To report two cases of type 1.5 split cord malformation (SCM), a subtype of SCM with combined characteristics of types I and II and to review the relevant literature and propose a new possible pathogenetic theory for type 1.5 SCM. A 52-year-old woman had hemicords within a single dural sac with a dorsal bony septum at the L5 level. A 9-year-old boy had hemicords within a single dural sac with a ventral bony septum and fibrous extension at the L3 level. Both patients underwent microsurgical treatments for removing the bony septum, detethering the spinal cord, and sectioning the filum terminale. The surgical procedure revealed an extradural partial bony septum and hemicords within an intact single dural sac in each patient. Both patients were discharged from the hospital without de novo nerve dysfunction. Published cases have validated that types I and II SCM can overlap. We recommend recent type 1.5 SCM as a normative terminology for this overlapping SCM and report two rare cases of this SCM. We propose an associated pathogenesis consisting of uneven distribution and regression to explain type 1.5 SCM. Furthermore, we postulate that the amount of condensing meninx primitiva might determine whether the left bony septum has fibrous extensions to the opposite dura in type 1.5 SCM.
Keywords: Embryogenesis; Meninx; Pathogenesis; Regression; Split cord malformation.