Split Cord Malformation: An Overview

Introduction Split cord malformation (SCM) is a type of occult spinal dysraphism, characterized by a longitudinal split in the spinal cord into two hemicords occurring during primary neurulation. It is a complex developmental disorder that, in most cases, leads to tethered cord syndrome and the development of neurological and orthopedic symptoms, which include anomalies of the spinal cord, meninges, and bony structures[1] The split cord malformation is the most common form of congenital myelodysplasia. Split cord malformation (SCM) is of two types:[2] Type I SCM (Diastematomyelia)[3] In this, two hemicords lie within separate dural sacs These sacs are separated by a rigid bony or cartilaginous septum This septum is derived from totipotent cells of the primitive steak. Type II SCM (Diplomyelia)[4] In this, two hemicords lie within a single dural sac The dural sac, in turn, is divided into two halves by a soft, nonrigid fibrous septum It is generally less severe than Type I. Three variants of Type II SCM exist as follows: Presence of an intervening fibrous septum Absence of a septum Partial cord splitting within a single dural tube housing both hemicords Further, diastematomyelia involves the division of one cord, while diplomyelia involves the duplication of the cord into two These can be identified by axial and coronal T2-weighted images as a thin hypointense strip interposed between the two hemicords SCM is often associated with other clinical conditions, namely spina bifida, vertebral anomalies such as hemivertebrae, butterfly vertebrae, and other neural tube defects.[5] Embryological Basis[6] The embryological processes involved in SCM are as follows: Nonobliteration of the neurenteric canal Notochord splitting Bifurcation of the neural plate Development of two hemicords Septum formation. The details are as follows:[6] The notochord secretes signaling molecules, namely Shh (Sonic hedgehog protein)[7] This is essential for the proper differentiation of the floor plate of the neural tube and the proper formation of adjacent vertebrae. This is facilitated by the transient canal phase Normally, the notochord forms a solid rod and separates from the endoderm (23–25 days) Normally, a single neurenteric canal exists during the 3rd week of gestation to provide a transient communication between the amniotic cavity and yolk sac in the region of Hensen’s node. Usually, it closes within 48 h If the process is disrupted, due to persistent adhesion between the ectoderm and endoderm, the developing notochord is prevented from separating completely from the endoderm. Rather, it splits into two heminotochords The overlying neural plate also splits into two hemineural plates, which go on to form two separate hemicords – a characteristic of SCM Nonclosure of the transient neurenteric canal during notochord formation leads to the formation of two hemineural chords. The paired notochordal processes are separated by primitive streak cells The resulting malformation depends upon the nature of the development of totipotent cells of the primitive streak, because being totipotent, these cells can differentiate into ectodermal, mesodermal, and endodermal lineages. For example, if these cells differentiate into cartilage or bone, the two hemicords contained within individual dural sacs will be separated by an osteocartilaginous bar/spur The septum is formed between hemicords derived from endomesenchymal tissue in the region that incorporates primitive meningeal tissue The neurenteric canal is a part of developmental process of notochordal process. The differences between notochondral canal and neurentric canal are as follows:[8] The notochordal canal and neurocentric canal are related but different embryonic structures. The notochordal canal is a lumen that forms within the notochordal process, while the neurocentric canal is a temporary opening between the amniotic cavity and yolk sac, which develops when the notochordal process fuses with the ectoderm. The notochordal canal is a part of the process of formation of the notochord. The neurenteric canal equalizes pressure between the amniotic cavity and the yolk sac. Its persistence can lead to split cord malformation.[8] Congenital Anomalies Associated with SCM[9] These are as follows: Other neural tube defects, namely spina bifida Tethered spinal cord syndrome, as well as various spinal and orthopedic deformities (such as scoliosis or kyphoscoliosis and abnormal vertebral bodies) In addition, SCM can be accompanied by Chiari malformations, syringohydromyelia, and intraspinal lipomas.[9] Clinical Presentation of Split Cord Malformation (SCM) Clinical signs and symptoms of SCM are not distinct and typically occur during infancy or adolescence, mostly between the ages of 6 months and 15 years. SCM can present clinically with scoliosis and tethered cord syndrome. A hairy tuft on the back of the patient may be seen in these patients. The common signs and symptoms include the following: Back and leg pain, weakness, or sensory loss in the lower limb and bladder or bowel dysfunction Other signs may include orthopedic deformities such as scoliosis and cutaneous stigmata such as a tuft of hair or birthmarks over the spine. Lower limb deformities, namely clubfoot and high arched/flat feet etc Children are more likely to have neurological defects, while in adults, pain is the most common symptom.[10] Treatment of SCM[11] The treatment of SCM is primarily surgical. It involves the following steps: Detethering of the spinal cord: Here, the spinal cord is released from its abnormal attachment to the spinal canal to reduce the tension. Further, the filum terminale is also cut to completely untether the spinal cord from the bottom Removal of the septum (bony or fibrous) dividing the two hemicords Removal of associated lesions at the same time, namely lipomas, dermoid cysts, or epidermoid cysts.[11]