The steeple sign on an X-ray is caused by croup, a viral infection formally known as laryngotracheobronchitis. It appears on a frontal (anteroposterior) neck X-ray as a tapered narrowing of the airway just below the vocal cords, creating a shape that resembles a church steeple. This finding has a 93% sensitivity and 92% specificity for croup, making it one of the more reliable radiographic signs in pediatric medicine.
What Creates the Steeple Shape
In a healthy airway, the trachea below the vocal cords has roughly parallel walls on an X-ray. During croup, the parainfluenza virus (the most common cause) triggers inflammation and swelling in the subglottic region, the narrowest part of a child’s airway. That swelling pushes inward from both sides, creating a gradual taper that comes to a point rather than maintaining the normal rectangular column of air. The result looks like a pointed steeple when viewed on a frontal neck film.
Children are especially vulnerable to this narrowing because their subglottic airway is already small and the tissue lining it is loose, making it prone to significant swelling from even mild inflammation. A small amount of edema that would barely register in an adult airway can dramatically reduce airflow in a toddler.
Who Gets Croup
Croup most commonly affects children between 6 months and 3 years old, though it can appear in kids as young as 3 months and occasionally up to age 15. It peaks in fall and early winter, coinciding with parainfluenza virus season, but cases occur year-round. It’s one of the most common respiratory illnesses in children under 6.
The classic presentation is a barking cough that often worsens at night, along with a hoarse voice and, in more significant cases, stridor (a high-pitched sound when breathing in). Most children with croup are diagnosed based on these symptoms alone. The steeple sign confirms the diagnosis on imaging, but a neck X-ray isn’t routinely ordered unless the presentation is unusual or the doctor needs to rule out other conditions.
Steeple Sign vs. Thumbprint Sign
Two airway emergencies in children produce distinctive X-ray findings, and they’re frequently compared. The steeple sign points to croup: subglottic narrowing with a normal-sized epiglottis. The thumbprint sign points to epiglottitis: an enlarged, swollen epiglottis with distension of the throat behind it. These are different diseases affecting different parts of the airway.
Croup sits lower, in the trachea below the vocal cords, and is almost always viral. Epiglottitis sits higher, at the flap of tissue that covers the airway during swallowing, and is typically bacterial. Epiglottitis has become far less common since widespread vaccination against Haemophilus influenzae type b, but it remains a more dangerous condition when it does occur. Knowing which sign you’re looking at on an X-ray immediately narrows the diagnosis.
Other Conditions That Can Mimic It
While the steeple sign is strongly associated with croup, a few other conditions can produce similar subglottic narrowing on imaging. Bacterial tracheitis, a less common but more serious infection of the trachea, can show the same radiographic pattern. The key difference is clinical: bacterial tracheitis tends to cause higher fevers, a more toxic appearance, and doesn’t respond to the standard croup treatments.
Other possibilities include a foreign body lodged in the airway, subglottic stenosis (a chronic narrowing, sometimes from prior intubation), angioedema, and rarely, airway tumors or deep neck infections. These are uncommon enough that in a child with a barking cough during fall or winter, the steeple sign almost always means croup. Context matters: age, symptoms, and season guide interpretation of what the X-ray shows.
How the X-ray Is Taken
The steeple sign is best seen on an anteroposterior (front-to-back) neck X-ray, which gives a direct view of the tracheal air column. Radiologists measure the narrowest tracheal width and compare it to the normal width to quantify the degree of narrowing. A lateral (side) view can also be taken to assess the airway from a different angle and to evaluate the epiglottis, helping distinguish croup from epiglottitis in ambiguous cases.
In practice, many children with straightforward croup never get an X-ray. The imaging is most useful when the diagnosis is uncertain, when the child isn’t responding to treatment as expected, or when symptoms suggest something other than a typical viral illness.
How Croup Is Treated
Most croup is mild and resolves on its own within a few days. Cool mist, keeping the child calm, and monitoring breathing are the basics of home care. For moderate to severe cases, a single oral dose of a steroid is the standard treatment, which reduces airway swelling and typically produces noticeable improvement within a few hours. Children with significant breathing difficulty may also receive a nebulized form of epinephrine in an emergency setting, which works within minutes to shrink the swollen tissue. Its effects are temporary, so children who receive it are usually observed for a period before going home.
Most kids recover fully within three to five days. The barking cough is often the last symptom to linger. Recurrent croup or croup that doesn’t follow the expected pattern may warrant further investigation for underlying airway abnormalities.