Stridor is a high-pitched, harsh breathing sound caused by a partially blocked airway, usually in the throat or voice box area. Unlike the whistling of a wheeze, which comes from the smaller airways deep in the lungs, stridor originates from the upper airway and produces a loud, musical tone of relatively constant pitch. It is always a sign that something is narrowing the airway, and in some cases it signals a medical emergency.
What Stridor Sounds Like
Stridor has been described as a harsh, strained, high-pitched sound, often compared to a seal bark or a creaking gate. It tends to be louder than a typical wheeze and is usually audible without a stethoscope. The fundamental frequency averages around 194 Hz, which places it in a range similar to a low-to-mid singing voice. The sound is generated by air vibrating the vocal cords or nearby soft tissue as it squeezes through a narrowed passage.
Wheezing, by contrast, is produced by the walls of smaller airways oscillating against each other. Both sounds indicate narrowed airways, but stridor points to a problem in the trachea or larynx (the windpipe or voice box), while wheezing typically points to the bronchial tubes inside the lungs. If you hear a single, prominent tone coming from the throat area rather than diffuse whistling across the chest, you’re likely hearing stridor.
Why the Timing Matters
Stridor can occur when breathing in, breathing out, or both. The timing tells clinicians where the blockage is.
- Inspiratory stridor (heard on the inhale) is the most common type and points to an obstruction at or above the vocal cords, such as the voice box or the area just below it.
- Expiratory stridor (heard on the exhale) suggests a problem lower in the airway, typically in the trachea below the vocal cords.
- Biphasic stridor (heard on both inhale and exhale) usually means the obstruction is right at the level of the vocal cords or in a fixed, rigid area that doesn’t change with the direction of airflow.
How a Narrowed Airway Creates the Sound
When part of the airway narrows, the physics of airflow change dramatically. Air speeds up as it passes through a tighter space, and the pressure drops on the other side. This is the same principle that lets an airplane wing generate lift. In the airway, that pressure drop pulls soft tissues inward, causing them to vibrate and produce the characteristic sound.
The relationship between narrowing and resistance is steep. A four-fold reduction in the airway’s cross-sectional area creates roughly a four-fold increase in resistance to airflow. This is why stridor can escalate quickly: a modest amount of additional swelling in an already narrow airway can cause a dramatic increase in breathing difficulty.
Common Causes in Children
Children are far more prone to stridor than adults because their airways are smaller and softer. Even a small amount of swelling can significantly narrow the passage.
Croup is the most frequent cause. It’s a viral infection of the voice box and windpipe that produces a distinctive barking cough along with inspiratory stridor. It typically affects children between six months and three years old, peaks at night, and usually resolves within a few days. Children with moderate to severe croup are often given an inhaled mist of medication to temporarily shrink the swollen tissue, along with a steroid to reduce inflammation over several hours.
Laryngomalacia, a condition where the tissue above the vocal cords is floppy and collapses inward during breathing, is the most common cause of chronic stridor in newborns. Most babies outgrow it by 18 to 24 months as the cartilage firms up. Other structural causes in children include narrowing below the vocal cords (subglottic stenosis), growths called hemangiomas, vocal cord paralysis, and laryngeal webs, which are thin membranes of tissue stretching across the airway.
Epiglottitis, a rapid swelling of the flap that covers the windpipe during swallowing, used to be a common and dangerous cause of stridor in children. It has become rare since widespread vaccination against the bacterium that most often caused it, but it still occurs and can deteriorate rapidly. A child with epiglottitis often drools, leans forward, and looks much sicker than a child with croup.
Common Causes in Adults
In adults, the causes tend to be different. Vocal fold dysfunction, where the vocal cords close inappropriately during breathing, is one of the most common culprits. It can mimic asthma and is sometimes triggered by stress, exercise, or irritants. The stridor it produces is typically intermittent and may resolve on its own between episodes.
Other adult causes include vocal cord paralysis (often from nerve damage during thyroid surgery, neck surgery, or intubation), swelling of the voice box after having a breathing tube removed in a hospital, allergic reactions that cause the throat to swell, and tumors of the larynx. Laryngeal tumors tend to cause stridor that gradually worsens over weeks or months as the growth enlarges. Unexplained stridor in an adult, especially one with a history of smoking or head and neck cancer, warrants prompt evaluation.
Severity and Warning Signs
Not all stridor is equally urgent. Mild stridor that only appears when a child is crying or agitated is less concerning than stridor heard at rest. In croup severity scoring, stridor at rest scores twice as high as stridor only with agitation, reflecting the greater degree of airway narrowing it represents.
Several signs suggest the airway is in serious trouble. Skin turning blue or gray (especially around the lips and fingertips) means oxygen levels have dropped significantly. Visible pulling of the skin between the ribs, above the collarbones, or at the notch of the throat with each breath indicates the body is working hard to move air past an obstruction. A child or adult who becomes confused, unusually sleepy, or stops making the stridor sound despite looking worse may be tiring out, which can precede complete airway collapse. These situations require emergency care.
How Stridor Is Evaluated
The first priority is always making sure the person can breathe. Beyond that initial assessment, doctors look at the timing of the stridor, how quickly it developed, and what other symptoms are present. A barking cough with a runny nose in a toddler points strongly toward croup. Sudden onset after eating in any age group raises concern for a foreign object. Gradual worsening over weeks in an adult smoker raises concern for a growth.
When the cause isn’t clear from the history and physical exam, a flexible camera passed through the nose to view the vocal cords and upper airway (laryngoscopy) is the most direct way to identify the problem. Imaging of the neck and chest can also help, particularly when structural abnormalities, masses, or compression from outside the airway is suspected. Children with recurrent episodes of stridor are often referred to an ear, nose, and throat specialist to look for underlying airway abnormalities that might not be obvious during a single episode.
Treatment Depends on the Cause
For croup, the most common scenario, treatment centers on reducing airway swelling. A single dose of a corticosteroid works over several hours to bring down inflammation. In more severe cases, an inhaled mist containing epinephrine provides rapid but temporary relief by constricting the blood vessels in the swollen tissue, shrinking it within minutes. The effect wears off in one to two hours, which is why children who receive it are monitored before going home.
For allergic reactions causing throat swelling, injectable epinephrine is the primary treatment. Vocal fold dysfunction is often managed with breathing techniques and speech therapy rather than medication. Structural problems like subglottic stenosis or tumors may require surgical intervention. Vocal cord paralysis sometimes recovers on its own over months, depending on the cause, but persistent cases can be treated with procedures to reposition or modify the vocal cords.
The common thread across all causes is that stridor itself is not a diagnosis. It is a physical sign that the upper airway is compromised. Identifying and addressing the underlying cause is what determines the treatment path and the outcome.