Split cord malformation with dorsally located bony spur: Report of four cases and review of literature.

Split cord malformations (SCM) with a dorsally located bony spur are a very rare entity. The authors report a series of four such cases. The literature is reviewed regarding the pathogenesis and management of this uncommon variant of SCM. The presenting features include - scoliosis with motor and autonomic dysfunction (n = 1), scoliosis with cutaneous patch (n = 1), hypertrichotic area (n = 1), and motor deficits alone (n = 1). The location of spur was thoracic and lumbar in two patients (50%) each. Low-lying conus was present in three patients. Long segment syrinx was present in one patient. With respect to the bony anomalies, two patients had a hypertrophied posterior arch (HPA) and one patient had a dysraphic spine. All patients underwent surgical excision of the spur with detethering, if a low-lying conus was present. Two patients developed transient worsening of the neurological status after surgery which recovered at the time of the last follow-up; in both these patients, there was a HPA and a low-lying conus. Surgical excision of the spur with detethering of filum, in cases of low-lying conus, is the treatment of choice. Risk of post-operative worsening of the neurological status is increased in cases in which there is concomitant presence of HPA.

Introduction

Split cord malformations (SCM) are rare, congenital spinal anomalies involving splitting of the cord. Here, a segment or whole of the spinal cord is divided into two parts by a rigid or a fibrous septum. Mahapatra et al.[1] proposed a new sub-classification for type I SCM′s wherein they divided type-1 SCM′s into four types, based on the intra-operative location of the bony spur causing the split. The bony septum usually originates from the vertebral body and proceeds posteriorly causing splitting of the cord. However, septum arising from the posterior vertebral elements is very rare. Individual case reports of nine such cases have been reported in the English literature.[2-5] The authors report four cases of SCMs with a dorsally located bony spur and the pertinent literature is reviewed.

Case Reports

Case 1

A 2-year-old male child presented to the neurosurgical outpatient services with features of scoliotic deformity of the dorsal spine including urinary and fecal incontinence. The neurological examination revealed dorsal kyphoscoliosis and mild asymmetrical paraparesis. There was no evidence of cutaneous stigmata of neural tube defects.

A magnetic resonance (MR) imaging of the whole spine showed features of SCM type I, with a bony spur at the D12–L1 vertebral level. A long-segment syrinx and low-lying conus at the L3 vertebral level was observed with tethering of the cord [Figure 1a–c]. Further, a plain computed tomography (CT) scan of the dorso-lumbar spine confirmed the bony origin of the spur with the spur arising from the posterior elements of the D12 vertebra and ending just short of the vertebral body [Figure 1d]. The other associated bony anomalies included a hypertrophied posterior arch (HPA) at the level of spur and dysraphic spinous process. Intra-operatively, the incomplete dorsal bony spur was removed completely, the two dural tubes were converted into a single tube and detethering of the filum was performed. Post-operatively, the child developed mild worsening of paraparesis. At follow-up of 1 year following surgery, the child had improvement in paraparesis and autonomic dysfunction.

Figure 1a-d

Sagittal, axial T1W, axial T2-W MRI images and axial CT image of a representative case showing split cord malformation type I with dorsally located bony spur at the D12-L1 vertebral level causing asymmetric division of spinal cord into two separate hemicords with separate dural sheaths. Also note low-lying conus and long-segment syrinx. The bony spur proceeds in a dorso-ventral direction and ends just short of the posterior surface of vertebral body

Case 2

A 2½-year-old female child presented with gradually progressive paraparesis. Neurological examination revealed lower-limb weakness of 4/5 in all muscle groups. MR imaging of the spine showed features of low-lying conus at the L5 vertebral level, split cord from L1-3 vertebral level and a bony spur at the L3 vertebral level. There was an associated HPA at the level of the spur. Detethering of the filum and microsurgical excision of the spur was performed in a single stage. Post-operatively, the child developed marked weakness (power 2/5 Medical Research Council grading). She recovered gradually and at the last follow-up 1 year following surgery, the child was ambulatory with support, with lower limb power returning to the pre-surgical state of 4/5.

Case 3

A 6-month-old male child, presented to our neurosurgical outpatient services with features of hypertrichotic area over the lower lumbar region. The neurological examination was normal. MR imaging showed features of split cord at L2-5 vertebral level with an incomplete bony spur located dorsally at the L4 vertebral level with a low-lying conus at the S1 vertebral level. Exploration, detethering of filum and removal of the dorsal spur was performed in single stage. At the last follow up after 14 months following surgery, the child was doing well and had no deficits.

Case 4

A 3-year-old female child, presented to us with features of deformity of spine and a pigmented patch over her lower lumbar region. Her neurological examination was essentially normal. Imaging showed features of kyphoscoliosis with split cords from the D4-D6 vertebral level and a dorsally located spur at the D4 vertebral level. Exploration and microsurgical removal of the spur was performed. The post-operative period remained uneventful and at 1 year after surgery, she had no fresh deficits.

Details of all the four patients have been summarised in Table 1.

Table 1

The summarised clinical data

Discussion

SCM is an uncommon congenital anomaly in which a segment of the spinal cord is divided into two parts by a fibrous or rigid bony spur. The bony septum usually originates from the vertebral body and passes posteriorly causing splitting of the cord. However, septum arising from the posterior vertebral elements is very rare. Individual case reports of nine such cases have been reported in the western literature.[2-5] Many hypotheses have been postulated for the genesis of these malformations. However, the most widely accepted theory about the embryogenesis of these complex malformations was originally proposed by Bremer in 1952,[6] and subsequently modified by Pang et al.[78] as the ′Unified theory of embryogenesis. The basic error appears to be the development and persistence of “Accessory Neurenteric Canal” (ANC). According to the theory, these lesions were divided into two types depending upon the type of the midline mesenchymal derivative and the dural investment of the hemicords. SCM-I is characterized by the presence of double dural sacs, rigid extradural bony/cartilaginous spur leading into symmetrical or asymmetrical division of the cord. In SCM-II, there is a single dural sac with a non-rigid fibrous spur and symmetrical division of the cord. SCM-II are slightly more common than SCM-I, constituting around 50-60% of SCMs.[78] Mahapatra et al.[1] further proposed a new subclassification for type I SCM′s depending upon the intraoperative location of the bony spur and the splitting of the cord. Accordingly, they were subclassified into Type Ia, Ib, Ic, Id and they concluded that type Id is the most difficult and challenging. SCMs can present with a myriad of clinical manifestations. These can range from asymptomatic ones to pain, gait disturbance, motor or sensory deficits, and autonomic dysfunction. In our study, we had one patient (25%) each presenting with dorsal scoliosis with autonomic and motor dysfunction, dorsal scoliosis with cutaneous pigmented patch, hypertrichosis, and two patients had progressive paraparesis.

MR imaging is the diagnostic modality of choice and imaging of the whole spine is essential to rule out other associated anomalies. CT scan is complimentary to MRI and is helpful in evaluating the nature of the spur and associated vertebral body anomalies. The location of the spur was thoracic and lumbar in two patients each. Low-lying conus was present in three patients. Long segment syrinx was present in one patient. With respect to the bony anomalies, two patients had a HPA and one patient had dysraphic spine.

Although the unified theory of Pang et al.[7] has resolved the pathogenetic classification and clinical significance of these conditions, this theory does not explain the unusual variant such as a dorsally located bony spur. In our study, the bony spur was situated dorsally with two hemicords in a single dural tube as illustrated. In type-I SCMs, there is rigid bony spur with two hemicords in two dural sacs. Accordingly, these lesions cannot be classified as type-I or type-II SCMs according to the unified theory. Chandra et al.,[3] in their reported case of dorsal bony spur in a lumbar SCM hypothesized two mechanisms for the occurrence of this rare variant: (1) passage of an abnormal cell population dorsally with subsequent loss of contact with the ventrally situated cell population and (2) migration of cells around the hemicords and subsequent passage between them in a dorso-ventral direction. Furthermore, Katoh et al.[9] reported a case of SCM-II without a fibrous spur which again is not explainable by the unified theory. Hence, although a hypothesis has been proposed for this uncommon variant by some authors, further embryological studies are necessary to validate such proposals.

Surgery is the treatment of choice wherein the bony spur should be excised microsurgically. Low-lying conus should also be addressed along with the excision of bony spur. In all our cases which were associated with a low-lying conus, exploration and excision of the bony spur was followed by filum detethering in the same stage using standard microneurosurgical principles. Midas Rex high-speed drill was used in all our cases with gradual drilling of the dorsally located spur in a dorso-ventral direction. The post-operative neurological status was as follows: Two patients had no fresh neurological deficits whereas the remaining two patients developed transient worsening of the neurological status (One developed mild paraparesis which recovered completely and other developed marked paraparesis with good functional recovery to the pre-surgical state; in both these patients, there was a HPA and low-lying conus).

The operative steps can be summarized as follows: (1) Midline posterior approach. (2) Laminoplasty should never be performed in these cases for fear of causing increased cord damage because of the dorsally situated spur. Hence, laminectomy is preferred and should be done at least one level above and one below the level of the spur. The level of the bony spur can be roughly assessed by the HPA. As the bony spur is attached to the laminae, they need to be handled with utmost care and should be gradually drilled off using a drill. (3) The dura is opened in a conventional linear manner with a gentle curve encircling the spur. The dural incision is extended about two levels above and below the level of lesion so that there remains sufficient dura for closure. (4) The initial work on the bony spur is done by a pneumatic drill and the thinned-out portion removed with microronguers. (5) The dura is mobilized both vertically and in a lateral direction. Watertight dural closure is of prime importance to avoid post-operative CSF leak. Dura is closed primarily with 5-0 vicryl suture and duraplasty is performed when primary dural closure is difficult. (7) The rest of the wound is closed in layers.

Conclusion

SCMs are uncommon, complex conditions of spinal dysraphism. Dorsally situated bony spur is a very rare entity. MR imaging is the diagnostic modality of choice complemented by CT scan. Other associated spinal anomalies should always be looked for, especially a low-lying conus and a syrinx. Surgical excision of the spur with detethering of filum, in cases of low-lying conus, is the treatment of choice in which laminectomy with gradual drilling of the dorsally located spur, is preferred. The risk of post-operative worsening of the neurological status is increased in cases in which there is concomitant presence of HPA and low-lying conus. A sound anatomical knowledge coupled with the principles enlisted above can help prevent spinal-cord damage and can help achieve satisfactory results.