Infection and Inflammation

Chapter 34

Infection and Inflammation

Bacterial, viral, fungal, and parasitic organisms are all causative factors in neurologic infection. Brain infection manifests as encephalitis, cerebritis, and meningitis. Encephalitis refers to diffuse infection of the brain parenchyma, whereas cerebritis is a more focal parenchymal infection. Meningitis refers to infection of the pia, arachnoid, and dural membranes, as well as the cerebrospinal fluid (CSF). Ventriculitis is often present in cases of meningitis. Infectious complications most often include abscess, empyema, or both. Infection in the setting of tumor often presents a diagnostic conundrum.

Imaging of central nervous system (CNS) infection is most often initially performed by means of computed tomography (CT) to assess for the possibility of hydrocephalus or increased intracranial pressure prior to the preparation of a lumbar puncture. Nonspecific parenchymal hypoattenuation indicative of edema on CT is sometimes appreciated in cases of more focal infection. CT is superior in the evaluation of bone erosion and destruction. Magnetic resonance imaging (MRI) is warranted in the assessment of infectious complications such as abscess, empyema, vasculitis, and ischemia in the setting of a worsening clinical condition or lack of clinical improvement despite appropriate therapy. Infection is most often manifested on MRI by abnormal hyperintense signal on T2-weighted (T2W), proton density, and fluid-attenuated inversion–recovery (FLAIR) sequences, with corresponding hypointense signal on T1-weighted (T1W) sequences. Postcontrast T1W images are essential in the evaluation for infectious collections and meningeal enhancement. Magnetic resonance venography (MRV) sequences may detect associated venous sinus thromboses. Diffusion-weighted imaging (DWI) may assist in localizing abscess collections, infection associated with ischemia, or both and may at times help distinguish lymphoma from abscess, especially in immunocompromised patients. DWI may also demonstrate lesions earlier than conventional sequences in viral infections such as herpes and West Nile virus (e-Fig. 34-1). Magnetic resonance spectroscopy has shown promise in distinguishing pyogenic abscesses from those caused by atypical organisms, with the former often demonstrating the presence of amino acids and lack of choline.

Bacterial Infections

Nearly two thirds of cases of bacterial meningitis in the United States occur in children. Routes of transmission include hematogenous, direct traumatic, congenital routes, as well as direct extension from adjacent sinus or mastoid disease (Figs. 34-2 through 34-4 and e-Fig. 34-5). Imaging plays a key role in determining the course of treatment for bacterial infection. It is essential to distinguish between focal cerebritis (Fig. 34-6), which tends to respond to antibiotics, and abscess, which often requires surgical intervention. Hypoattenuation on CT, indicative of edema, and corresponding hyperintense T2W and hypointense T1W signal on MRI, with patchy nonspecific postcontrast enhancement, are typical imaging characteristics of cerebritis. Mild to moderate mass effect is often present. Sequential imaging is essential in the assessment of the response to antibiotics as well as for progression to an abscess. Progression from cerebritis to abscess generally takes 1 to 2 weeks but may progress more quickly in neonates. Citrobacter, Serratia, and Proteus are the most common causes of neonatal brain abscess. Citrobacter and Serratia infection may cause medullary vein thrombosis and associated hemorrhage (Fig. 34-7). In general, abscesses tend to be situated at gray-white matter junctions, where the diameter of the end arterioles decreases (Fig. 34-8). Opportunistic organisms are common in immunocompromised neonates.

Differential diagnostic considerations of a peripherally contrast-enhancing fluid-filled structure in the brain includes infectious abscess and tumor. On DWI, abscesses will appear hyperintense (and dark on apparent diffusion coefficient, indicating restricting material within the capsule. On both MRI and CT, abscesses tend to have smooth regular inner margins and are often thinner walled along their medial edge than along their lateral margins. Intraventricular rupture of an abscess portends a poor outcome. Spectroscopy of abscesses is notable for the presence of amino acids and lactate (Fig. 34-9) and the absence of normal metabolite peaks. In neonates, ultrasonography may depict a hypoechoic abscess with peripheral hyperechogenicity, which may contain dependent echogenic debris.

Bacterial meningitis is the most common form of pediatric CNS infection. Although not diagnosed by imaging, imaging is warranted if a diagnosis is unclear, persisting seizures are present, and symptoms persist despite treatment. It is more common in preterm infants and full-term infants within the first month of life. The subarachnoid space tends to resist infection in older children, making meningitis in this age group a rarity. Most cases of neonatal meningitis in the United States are caused by group B Streptococcus (Fig. 34-10) and Escherichia coli. Other less common organisms (e.g., Serratia, enterococci, and Listeria) tend to inflict more extensive destruction. In infants older than 1 month, the most common causative organisms are Haemophilus influenza type B, Streptococcus pneumonia (Fig. 34-11), Neisseria meningitides, and Escherichia coli. Complications of meningitis include cerebritis, abscess, empyema, hydrocephalus, venous thrombosis, infarction (venous and arterial), ventriculitis (e-Fig. 34-12), mycotic aneurysms (Fig. 34-13), and sensorineural hearing loss (Fig. 34-14).

Dec 20, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on Infection and Inflammation

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