Sachin A. Borkar, Mohit Agrawal, and Ashok K. Mahapatra

Introduction

Split cord malformation (SCM) is a congenital anomaly of the spinal cord, wherein there is splitting of the spinal cord over part of its length to form two neural tubes. The etiopathogenesis of this condition was unknown before Pang et al proposed their theory of “unified embryogenesis.”2^–2 They described two varieties of SCM—type I which has a rigid bony septum between two hemicords, each within its own dural tube, and type II SCM comprising two hemicords in a single dural sac separated by a fibrous septum. This midline osseocartilaginous septum was described to always arise from the ventrally placed vertebral body. It was in 1999 when Chandra et al² described a new entity where the midline septum was seen to arise from the dorsal arch of the vertebrae. They named this as a dorsal spur.

Embryology

The earliest recognition of SCM as an abnormality was made by Ollivier in 1837 when he coined the term “diastematomyelia.”² He described a spinal cord divided into two sleeves separated by a sagittal bony or fibrous spur. Bruce et al advised the use of the term diastematomyelia to only describe a spinal cord split by a midline bony spur. They proposed the term “diplomyelia” for true doubling of the cord without a spur.² These were just anatomical descriptions of the condition without elaborating on the etiopathogenesis of the condition. Feller and Stenberg were the first to indicate the embryological basis of this condition as a persistence of the midline cell rest and formation of a notochordal cleft.² Since then, several theories have been proposed in order to explain the genesis of SCM like the hydromyelic theory by Gardner in which he proposed that cerebrospinal fluid (CSF) causes the rupture of the neural tube, causing a split.² Primary mesodermal abnormality theory was advocated by Lichtenstein,² while Hendrick proposed the accessory neurenteric canal hypothesis.² Others proposed the mesodermal invasion of the neural tube.^2–2

All these theories were laid to rest when Pang et al in 1992 proposed a unified theory of embryogenesis and recommended that the term “SCM” be used for all double spinal cords.^2–2 This theory essentially stated that a single error occurs at the time of closure of the primitive neurenteric canal, which leads to the persistent communication between the yolk sac and amnion through an “accessory neurenteric canal.” This allows for a continued contact between the ectoderm and endoderm within the spinal canal. Subsequent mesenchymal infiltration and the timing of the formation of the endomesenchymal tract determines whether the neural tube splits into two separate hemicords each with their own dural sac with an intervening fibrocartilaginous or bony septum (diastematomyelia) or into two hemicords housed in a single dural sac with a fibrous septum dividing the two (diplomyelia). The important point to note here is that the development of the midline septum was described to always occur from a ventral direction, from the region of the vertebral body toward the posterior elements, splitting the canal in the middle. Chandra et al challenged this unified theory in 1999, when they described the entity known as dorsal spur.² They described a spur attached to a hypertrophied posterior arch (HPA), without any ventral attachment, thus contradicting the unified theory proposed by Pang et al. To explain this unusual finding, the authors proposed two possible hypotheses. The first was that during the process of abnormal migration of the meninx primitiva cells in between the split spinal cords, a sizable portion might get disconnected from the ventral tract and thus start accumulating dorsally. This abnormal mass would explain the formation of a dorsal spur with HPA. The second proposition was that the cells of meninx primitiva, instead of migrating in between the split cords, would pass around it to accumulate dorsally. These cells then pass between the two hemicords from dorsal to ventral direction, thus causing the formation of the dorsal spur. The rarity of this condition has precluded any definite proof of these theories.

Demography

The reported cases in literature have been summarized in Table 6.1. The occurrence of dorsal spur shows an equal predilection for both males and females (out of the available data). The relatively recent description of this condition might have led to it being missed in most of the large surgical series in literature. A probable estimate might be drawn from the existing data.

Abbreviations: HPA, hypertrophied posterior arch; LLTC, low-lying tethered cord; MRC, medical research council; SCM, split cord malformation.

Ersahin et al in their analysis of 74 patients with SCM reported 46 girls (62%) and 28 boys (38%).2 Mahapatra in his study of 300 cases showed that clinical presentation had two peaks, 1 to 3 years of age and also 12 to 16 years of age.² Mahapatra and Gupta, in their study of 254 patients with SCM, reported that the patients’ ages ranged from 16 days to 35 years (mean age 7.3 years). The mean age of the patients with neurological deficits was 6.66 years, whereas asymptomatic patients presented at a mean age of 6.7 years. As many as 23 of their patients were adults older than 18 years of age, and 60.3% of their patients were female. The common site of SCM was in the lumbar spine, followed by the dorsolumbar area.^2–2 The most common site of dorsal spur is lumbar followed by dorsal, with composite spurs seen in two of the reported cases.

Clinical Features

Of all the reported cases of dorsal spur, half of the patients had no neurological deficits at the time of presentation. Two presented with progressive scoliosis. Of the three patients with neurological deficits in the form of paraparesis, only one had preoperative bowel/bladder involvement (Table 6.1). There are no unique clinical features associated with a dorsal spur. The clinical features are similar to other SCM types reported in literature.

These include either cutaneous markers such as hypertrichosis, capillary hemangioma, hyperpigmentation, dimple, scoliosis/kyphoscoliosis, musculocutaneous deformities of the lower limb such as congenital talipes equinovarus (CTEV), or neurological sensorimotor deficits and autonomic disturbances. Weakness and atrophy of limbs, gait disturbances, dysesthetic pain, hypoesthesia, or trophic ulcers may be seen. Bladder and bowel disturbances are also noted.^2–2 Few of the patients might be asymptomatic at presentation.

Imaging Studies

Imaging forms the mainstay of diagnosis of a dorsal spur. Recent advances in the form of high-resolution MRI can accurately define the location, extent, orientation, and attachments of the midline septum (Fig. 6.1 and Fig. 6.2). Whole spine MRI should be performed in all suspected patients to find out the number of involved segments, any concurrent sites of defects, and other associated anomalies like lipoma, thick filum, dermoid and epidermoid tumors, neurenteric cyst or dermal sinus. Whole spine MRI is especially important to detect cases of composite spurs. The cervical spine should be evaluated to rule out any associated Chiari malformation and craniovertebral junction anomalies. A CT scan can be helpful to define the exact bony extent of the spur. Dorsal spur starts at the ventral surface of the posterior elements and is partial in nature, that is, it does not go all the way up to the vertebral body with which it might have a fibrocartilaginous attachment. Further, CT scan helps in demonstration of HPA, with which half the reported cases of dorsal spur are associated with.

Related posts:

Introduction to Split Cord Malformations Split Cord Malformations Type I Anesthesia for Neural Tube Defects Prophylactic Surgery in Split Cord Malformations Protocol for Management of Split Cord Malformations Composite Split Cord Malformations or Multisite Split Cord Malformations

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