10 Infection and Inflammation


10 Infection and Inflammation

10.1 Introduction

Inflammatory conditions of both infectious and noninfectious origin can result in neurologic symptoms and have a variety of appearances on imaging. The appearances on imaging of some of these disorders are characteristic, whereas others can have nonspecific appearance requiring a differential diagnostic approach. In some instances, the ability to narrow the differential diagnosis can by itself aid in determining the appropriate treatment and/or additional diagnostic tests for an inflammatory condition. Familiarity with the laboratory profiles of CSF abnormalities in different infectious and inflammatory processes is important.

10.2 Infection

10.2.1 Meningitis

The most common infectious condition of the central nervous system (CNS) is meningitis. Meningitis is not a diagnosis based on imaging, and imaging is generally not indicated in this condition unless there are atypical features or focal neurologic deficits. Testing of cerebrospinal fluid (CSF) is the primary means of confirming the diagnosis of meningitis. In patients with known meningitis, there are two primary reasons why imaging may be performed. The first is to seek signs of a source of the infection, such as osseous erosion from the mastoid air cells or paranasal sinuses, and the second is to seek signs of an abscess. Although computed tomography (CT) can reveal signs of osseous dehiscence, an abscess is best seen on contrast-enhanced magnetic resonance imaging (MRI) with diffusion-weighted imaging (DWI).

On MRI, meningitis will appear as leptomeningeal enhancement, usually without any parenchymal signal abnormality. Leptomeningeal enhancement is most commonly evaluated on post contrast T1 W images, although post contrast fluid-attenuated inversion recovery (FLAIR) images are especially sensitive to it. Contrast-enhanced CT is rarely indicated in meningitis, because in most situations a high-quality unenhanced CT scan will identify any extra-axial collections or parenchymal edema, and contrast-enhanced MRI is more sensitive than contrast-enhanced CT in identifying inflammation. Computed tomographic examination without and with contrast enhancement will double the radiation dose delivered to the patient while yielding only slightly more information than will unenhanced CT. Performing only a post-contrast CT scan will limit the ability to detect hemorrhage. Therefore, the preferred diagnostic evaluation in suspected meningitis would be an unenhanced CT scan and, if there are abnormalities or further clinical concern, subsequent MRI without and with contrast enhancement. Ultimately, it should be remembered that a lumbar puncture is the most important tool in diagnosing meningitis. This can also be associated with a callout to (s. Tab.). 1

Table 10.1 Results of lumbar puncture in inflammatory and infectious disorders of the central nervous system




Protein 15 to 60 mg/100 mL of CSF, glucose 50 to 80 mg/100 mL, cell count 0 to 5 white cells, no red blood cells

Bacterial meningitis

Elevated neutrophil count, decreased glucose concentration; protein concentration can be elevated

Viral meningitis

Elevated lymphocyte count, normal to mildly decreased glucose concentration

Noninfectious inflammatory process

Minimally elevated lymphocyte count, normal glucose concentration, mildly elevated protein concentration

Neoplasm with cerebrospinal fluid dissemination

Markedly elevated protein concentration; perform cytology on cerebrospinal fluid sample if concern exists about possible neoplasm

*Note that normal ranges will vary slightly among laboratories.

10.2.2 Abscess

An abscess will typically have a peripheral contiguous rim of enhancement and centrally will demonstrate reduced water diffusion (Fig. 10.1). When the abscess is intra-axial, there will be surrounding edematous changes. An abscess, also known as an empyema, can occur in the subdural or epidural space (Fig. 10.2), in which case it is usually secondary to conditions like sinonasal/mastoid disease or penetrating trauma, or is postoperative.

Fig. 10.1 Abscess. (a) Axial computed tomographic image of the head of a 10-year-old girl shows a round lesion (red arrowhead) with a surrounding rim of edema (red arrow). (b) Coronal fluid-attenuated inversion recovery image shows an ovoid lesion with a rim of hyperintense signal. (c) Axial ADC map shows a hypointense area centrally (red arrow) representing restricted diffusion, with a peripheral rim of hyperintense signal (red arrowhead) representing facilitated diffusion. (d) Coronal T1 W plus contrast image shows a peripheral rim of enhancement and no central enhancement. This represents an abscess with a peripheral rim of vasogenic edema.
Fig. 10.2 Pott’s puffy tumor/frontal empyema. (a) Axial computed tomographic image of the head of a 16-year-old boy with headache, fever, and new forehead swelling shows extracranial soft tissue swelling in the scalp (red arrow) and an extra-axial collection overlying the right frontal pole (red arrowhead). (b) Bone algorithm image shows a focal area of dehiscence in the outer cortex of the right frontal sinus (red arrowhead). (c) Sagittal T1 plus contrast image shows a peripherally enhancing fluid collection overlying the right frontal sinus (red arrow), representing an extracranial abscess (Pott’s puffy tumor). There is pachymeningeal thickening overlying the right frontal lobe, with focal areas of fluid collection consistent with empyemas (red arrowheads). There is also fluid within a peripherally enhancing frontal sinus (green arrowhead). (d) Axial diffusion-weighted image shows diffusion restriction within the extracranial fluid (red arrow) and intracranial collections (red arrowheads), confirming that these findings are empyemas/abscesses. The collection extends along the falx cerebri, indicating that it has a subdural component as well as a presumed epidural component.

10.2.3 Encephalitis

Encephalitis is a condition in which there is inflammation of the brain parenchyma itself, of either infectious or noninfectious origin. Because bacterial infection of the brain will usually lead to necrosis/abscess, encephalitis of infectious origin is typically related to a viral infection. A special consideration among viral encephalitides is herpes simplex virus (HSV) encephalitis, which most commonly presents as a febrile seizure. Such encephalitis is most often related to HSV-1 virus involved in an orofacial infection (such as a cold sore), in which there is reactivation of the virus in the trigeminal ganglion (also known as the gasserian ganglion) (refer to Fig. 9.6). Accordingly, the adjacent portion of the medial temporal lobe will be preferentially involved, as evidenced on imaging through a hyperintense T2/FLAIR signal, reduced water diffusion, and possibly postcontrast enhancement, with the eventual development of hemorrhagic necrosis. If this diagnosis is suspected, treatment with acyclovir should be started immediately (not waiting for confirmation of the diagnosis). Only upon a negative result of polymerase chain reaction (PCR) for HSV on a CSF specimen should acyclovir therapy be discontinued.

In the neonatal period, HSV infection is more commonly related to the HSV-2 virus, may not involve the temporal lobes, and can have a scattered and random distribution (see Chapter 5) from hematogenous and/or CSF dissemination.

Other viral encephalitides tend to cause nonspecific edema predominantly in the gray matter, most often without abnormal enhancement, hemorrhagic changes, or diffusion abnormality.

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May 28, 2020 | Posted by in NEUROLOGICAL IMAGING | Comments Off on 10 Infection and Inflammation

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