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.
Highlights
Magnetic resonance imaging (MRI) signal intensity of normal skeletal muscle is generally slightly higher than that of water and much lower than fat on T1-weighted (T1W) images and much lower than both fat and water on T2-weighted (T2W) images [1]
T1W images were available in 35 dogs (95%) with hypointense signal alterations observed in six dogs (17%)
Paraspinal muscle signal alteration was observed in 36% of dogs with acute intervertebral disk extrusion and is, considered a common finding on MRI examination
Summary
Magnetic resonance imaging (MRI) signal intensity of normal skeletal muscle is generally slightly higher than that of water and much lower than fat on T1-weighted (T1W) images and much lower than both fat and water on T2-weighted (T2W) images [1]. Alterations of signal intensity of skeletal muscles due to pathologic conditions is identified on inversion-recovery and fat-suppressed T2W images [1, 2]. The potential causes are diverse, but usually the abnormal signal intensity identified on MRI falls into one of three recognizable patterns: edematous lesions, mass lesions, or fatty infiltration [1]. Edematous lesions of the paraspinal muscles have been observed in immune-mediated polymyositis of the sublumbar muscles [3], associated with meningoencephalitis of unknown origin in the cervical musculature, or due to paraspinal infection in the ventral cervical and lumbar musculature [4]. A third pattern consists of fatty infiltration of paraspinal muscles and has been observed in chondrodystrophic and nonchondrodystrophic dogs, probably driven by a combination of chronicity and severity of spinal cord pathology [5]
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