Spinal arachnoid diverticulum: how it looks on MRI
6-year-old dog with pelvic limb ataxia and paresis that have slowly worsened over months, with no spinal pain. T2-weighted thoracolumbar MRI: sagittal slice (top) and transverse slice at T13 (bottom):
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What does the MRI show?
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A. Dorsal disc extrusion compressing the cord at T13
Extruded disc material is usually hypointense on T2 and arises from the disc space, generally ventrally or ventrolaterally. Here the lesion is dorsal, has fluid signal and lies over the vertebral body.
B. Teardrop dilatation of the dorsal subarachnoid space
Correct. This is an arachnoid diverticulum: a focal accumulation of cerebrospinal fluid in the subarachnoid space, teardrop-shaped, compressing the cord dorsally. It causes slowly progressive, non-painful signs.
C. Syringomyelia: a fluid cavity in the centre of the cord
A syrinx lies within the parenchyma, around the central canal. On the transverse slice, the fluid of this lesion lies outside and above the cord, which it flattens.
D. Acute epidural haematoma displacing the cord ventrally
An epidural haematoma causes acute, painful signs and a heterogeneous signal that changes over time. It does not fit a lesion with fluid signal and a slow, painless course.
lightbulb Key takeaway
An arachnoid diverticulum appears on MRI (or myelography) as a teardrop-shaped dilatation of the subarachnoid space, with a compressed cord and sometimes oedema or a syrinx beyond it. It is typical of Pugs and French Bulldogs in the thoracolumbar region and of Rottweilers in the cervical region.
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Updated: November 1, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.