Page 851 - Clinical Small Animal Internal Medicine

76 Myelopathy 819

A traction injury of the spinal cord involving tethering dural/extramedullary, or intramedullary location can be
VetBooks.ir of nerves often is traumatic in origin. This injury com- determined by the type of distortion occurring in the con-
trast column.
monly involves the nerves associated with the sacro-
Cross‐sectional imaging includes computed tomogra-
coccygeal and cervicothoracic spinal cord segments.
Traction of nerves/nerve roots causes injury of associ- phy (CT) and magnetic resonance imaging (MRI) and is
ated spinal cord segments such as tail‐pull and brachial more likely to provide a definitive diagnosis. These imag-
plexus avulsion. For example, in cats with tail‐pull injury, ing modalities are now available at many specialty
injury to the sacral segments affects the pelvic nerve and practices. Lesion descriptions affecting the spinal cord
its innervation of the bladder, resulting in loss of bladder are similar to those described for myelography.
contraction. Administration of contrast media can improve visualiza-
Ascending and descending hemorrhagic myelomalacia tion of pathology on MRI and CT. Intravenous adminis-
should be suspected in dogs with thoracolumbar tration of contrast media can increase the conspicuity of
intervertebral disc disease (IVDD) that have an ascend- a lesion due to breakdown in the blood–brain barrier and
ing loss of the cutaneous trunci reflex. Other neurologic better define its borders by creating greater contrast
signs of myelomalacia include loss of nociception caudal between the lesions and surrounding tissue.
to the lesion, ascending and descending flaccidity, weak- Computed tomography is a cross‐sectional imaging
ness and areflexia, which can lead to tetraplegia, hyper- modality in which images are constructed based on the
thermia, and respiratory distress. Death results from attenuation of X‐rays through tissue. The main benefit of
asphyxia from intercostal and diaphragmatic muscle CT over conventional radiography is that with CT, there
paralysis. Clinical signs of ascending and descending is greater soft tissue differentiation and lack of superim-
myelomalacia may manifest in hours to several days from position of overlying structures which greatly improves
onset of paraplegia. the evaluation of the vertebral column. CT also can assist
with determining lesion extent after myelography. CT
images should be evaluated for alterations in attenuation
Diagnostic Approach of tissues. Dark or relatively black areas are hypodense or
hypoattenuating while light or relatively white areas are
Diagnostic evaluation of an animal with spinal cord dis- hyperdense or hyperattenuating. CT may provide more
ease begins with bloodwork (complete blood count, useful information than MRI in cases of vertebral frac-
serum biochemistry), urinalysis, and survey spinal radio- tures or subluxation where bony detail is important.
graphs. Thoracic radiography is recommended in Taking advantage of the density of dystrophic mineral-
animals older than 5 years or when neoplasia is consid- ized intervertebral disc material, CT can be used in the
ered a disease differential. If a diagnosis cannot be deter- evaluation of chondrodystrophic dogs with suspected
mined through these routine procedures, referral to a IVDD. Due to the inferior soft tissue contrast, assess-
specialty practice is recommended. ment of soft tissues within spinal cord tissue is limited.
Conventional radiography of the vertebral column Magnetic resonance imaging is the preferred imaging
can assist with recognition of obvious abnormalities modality for assessment of the spinal cord. The primary
such as congenital malformation, discospondylitis, reason for this is the superior soft tissue contrast that
fracture/luxation, and bone neoplasia. Meticulous MRI provides, allowing differentiation of anatomic
technique, collimation, and proper patient positioning, structures. No other imaging procedure provides con-
usually under anesthesia or heavy sedation, are essen- spicuity of soft tissue discrimination along with excep-
tial to detection of subtle changes that are often the key tional resolution. Imaging can be performed in different
to a diagnosis. Although many lesions are obvious with planes (transverse, sagittal, dorsal) without loss of reso-
cursory inspection, subtle changes may be difficult to lution, enabling examination of complex anatomic
appreciate. regions. MRI is based on the magnetic properties of
However, in the majority of instances, the diagnostic hydrogen atoms, which are extremely abundant in tis-
capability of plain radiography of the vertebral column is sues containing water, protein, and fat. Lesions are char-
limited. The diagnostic capabilities are improved with acterized based on number (single or multiple), intensity
myelography, injection of iodinated contrast media into (hypo‐, hyper‐, isointense in relationship to an adjacent
the subarachnoid space. Myelography is accurate and sen- normal area), distribution of the intensity (homogeneous
sitive for identifying compressive lesions. Dynamic stud- or heterogeneous), lesion borders (well or poorly
ies (traction, extension, flexion) also can be performed. defined), anatomic location of the lesion as well as lesion
Masses that occupy space in the vertebral canal (e.g., location in reference to the spinal cord and meninges.
tumors, abscesses, disc herniations) cause alterations in Similarly, lesions are characterized based on pattern of
the myelographic contrast column. The extradural, intra- contrast enhancement.