Images # 1, 2 and 3

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Findings:Images #1 and 2 are axial CT images after contrast administration obtained upon admission which demonstrates diffuse meningeal enhancement consistent with meningitis. Image #3 is a post contrast-enhanced CT image obtained three days later, which now demonstrates small bilateral frontal extra-axial fluid collections. Note that there is adjacent meningeal enhancement on the right.

Diagnosis: Subdural empyema.

Discussion: Subdural empyemas are essentially collections of pus located between the dura and the leptomeninges. They are most commonly the result of direct extension secondary to sinus infection (frontal most commonly), but may also result from meningitis with organisms entering the subdural space through dural sinus or bridging veins. Imaging studies characteristically demonstrate a crescentic or lentiform extra-axial fluid collection that is low density on CT, but may be slightly higher density than CSF. Enhancement of the medial rim is invariably seen (such as in this case). In addition, MR imaging will generally demonstrate the extra-axial fluid collection to mildly hyperintense compared to CSF on both T1 and T2 weighted images. Potential complications include parenchymal extension (cerebritis), venous thrombosis and infarction. It is extremely important to quickly recognize a subdural empyema, as the mortality rate is approximately 8 to 12%.

References:

1. Haaga JR. Computed Tomography and Magnetic Resonance Imaging
   of the Whole Body. Mosby, St. Louis; 1994:239-241.
2. Osborn AG. Diagnostic Neuroradiology. Mosby, St. Louis; 1994:680-686.
3. Mathews VP. GD-DTPA Enhanced MR Imaging of Experimental Bacterial Mengitis:
   Evaluation and Comparison with CT. AJNR 1988:1045-1050.