MRI Pitfall in Canine Cervical HNPE: Why the Dorsal Subarachnoid Space May Remain Visible

Persistent Visibility of the Dorsal Subarachnoid Space in Dogs With Compressive Cervical Myelopathy Can Be Explained by the Interarcuate Space

Alexis Tolbert, Silke Hecht, Talisha Moore

Background

Intervertebral disc herniations in dogs frequently require advanced imaging for diagnosis. Magnetic resonance (MR) myelography evaluates the subarachnoid space and helps identify extradural spinal cord compression by detecting attenuation or deviation of cerebrospinal fluid (CSF) signal. In hydrated nucleus pulposus extrusion (HNPE), a type of acute cervical disc extrusion characterized by T2-hyperintense extradural material, the extruded material may be isointense to CSF. This can obscure attenuation of the ventral subarachnoid space and potentially mask the severity of compression. Clinically, the authors observed that the dorsal subarachnoid space sometimes remains visible at the level of disc extrusion, which may contribute to misinterpretation of MR myelograms.

Methods

This retrospective descriptive study reviewed medical records and cervical MRI studies of dogs presented to the University of Tennessee between 2013 and 2023 with an MRI diagnosis of cervical HNPE. Inclusion criteria included acute neurologic signs consistent with C1–T2 myelopathy and imaging findings characteristic of HNPE. MRI studies were evaluated for the presence or attenuation of the dorsal subarachnoid space on sagittal MR myelographic sequences. The degree of spinal cord compression (SCC) was calculated by comparing the cross-sectional area of the spinal cord at maximal compression with a nearby normal segment. When available, CT scans were also reviewed to assess vertebral anatomy and identify structural explanations for maintained dorsal CSF signal.

Results

Thirty-four dogs were included, with one dog contributing two cases, resulting in 35 MRI studies. The most common sites of HNPE were C4–5 and C3–4. The dorsal subarachnoid space remained visible on MR myelograms in 25 of 35 cases (71.4%), while attenuation occurred in 10 cases (28.6%). The degree of spinal cord compression was similar regardless of whether the dorsal subarachnoid space remained visible. In cases with available CT imaging, the retained CSF signal corresponded anatomically to the interarcuate space—the gap between adjacent vertebral arches—suggesting this structure allows focal dorsal expansion of the subarachnoid space.

Limitations

The study was limited by its retrospective design and relatively small sample size. Only cervical HNPE cases were evaluated, and other forms of cervical compressive myelopathy or thoracolumbar HNPE were not included. CT imaging was available for only two dogs, limiting confirmation of the proposed anatomic explanation. Additionally, some dogs were managed medically rather than surgically, so definitive confirmation of HNPE was not obtained in all cases. The effects of neck positioning, breed variation, and patient signalment on the appearance of the interarcuate space could not be assessed.

Conclusions

Failure of the dorsal subarachnoid space to attenuate on MR myelography is common in dogs with cervical HNPE and does not correlate with the severity of spinal cord compression. This phenomenon is likely explained by displacement of the dorsal subarachnoid space into the interarcuate space between adjacent vertebral arches. Recognizing this imaging pitfall is important, as reliance on circumferential CSF attenuation alone may lead to underestimation or missed diagnosis of compressive cervical myelopathy. Careful evaluation of other MRI indicators of spinal cord compression remains essential.

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