Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

670 Small Animal Radiolo g y and Ultrasono graphy
Extramedullary-Intradural. Extramedullary-intradural lesions arise within the dural
tube but external to the substance of the spinal cord and result in widening of the cord in
one view and deviation to the side on the other view (Fig. 6-5). The mass within the dural
tube will be within the subarachnoid space or will impinge upon it and may be outlined
by the contrast material. A “golf tee” appearance, caused by focal widening of the subarachnoid
space and contrast displacement around the mass, may be identified.
Extramedullary-intradural lesions may be due to tumors such as meningioma or neurofibroma.
Rarely, a prolapsed intervertebral disc may penetrate the dura and produce an
extramedullary-intradural lesion, or there may be an unusual primary or metastatic
tumor in this site. 24
Extradural. Extradural lesions arise external to the dural tube and usually will produce
widening of the cord on one view and displacement away from the lesion with resultant
compression of the spinal cord on the opposite view (Fig. 6-6). When extradural lesions are
not centered on the midline they may result in “elevation” of the subarachnoid space on
one side of the spinal canal and result in a “double-line” sign on the lateral view. In rare
instances, an extradural lesion will be contained focally on the midline and will demonstrate
a double-line sign on the lateral view, or the lesion may be distributed circumferentially
around the spinal cord, producing cord narrowing on both views. 25-27 The most
common cause for an extradural lesion is a prolapsed intervertebral disc. Tumors arising
from the vertebral body, tumors arising from the dura, neurofibromas, abscess, foreign
bodies, hemorrhage, aberrant parasites, and metastatic tumors also may produce
extradural lesions. 28-33
C O N G E N I TA L A B N O R M A L I T I E S
Abnormal Numbers of Vertebrae. Numerous anomalies of the spine have been reported.
A relatively common anomaly is an abnormal number of vertebrae for a specific region
(e.g., cervical, lumbar). This has been reported in 7% to 16% of dogs in studies. 34,35
Transitional Vertebra. Vertebral body anomalies are readily recognizable, and most
cause no clinical signs. Transitional segments frequently are seen at the lumbosacral and
thoracolumbar junctions. They are less common at the cervicothoracic junction. These
transitional vertebral segments have characteristics of one portion of the vertebral column,
yet when the vertebrae are counted they belong in another segment. One of the
most common of these anomalies is referred to as sacralization of L7 or lumbarization
of S1 (Fig. 6-7). 36,37 In this anomaly, either L7 articulates with the ilia in a manner similar
to the sacrum, or the first sacral segment will have transverse processes and may be
separated distinctly from S2 and S3. The articulation between L7 or S1 and the ilium may
be asymmetric, with a transverse process on one side and a sacroiliac joint on the other.
This asymmetry has no clinical significance, but it may interfere with symmetric positioning
of the pelvis for hip radiographs. A relationship between transitional lumbosacral
vertebrae and hip dysplasia or cauda equina syndrome has been suggested. 37-39
Incomplete or transitional ribs involving T13 or L1 also are very common and are identified
most often on the ventrodorsal view. The incomplete ribs are linear bony structures
that may lack apparent rib heads but have a junction with the vertebral body similar to
that of the lumbar transverse processes. The incomplete ribs extend beyond the normal
length of the transverse processes but usually are shorter than normal ribs and have a
greater curvature than the normal lateral process (Fig. 6-8). A transitional vertebra may
be seen at the cervicothoracic junction. This most often takes the form of ribs on C7.
These ribs often are wider than normal ribs and frequently are fused with the first thoracic
ribs distally.
Hemivertebrae. Hemivertebrae, vertebral bodies that are the result of incomplete
formation and appear wedge shaped, and butterfly vertebrae, which are cleft in the
sagittal plane, may be seen in any breed of dog but are common in brachycephalic