The key differentiator is location and displacement vector. A retropharyngeal abscess (RPA) fills the retropharyngeal space in the midline, spans side-to-side, and pushes the pharynx anteriorly, with a thick enhancing wall. A parapharyngeal abscess (PPA) is a unilateral, lateral collection that displaces the pharynx laterally, typically with a thinner enhancing rim. When the distribution is ambiguous, the volume of low-attenuation/necrotic content — not rim thickness — best predicts which collections need surgical drainage versus IV antibiotics alone [1].
Anatomical Location
Retropharyngeal abscess (RPA) — retropharyngeal space, between the pharyngeal constrictor/buccopharyngeal fascia and the prevertebral (alar) fascia. Fills the space midline, side-to-side, spanning clivus to mediastinum via the danger space [1].
Parapharyngeal abscess (PPA) — parapharyngeal (lateral pharyngeal) space, a pyramidal space from skull base to hyoid. Presents as a unilateral, laterally located collection [1,2].
Four-Step CT/MRI Approach
| Step | Retropharyngeal Abscess | Parapharyngeal Abscess |
|---|---|---|
| 1. Distribution | Fills space side-to-side, midline | Unilateral, lateral |
| 2. Configuration / mass effect | Oval/rounded; moderate–marked mass effect; anterior pharyngeal displacement; flattens prevertebral muscles | Oval/rounded; mass effect scales with size; lateral pharyngeal displacement |
| 3. Wall enhancement | Thick enhancing wall | Thin hyperdense/enhancing rim |
| 4. Ancillary findings | Look for airway compromise, mediastinal extension, vascular involvement | Look for otitis media/tonsillitis as primary source; adjacent retropharyngeal edema may coexist |
Pearl: volume of the hypodense/necrotic focus predicts need for surgical drainage better than rim enhancement alone — a thin enhancing rim around a large low-attenuation collection is not reassuring by itself [1].
CT and MRI Findings
- CT: low-attenuation collection with rim enhancement in both; RPA is classically described as a “bow-tie” shaped collection; gas is uncommon but suggests gas-forming organisms or perforation.
- MRI: T2 hyperintense, T1 hypointense collection with peripheral post-contrast enhancement — useful when CT is equivocal or in pregnant/pediatric patients where radiation matters.
- Suppurative retropharyngeal adenitis (rim-enhancing nodes with central low attenuation) is a distinct, often medically-managed entity from a true retropharyngeal abscess — don’t conflate the two on the report [1].
Complications to Actively Search For
- Airway compromise — stridor reported in a minority of cases but changes management immediately.
- Mediastinal extension via the danger space (RPA more than PPA, given midline/vertical distribution).
- Internal jugular vein thrombosis (Lemierre syndrome) and carotid sheath involvement — check both abscess types, more common with lateral (PPA) spread toward the carotid space [2].
References
- Hoang JK, Branstetter BF 4th, Eastwood JD, Glastonbury CM. Multiplanar CT and MRI of collections in the retropharyngeal space: is it an abscess? AJR Am J Roentgenol. 2011;196(4):W426-32.
- Esposito S, De Guido C, Pappalardo M, Laudisio S, Meccariello G, Capoferri G, et al. Retropharyngeal, parapharyngeal and peritonsillar abscesses. Children (Basel). 2022;9(5):618.
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