Composite Split Cord Malformations or Multisite Split Cord Malformations

Split cord malformations (SCMs) are an uncommon form of spinal dysraphism. The spinal cord is divided to form two neural tubes in SCM.2^–² The length and number of places where the spinal cord is split varies significantly among cases. These malformations result due to abnormalities during early embryonic period (2–6 weeks of gestational age).² Early embryonic period is divided into three periods—gastrulation period (2nd to 3rd week), primary neurulation period (3rd week), and the secondary neurulation period (3rd to 4th week). Diastematomyelia and neurenteric cyst result from disorders of gastrulation, while meningocele and myelomeningocele arise from abnormalities of primary neurulation.² Conus and filum region abnormalities arise as a result of secondary neurulation disorders.² The fact that we get to see so many rare combinations of dysraphic abnormalities and so many variations in a single entity emphasizes the complexity of the underlying embryological processes,²^,²^–² which cannot be explained by simplified theory put by Pang et al in 1992.²

Complex Dysraphic Spine

Patnaik and Mahapatra defined complex dysraphic forms as “the conditions where there is more than one type of spinal dysraphism in the same patient involving various phases of embryogenesis.”² These include more than one type of lesions present in one patient at one or different locations, or multiple similar type of lesions present at different locations in the same patient. They usually arise when multiple developmental periods are affected. They may or may not be associated with Chiari malformations, syrinx or hydrocephalus. However, any malformation arising during the gastrulation period is termed as a complex dysraphic state from the radiological perspective² (Fig. 8.1).

Fig. 8.1A complex SCM with a type II split (left) with fibrous septum and a dermal sinus associated with a type I split (right).

Composite Split Cord Malformations

Composite SCM is a type of complex spinal dysraphic abnormality. In composite SCM, there is single or different type of SCM in the same patient separated by intervening normal cord. SCM type I are characterized by a bony septum dividing the spinal canal into two halves, with two hemicords covered by a separate dural sheath, while the septum is fibrous and a single dural sheath cover for the hemicords in type II SCM.²^,² Rest of the discussion in this chapter pertains to composite split cord malformations.

Incidence

The true incidence of composite SCM is not known, as most of the reports come in the form of case reports only.2^,²^,²^–² Harwood-Nash and McHugh reported that the incidence of composite SCM is less than 1%.² Erşahin reported 6 cases of composite SCM in his personal series of 131 patients.² Another surgeon reported only 2 cases of composite-type SCM out of a total of 39 cases in his series.² None of the other large series mention the incidence of composite SCM.²^,² It emphasizes that composite SCM are grossly underreported.

Embryogenesis

Origin of composite-type SCM can be explained by multiple neurenteric canal theory, that is, multiple separate foci of ectoendodermal adhesions and endomesenchymal tracts, leading to development of SCM with intervening normal cord in the same patient.²^,² The accessory neurenteric canal, which may be single or multiple, is the cause of SCM as per Pang’s unified theory of embryogenesis. The presence of multiple accessory neurenteric canals results in two or more septa that form the endomesenchymal tracts and divide the neural tube resulting into two hemicords² (Fig. 8.2).

Fig. 8.2 A composite SCM with two type I splits interspersed by a type II split.

Clinical Significance

Composite SCM is rare but this fact should not preclude a neurosurgeon from performing whole spine MRI to find split at multiple locations, as missing SCM might be devastating for the patient.

Distribution of Spurs in Reported Cases

Most of these cases had two spurs at different levels, and one case having two different types of spurs at three different levels.2^,² In most of these cases, the spurs were not more than three vertebral levels apart. Only one case had widely separated spurs present at T2 and L3 levels.²^,²^,²^,² The spur is most commonly found in the lower thoracic or the upper lumbar region, and only four case reports of septum above T3 are available in the published literature (Fig. 8.3).