Magnetic Resonance Imaging Signal Alterations in Paraspinal Muscles in Dogs with Acute Thoracolumbar Intervertebral Disk Extrusion

Abstract

Muscle signal alteration detected on MRI is seen in diverse pathologic conditions. We observed signal alterations within the paraspinal muscles in dogs with acute thoracolumbar intervertebral disk extrusion. The aim of this retrospective study was to describe MRI features of paraspinal muscle signal alteration in dogs with acute thoracolumbar intervertebral disk extrusion and to investigate an association of the signal alterations with neurological grade, type and location of intervertebral disk extrusion, degree of spinal cord compression, and presence of epidural hemorrhage. Medical records of dogs undergoing MRI because of thoracolumbar intervertebral disk extrusion between August 2014 and June 2016 were reviewed. MRI was evaluated for SI changes within the paravertebral musculature, their location, extension, affected muscles, contrast enhancement, and signal void in T2* sequences. Intervertebral disk herniation was categorized as acute non-compressive nucleus pulposus extrusion (ANNPE) or compressive intervertebral disk disease. In five patients, muscle biopsies of areas with signal intensity changes were taken during surgery. In total, 103 dogs were enrolled in the study. Paraspinal muscle signal alterations were visible in 37 dogs (36%) affecting the epaxial musculature (n = 17), hypaxial musculature (n = 12), or both (n = 8). All signal alterations were hyperintense on T2-weighted images and iso- or hypointense in T1-weighted images. Signal void in T2* was not observed in any dog. Postcontrast sequences were available in 30 of the 37 dogs and showed enhancement in 45%. There was neither an association with degree of compression nor epidural hemorrhage. Intervertebral disk extrusion caudal to L1 and a higher neurological grade was associated with the presence of muscle changes. Histopathology revealed mild to moderate acute muscle fiber degeneration with edema and necrosis in three of five samples. The MRI, as well as the muscle samples, show rather unspecific changes. The underlying pathomechanism might be related to ischemia or muscle spasm, but also denervation edema may explain the signal alteration.

Keywords: dog; intervertebral disk extrusion; magnetic resonance imaging; paraspinal muscle; signal alteration; thoracolumbar.

Figure 1

Dorsal T2-weighted fat-suppressed images of two different dogs (A,B). In (A), only a mild signal alteration is visible in the Mm. multifidi of a French Bulldog with acute intervertebral disk herniation at the level L2/L3. More extensive signal alterations are visible in the Mm. multifidi of a Basset hound (B) with intervertebral disk extrusion at the level T12/T13. Both dogs had a neurologic grade of IV.

Figure 2

Dorsal T2-weighted (T2W) fat-suppressed image of a French bulldog presented with intervertebral disk extrusion at the level L5/L6. (A) bilateral signal alterations are visible in the hypaxial muscles (black arrows). On the T2W fast spin echo transverse image (B), only a focal area of increased signal intensity is visible in the M. psoas major on the right side (white arrow).

Figure 3

Graph illustrating the location of the intervertebral disk extrusion of all dogs, dogs without and with muscle signal alteration. Dogs with a disk extrusion caudal to L1 had a higher risk for muscle signal changes (OR = 2.8, P = 0.03) than dogs with a disk extrusion cranial to L1.