Post-meningitis empyema: true or false?

59-year-old female with history of hospital admission one month ago due to bacterial meningitis who was taken to the Emergency Department with left hemispheric symptoms. The patient was explored and primary brain computed tomography (CT) scanning was performed. Non-enhanced brain CT showed increased cerebrospinal fluid (CSF) density in left parasagittal frontal space which make us think about a meningeal complication, as empyema. Hence, contrast brain CT was acquired proving leptomeningeal uptake (typical of meningitis) but no other pathological contrast enhancement.

Meningitis is a both meningeal and cerebrospinal fluid inflammation. It could be acquired by blood, due to near adjacent infectious processes (sinusitis, otitis or mastoiditis), fractures or penetrating wounds. Bacteria are the most commonly infectious agents found, being Streptococcus pneumoniae the most frequent pathogen. These microorganisms as well as pus and detritus invade CSF, which is aseptic and clear in a normal individual. It disturbs subarachnoid vessels and brakes blood-brain barrier. Once meningitis is treated, patients usually get better without problems. However, if damage persists and there is no clinical improvement they could develop some complications, such as: hydrocephalus, cerebritis, abscess, cerebrovascular alteration, ventriculitis or empyema. Empyema is an extra-axial, subdural or epidural, encapsulated, thick and purulent collection. It is hypodense and causes effacement of adjacent sulci at non-enhanced CT. Even though empyema has low density, it is slightly more dense in comparison with normal CSF. Both sulcal effacement and a more hyperdense collection than CSF were seen in our patient. Empyemas typically show a thick enhancement rim at contrast studies. Therefore, enhanced CT was performed showing meningeal uptake but not thick rim enhancement in subdural or epidural spaces. Thus, the extra-axial collection was studied by Magnetic Resonance Imaging (MRI). There was partial occupation of left frontal parasagittal sulcus by an unspecific heterogenous material with neither brightness at diffusion sequences on MRI nor rim enhancement on contrast sequences. It all was found to be detritus and rests of meningeal inflammation due to the leaky blood-brain barrier.

Meningitis is a common disease with serious complications, such as empyema. Correct diagnosis of complications is decisive to improve patient prognosis. Nevertheless, knowing the pathophysiology of this disease is also important in order to avoid imaging pitfalls.

- Shih RY, Koeller KK. Bacterial, Fungal, and Parasitic Infections of the Central Nervous System: Radiologic-Pathologic Correlation and Historical Perspectives: From the Radiologic Pathology Archives. RadioGraphics, 2015, 35(4):1141-69. - Smirniotopoulos