Page 172 - Atlas of Small Animal CT and MRI

2.2

Brain edema

Introduction Vasogenic edema

Brain edema may result from a wide array of causes, Vasogenic edema occurs because of a disruption of
which can be divided into the four principal forms listed the tight junctions of the blood–brain barrier, resulting
in Table 2.2.1. Clinically, multiple forms of brain in extravasation of high‐protein fluid into the brain.
1–3
edema can occur simultaneously, and often the predom- Vasogenic edema is extracellular, so it tends to preferen-
inating form depends on the inciting cause as well as tially accumulate in white matter, which has a sparser
the time course of the disease. Whether intracellular cellular density and therefore more potential space for
or extracellular, edema appears mildly to moderately fluid distribution compared to highly cellular gray mat-
hypoattenuating to normal brain parenchyma on CT ter. Depending on the initiating cause and volume of
3
images and T1 hypointense and T2 hyperintense on MR fluid, edema can distribute widely (Figures 2.2.2, 2.2.3).
images. Because edema fluid is distributed within a
microenvironment of cells and macromolecules, it will Interstitial or hydrocephalic edema
also appear hyperintense on FLAIR and other pure Interstitial edema most often occurs in association with
water‐nulling sequences.
obstructive hydrocephalus when intraventricular pres-
sure increases, causing transependymal CSF migration
Cytotoxic edema into adjacent brain parenchyma. As a result, hydrostatic
edema preferentially occurs within periventricular
Cytotoxic edema occurs as a result of ischemia resulting
in cell membrane Na/K pump dysfunction, increased parenchyma and is extracellular (Figure 2.2.4). Unlike
vasogenic edema, interstitial edema fluid is a transudate
intracellular fluid volume, and cell swelling. Because 3
of the underlying cause and the intracellular nature of containing little in the way of cells or macromolecules.
this form of edema, white and gray matter may both be Osmotic edema
affected, and the distribution of edema roughly conforms
to the geographic distribution of ischemia (Figure 2.2.1). Osmotic edema occurs rarely and is caused by reduced
3
In most instances, cytotoxic edema occurs in combina- plasma osmolality resulting from water intoxication,
tion with vasogenic edema. Diffusion‐weighted imaging hemodialysis, or metabolic disorders that reduce plasma
has been used to discriminate between the two forms sodium or glucose concentration. The imbalance in
following acute episodes of ischemia, with reduced brain extracellular fluid osmolality and plasma osmolal-
apparent diffusion coefficient (ADC) intensity reflecting ity results in a fluid shift to the brain leading to formation
predominantly cytotoxic edema. 4 of extracellular edema. 3

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
162