Magnetic resonance imaging features of canine intradural/extramedullary intervertebral disc extrusion in seven cases

Abstract

Intervertebral disc disease, including intervertebral disc extrusions and protrusions, is the most common spinal cord disorder in dogs. Atypical and uncommon intervertebral disc herniations include intradural/intramedullary disc extrusion, intervertebral foraminal disc extrusion and intervertebral disc herniation (Schmorl's node). Intradural/extramedullary disc extrusion is the least common type of intervertebral disc herniation in veterinary medicine, characterized by extruded disc material within the intradural space. To date, only one study has been published in veterinary medicine reporting intradural/extramedullary disc extrusions. In this study, low field MRI was used, and the authors could not find any MRI features to diagnose with confidence an intradural/extramedullary disc location of the extruded disc material. The aim of this study was to describe the high field (1.5T) MRI characteristics of surgically confirmed intradural/extramedullary disc extrusions. This is a retrospective, multicentric and descriptive study. Inclusion criteria was surgical confirmation of intradural/extramedullary disc extrusion by durotomy and complete MRI study of the spine. Seven cases were included. Images were reviewed by a radiology resident and a certified radiologist, with emphasis on the following signs: "Golf-tee sign" (widening of the subarachnoid space cranial and caudal to the lesion), "Beak sign" (pointed and sharp compressive lesion) and "Y sign" (division of the dura and arachnoid layers). MRI showed a "Y sign" in all the cases (7/7) seen from the T2-weighted sagittal views, while "Golf-tee sign" was not recognized in any of the cases (0/7). Additionally, "beak sign" was present in half of the cases (4/7). "Y sign" maybe a reliable MRI feature for identifying intradural/medullary disc extrusions from the MRI study. As the arachnoid is peeled from the dura by the disc herniation there is a splitting of the arachnoid mater and the ventral dura. The intradural disc material will be surrounded by CSF signal intensity margin, giving the appearance of a Y, which can be identified from the T2-weighted sagittal images.

Keywords: MRI; Y sign; disc extrusion; dog; extramedullary; intradural.

Figure 1

Examples of “Y sign” on T2-weighted sagittal magnetic resonance images of dogs with intradural/extramedullary disc extrusion (A–C). The red line in (D–F) highlights the “Y sign” appearance. (A,D) T2W sagittal sequence showing a L3-L4 intradural/extramedullary disc extrusion with the presence of a “Y sign” (Case 4). The T2-weighted homogenous hypointense disc material is appreciated surrounded by the T2W hyperintense CSF within the subarachnoid space giving a “Y” shape. This is caused by the division of the dura and the arachnoid layers as the latter is peeled off from the dura by the disc herniation which occurs between the dura and the arachnoid. (B,E) T2-weighted sagittal sequence showing a T11-T12 intradural/extramedullary disc extrusion (Case 6) with the presence of a “Y sign.” This is represented by T2-weighted hyperintense CSF in the caudodorsal and caudoventral aspects of the disc material. This “Y” shape is given by the separation of the dura and the arachnoid layers by the intradural disc material. Consequently, T2-weighted hyperintense signal (cerebrospinal fluid) is appreciated at the edges of the lesion displayed as a “Y” shape. (C,F) T2-weighted sagittal sequence showing a T11-T12 intradural/extramedullary disc extrusion (Case 7) with the presence of a “Y sign.” The presence of disc material within the subdural space creates a “Y” shape which corresponds to the splitting of the dura matter and the arachnoid matter in two separate lines “Y sign.”

Figure 2

(A,C) T2-weighted transverse sequence showing a L3-L4 intradural/extramedullary disc extrusion (Case 4). The T2-weighted homogeneous hypointense disc material is appreciated on the left side compressing moderately the spinal cord ventrally and to the right. This is appreciated as a beak-like shaped and sharply marginated lesion (“beak sign”). The T2-weighted hyperintense signal within the subarachnoid space representing CSF is appreciated surrounding the intradural disc material. (B,D) T2-weighted transverse sequence displaying a T11-T12 intradural/extramedullary disc extrusion with the presence of a “beak sign” (Case 3). This is appreciated as a sharply marginated T2-weighted hypointense lesion that is compressing the spinal cord to the right side.