Page 165 - Atlas of Small Animal CT and MRI

2.1

Ventricular system and hydrocephalus

Normal ventricular system be more appropriate when passive ventricular enlarge-
ment occurs as a result of diminished brain parenchyma
The normal ventricular system consists of the third, volume (also known as hydrocephalus ex vacuo)
fourth, and lateral ventricles. The third ventricle (Figure 2.1.3). Normal CSF has a density near that of
communicates with the lateral ventricles via the inter- pure water and will therefore have a HU value of close to
ventricular foramina, and the mesencephalic aqueduct 0 on unenhanced CT images and will be hypoattenuat-
connects the third and fourth ventricles (Figures 2.1.1, ing to surrounding brain parenchyma. On unenhanced
2.1.2). Caudally, the fourth ventricle communicates MR images, normal CSF will appear T1 hypointense and
with the central canal. Cerebrospinal fluid (CSF) is T2 hyperintense to brain parenchyma and will have no
produced by the choroid plexus, located on the floor of or low signal on water‐nulling sequences, such as FLAIR.
the lateral ventricles and on the dorsal margins of the In patients with abnormal CSF due to hemorrhage,
third and fourth ventricles. CSF circulates throughout inflammation, or neoplasia, signal intensity may be
the ventricular system and exits into the subarachnoid significantly increased on T1 and pure water‐nulling
space through the lateral apertures of the fourth ventri- sequences depending on cellular and macromolecular
cle. CSF is resorbed primarily through arachnoid villi content (Figure 2.1.4).
that extend into the dural venous sinuses and secondar-
ily through lymphatic drainage of the meningeal sheaths Congenital hydrocephalus
1
surrounding nerve roots. The ventricular system also
includes a number of anatomic recesses that may appear Congenital hydrocephalus occurs predominantly in
more prominent in the presence of hydrocephalus. brachycephalic and toy breeds (Figure 2.1.5). In some
instances, mechanical obstruction from such entities as
Hydrocephalus mesencephalic duct stenosis or Chiari malformation
explains the presence of hydrocephalus; in other cases,
Hydrocephalus is defined as an abnormal distension of no underlying cause is recognized. 4–6
all or part of the ventricular system with CSF. Ventricular
distension is typically caused by constant or intermittent Obstructive hydrocephalus
increased hydrostatic pressure. The term hydrocephalus
denotes the anatomic status of the ventricular system Obstructive hydrocephalus may be due to intraluminal
rather than an underlying cause. Hydrocephalus can be or extraluminal masses or other lesions that impair
developmental or acquired and can result from obstruc- CSF flow within the ventricular system. The underlying
tion of CSF drainage, from impaired CSF resorption, or causes of obstructive hydrocephalus vary widely, as do
from CSF overproduction. The latter two forms are the imaging features of the ventricular system.
2,3
referred to as communicating or nonobstructive hydro- Depending on the source and location of obstruction,
cephalus. Use of a related term, ventriculomegaly, may hydrocephalus may be uniform or regional. Obstruction

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.
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