The Pathogenesis of Ventral Idiopathic Herniation of the Spinal Cord: A Hypothesis Based on the Review of the Literature

Abstract

Idiopathic ventral herniation of the spinal cord (SC) is not often encountered in daily practice. Its clinical prevalence, however, will increase through increasing awareness and more frequent use of MRI. A clear explanation of its pathophysiology has never been formulated. It was hypothesized that the findings during surgery might indicate the real causative mechanism. An extensive literature search was performed, using Embase, PubMed, and Google Scholar. Titles and abstracts were screened by two investigators, using strict inclusion and exclusion criteria. Reference lists of the full paper versions of each included article were checked. The following data were registered for the articles included: age, gender, level of herniation, relation to intervertebral disk, duration of symptoms, findings from surgery, and outcomes. Nine cases treated at our department were added. A total of 117 articles reporting on 259 patients were included. Including our cases, 268 patients were reviewed. Females outnumbered males (160/100). The mean age was 51.3 ± 12.0 years. In 236 patients, the duration of symptoms was reported: 55.5 ± 55.6 months. In 178 patients, the intraoperative findings for the herniated part of the SC were not mentioned. In 59 patients, a tumor-like extrusion was seen, without any alteration to the SC. Deformation of the SC itself was never observed. Biopsies of these structures were without clinical consequence. Based on the intraoperative findings reported in literature and the cases presented, acquired causes, such as trauma and erosion of the dura due to a herniated disk, were not plausible. We hypothesize that a non-functioning appendix to the SC can only develop during an early embryologic phase, in which several layers separate. We propose renaming this entity as congenital transdural appendix of the SC.

Keywords: congenital; embryology; review; spinal cord herniation; transdural appendix.

Figure 1

MRI of case 1 showing signs of a herniated spinal cord at T4–T5 at a sagittal T2-weighted image (A), sagittal T1-weighted image (B), and an axial T2-weighted image (C).

Figure 2

Intraoperative view from the left side. Spinal cord (SC) was slightly moved to the right side with a spatula. The edge of the dural defect (V) can clearly be seen as well as the appendix from the SC (*).

Figure 3

MRI of second patient disclosing a herniated spinal cord at T8 at sagittal T2 weighted image (A), at a STIR weighted image (B), and an axial T2 weighted Image (C).

Figure 4

Intraoperative view from the right side. Spinal cord (SC) was disclosed and a clear tumor-like appendix was seen (*), as was the sharp edge of the ventral dural defect (V) in (A). More caudally, (B) a yellow globule (o) was apparent that was adherent to the above-mentioned appendix.

Figure 5

Flow of information.

Figure 6

Schematic presentation of proposed hypothesis. In the upper left corner, the neural tube is shown (black) with adjacent somites (red). The neural crest (green) is formed at the dorsolateral aspects of the neural tube and migrates to the ventral aspect of the neural tube (arrows). In the middle, the neural crest cells have been intermingled with mesenchymal cells from the somites forming the meninx primitiva. (A) The normal embryologic development is shown with the formation of the pia mater (yellow), the dura mater (blue), the posterior longitudinal ligament (violet), and the vertebral mesenchyme (red). The formation of the congenital transdural appendix of the spinal cord is depicted in panel (B). A local aggregation of neural crest cells is formed (green without red dashes) and the transdural appendix is formed, while perforating the dura, sometimes the posterior longitudinal ligament, and on rare occasions a little cavity within the vertebral body. Color legend: black: neural tube; red: somites (later vertebral mesenchyme); green (neural crest); yellow (pia mater); blue (dura mater); and violet (posterior longitudinal ligament).