general term "split cord malformation" is preferable to the potentially confusing terms used to describe the conditions often referred to as diastematomyelia and diplomyelia. The abnormality may take the form either of a division of the spinal cord within a single dural sac (diplomyelia, type II split cord malformation) or of splitting of two hemicords, each with its own dural covering, by a septum, usually made up of a bony spur and/or fibrocartilagenous band (type I split cord malformation).
The embryological origin of split cord malformations remains unclear. The presence of two hemicords within a single dural tube has been considered to occur as a result of a doubling of the neural tube without fibrous tissue dividing the cords and therefore without the potential for causing tethering. On the other hand, two hemicords, each with its own dural sheath and surrounding bony structures, have been thought to be due to splitting of the noto-chord by an interposed adhesion between the primitive endodermal and ectodermal layers or of incomplete persistence of an accessory cranial neurenteric canal. This leads to a septum splitting the cord and, with growth, tethering as the bony spine lengthens relative to the cord itself. However, Pang has considered that all split cord malformations represent a spectrum of a single abnormal embryological process [21,22]. This is characterized by the formation and persistence to a variable degree of an abnormal connection between the endodermal and ectodermal layers of the embryo, resulting in an endomesenchymal tract. The endomes-enchymal tract results in the independent development of two heminotochords and two hemineural tubes. The degree to which the invading mesoderm (future meninges) is associated with the endomesenchymal tract between the two developing neural tubes affects the extent to which the two future hemicords are split. If both hemicords are each surrounded by invading mesoderm, then two separate dural tubes result, with associated bony and fibrocar-tilaginous tissue producing the dividing spur. Pang has termed this the "split cord malformation type I". If the mesoderm of the future meninges is not associated with the endomes-enchymal tract dividing the hemicords, then a single dural sac is created, with only thin sagittal fibrous bands, representing the remains of the endomesenchymal tract, attaching the hemicords to the dural sleeve. This Pang terms the "split cord malformation type II". The implication here is that both types of split cord malformation represent tethering lesions and therefore both require surgical exploration in order to prevent subsequent neurological deterioration.
Split cord malformations are characteristically associated with a midline hairy patch or "horse's tail". This is much more commonly found in split cord malformations than with other forms of "occult" dysraphism (Fig. 27.3). There may occasionally be an associated dermal sinus or pigmented skin lesion. The spur is generally found in the lumbar or lower thoracic region. Clinically, this may produce one leg with muscle wasting or sensory loss, with or without orthopedic manifestations, such as pes cavus. One or both ankle jerks are often absent. There may be involvement of the bladder and bowels, particularly in those children presenting in early adolescence. Scoliosis may also occur, as may other vertebral anomalies. The anatomy of the defect is demonstrated on MRI scans; particularly well seen on axial imaging are the two hemicords. Once the level has been established,
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