or subacute onset' above):

•Laryngotracheitis (croup) is the most common cause of stridor in children and tends to have a subacute pattern, typically preceded with upper respiratory tract infection, and can develop over hours to days.

•Acute onset of symptoms with fever suggest epiglottitis or bacterial tracheitis. These serious bacterial infections typically present with respiratory distress, drooling, and gasping for air. These findings, which suggest a critically narrowed airway, signify an emergency requiring prompt medical attention. (See "Emergency evaluation of acute upper airway obstruction in children", section on 'Infection'.)

•Acute onset of symptoms without fever suggests foreign body aspiration or anaphylaxis.

•Peritonsillar and retropharyngeal abscesses generally develop after symptoms of pharyngitis or tonsillitis.

●Intermittent/recurrent stridor – Episodes of stridor in which the onset and cessation are abrupt may indicate spasmodic croup, which typically occurs at night, or inducible laryngeal obstruction (vocal cord dysfunction), which typically worsens with activity (see 'Acute or subacute onset' above). Other entities with an intermittent pattern are hypocalcemic laryngeal spasm or papillomatosis (See 'Acquired' above.)

●Chronic stridor – Chronic episodes of stridor may suggest a subglottic stenosis or exogenous compression of the airway caused by a vascular ring or a tumor. The timing of symptom onset (congenital, gradual, or after an acute event) and associated symptoms help to distinguish among these possibilities. (See 'Chronic' above.)

Associated symptoms — Additional symptoms that provide important clues are:

●Fever – Fever with toxic appearance suggests a serious bacterial infection (eg, epiglottitis, peritonsillar or retropharyngeal abscesses, or bacterial tracheitis). However, patients with a serious bacterial infection occasionally are afebrile on presentation to the clinician. Fever is also common among children with laryngotracheitis (croup) but tends to be lower than in bacterial infections [27,28].

●Mental status – Alteration in mental status (lethargy, anxiety) with increased work of breathing can point to an impending airway obstruction.

●Drooling – Drooling and a muffled voice usually suggest that the obstruction is supraglottic, including retropharyngeal abscess and epiglottitis. Drooling and dysphagia can occur with epiglottis, a foreign body in the trachea, or a mass compressing the anterior esophageal wall.

●Cough – A barking cough is almost always present in laryngotracheitis (croup) and typically absent in children with acute epiglottitis.

●Voice tone – A change in the voice tone (hoarseness or pitch) suggests a laryngeal lesion such as vocal cord injury due to inflammation (eg, croup) or paralysis [29]. A muffled voice suggests supraglottic obstruction, such as in retropharyngeal or peritonsillar abscess or epiglottitis.

●Stridor during feeding – Respiratory distress and stridor related to feeding can be caused by aspiration secondary to swallowing dysfunction, certain types of tracheoesophageal fistula, or gastroesophageal reflux.

●Hives – Presence of rash, hypotension, and wheezing with acute onset of stridor indicates an allergic reaction with angioedema.

Past medical history — A thorough past medical history should be elicited and should include the prenatal and perinatal period, including infections, prematurity, complicated delivery, necessity of intubation, and mechanical ventilation. Any significant surgical history (patent ductus arteriosus ligation, cardiac, or neck surgery) should be reviewed. It is important to learn whether there have been previous admissions secondary to respiratory diseases and whether the patient ever required prolonged intubation. A history of food allergy or potential exposure to food or environmental allergens should be explored, especially in infants or children with symptoms suggestive of anaphylaxis.

Physical examination — If the initial rapid assessment determines that the patient is stable and immediate therapy is not required, then a detailed physical examination and diagnostic testing can be conducted.

●General – The height and weight should be measured and compared with past data, if available. Failure to thrive suggests a chronic process, while recent weight loss suggests a subacute process such as infection. Obesity is a risk factor for obstructive sleep apnea.

●Skin and extremities – The skin should be inspected for hemangiomas, which, if present, suggest the possibility of a hemangioma in the airway. Café au lait spots are associated with head and neck neurofibromas, which can involve the airway. Lymphadenopathy suggests an intrathoracic process, such as infection or tumor compressing the airway. Clubbing may suggest an underlying congenital heart disease or bronchiectasis.

●Head, eyes, ears, nose, and throat (HEENT) – Special notice should be given to the size of the tongue and mandible and the presence of any craniofacial malformation. Surgical scars around the upper chest or neck are clues to a past procedure that might have contributed to vocal cord paralysis or tracheal or subglottic stenosis. Neck edema accompanied by fever suggests a retropharyngeal or peritonsillar abscess.

●Respiratory – Breathing should be observed both during rest and after activity. The patient's work of breathing should be assessed, including the presence of cyanosis, nasal flaring, and retractions. Children with epiglottis prefer to sit up in a "tripod" or "sniffing" position (picture 2); this posture can also be seen in severe asthma or any cause of airway obstruction. Infants may instead extend their neck backwards in an effort to maximize upper airway patency.

For auscultation, the stethoscope bell is placed over the child's mouth and neck to determine the origin of the stridor. Stridor is a high-pitched monophonic sound made during breathing that is usually inspiratory and is heard best over the anterior neck. The character and timing of the stridor with the respiratory cycle provides a clue to its origin (figure 1B) (see 'Auscultatory clues' above):

•Inspiratory stridor alone is the most common finding and usually is caused by obstruction above the glottis (extrathoracic)

•Expiratory stridor is more likely to be the result of an intrathoracic obstruction

•Biphasic (inspiratory and expiratory) stridor suggests either critical or fixed obstruction (at any level) or an obstruction in the area between the glottis and subglottis

•Nasal, nasopharyngeal, or oropharyngeal obstruction results in a snoring, low-pitched sound (stertor) that may be identified with or without a stethoscope

DIAGNOSTIC TESTING

Laboratory — If an infectious process is suspected, the presence of viral or bacterial infection may be investigated by performing complete blood count (CBC) with white blood cell (WBC) count and differential. For children with a typical presentation of croup, confirmation of a viral cause is not necessary. When an etiologic diagnosis is necessary to make decisions regarding isolation for patients requiring hospitalization, viral culture and/or rapid diagnostic tests that detect viral antigens are performed on secretions from the nasopharynx or throat, especially for parainfluenza types 1, 2, and 3 [30]. (See "Croup: Clinical features, evaluation, and diagnosis", section on 'Etiologic diagnosis'.)

Radiography — Radiography is not necessary for a child with a typical presentation of croup. However, it can be a useful tool in evaluating a child with severe or atypical features or other suspected causes of stridor, including foreign body aspiration. Prior to obtaining any radiographs, the clinician should evaluate the child and assess the risk of developing complete airway obstruction during the procedure. If any life-threatening situation is suspected, the patient must be accompanied to the radiology suite by appropriate medical personnel with the equipment necessary for airway management, including emergency tracheotomy or cricothyroidotomy. For children with suspected epiglottitis and stable respiratory status, radiography usually can be performed safely with close supervision. If the patient is unstable, an emergency evaluation of the airway in a controlled setting such as the operating room or the intensive care unit is indicated. (See "Epiglottitis (supraglottitis): Clinical features and diagnosis", section on 'Diagnosis'.)

Neck radiographs — Plain neck films are usually nonspecific but may reveal changes associated with retropharyngeal abscess, epiglottitis, croup, or a radiopaque foreign body.

●Retropharyngeal abscess – Radiographic findings include a retropharyngeal space greater than 7 mm anterior to the inferior border of the second cervical vertebral body or a retrotracheal space greater than 14 mm in children anterior to the inferior border of the sixth cervical vertebral body (image 1) [31]. Other findings include soft tissue air-fluid levels and cervical retro flexion. Overexposed anteroposterior and lateral projection views obtained may help to identify soft tissue abnormalities. (See "Retropharyngeal infections in children", section on 'Imaging'.)

●Croup – Anteroposterior and lateral neck radiographs are sometimes used in evaluating children with suspected laryngotracheitis (croup) [32]. Typical radiographic features include the "steeple" sign, which occurs when the subglottic arch becomes edematous and shows an inverted "V" in the anteroposterior view (image 2). In the lateral view, the hypopharynx is distended and there is subglottic haziness. The epiglottis is normal (image 3). Unfortunately, these signs are not consistently present, so the plain radiograph has limited value in distinguishing between croup and epiglottitis [33]. (See "Croup: Clinical features, evaluation, and diagnosis", section on 'Radiographs'.)

●Epiglottitis – If lateral neck radiographs show an edematous epiglottis with the thumb sign (image 4), enlarged aryepiglottic folds, and a ballooned hypopharyngeal airway, epiglottitis should be considered. However, these findings are subjective, and as many as 70 percent of all patients with epiglottitis have radiographic findings interpreted as normal [34,35].

Of note, disorders affecting the glottis (eg, vocal cord paralysis), which typically present with hoarseness, usually cannot be identified by radiography. Direct visualization of the larynx is the most important diagnostic tool when these disorders are suspected. (See 'Airway examination' below and "Hoarseness in children: Evaluation".)

Chest radiographs — In cases where an intrathoracic problem is suspected, a plain chest radiograph can help evaluate for mediastinal lymphadenopathy or masses. Findings suggestive of vascular rings, including a right aortic arch, may also be identified on plain films (table 4). Findings suggestive of foreign body aspiration include mediastinal shift, unilateral hyperinflation, atelectasis, or the actual foreign body, if it is radiopaque. However, a normal chest radiograph cannot completely rule out an airway foreign body, because most of the objects aspirated by children are radiolucent [36,37]. (See "Airway foreign bodies in children".)

Computed tomography — Chest and neck computed tomography (CT) with contrast can be helpful in the diagnosis of retropharyngeal cellulitis, in which compression of the pyriform sinuses and an anterior deviation of the trachea may be seen. It is also used to look for enlarged lymph nodes, tumors, aberrant arteries, and vascular rings. In addition, it may show narrowing of the airway, suggestive of tracheal stenosis.

●Three-dimensional reconstruction of the airway from CT scans may also better define the anatomy of the airways and can be done on request in many institutions. Children with suspected tracheomalacia can be evaluated with studies that show airway dynamics during breathing, including CT scan with airway reconstruction and inspiratory/expiratory imaging, a dynamic airway study, and/or bronchoscopy via laryngeal mask during spontaneous breathing. (See 'Airway examination' below and "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Tracheomalacia'.)

●Multidetector CT is one of the most recent diagnostic technologies. It is able to capture detailed images with different tissue thickness. A study showed how this technology is useful in assessing persistent stridor in children. They were able to detect large airway abnormalities as well as non-airway abnormalities in 82 percent of their study cases [38].

Other imaging

●Magnetic resonance angiography is most valuable to define vascular anatomy in children if a vascular ring is suspected after plain radiography. (See "Radiology of the trachea" and "Vascular rings and slings", section on 'Evaluation'.)

●Contrast esophagram is a useful test to identify swallowing dysfunction or esophageal abnormalities [39]. It can also show evidence of vascular rings or mediastinal masses compressing on the esophagus but is not the test of choice for the diagnosis of these disorders.

Spirometry — Spirometry is difficult to perform in infants and young children, the age groups most likely to have stridor. Spirometry usually can be performed in children over six years of age. It is useful for those with chronic stridor in whom the location or nature of the obstruction cannot be localized by the radiographic evaluation described above.

The configuration of the flow-volume loop may help distinguish the site of the airway obstruction, its variability, and its response to bronchodilators. The airways can be divided into intra- and extrathoracic components by the thoracic inlet, which projects 1 to 3 cm above the suprasternal notch on the anterior chest at the level of the first thoracic vertebra. (See "Flow-volume loops", section on 'Abnormal inspiratory loop'.)

Three patterns of obstruction are identified by the flow-volume loop, providing a clue to the type and location of the obstruction (figure 3 and table 1B):

●Variable extrathoracic obstruction – The presence of a non-rigid extrathoracic obstruction (ie, vocal cord paralysis) leads to an increase in obstruction during inspiration (figure 1A), with flow-volume loop showing flattening of the inspiratory component.

●Variable intrathoracic obstruction – The presence of intrathoracic obstruction (eg, due to foreign body in the mainstem bronchi, vascular rings, or bronchial webs) is more pronounced during expiration, resulting in compression of the lower trachea or main bronchus (figure 2). Flow-volume loops will show a flat expiratory component.

●Fixed upper airway obstruction – As there is limited flow when there is a fixed obstruction (foreign body, tracheal stenosis, or endobronchial tumors), the curve will show flattening of both inspiratory and expiratory components.

Airway examination — Visualization of the airways with nasopharyngoscopy, laryngoscopy, and bronchoscopy allows definitive diagnosis of the cause of stridor in children. More than one airway abnormality may be present in the same child, and a thorough evaluation of the upper and lower airways may be warranted.

Airway visualization with the intention of intubation should be promptly performed if the underlying cause of stridor in an acutely ill child is thought to be epiglottitis (picture 3) or bacterial tracheitis (picture 4). Patients with suspected or known foreign body aspiration also require examination of the airway, usually as part of a procedure to remove the foreign body. These procedures should be performed by clinicians with experience in intubation and manipulating pediatric airways.

●Nasopharyngoscopy is appropriate for the patient with a stable airway and can be performed in an unsedated and awake child at the bedside. It is useful in the diagnosis of laryngomalacia and anatomic defects between the nose and pharynx, as well as in the assessment of vocal cord movement. (See "Inducible laryngeal obstruction (paradoxical vocal fold motion)".)

●Flexible or rigid laryngoscopy and/or bronchoscopy are indicated for patients with an unstable airway or for those with suspected foreign body aspiration. These procedures should be performed in the operating room, with anesthesia, sedation, and continuous monitoring.

•In cases of suspected epiglottitis, direct laryngoscopy usually can be performed safely by experienced clinicians with little risk of complications such as laryngospasm [40,41]. (See "Emergency evaluation of acute upper airway obstruction in children".)

•If foreign body aspiration is suspected, rigid bronchoscopy is performed to confirm the diagnosis and remove the foreign body. (See "Clinical presentation, diagnostic evaluation, and management of malignant central airway obstruction in adults" and "Airway foreign bodies in children", section on 'Suspected foreign body aspiration'.)

•For many patients with chronic or intermittent stridor, airway visualization is indicated to evaluate the airway and establish a diagnosis. Rigid laryngoscopy and bronchoscopy are the gold standards for evaluation and diagnosis of subglottic, tracheal, and central airway lesions. The rigid bronchoscope allows the clinician to take biopsy samples and dilate stenosed airways, if necessary. If a foreign body is not suspected, a flexible bronchoscope is usually used and the examination is performed during spontaneous breathing to help identify abnormalities in airway dynamics, including laryngomalacia.

SUMMARY

●Characteristics of stridor

•Stridor is a monophonic, high-pitched sound that is loudest over the anterior neck (movie 1). It is typically inspiratory but occasionally heard on expiration or throughout the respiratory cycle. The sound is caused by narrowing of the upper or central airway. (See 'Pathophysiology' above.)

•Stridor originating from the extrathoracic area is more pronounced during inspiration and tends to abate during exhalation. Stridor caused by obstruction at the glottis (vocal cords) may occur during inspiration only or during both inspiration and expiration (figure 1B). Critical and fixed airway obstruction typically results in biphasic inspiratory and expiratory stridor. Compression of the proximal trachea can result in expiratory stridor, but most other types of intrathoracic airway compression usually give rise to coarse wheezing rather than stridor. (See 'Auscultatory clues' above.)

●Causes – Stridor may result from a variety of conditions that can be either congenital or acquired (table 1A-B). The timing, acuity of onset, and associated symptoms help to narrow the differential diagnosis. (See 'History' above.)

•The most common cause of stridor in children is laryngotracheitis (croup), which tends to have gradual onset of symptoms (hours to days) with associated congestion, hoarseness, and barking cough, with or without fever. (See 'Acute or subacute onset' above.)

•Severe or rapidly progressive symptoms with fever suggest epiglottitis or bacterial tracheitis. These disorders are rare but may be life-threatening. Acute onset of symptoms without fever can occur in spasmodic croup, foreign body aspiration, airway burns, or anaphylaxis (table 1B). (See 'Acute or subacute onset' above.)

•Congenital anomalies can cause stridor through intrinsic or extrinsic obstruction of the upper airway. Most present during the first few weeks of life, but symptoms may be intermittent, and some anomalies such as bronchogenic cysts and laryngeal clefts may present later in infancy or childhood (table 1A). (See 'Chronic' above.)

●Evaluation

•The initial evaluation of children with stridor must begin with a rapid assessment to identify patients who need intervention or are in danger of imminent decompensation. Key steps are evaluation of upper airway patency, degree of respiratory effort, and effectiveness of respiratory function. (See "Emergency evaluation of acute upper airway obstruction in children", section on 'Evaluation'.)

•Once the clinical history and physical examination narrow the diagnostic possibilities, radiographic studies and/or direct examination of the airway are selected for further evaluation. The diagnosis of laryngotracheitis (croup) is clinical; radiography and laboratory studies usually are not needed for patients with typical features. (See 'Clinical assessment' above and 'Diagnostic testing' above.)