Split cord malformations, although rare, can cause progressive neurologic impairment if not properly identified and surgically treated. These malformations can be associated with tethered cord, congenital scoliosis, and other spinal dysraphisms. In this article, the authors describe the various signs and symptoms that are associated with these malformations, the imaging modalities best used for diagnosis, and the surgical treatment options and prognosis. They also provide updated information about the etiology, evaluation, and treatment of this interesting group of disorders.
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• Split cord malformations are associated with spinal dysraphisms, congenital scoliosis, and tethered cord.
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• Split cord malformations usually present in childhood but can present in adulthood, especially in patients with the above conditions who have new-onset neurologic symptoms.
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• Split cord malformations can be missed by routine imaging, especially if that imaging was done in the past with older technology; thus, a prior normal scan does not rule-out this condition.
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• Early identification and surgical correction can arrest or improve neurologic symptoms.
Historical note and terminology
Diastematomyelia was first described in 1837 by Ollivier. The term is used to describe a developmental malformation of the spinal cord that is characterized by a splitting of the cord into at least 2 independent segments. The term is from the Greek "diastema" (cleft) and "myelos" (marrow or medulla). Usually, the cord is split by some type of mesenchymal derivative such as bone or cartilage. This term is meant to differentiate the malformation from diplomyelia, which signifies a twinning of the spinal cord.
The major distinction made by most authors between these 2 entities had to do with the presumed embryologic pathogenesis (33). Diastematomyelia was believed to be a disruptive embryological process where the developing neural tube and related structures are split by abnormal tissue forces. Diplomyelia was believed to be the result of some type of local twinning phenomenon. The clinical impression of diastematomyelia was always that there is a high probability of clinical deterioration in individuals with this condition (58; 25). Several clinical series were published that emphasized the therapeutic role that neurosurgical intervention could play (14; 15). Pang and colleagues published 2 papers that cast doubt on the distinction between these 2 conditions by presenting a unified theory of pathogenesis and management. They proposed grouping together these 2 conditions and reterming them "split cord malformations" (43; 43). They are now referred to as split cord malformations type I SCM type 1, formerly diastematomyelia) and split cord malformations type II (SCM type 2, formerly diplomyelia).
Split cord malformations usually occur in the lower thoracic or upper lumbar area, although they have been reported in other locations. The resulting hemicords usually have intact pia mater and often separate dorsal and ventral horns. In SCM type 1, the hemicords are split by a midline osseocartilaginous septum and reside in separate (thus, double) dural sacs. In SCM type II the hemicords are separated by a nonrigid fibrous median septum within a single dural sac and often reunite caudally.