The lateral neck x-ray shows widened prevertebral soft tissue space anterior to C1-C4. The epiglottis appears swollen but this is likely projectional. The patient appeared toxic with high inflammatory markers and hence went on to have a CT scan of the neck which revealed a retro pharyngeal abscess.

The lateral neck radiography, in the diagnosis of a retro pharyngeal abscess is not a highly sensitive test.

Prevertebral soft tissue diameter should be less than 7 mm in front of C2 and less than 21 mm in front of C6. Any widening beyond these measurements should raise suspicion.

In addition to showing widening of the prevertebral space, lateral neck x-ray may show gas-fluid level, gas in the tissues or a foreign body.

Reference:https://emedicine.medscape.com/article/764421-workup#c6