Page 830 - Clinical Small Animal Internal Medicine

798 Section 8 Neurologic Disease

extradural, intradural‐extramedullary or intramedullary pulse and capture of the resulting signal can be adjusted
VetBooks.ir locations. Although myelography may be abnormal with to create many different sequences, which can highlight
or remove signals associated with certain tissues or
meningomyelitis if a mass effect or spinal cord swelling is
a component of the disease process, these abnormalities
hypointense on T1‐weighted images and hyperintense
are nonspecific and rarely facilitate a definitive diagnosis. pathologies. Inflammatory lesions generally appear as
In addition, myelography itself results in inflammation on T2‐weighted images, with some degree of enhance-
of the meninges and spinal cord, which can worsen clini- ment after contrast administration. Enhancement of
cal signs, and so this intervention must be pursued with both parenchymal lesions and meninges can be seen
caution. Therefore, more advanced imaging modalities after intravenous contrast administration. This enhance-
are preferred for patients with potential inflammatory ment varies in intensity and uniformity, and may be
CNS disease, if available. influenced by a variety of factors including the degree of
Computed tomography (CT) is a technique that uti- inflammatory cell infiltrate, associated tissue edema and
lizes X‐rays to produce cross‐sectional images of desired the presence of necrotic tissue within the lesion.
anatomic structures. Software packages allow manipu- The analysis of CSF is an integral part of the diagnostic
lation of images to highlight tissues of varying densities process for patients with inflammatory CNS disease.
(“windowing”) and images can be obtained both before Cerebrospinal fluid can be collected from the cerebel-
and after the administration of intravenous contrast lomedullary cistern or the lumbar subarachnoid
material, which can highlight areas within the CNS space. Normal CSF has very few leukocytes (0–5/μL), an
where normal vascular barriers are compromised (e.g., absence of erythrocytes, and a low protein concentration
neoplastic tissue, areas of inflammation). Computed (0–20 mg/dL). Meningeal, brain or spinal cord inflamma-
tomography can also be combined with myelography to tion usually leads to increases in leukocytes (pleocytosis)
improve visualization of contrast within the subarach- and protein concentration within the CSF. Occasionally,
noid space, which increases the diagnostic utility of this an infectious (e.g., canine distemper) or noninfectious
latter technique. In animals with meningoencephalitis meningoencephalitis or meningomyelitis will lead to an
or meningomyelitis, CT may show areas of hypodensity increased protein without a corresponding increase in
within the brain or spinal cord parenchyma respectively, white blood cell numbers, which is known as albumino-
corresponding to inflammation and associated edema; cytologic dissociation. Cytologic evaluation of the CSF
these areas often show enhancement after contrast can also provide valuable information to the clinician.
administration. Meningeal enhancement, either occur- The composition of leukocytes within the CSF can
ring alone or in conjunction with parenchymal enhance- provide clues to an infectious etiology in some cases
ment, may also be noted. There are some limitations (e.g., eosinophilic pleocytosis with parasitic and some
associated with CT imaging, including a relative lack of fungal infections, lymphocytic pleocytosis with viral
sensitivity for inflammatory lesions when compared infections) or can be associated with presumed immune‐
with magnetic resonance imaging (MRI) and the occur- mediated conditions in others (e.g., neutrophilic pleocy-
rence of a phenomenon known as beam hardening tosis with SRMA, mononuclear pleocytosis with NME).
artifact, which compromises imaging of the medulla In rare cases, infectious organisms can be visualized on
oblongata due to the thick nature of the adjacent petrous CSF cytology (e.g., Cryptococcus neoformans).
temporal bone. It must be noted that CSF pleocytosis is not exclusive
Magnetic resonance imaging is currently the gold to inflammatory disease, as such a pattern can also
standard for visual evaluation of CNS anatomy and asso- be seen with other conditions, including neoplasia or
ciated disease states. Compared with CT, it has improved acute trauma. In addition, albuminocytologic dissocia-
sensitivity for the detection of most CNS pathology, tion is a common and relatively nonspecific change
including inflammatory diseases. It utilizes a completely seen with compressive, vascular, neoplastic, and
novel technology that does not rely on ionizing radiation degenerative CNS diseases. As a result, CSF evaluation
but rather on placing the patient in a high magnetic field is ideally performed in conjunction with advanced
and delivering radiofrequency pulses, which change the diagnostic imaging (preferably MRI) in order to rule
movements and behavior of hydrogen atoms (protons) out these other conditions.
within the patient. As these protons move, they create a Despite utilizing both CNS imaging and CSF analysis,
magnetic signal that is captured to generate images. the diagnosis can remain elusive in some cases. The most
As protons are primarily concentrated within water in common scenario is difficulty in distinguishing inflam-
biologic tissues, the differences in water content between matory from neoplastic disease. In these situations,
different tissues and different pathologic processes are obtaining meningeal and/or brain tissue for histologic
what determine the contrast within the images generated. evaluation may be the best way to arrive at a definitive
The parameters controlling delivery of the radiofrequency diagnosis. Brain biopsy is rarely performed for suspected