Laboratory Studies
Laboratory findings in cases of retropharyngeal abscess (RPA) are nonspecific. White blood cell (WBC) counts can be elevated (mean, 17,000/μL; range, 4000-45,000/μL). In a study of 2181 children with cervical abscesses, Harounian et al found the younger cohort (< 2 years of age) to have a higher preoperative WBC count (20.7 vs 17.5). [12]
Radiography
Lateral soft-tissue neck radiography may be helpful in making the diagnosis of RPA. This study is best obtained during inspiration with the neck held in normal extension. An RPA occupies the soft-tissue space that can be observed between the radiolucent airway (ie, pharynx and trachea) and the spine. Widening of these soft tissues (see the image below) is pathologic until proved otherwise.
Plain lateral neck x-ray. Top radiograph reveals widening of soft tissues, with anterior airway displacement. Careful examination of this film also reveals gas in soft tissues. Bottom radiograph reveals soft tissue widening at C2.
Measured at the level of C2, the distance from the anterior surface of the vertebra to the posterior border of the airway should be less than 7 mm, regardless of the patient's age. At C6, this distance should be less 14 mm in children younger than 15 years. A distance of 22 mm is considered normal in an adult. A simpler (but less precise) rule is that the soft-tissue plane should be less than half the width of the corresponding vertebral body.
A plain film may also demonstrate gas or a foreign body in the retropharyngeal space. The normal spinal lordosis may also be reversed. With a child's head extended, the width of the soft tissue is no more than a vertebral body width in an average child.
Unfortunately, lateral plain radiography is not very sensitive or specific, with a false-negative rate as high as 33%. Poor neck extension or an expiratory view that results in a falsely narrowed airway can also produce a false-positive result. One study detailed three cases in which plain radiography missed three RPAs because of nasopharyngeal location. [16]
Chest radiography should be performed if mediastinal or pulmonary involvement is suspected. Furthermore, patients who undergo transcervical or transoral drainage of RPA and do not completely recover as expected should undergo chest imaging to rule out a developing mediastinitis or pneumonia.
Computed Tomography
Computed tomography (CT) is currently the imaging modality of choice for pediatric RPA. CT of the neck with intravenous (IV) contrast should be obtained to help demarcate the lesion and determine whether vascular involvement is present. The radiologist should be informed of the purpose of the study in advance because a standard CT scan of the neck may not employ thin enough slices (3-5 mm) and may not scan the entire extent of the retropharyngeal space (skull base to T2).
CT may be able to determine the presence of an abscess and help distinguish cellulitis from abscess because there may be an area of central lucency. It is typically an area of low attenuation (see the images below), surrounded by an enhanced ring. Gas is sometimes present within the abscess cavity, and the nearby soft tissues are edematous with obliteration of fat planes. Neighboring structures, including the airway, can also be compressed. This study can also assist in determining the location and extent of the abscess and the presence of infectious complications.
Contrast axial CT scan demonstrates left-side retropharyngeal abscess with central hypoattenuation.
Contrast sagittal CT scan demonstrates central hypodensity with prevertebral soft-tissue stripe widening and oropharyngeal air column narrowing consistent with retropharyngeal abscess formation.
CT also provides much more information than plain radiography. Depending on the study, its sensitivity can exceed 90%. [9] The positive predictive value has been estimated at 82%, and the negative predictive value has been estimated at 100%. [17]
Shefelbine et al, in a case series of 30 pediatric patients with RPA, [18] concluded that a low attenuation focus with rim enhancement on CT indicates a presuppurative or suppurative lymph node. They also suggested that children with a hypodense focus diameter smaller than 2 cm often respond to IV antibiotic therapy, whereas children with a hypodense focus diameter larger than 2 cm likely require surgical intervention.
In contradistinction, Malloy et al reported a lack of correlation between CT findings and the presence of purulence at the time of surgical drainage. [19] They found no statistically significant differences in size, rim enhancement, and prevertebral edema between the purulent and nonpurulent groups.
Kirse and Roberson's series reported that scalloping is the most useful characteristic on CT imaging. [20] Although rim enhancement was helpful, scalloping (irregularity of the abscess wall) had a stronger association with finding purulence at the time of surgery. The investigators also found that retropharyngeal edema was present in all patients in their series and was not helpful in distinguishing abscess formation from phlegmon.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) produces images superior to those produced by the other studies; however, it is usually unnecessary and is rarely used unless there is concern that the abscess has spread to the central nervous system (CNS). Additionally, this study requires a protracted time during which the patient is in an unmonitored setting. Finally, children usually require sedation for this test, which is also dangerous in any patient with a potentially unstable airway.
Ultrasonography
Ultrasonography (US) has been suggested for use in deep neck space infections. The advantages of this modality are portability and lack of radiation exposure. It can also be less traumatic to children, rarely requiring the use of sedation. In experienced hands, US has the potential to determine the presence and location of an abscess and to allow the clinician to distinguish abscess formation from cellulitis. For surgical planning, however, US provides only limited assistance.
Histologic Findings
Purulent abscess contents should be cultured sent for Gram stain and acid-fast stains so that the etiology of the infection can be established; this permits a targeted and selective choice of antibiotic as well as assists in the determination of the appropriate duration and route (IV or oral) of antibiotic therapy.
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Schematic of neck deep space anatomy, as illustrated in lateral and cross-sectional views. Fascial planes (see color key) surround potential spaces. Anteriorly, retropharyngeal space is bounded by buccopharyngeal fascia, which invests pharynx, trachea, esophagus, and thyroid; posteriorly by alar fascia; and laterally by carotid sheaths and parapharyngeal spaces. Retropharyngeal space extends from skull base to mediastinum at level of tracheal bifurcation. Note danger space located between alar fascia and prevertebral fascia.
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Plain lateral neck x-ray. Top radiograph reveals widening of soft tissues, with anterior airway displacement. Careful examination of this film also reveals gas in soft tissues. Bottom radiograph reveals soft tissue widening at C2.
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Contrast axial CT scan demonstrates left-side retropharyngeal abscess with central hypoattenuation.
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Contrast sagittal CT scan demonstrates central hypodensity with prevertebral soft-tissue stripe widening and oropharyngeal air column narrowing consistent with retropharyngeal abscess formation.
