Case Report: Filum terminale extradural arachnoid diverticulum associated with tethered cord syndrome in a French Bulldog.

Extradural arachnoid diverticula and tethered cord syndrome are rare disorders in dogs, and their simultaneous occurrence with a possible mechanistic association has not been previously reported. A 2-year-old male intact French Bulldog was evaluated for chronic lumbar pain, presumed secondary aggression, and subtle pelvic limb neurological deficits. Magnetic resonance imaging (MRI) revealed a well-defined, extradural, CSF-isointense, cyst-like lesion located at the origin of the filum terminale externum and dural sac termination. An additional intradural, elongated fluid-filled lesion positioned within the subarachnoid space, dorsal to the conus medullaris termination in the region of the filum terminale internum was also identified. Dynamic lumbosacral MRI in neutral, flexed and extended positions demonstrated a fixed position of the conus medullaris and dural sac terminations, raising suspicion for associated tethered cord syndrome. Mild dorsal sacral laminar telescoping with ventral bulging of the interarcuate ligament and dynamic vertebral canal stenosis was also noted, without cauda equina compression. Surgical treatment included a left dorsolateral hemilaminectomy at L5-L6 for en bloc removal of the extradural lesion, excision of a dural-vertebral fibrous adhesion identified intra-operatively, and durotomy with transection of the filum terminale internum, which did not appear abnormal intra-operatively. A dorsal laminectomy at S1 was also performed to resect the interarcuate ligament. Histopathology revealed a diverticulum composed of dural and arachnoid elements and confirmed the identity of the filum terminale internum. A final diagnosis of a type Ib extradural arachnoid diverticulum associated with tethered cord syndrome was made, although concurrent dynamic lumbosacral stenosis due to sacral laminar telescoping could not be excluded. The dog achieved full clinical recovery, with resolution of pain and behavioral abnormalities, and remained normal at 1-year follow-up. This case documents a previously unreported extradural arachnoid diverticulum arising from a pedicle of the meninges of the filum terminale externum and dural sac termination. It highlights the potential role of dynamic MRI in assessing conus medullaris mobility in cases with filum terminale pathology and shows a favorable outcome following surgical intervention.