Neuroimaging of the Most Common Meningitis and Encephalitis of Adults: A Narrative Review

Abstract

Meningitis is the infection of the meninges, which are connective tissue membranes covering the brain, and it most commonly affects the leptomeninges. Clinically, meningitis may present with fever, neck stiffness, altered mental status, headache, vomiting, and neurological deficits. Encephalitis is an infection of the brain, which usually presents with fever, altered mental status, neurological deficits, and seizure. Meningitis and encephalitis are serious conditions which could also coexist, with high morbidity and mortality, thus requiring prompt diagnosis and treatment. Imaging plays an important role in the clinical management of these conditions, especially Magnetic Resonance Imaging. It is indicated to exclude mimics and evaluate the presence of complications. The aim of this review is to depict imaging findings of the most common meningitis and encephalitis.

Keywords: computed tomography; encephalitis; magnetic resonance imaging; meningitis.

Figure 1

Axial FLAIR (A), T1 (B), coronal (C), and sagittal FLAIR (D) show diffuse hyperintensity on FLAIR and T1 of the subarachnoid spaces mainly in the cerebral convexity (arrows in A,B) in a patient with Streptococcus Pneumoniae meningitis. Note also concomitant cerebellitis, which appears as diffuse cortical hyperintensity on FLAIR of the cerebellum (arrowheads in C,D), with herniation of the cerebellar tonsils (arrow in D).

Figure 2

Axial DWI (A), ADC (B) shows multiple foci of restricted diffusion in the subarachnoid spaces at convexity bilaterally (arrows in A,B) in a patient with newly diagnosed Streptococcus Pneumoniae meningitis.

Figure 3

Axial DWI (A,E), FLAIR (B), enhanced FLAIR (C,D), show multiple recent ischemic areas (arrowheads in A,B,D) in a patient with Neisserria Meningitidis infection. Note also pachymeningeal (arrows in C) and leptomeningeal enhancement (arrows D) and purulent material in the subarachnoid spaces in the left insular region (arrowhead in E) and in the lateral ventricle bilaterally (arrows in E).

Figure 4

Axial FLAIR (A), T2 (B), DWI (C), ADC (D), enhanced T1 (E), and FLAIR (F) of a patient with Streptococcus Pneumoniae depict multiple recent ischemic areas in the white matter in the fronto-parietal regions bilaterally (arrows in A–E), which are hyperintense on FLAIR and T2, show restricted diffusion and faint contrast enhancement. There is also a leptomeningeal enhancement in the right frontal and parietal regions (arrowheads in E,F). Note also a gliotic area in the right frontal region (*) due to a known previous ischemic event.

Figure 5

Axial DWI (A), ADC (B), T2 (C), FLAIR (D), and enhanced T1 (E) show purulent material in the lateral ventricles bilaterally (arrowheads in A–E), with restricted diffusion, in a patient with Streptococcus Pneumoniae meningitis. Note also the recent ischemic lesions in the left nucleo-capsular region, which show faint contrast enhancement after contrast injection (arrows in A–E). There is also enlargement of the lateral ventricles due to hydrocephalus.

Figure 6

Axial DWI (A), FLAIR (B), T2 (C,G), enhanced FLAIR (D), T1 with fat-suppression (E), enhanced T1 with fat-suppression (F) and axial 3D-CISS show multiple recent ischemic lesions (arrows in A–C) and pachymenangeal enhancement (arrowheads in D) in a patient with Streptococcus pneumoniae meningitis. There is also cochlear hemorrhage on the right side (arrow in E), with enhancement of the structures of the inner ear (arrow in F). Axial T2 (G) and 3D-CISS (H) performed after one month depict subdural hygroma in the frontal regions bilaterally (arrowheads in G) and loss of the normal fluid signal of the internal right ear due to ossificans labyrinthitis (arrow in H).

Figure 7

Coronal FLAIR (A) and enhanced T1 (B) show thrombosis of the right sigmoid sinus in a patient with Streptococcus pneumoniae meningitis (arrows in A,B).

Figure 8

Axial DWI (A), ADC (B), T2 (C), FLAIR (D), and enhanced T1 (E) show purulent material in the lateral ventricles bilaterally (arrows in A–D) with linear subependymal enhancement (arrowheads in E) in a patient with Hemophilus influenzae meningitidis.

Figure 9

Axial FLAIR (A), T2 (B), and enhanced T1 (C) shows a tuberculoma in the right frontal region, which is hypointense in FLAIR and T2, and has intense contrast enhancement (arrows in A–C). Note also the surrounding vasogenic edema (arrowheads in A,B).

Figure 10

Axial FLAIR (A,B) and enhanced T1 (C,D) depict multiple tiny foci of enhancement, some of which have a ring appearance (arrows in C,D) in a patient with miliary tuberculosis. They are surrounded by vasogenic edema (arrowheads in A,B).

Figure 11

Axial T2 (A), FLAIR (B), and enhanced T1 (C) show a cryptococcoma in the head of the right caudate nucleus, which is slightly hypointense on T2 and FLAIR, and show inhomogeneous enhancement (arrows in A–C).

Figure 12

Axial DWI (A), ADC (B), FLAIR (C), and enhanced T1 with fat-suppression depict an area of cerebritis in the left rectus gyrus (arrows in A–C), which shows restricted diffusion and hyperintensity on FLAIR (arrow in C). Also note the sinusitis in the left anterior ethmoidal cells, with necrosis of the mucosa (arrowheads in D).

Figure 13

Axial DWI (A,B), FLAIR (C,D), and enhanced T1 (E,F) show fronto-temporo-insular signal alterations due to Herpes virus simplex type 1 encephalitis, which determines restricted diffusion (arrows in A,B), hyperintensity on FLAIR (arrows in C,D) and gyriform contrast enhancement (arrows in E,F).

Figure 14

Axial T2 (A), FLAIR (B), susceptibility-weighted imaging (C), and enhanced T1 (D) show hyperintensity of the temporal lobe on T2 and FLAIR (arrows in A,B) in a patient with Herpes virus simplex type 1 encephalitis in the subacute phase. There are also some hemorrhagic foci (arrowheads in C) and contrast enhancement (arrow D). Note also concomitant temporo-polar arachnoid cyst (asterisk in A–D).

Figure 15

Enhanced T1 shows enhancement of the right VII-VIII nerves (arrow) in a patient with Herpes virus simplex in the cerebrospinal fluid.

Figure 16

Axial enhanced T1 shows enhancement of the left acoustic nerve and homolateral facial nerve (arrows) in a patient with Ramsay–Hunt syndrome.

Figure 17

Axial computed tomography shows a small hemorrhage in the subcortical region of the left parietal region (arrow) in a patient with varicella zoster virus encephalitis.