Split Cord Malformation (Diastematomyelia)

Split cord malformation (SCM) is an occult spinal dysraphism that frequently coexists with congenital scoliosis. Missed before a corrective osteotomy or instrumentation, it can produce catastrophic cord injury when the hemicords are stretched across a fixed septum. Early recognition allows safe detethering and dramatically reduces operative risk.

The mandate. MRI is mandatory before any instrumentation or osteotomy in every child with congenital spinal deformity, even when the neurological examination appears normal. It is obtained before the operating theatre is booked.

Who. SCM is rare, estimated at 1 in 5,000-10,000 live births, with a slight female predominance. It presents anywhere from infancy to adolescence, and many cases are diagnosed during a scoliosis work-up.

Embryology. The error is a failure of primary neurulation: a split notochord, or an adhesion between ectoderm and endoderm. Pang's unified theory holds that both types result from a similar embryological error at days 18-25. Associated anomalies (hemivertebra, block vertebra, rib fusions) lie at the same level as the split or at a remote one.

The scoliosis association, in the right direction. About 60-80% of patients with an SCM have a congenital scoliosis. The reverse runs much lower: across 251 congenital scoliosis patients, 18.3% had an occult intraspinal anomaly, most often a split cord. That risk is pattern-dependent and peaks at 52% only for a unilateral unsegmented bar with a contralateral hemivertebra in the lower thoracic or thoracolumbar spine. Quoting 60-80% as the yield of screening a scoliotic spine reads the figure backwards.

Tethering. The split creates two hemicords that are mechanically tethered, and the split itself acts as a tether point. In Type I the rigid bony septum fixes the cord and prevents its normal ascent during growth; in Type II the fibrous band produces similar tethering. A thickened filum terminale or scar may add further tethers.

From stretch to deficit. Progressive stretch produces ischaemia, axonal loss and syringomyelia. Growth spurts, or spinal lengthening during scoliosis surgery, increase the tension and produce an acute or progressive deficit.

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The second tether. A low-lying conus (below L2) or a thickened filum over 2 mm is common, and how common depends sharply on level. In Pang's series every lumbar SCM had both a low-lying conus and at least one additional tethering lesion, whereas only 1 of 7 cervical and high thoracic lesions did.

Why that matters. A lumbar split cord obliges you to hunt for the second lesion and plan to deal with it. Releasing the septum alone leaves the cord tethered.

Why deformity correction is dangerous. Correction of congenital scoliosis often requires distraction, derotation or vertebral column resection. An unrecognised bony septum acts as a fulcrum and tears the hemicords, and even a Type II fibrous band can produce significant stretch injury once the spine is lengthened. Any manoeuvre that lengthens the column, distraction or osteotomy, can acutely worsen the tether if the septum has not been removed first.

Examiners expect you to place split cord malformation within the wider spinal-dysraphism framework rather than treat it in isolation. The first division is open versus closed, and the closed (occult) group is then split by whether there is a subcutaneous mass.

SCM is a closed (occult) dysraphism without a subcutaneous mass. There is no lump to prompt imaging and the cord is fully skin-covered, which is exactly why the subtle cutaneous stigmata (hairy patch, sinus, dimple) matter so much. The unifying functional theme across the whole closed group is tethering of the cord.

Pang's classification is built on the dural tubes. Type I has two hemicords in two separate dural sacs, divided by a rigid bony or osteocartilaginous septum. Type II has two hemicords in a single shared dural sac, divided only by a fibrous band. The split usually occupies one level.

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Type predicts the operation, not the course. It is tempting to assume the rigid bony septum of Type I is the more dangerous lesion, and examiners like that assumption. Pang's series does not support it: symptomatic progression was independent of lesion type, with a Type I to Type II ratio of 13:11, and independent of level, at 67% for cervical against 64% for thoracolumbar lesions.

What type does decide. Type I is technically harder and carries slightly higher surgical morbidity. It is more dangerous still when an oblique septum has divided the cord asymmetrically into one large and one small, delicate hemicord.

What decides the risk of deterioration. Age and symptoms, not type. All 8 adults in the series were symptomatic at diagnosis, against only 16 of 31 children.

Type II still tethers. The hemicords remain tethered by the band, tethered-cord signs may develop later, and the cord may appear unified on axial views if the images are not read carefully.

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Associated anomalies. Other lesions travel with the split; the table gives their frequency and what each means for the operation.

The skin. Hypertrichosis (a hairy patch, classically over the lumbar spine) is the classic sign and the best single predictor, but quote it honestly: it was present in 56% of Pang's 39 patients, so nearly half had no hairy patch. What is close to universal is a cutaneous marker of some kind: occult or open dysraphic stigmata were present in all but 3 of the 39. Look also for a dermal sinus, lipomatous mass, capillary malformation or an asymmetric gluteal cleft.

A normal back does not clear the spine. Any midline cutaneous lesion below L1 demands MRI, yet a normal-looking back does not exclude an SCM. The 3 of 39 with completely normal skin are about 8 percent, and later series report higher rates.

The neurology. Look for asymmetric leg weakness, cavus feet or bladder dysfunction in a child with scoliosis; the foot may be cavus or equinovarus on one side. The neurological level often does not match the level of the bony deformity.

History. The answers that point to a split cord:

• A cutaneous lesion noted at birth or during growth
• Leg weakness, numbness or a change in gait, and whether it is progressing
• Early bladder or bowel dysfunction: incontinence, recurrent UTI, constipation
• A curve noticed by the parents or at school screening
• Any previous spinal procedure performed without an MRI

Examination. Beyond the skin:

• Legs: asymmetric atrophy, cavus feet, trophic ulcers
• Neurology: asymmetric reflexes, a sensory level, Babinski sign
• Spine: the congenital curve, kyphosis, local tenderness

MRI makes the diagnosis. No plain film or CT finding replaces it. Plain radiographs show the scoliosis and vertebral anomalies but never confirm or exclude SCM.

The protocol. Image the entire spine, foramen magnum to sacrum, because multiple levels or additional anomalies (syrinx, lipoma, dermal sinus) are common. Sagittal and axial T1, T2, STIR and post-contrast T1 are standard. Read them for the hemicords, the septum and whether it is bony or fibrous, the conus level, filum thickness, and a syrinx, lipoma or Chiari.

Read the axials. The question is whether the hemicords lie in separate or shared dural sacs, and whether a bony, cartilaginous or fibrous septum is present. Axial images are essential: sagittal views alone can miss the split.

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Type on the dural tubes, not on the septum you can see. The two septa are not equally visible. In Pang's series every Type I bone septum was identified before surgery, but only 5 of the 18 Type II fibrous septa appeared on preoperative imaging, while at operation a fibrous septum or other tethering bands were found in every Type II case. A single dural tube with two hemicords and no visible septum is therefore still a Type II SCM with something tethering it, and it should be counselled and explored as one. The number of dural tubes is the criterion that never fails; the septum is the one that hides.

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CT is for surgical planning of a bony septum, not diagnosis. It shows the location and extent of the bony bar and the associated vertebral anomalies, and it helps anticipate a difficult dural closure.

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Urodynamics are indicated for any bladder symptom, including a history of incontinence, and before scoliosis surgery. They may show detrusor-sphincter dyssynergia, high-pressure voiding or reflux, and they give a baseline for monitoring neurological recovery after surgery.

Neonatal spinal ultrasound. In the neonate and young infant, up to about 3 to 6 months, the posterior vertebral elements are incompletely ossified and leave an acoustic window, which makes ultrasound the first-line screen for a cutaneous stigma (sacral dimple, hairy patch, lipoma, dermal sinus). It can show the conus level, cord pulsation and movement, a thickened filum and even the hemicords of an SCM, with no sedation and no radiation.

When ultrasound hands over to MRI. A confidently normal study in a low-risk marker can avoid MRI. An abnormal, equivocal or high-risk finding, and any child beyond the ossification window, proceeds to MRI, which remains the definitive test. This newborn-clinic pathway for an isolated skin marker is distinct from the pre-deformity-surgery MRI mandate.

Asymptomatic split cord with scoliosis. The goal is to protect the cord during planned deformity correction, and the plan follows the type:

• Type I bony septum: plan simultaneous or staged detethering and septum resection
• Type II fibrous band: divide the band during the same anaesthetic as the scoliosis correction
• Always section the filum terminale if it is thickened or the conus is low
• Intraoperative neuromonitoring is mandatory

Progressive neurological deficit or bladder dysfunction. The goal is to release every tethering point before the damage becomes irreversible. The sequence:

1. Positioning. Prone on a Jackson table, head neutral, all pressure points padded, Foley catheter.
2. Exposure. Midline incision; expose the laminae one level above and below the split; laminectomy preserving the facets.
3. Dural opening. Open the dura in the midline and identify both hemicords and the septum or band.
4. Septum. Type I: resect the bony septum with a high-speed burr and Kerrison, protecting both hemicords. Type II: divide the fibrous band sharply under magnification.
5. Tether release. Section the filum terminale, release any scar or lipoma, and confirm free cord movement with a Valsalva manoeuvre.
6. Closure. Watertight dural closure, layered wound closure, and a drain if there is a large dead space.

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Complications arise from the natural history of an untreated split cord, from a missed diagnosis during deformity surgery, and from the detethering operation itself.

Of the untreated or missed split cord. The single most important complication to prevent is acute cord injury from operating on congenital scoliosis without a preoperative whole-spine MRI. Distraction or derotation across an unrecognised bony septum can transect the hemicords and cause immediate, permanent paraplegia: the feared, preventable disaster of congenital-scoliosis surgery. The others:

• Progressive tethered-cord syndrome: worsening leg weakness, sensory loss, cavus foot deformity and gait deterioration with growth
• Neurogenic bladder and bowel: detrusor-sphincter dyssynergia, recurrent UTI, incontinence, which may be irreversible if late
• Syringomyelia extending into the hemicords
• Curve progression while definitive correction is deferred

Of detethering and septum resection.

• New or worsened neurological deficit (motor, sensory, bladder) from manipulation of the hemicords
• CSF leak or pseudomeningocele, especially after the difficult dual-tube dural closure in Type I
• Wound infection or meningitis
• Re-tethering from scar, which may present as late deterioration after initial improvement
• Incomplete release leaving a residual tether (an unsectioned thick filum, for example) with persistent symptoms