Persistent absence of ring-enhancement on CT with an encapsulated brain abscess.

Ring-enhancement on CT (RECT) is generally considered a sine qua non in diagnosing a cerebral abscess. We describe a 16-year-old female who presented with headaches, vomiting and drowsiness, which over 2 weeks rapidly progressed to coma. CT demonstrated a moderately large left frontal extradural abscess, associated with contiguous left frontal osteomyelitis, and underlying frontal and ethmoidal sinusitis. In addition, there was a large circular low density area within the left frontal lobe associated with midline shift that, owing to negative RECT, was assumed to represent nascent ischaemic cerebritis. Despite emergency twist-drill drainage of the extradural abscess, and antibiotic/corticosteroids administration, her clinical condition continued to deteriorate and two episodes of uncal herniation were reversed medically. Repeated CT, however, continued to demonstrate negative enhancement within the left frontal low density, although significant enhancement continued to be apparent with recurrent contiguous extradural suppuration. At definitive craniotomy, a large, well-encapsulated abscess cavity was excised from the left frontal lobe corresponding precisely to the area of previously negative enhancement, along with drainage of the recurrent extradural abscess. Thus, in addition to well-known 'false-positives' for RECT with a cerebral abscess, our case highlights the rare occurrence of a 'false-negative'. A low density mass lesion on CT with persistent negative RECT can neither be assumed to represent early cerebritis nor to exclude a mature abscess.