Restricted Diffusion of Pus in the Subarachnoid Space: MRSA Meningo-Vasculitis and Progressive Brainstem Ischemic Strokes - A Case Report

Abstract

Extra-axial restriction on diffusion weighted imaging (DWI) is an unusual finding on brain magnetic resonance imaging (MRI). Intra-axial restriction on DWI, however, is common, and can represent brain parenchymal infarction, tumor, abscess, or toxic-metabolic process. The infrequency of extra-axial DWI restriction and the paucity of clinico-pathological correlation in the literature limit its differential diagnosis. Scant case reports suggest that extra-axial DWI restriction could be a lymphoma, neurenteric cyst, or, in one patient, subdural empyema [1,2,3]. We postulate that pus formation must be excluded first, because it can provoke an aggressive meningo-vasculitis with rapidly fatal, intra-axial infarctions. Our patient was a 45-year-old man, presenting to our hospital with left facial droop and right (contralateral) arm and leg weakness. Initial MRI revealed DWI restriction in the left lateral pons, consistent with a classic Millard-Gubler stroke. Also noted was a subtle, extra-axial area of curvilinear diffusion restriction in the left cerebellar-pontine angle's subarachnoid space. Days later, the patient had a headache, and repeat MRI revealed extension of the two DWI lesions - both the intra-axial pontine infarction and the extra-axial area of restricted diffusion in the subarachnoid space. The patient became comatose, a third MRI revealed more extensive DWI restrictions, and he expired despite aggressive care. Autopsy revealed massive brainstem infarcts, a thick lymphoplasmacytic infiltrate, copious Gram-Positive cocci (likely MRSA) and arteries partially occluded with fibrointimal proliferation. This emphasizes the concept that extra-axial DWI restriction can represent pus development in the subarachnoid space - a radiographic marker to identify a patient at risk for demise due to septic, meningo-vasculitic infarctions.

Fig. 1

On MRI (top panel), a focal left pontine intra-axial infarct is seen as a high FLAIR signal (left) and DWI restriction (right). Slightly more caudal on this same MRI (bottom panel), there is seen a subtle, extra-axial, curvilinear, FLAIR (upper left) and T1 (upper right) signal abnormality, also showing restricted diffusion with high signal intensity on DWI (lower left) and matching low signal intensity on ADC map (lower right) in the subarachnoid space (red arrows).

Fig. 2

Repeat MRI (top panel) revealed fourth ventricle effacement and infarct extension into the brainstem and cerebellum on FLAIR (left) and DWI (right). On this same MRI (bottom panel), the previously subtle cerebello-pontine angle signal has increased in size on FLAIR (upper left), T1 (upper right), DWI (lower left) and ADC (lower right).

Fig. 3

A third and final MRI revealed considerable disease progression with marked increase in size of signal abnormalities on FLAIR (upper left) and T1 (upper right) in the brain parenchyma itself. In the subarachnoid space, pons, medulla and cerebellum, the high signal intensities also increased in size on DWI (lower left) with matching low signal intensity on ADC map (lower right). The restricted diffusion in the subarachnoid space represented pus from bacterial meningitis and the restricted diffusion in the brainstem and cerebellum represented acute infarctions.

Fig. 4

MRA initially appeared unremarkable (a), but later it appeared vasculitic (b) in the basilar and posterior cerebral arteries, with left vertebral artery dropout. Autopsy revealed patency of the basilar, posterior cerebral and cerebellar arteries (c) but variable arteritis with partial occlusion by fibrointimal proliferation was seen on high-power light-microscopy (d).

Fig. 5

Base of brain gross autopsy reveals basilar meningitis that partially obscures pontine circumferential arteries (arrow). Recent, severe necrosis of the left cerebellar hemisphere (star) and pons is seen.

Fig. 6

In the pons, copious Gram-Positive cocci in clusters (arrow), macrophage infiltrates, and capillary proliferation were seen on post-mortem microscopy.