What causes the thumb sign on the lateral neck radiograph?
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Answer:
The thumb sign is a thickened, rounded epiglottis on a lateral soft-tissue neck radiograph, replacing the normal thin, curved leaf-like shadow and resembling an adult thumb held up in profile. It indicates acute epiglottitis, now more often seen in adults than children following Hib vaccination. It is accompanied by the vallecula sign, loss or poor definition of the normally deep air-filled vallecula, and by thickening of the aryepiglottic folds and ballooning of the hypopharynx. It is the supraglottic counterpart of the steeple sign of croup, which shows subglottic tracheal narrowing on the frontal view. The critical caveat is one of safety rather than of interpretation: in a child with stridor and suspected epiglottitis, sending the patient to the radiology department, or positioning them supine, may precipitate complete airway obstruction. The correct pathway is direct visualisation by fibreoptic laryngoscopy in a controlled setting with immediate intubation and surgical airway capability, and radiography should be reserved for the stable patient in whom the diagnosis is uncertain.
Why is it called so?
The swollen epiglottis projects into the airway with the broad rounded outline of a thumb.
Pathophysiology
Bacterial infection, historically Haemophilus influenzae type b and now more often streptococci or staphylococci, invades the loose supraglottic submucosa. That tissue plane is only loosely attached and offers little resistance, so inflammatory oedema accumulates rapidly and expands the epiglottis and aryepiglottic folds in all directions. The epiglottis therefore loses its thin leaf shape and becomes short, broad and rounded, and the swelling obliterates the vallecula in front of it. Because the swollen supraglottis both narrows the airway and can prolapse into it, obstruction can be sudden and complete, which is the basis of the management caution.
Alternative names: Thumbprint sign of the epiglottis
Other associated named signs: Vallecula sign, steeple sign of croup, retropharyngeal soft-tissue widening of retropharyngeal abscess, and the ballooned hypopharynx
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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