Jun. 18, 2022
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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.
• Split cord malformations are associated with spinal dysraphisms, congenital scoliosis, and tethered cord.
• 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.
• 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.
• Early identification and surgical correction can arrest or improve neurologic symptoms.
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.
The clinical presentations of split cord malformations can be classified as asymptomatic or symptomatic. Asymptomatic presentations are similar to those of other forms of occult spinal dysraphism, the most prominent presentation being external cutaneous manifestations. The most predictive lesions for an underlying split cord malformation occur as hypertrichosis followed by capillary hemangiomas, often occurring together. Hypertrichosis is by far the most common manifestation of the split cord malformations, seen in over 50% of patients; the hypertrichosis includes a variety of presentations such as “faun’s tail,” “silky down,” or even just focal normal texture but excessive hair (15; 11; Giggisberg et al 2004; 20; 26). Common vertebral malformation such as spina bifida, butterfly vertebra, hemivertebra, and fused or absent ribs can also be seen (11; 48; 36). Other cutaneous markers include subcutaneous lipoma, dermal sinus, dimple or gluteal cleft abnormality, or hyperpigmented patch (Giggisberg et al 2004; 59; 26). Other associated manifestations include tethered cord, syringohydromyelia, and Chiari malformation (41; 52).
Symptomatic cases often present with progressive signs and symptoms. The most common presenting symptoms are scoliosis and tethered spinal cord syndrome (43; 48; 36). Progressive motor and sensory symptoms are especially common. Bladder and bowel symptoms are also common, although sometimes signs of neurogenic bladder may only be detected after urodynamic testing (29; 48; 36). Scoliosis or kyphoscoliosis and orthopedic lower-limb deformities (most commonly congenital talipes equinovarus, followed by shortening of one limb) are findings often seen in patients with split cord malformations (24; 48; 36). Finally, back and leg pain is often seen in this condition, particularly in adults (45; 05; 39). Pain is often initially described as dull and localized over the affected region of the back or as ill-localized dysesthetic pain diffusely involving both legs. Others have "boring" or "searing" pain in the perineal and anal regions. Most of these signs and symptoms are related to lesions in the lumbosacral area, the region most commonly involved in split cord malformation (45; 43; 48; 36). Rarely, split cord can occur in the cervical region, where the signs and symptoms affect the arms and hands in addition to the legs, bladder, and bowels (48; 36; 40). Also, it has been reported in association with other complex central nervous system malformations (46; 02).
In children, the clinical presentation is more likely to be scoliosis, weakness, or cutaneous, lower-limb abnormality, but pain is a rare complaint (Schijiman 2003; 17; 62). Furthermore, split cord malformation is commonly seen in children with congenital spine deformities, such as myelomeningocele and congenital scoliosis (43; 43; 30; 51; 19). Also, tethered cord is commonly found in children with split cord malformation or other congenital spinal malformations (01). Identification of these situations is important because of the real potential for neurologic deterioration that exists with split cord pathology, especially if the congenital spinal anomaly is corrected. This deterioration is likely secondary to traction on the cord and underlying tethered cord pathophysiology. Therefore, imaging of the entire neuroaxis is recommended for patients with congenital spine abnormalities (30; 18; 03; 35). Split cord malformation has also been reported with situs inversus (10; 60) and intraspinal teratomas (37).
In a retrospective report of 266 patients with congenital scoliosis and coexisting split cord malformations, 83.5% of cases presented with multiple malformations. As compared to the Type II group, patients in the Type I group showed higher prevalence of multiple malformations and mixed anomalies of vertebral deformations. Additionally, the length of spine involved was also longer in the Type I group patients. This study also showed a 62.8% prevalence of rib anomalies among patients with congenital scoliosis and spinal cord malformations. They also identified multiple intraspinal anomalies with high prevalence in association with congenital scoliosis with comorbid split cord malformations (13).
A 6-year-old female presented to a neurology clinic for a recent history of increased fatigue when walking more than a block. An area of hypertrichosis with a capillary hemangioma over the lumbosacral area was noted. She also was noted to have moderate scoliosis, and scoliosis films demonstrated a lumbar hemivertebra. No other problems were reported.
Her exam showed some weakness of both ankle dorsiflexors. She had difficulty toe walking. Her patellar deep tendon reflexes and ankle reflexes were hyperactive. Toes were upgoing bilaterally. The tone in her lower extremities was moderately increased. No sensory abnormalities were found. MRI of her spine demonstrated a thick filum terminale and a split cord malformation with an osseous septum in the lower lumbar area. Urodynamic study showed evidence of bladder instability with upper motor neuron findings.
She was referred to pediatric neurosurgery and underwent surgical exploration of the malformation, which demonstrated the site of tethering as the bony septum. The septum was excised, and the filum was divided. Her clinical course has remained stable. There has been neither increased neurologic impairment nor any improvement. Her urodynamics returned to normal after the surgery. She was diagnosed with split cord malformation type I with associated tethered cord.
Diastematomyelia has been reported in sisters, suggesting a possible X-linked inheritance as the cause (04). It also has been reported in association with Angelman syndrome (38). It should be emphasized that split cord malformation is usually identified as a nonsyndromal, sporadic disorder.
The split cord malformation begins early in nervous system development. Formation of the spinal cord begins during the third week of development. The notochord develops from endoderm and constitutes the axis around which the other structures later form. The notochord induces the neuroplate to form from ectoderm, and this then invaginates to form the neural tube, the source of the spinal cord. The underlying defect has been proposed to be the formation of an accessory neurenteric canal (43). The normally present primitive neuroenteric canal forms between embryonic days 16 and 24 and provides a conduit of communication between the amnion and the yolk sac via the primitive knot. The accessory canal provides an abnormal fistula, enabling continued contact between ectoderm and endoderm within this space. The abnormal canal causes regional "splitting" of the notochord and overlying neural plate, allowing mesenchymal tissue to fill the gap, resulting in the endomesenchymal tract. The formation of the abnormal fistula is the critical step because it allows the multipotent mesenchymal tissue to permanently bisect the notochord, forcing each overlying hemineural plate to neurulate against its own hemicord. This tract can result in bony and cartilaginous clefts along with neuroenteric cysts in an area of split cord. The final appearance of the matured split cord malformation ultimately depends on the ability of the embryo to heal around the endomesenchymal tract, the variable extent to which the endomesenchymal tract persists, and the ultimate developmental fates of the dislocated midline mesenchyme and endoderm. The interposed septum derives from the mesenchymal tissue, resulting in either a bony (type I) or fibrous (type II) composition (43; 26). In reality, however, many patients have variations that do not fit either category exactly; thus, the process is likely much more complex and still not completely understood. Intermediate or overlapping split cord malformations have been reported, suggesting that the distinction between the 2 types of split cord malformation may be artificial and that they instead represent a spectrum of malformations due to an underlying ontogenetic error (36; 61; 21; 35; 28; 54).
There is little information regarding epidemiology, although the condition is rare. The male to female ratio has been reported at 1:1.5 (43; 27; 48; 36; 59). Case reports show that this condition can become symptomatic even in the elderly (31; 42; 49), although the vast majority of patients are detected in childhood due to symptoms secondary to tethered cord syndrome or associated malformations such as congenital scoliosis, myelomeningocele, or lower-extremity orthopedic malformations, especially with increased availability of early MRI imaging. Type I split cord malformation is more common, representing about two thirds of cases in most case series (48; 36; 59).
Folic acid supplementation can decrease the risk for myelomeningocele, which is associated with split cord malformations (51). Other preventative measures are not available at this time.
The differential diagnosis of this condition depends on whether or not external cutaneous manifestations are present, thus, narrowing the likely diagnoses to some sort of spinal dysraphism. Two or more skin lesions in the midline lumbosacral area are a particularly strong indication on an underlying lesion (16; 36). When the skin of the back is without markings, the differential diagnosis of back pain and progressive neurologic signs and symptoms becomes considerably broader. In this situation, one must consider tumors, infectious processes, demyelinating disorders, arachnoiditis, discitis, rheumatoid disorders, and degenerative vertebral disease as potential causes.
When signs of occult spinal dysraphism are present, including hypertrichosis, cutaneous capillary hemangiomas, dimples or sinuses, or spina bifida occulta noted on plain x-ray, the possibility of a split spinal cord malformation becomes real. However, it should be emphasized that other pathologies that cause the tethered spinal cord syndrome can also be seen in dysraphic states. These conditions may be the sole or contributing causes to the presenting symptoms and signs. These conditions include: thickened filum terminale, lipomas, and dermal sinuses with and without dermoid cysts (41; 52).
Finally, split cord malformation should be considered in the differential diagnosis of neurologic deterioration in individuals with various forms of myelodysplasia or scoliosis, and progression of symptoms can occur after surgical repair if the associated split cord malformation was not detected and corrected simultaneously (31; 36; 35).
Diagnostic workup centers on the proper radiologic imaging of the underlying abnormal anatomy. Particular attention needs to be paid to identifying potential secondary tethering lesions. Plain x-ray studies of the spine can show the bony abnormalities within the spinal canal, but this is not always the case, and overall the diagnostic yield for detecting split cord malformations is low; however, plain films can be helpful for identifying other associated body abnormalities (59). MRI is a good screening study to identify the presence and extent of any split cord malformation and any associated lesions such as tethered cord or low-lying cord; also, it can identify non-adjacent abnormalities, such as syringohydromyelia and Chiari type 1 malformation (36). However, MRI does not always identify all of the pathologic anatomy of importance to anyone planning neurosurgical intervention, such as vertebral body abnormalities often present in this condition. It is particularly important to define the anatomy of any osseous or osseocartilaginous midline septa because the presence of a midline septa requires special surgical consideration. For this reason, CT myelography or 3-D spiral CT scans as a complementary study to MRI in the evaluation of split cord malformations is recommended. Firstly, CT allows the differentiation of split cord lesions into types I and II. A type I split cord malformation consists of 2 hemicords, each contained within its own dural tube and separated by a dura-sheathed rigid osseocartilaginous median septum. A type II split cord malformation consists of 2 hemicords housed in a single dural tube separated by a nonrigid, fibrous median septum. Secondly, CT is superior to MRI in delineating the size, shape, and configuration of bony septa as well as the delineation of hypertrophic neural arches and other vertebral body abnormalities. Some lesions, however, are not identified until the time of surgery, although as the quality of imaging continues to improve, this becomes less likely over time (41; 52).
It has become possible to diagnose split cord malformations prenatally by ultrasound, usually in the second and third trimester. There has been one case of diastematomyelia diagnosed in the first trimester (34). The main diagnostic clue for prenatal ultrasound is the presence of an extra echogenic focus in the midline between the posterior ossification centers of the spine.
Another important area of diagnostic workup is related to bladder function. Many authors stress the importance of a urodynamic study to determine whether or not the bladder is neurologically involved (29; 48; 36). Although many cases show such neurologic symptoms as incontinence, recurrent urinary tract infections, or renal deterioration, others have more subtle findings. These findings are present on urodynamic testing and do not correlate well with the neurologic examination. Nonetheless, these individuals have neuropathic bladder changes and may require urologic intervention because of a high risk for renal deterioration. Sometimes, urodynamic findings alone are the only sign of current neurologic involvement from split cord malformation and may be the only objective data available for helping to make decisions about whether or not to intervene surgically or to follow the patient, looking for deteriorations in symptoms or the neurologic examination (47).
The role of somatosensory evoked potentials is less clear. Some authors have advocated using the study to follow asymptomatic patients. Somatosensory evoked potentials testing may be helpful if performed in neurologically-normal patients at regular intervals (25). In this situation, the test would be used to give advanced warning of changes in nerve conduction before there is clinical evidence of neurologic deficit. This test, however, is not widely used for this purpose, and there are no clear data that it adds useful information.
Prenatal ultrasound diagnosis of isolated diastematomyelia has been reported (56; 02; 32).
State-of-the-art management of split cord malformation is similar to that of the tethered cord syndrome and includes early identification (particularly in high-risk individuals with external cutaneous manifestations of spinal dysraphism, those with congenital scoliosis or lower-extremity orthopedic malformations, or those with myelodysplasia), appropriate decisions concerning whom to operate on, and good postoperative follow-up. Clear indications for surgery include neurologic progression and scoliosis as well as congenital dysraphisms, such as myelomeningocele. Surgical treatment of asymptomatic individuals is more controversial, and the decision whether to operate is a balance between the skill of the surgeon and the risk of associated complications, such as meningitis, CSF leak, worsening of neurologic symptoms immediately after surgery, or late neurologic deterioration because of retethering in the future due to scar tissue, or incomplete removal or regrowth of the septum. Furthermore, many patients continue to have progression of their scoliosis after split cord repair and require later spinal fusion. In cases of split cord malformation manifesting in adults, surgery versus expectant management remains debatable given limited presentations and literature (63).
In terms of the surgical procedure, primary attention is paid to untethering the hemicords by resecting the midline bony or cartilaginous spur, the dural sleeves enwrapping the spur, and any soft tissue band or septum attaching any part of the hemicords to the surrounding dura or bone (43; 01). Split cord malformations are frequently associated with other tethering lesions. These other lesions, usually caudal to the split cord malformation, need to be released to relieve the symptomatically stretched spinal cord. The associated lesions include thickened filums, dermoids or dermal sinus tracts, intradural lipomas, and neuroenteric cysts that can be other causes of tethered cord and, thus, also need be surgically corrected. Myelomeningocele manque is an occult spinal cord lesion secondary to failed meningocele formation; it consists of taut fibrous bands admixed with dorsal paramedian nerve roots and blood vessels that tether the hemicords to where the fibroneurovascular stalk penetrates the dura. Myelomeningocele manque is commonly found and, because it contributes to cord tethering, necessitates repair. Hemicords sometimes occur in tandem in the same spinal cord (so-called composite split cord malformations). Also, hydrocephalus and syringohydromyelia can be associated, so care should be taken to image the entire neuraxis. These points are made to emphasize the importance of having a neurosurgeon with operative experience with split cord malformation involved in managing the case (01).
Most surgeons continue to advise surgery for all individuals with split cord pathologies. This is based on data that neurologic outcome worsens with age of the patient at time of surgery (11; 62). Furthermore, surgery during the asymptomatic period is advocated by some due to fears that neurologic symptoms may not improve after surgery (17; 62). Some others propose that split cord malformations may contribute to scoliosis progression and should be repaired before scoliosis surgery (35). Others advocate that split cord malformation type I may require surgical repair before scoliosis surgery, but that type II may not need corrective surgery (03; 50). Alternatively, some studies suggest that patients with congenital scoliosis associated with split cord malformation, regardless of split cord malformation type, can safely undergo spinal deformity correction without neurologic intervention or prophylactic tethering in patients with intact or stable neurologic status (12; 57). One 2019 study of 32 patients with congenital scoliosis and type 1 split cord malformation suggested that not resecting the bony spur prior to spinal curve correction may also achieve satisfactory radiographic and clinical outcomes with potentially fewer complications (64).
Use of endoscopic surgery may reduce complications and increase the benefit-to-risk ratio for surgical repair (08). One study found that 1-stage major spinal osteotomy, including simultaneous bony septum excision in patients with scoliosis with type I spinal cord malformation, and spinal osteotomy alone in patients with scoliosis and type II is effective for correction of severe, rigid scoliosis in split cord malformations without increasing the rate of surgical complications (07).
In high-risk surgical procedures, such as those patients with multiple spinal conditions, use of intraoperative neurophysiological monitoring is advised. In the case of a female pediatric patient who underwent 12 operations for the correction of tethered cord, scoliosis, and split spinal cord, the use of intraoperative neurophysiological monitoring was crucial because it detected sudden significant loss of transcranial electrical motor-evoked potentials, which caused cancellation of a surgical procedure. Had they performed the procedure without intraoperative neurophysiological monitoring, the patient may have experienced paraplegia (22).
Management also must address the potential deficits that may be present prior to operation. Often, these do not completely improve but, rather, stabilize after operation. The surgery should be considered prophylactic rather than curative. Orthopedic deformities such as rigid ankle deformities and scoliosis may require management and need to be followed. Also, neurogenic bladder should be managed aggressively as this condition can lead to life-threatening renal deterioration. Management includes treatment of urinary tract infections, clean intermittent catheterization, and the use of anticholinergic medications; furthermore, post-operative urodynamics are helpful for establishing a baseline. Patients should be followed closely by a multi-disciplinary team both before and after surgery (01).
Surgery is particularly effective for pain as well as neurologic deterioration; the majority of patients have improvement, and most of the rest have stabilization of symptoms (48; 36; 59). As an exception, pre-existing vertebral column anomalies can progress and require further surgery in the future. Also, some cases of later neurologic deterioration are seen, usually because of retethering due to scar tissue, although this also responds to surgical correction. The role for surgery in asymptomatic patients, however, is controversial, with some authors arguing for surgical intervention at the time of diagnosis (11; 36; 59) or before other corrective spinal surgeries (35) and others arguing for careful monitoring and surgery only if neurologic deterioration occurs (23; 02; 03). Although some series have demonstrated detrimental effects of delayed diagnosis and treatment in terms of neurologic and urologic function (55; 11; 47; 36), others have not (23; 02). Patients with split cord malformations associated with meningomyelocele may have worse outcome than those without meningomyelocele (30) although neurologic status may be stabilized following surgery for both conditions (19).
One retrospective, longitudinal study of 65 patients showed that in a subgroup of patients who underwent tethered spinal cord surgery with the use of intraoperative neurophysiological monitoring at 12 years of age or younger exhibited a high rate of progressive scoliosis (09). It is unknown whether the progressive scoliosis is due to the tethered cord or other reasons; however, other studies have shown that patients with spinal dysraphism associated with tethering are predisposed to scoliosis (06).
Symptoms of tethered cord syndrome precipitated after childbirth in the lithotomy position has been reported (45). Thus, extra care of obstetric patients with known spinal dysraphisms is warranted.
Neuraxial anesthesia must be carefully considered in patients with spinal cord malformations. In the case of vertebral anomalies, epidural space may be absent or compromised, complicating access to it during needle placement (53).
Michael V Johnston MD
Dr. Johnston of Johns Hopkins University School of Medicine has no relevant financial relationships to disclose.See Profile
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