Occult tethered cord syndrome: insights into clinical and MRI features, prognostic factors, and treatment outcomes in 30 dogs with confirmed or presumptive diagnosis

Abstract

Occult tethered cord syndrome (OTCS) is poorly documented in dogs. This retrospective multicenter study evaluated the clinical presentation, MRI findings, treatment outcomes, and prognostic factors in 30 dogs diagnosed with OTCS managed surgically (n = 11) or medically (n = 19). Novel clinical severity scoring and neurological grading systems were developed to assess prognostic utility. The median age at clinical onset was 11 months (range 2-65), with a median duration of clinical signs of 13 months (range 1-60). Pain/dysesthesia in the lumbosacral region/tail/pelvic limbs was the most common presenting complaint (97%), followed by pelvic limb gait abnormalities (70%), behavioral changes (67%), impaired physical activity (63%), and urinary/fecal incontinence (17%). Neurological deficits were present in 90% of dogs. MRI findings showed variability in conus medullaris and dural sac termination, with no physiological translocation detected in available dynamic studies. Electrodiagnostic abnormalities were identified in four of nine tested dogs (44%). Clinical severity scores strongly predicted response to medical treatment, with responders having significantly lower scores than non-responders (3.25 ± 2.09 vs. 7.78 ± 3.15, p < 0.001). Higher neurological grades (p = 0.006), presence of behavioral abnormalities (p = 0.045), and worsening clinical evolution prior to referral (p = 0.009) were also associated with poor medical therapy outcomes. Surgical intervention was significantly associated with full recovery (p = 0.015) and discontinuation of medical treatment (p = 0.023) at last follow-up (median: 9 months, range: 2-108). Three surgically treated dogs experienced partial relapse within 6 months, with two undergoing reintervention and improving postoperatively. This study highlights the clinical and MRI characteristics of canine OTCS, introduces novel prognostic factors, and supports surgical detethering as a key intervention for optimizing outcomes. Larger prospective studies are needed to validate these findings, refine the proposed scoring systems, and establish evidence-based guidelines for managing canine OTCS.

Keywords: behavior; conus medullaris; dural sac; dynamic MRI; filum terminale; incontinence; lumbosacral; pain.

Figure 1

T2w 3D fast imaging employing steady-state acquisition (FIESTA) high resolution mid-sagittal images at the level of the caudal lumbar, sacral, and cranial coccygeal vertebral column and associated structures, on neutral (A) and flexed (B) lumbosacral angles (dynamic lumbosacral study), demonstrating the possible termination levels of the conus medullaris and dural sac. The red line is drawn perpendicularly in the middle of a white line connecting the mid-cranial and mid-caudal endplates of the same vertebra. The green lines are drawn over the margins of the vertebral endplates. In this case, the conus medullaris terminates at cranial L7 (yellow arrow) on both neutral and flexed lumbosacral angles (confirmed on transverse images). The dural sac terminates at the level of the intervertebral disc space L7-S1 (light blue arrow) in both neutral and flexed angles. IVD = intervertebral disc space, CrV = cranial vertebral body, MV = middle vertebral body, CaV = caudal vertebral body, IA = interarcuate space.

Figure 2

T2w 3D FIESTA high resolution mid-sagittal images of the lumbosacral junction on neutral (A) and flexed (B) position demonstrating a change in lumbosacral angle (LSA) from 150^o on neutral to 175^o on flexion.

Figure 3

Study flow diagram illustrating case selection. A total of 41 candidate cases were initially identified from the clinical records of 12 referral hospitals based on the inclusion criteria. Eleven cases were excluded for various reasons: five had concurrent lumbosacral abnormalities on MRI, such as congenital malformations and degenerative changes; three conservatively managed cases showed conus medullaris (CM) physiological translocation on dynamic MRI; two surgically treated cases lacked documentation of transection of the filum terminale (FT) in the clinical notes; and one case had inadequate follow-up data. As a result, 30 cases met the study criteria: 19 were managed conservatively, while 11 underwent surgery.

Figure 4

Anatomical distribution of conus medullaris and dural sac termination (CMt, DSt) levels in dogs with C-OTCS and P-OTCS. The plot illustrates individual termination levels for both structures, highlighting case-specific anatomical variations. The range and median termination levels for each group are also shown.

Figure 5

Dorsal laminectomy from L7 to S2 in a dog with C-OTCS showing normal dural sac (cyan arrow), thickened and more opaque dura (yellow arrow) near the termination of the dural sac (interrupted green line), and the filum terminale externum (white arrow).

Figure 6

T2w 3D FIESTA high resolution mid-sagittal image of the caudal lumbar, sacral, and cranial coccygeal vertebral column in a dog with OTCS showing no visible structural abnormalities (A). The conus medullaris terminates at mid-L7 (red arrow), the dural sac terminates at the cranial sacrum (yellow arrow), and the filum terminale internum (FTi) is marked with cyan interrupted lines. Panel (B): Dorsal microsurgical view after dorsal laminectomy at L7-S2 in the same dog. The dural edges are retracted after durotomy, revealing the FTi (white arrow), its width (blue lines), and the dorsal spinal vein of the filum terminale (interrupted white arrow). Abnormal infiltrative soft tissue (white to gray) adheres to the distal FTi (white arrowheads) and extends caudally to the termination of the dural sac.

Figure 7

Dorsal laminectomy at L7-S3 in a dog with C-OTCS followed by durotomy from the cranial L7 vertebra to the termination of the dural sac in the sacrum (green arrow) for exposure of intradural structures (A). The filum terminale internum is highlighted by the white arrow before (B) and after (C) dissection. The cyan arrow marks the proximal segment of the filum terminale internum, while the yellow arrow points to the distal remaining part. The interrupted black line indicates the distance created after dissection and cranial translocation of the conus medullaris.

Figure 8

Panel (A): Boxplot comparing clinical severity scores between responders and non-responders to medical therapy in dogs with OTCS. Responders exhibited significantly lower clinical severity scores (mean = 3.25) compared to non-responders (mean = 7.78), with p < 0.001. In panel (B), a stacked bar chart showing the relationship between neurological grades (I-IV) and medical treatment response can be seen. Grade I patients had the highest response rate (75%, 9/12), followed by Grade II (27%, 3/11), with no response observed in Grade III (0/5) or Grade IV (0/2) patients. The chart highlights a significant association between lower neurological grades and positive medical treatment response (p = 0.006). In panel (C), the proportion of recovery outcomes (full recovery vs. no full recovery) at last follow-up, stratified by treatment modality (surgical vs. medical) is also shown. Surgical treatment is associated with a significantly higher proportion of full recovery compared to medical treatment (p = 0.015).