Use of prone position magnetic resonance imaging for detecting the terminal filum in patients with occult tethered cord syndrome

Clinical article

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Object

The pathophysiology of occult tethered cord syndrome (OTCS) with no anatomical evidence of a caudally shifted conus and a normal terminal filum is hard to understand. Therefore, the diagnosis of OTCS is often difficult. The authors hypothesized that the posterior displacement of the terminal filum may become prominent in patients with OCTS who are in a prone position if filum inelasticity exists, and they investigated prone-position MRI findings.

Methods

Fourteen patients with OTCS and 12 control individuals were examined using T2-weighted axial MRI with the patients in a prone position on a flat table. On each axial view, the distance between the posterior and anterior ends of the subarachnoid space (A), the distance between the posterior end of the subarachnoid space and the terminal filum (B), the distance between the posterior end of the subarachnoid space and the dorsal-most nerve among the cauda equina (C), and the distance between the posterior end of the subarachnoid space and the ventral-most nerve (D) were measured. The location ratios of the terminal filum, the dorsal-most nerve, and the ventral-most nerve were calculated by the ratio of A to B (defined as TF = B/A), A to C (defined as DN = C/A), and A to D (defined as VN = D/A), respectively. Patients underwent sectioning of the terminal filum with the aid of a surgical microscope. The low-back pain Japanese Orthopaedic Association score was obtained before surgery and at the final follow-up visit.

Results

On prone-position axial MRI, the terminal filum was separated from the cauda equina and was shifted caudally to posterior in the subarachnoid space in all patients with OTCS. The locations of the caudal cauda equina shifted to ventral in the subarachnoid space. The TF values in the OTCS group were significantly lower than those in the control group at the L3–4 (p = 0.023), L-4 (p = 0.030), L4–5 (p = 0.002), and L-5 (p < 0.001) levels. In contrast, the DN values in the OTCS group were significantly higher than those of the control group at the L-2 (p = 0.003), L2–3 (p = 0.002), L-3 (p < 0.001), L3–4 (p < 0.001), L-4 (p = 0.007), L4–5 (p = 0.003), and S-1 (p = 0.014) levels, and the VN values in the OTCS group were also significantly higher than those of the control group at the L2–3 (p = 0.022), L-3 (p = 0.027), L3–4 (p = 0.002), L-4 (p = 0.011), L4–5 (p = 0.019), and L5–S1 (p = 0.040) levels. Sections were collected during surgery for histological evaluation, and a decreased elasticity within the terminal filum was suggested. Improvements in the Japanese Orthopaedic Association score were observed at the final follow-up in all patients.

Conclusions

The authors' new method of using the prone position for MRI shows that the terminal filum is located significantly posterior and the cauda equina is located anterior in patients with OTCS, suggesting a difference in elasticity between the terminal filum and cauda equina.

Abbreviations used in this paper:JOA = Japanese Orthopaedic Association; OTCS = occult tethered cord syndrome.

Article Information

Address correspondence to: Kazuyoshi Nakanishi, M.D., Ph.D., Department of Orthopaedic Surgery, Programs for Applied Biomedicine, Division of Clinical Medical Science, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan. email: kazn@hiroshima-u.ac.jp.

Please include this information when citing this paper: published online November 9, 2012; DOI: 10.3171/2012.10.SPINE12321.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Axial MR image showing the distance between the posterior and anterior edges of the subarachnoid space (A), the distance between the posterior end of the subarachnoid space and the terminal filum (B), the distance between the posterior end of the subarachnoid space and the dorsal-most nerve among the cauda equina (C), and the distance between the posterior end of the subarachnoid space and the ventral-most nerve among the cauda equina (D). The locations of the terminal filum, the dorsal-most nerve, and the ventral-most nerve were calculated by the ratio of A to B (defined as TF = B/A), A to C (defined as DN = C/A), and A to D (defined as VN = D/A), respectively.

  • View in gallery

    A–C: Views of the surgical field after the dura mater and arachnoid membrane are opened, reflected, and secured with stay sutures. The cranial end is toward the top in these pictures. The terminal filum is located posterior in the subarachnoid space (A). The cranial end of the terminal filum is drawn cranially and disappears after cutting (B and C). D: Histological findings of the terminal filum, which had a normal appearance during surgery and on MRI. Abnormally large amounts of connective tissue with dense prominent collagen fibers, dilated capillaries (Ca), neural tissue (N), and some areas of hyalinization are seen. E: Histological findings of the terminal filum that was greater than 2 mm in diameter and with fatty tissue inside, as noted during surgery and on MRI. Increased connective tissue, hyalinization (H), and large clumps of fat cells (Ft) are seen. Bar = 100 μm.

  • View in gallery

    Left: Axial T2-weighted MRI studies obtained in an 18-year-old individual in the control group between vertebral levels L-1 and L-5. The dot of the terminal filum is not clearly separate from the dots of the cauda equina, and was not clearly observed. Right: Axial T2-weighted MRI studies obtained in an 18-year-old patient in the OTCS group between vertebral levels L-1 and L-5. The arrowheads indicate the dot of the terminal filum. The dot of the terminal filum separates from the dots of the cauda equina at the L-2 level, and the location of the dot shifts caudally to posterior in the subarachnoid space, and touches to the dorsal dura at the L3–4 level.

  • View in gallery

    Results of the TF values (upper) and the DN and VN values (lower) in the OTCS and control groups. *p < 0.05, †p < 0.01, ‡p < 0.001 versus control group.

  • View in gallery

    Sagittal (A) and axial (B) T2-weighted MRI studies obtained at the L-3 level of the lumbar spine in the supine position. Axial view (C) T2-weighted MRI obtained at the L-3 level of the lumbar spine in the prone position. Intraoperative microscopic view (D).

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