Address correspondence to: Vincenzo Suma, MD, Ultrasonic Service, Division of Obstetrics and Gynaecology, Civilian Hospital,Via Giustiniani 3. 35128 Padua, Italy. Ph.: 39-49-821-3481 Fax: 39-49-821-3493

Synonyms: Split cord, occult spinal dysraphism, diplomyelia with bony spur.

Definition: Primary developmental malformation leading to the division of the spinal cord into two hemi-cords¹.

Prevalence: Unknown. More than 100 postnatal cases described to date; one report of prenatal diagnosis ².

Etiology: Unknown, rare case of autosomal dominance⁹.

Pathogenesis: Partial obliteration or retention of the neurenteric canal³ or secondary to a dorsal and ventral cleft that severs the neural plate, resulting in separate closure of the two halves ⁴; otherwise, excessive neural tube dilatation after formation of the spinal cord may determine its subdivision and internal penetration by mesodermal tissue of vertebral origin ⁵.

Associated anomalies⁶:Cutaneous manifestations at the midline level of the dorsal region, including teleangiectasias, atrophic skin, hemangiomas, subcutaneous lipomas, pigmented nevi and hypertrichosis. Vertebral anomalies such as congenital scoliosis, kyphosis, spina bifida with myelomeningocele and hemivertebrae. Orthopedic deformities of the foot, especially clubfoot ⁷.

Differential diagnosis: Diplomyelia, tethered spinal cord, thickened filum terminale, neurenteric cysts, congenital dermal sinus tract, anterior sacral meningocele, spinal lipomas, dermoid and epidermoid tumors.

Prognosis: Variable; good if it presents as a closed defect, and poor if associated with spina bifida.

Recurrence risk: Unknown.

Management: Standard obstetrical care.

MESH Diastematomyelia BDE 0292 ICD9 742.51 CDC 742.52

Introduction

Spinal dysraphism⁶ is a general term to describe congenital malformations involving an abnormal closure and development of the neural tube, its neighboring structures and the posterior midline anatomy. The open forms of spinal dysraphism (meningocele, myelomeningocele and myeloschisis) may be diagnosed in utero, are often associated with Arnold-Chiari malformation or hydrocephalus, and are visible at birth. The occult forms (diastematomyelia, neurenteric cysts, spinal lipomas, thickened filum terminale, tethered spinal cord and anterior sacral meningocele) do not present a visible lesion at birth, are rarely associated with Arnold-Chiari malformation or hydrocephalus and manifest clinically with a congenital, progressive neurological deficit. The recognition of this group of disorders is important because prophylactic surgical correction can halt the progression of neurological damage. We describe a case of diastematomyelia that was identified in utero as occult spine dysraphism.

Case report

A 28-year-old woman, G₂P₀₁₀₁ was referred at about 26 weeks of gestation for further ultrasound evaluation of a triplet pregnancy; prior examination elsewhere had disclosed the presence of two sonographically normal fetuses and a suspect spina bifida in the third. A preceding triplet pregnancy had terminated at the 29^th week of gestation with the survival of a single fetus. Sonographic evaluation confirmed the triplet pregnancy. The biometry (biparietal diameter, skull and abdominal perimeters, femur and humerus length) of the three fetuses (1 female and 2 males) were consistent with a gestational age of 26 weeks. With the exception of the female, fetal anatomy was entirely normal.

A sagittal scan of the thoraco-lumbar spine evidenced the presence of a hyperechogenic structure of 12 x 9 mm, that involved the ossification centers of the last dorsal vertebrae and deformed the morphology of the posterior arches by protruding towards the skin. There was no myelomeningocele, nor disruption of the overlying integument (fig. 1,2). Transverse scans showed that the echoes corresponding to the bony processes of the vertebral body were bifid and protruding grossly towards the skin (fig. 3,4). The soft tissues in correspondence with the vertebral lesion were intact. Sonographic evaluation of the skull excluded the presence of associated cranial findings, such as ventricular dilatation, the "lemon sign" and obliteration of the cisterna magna ("banana sign"). Finally, no neuromuscular deformities were evidenced in the lower limbs. The diagnosis of occult spinal dysraphism was formulated.

Figure 1: A sagittal scan of the spine at 26 weeks demonstrates a hyperechoic structure at the level of the ossification centers of the last thoracic vertebra.

Figure 2: Similar sagittal scan as in fig 1, obtained at 30 weeks demonstrates a hyperechoic structure at the level of the ossification centers of the last thoracic vertebra.

Figure 3: A transverse scan of the spine at 26 weeks shows a central bright linear echo within the vertebral canal. The overlying soft-tissues are intact.

Figure 4: Transverse scan of the spine at 30 weeks. The echoes corresponding to the bony processes of the vertebral body appear bifid and to be protruding towards the skin.

Follow-up sonography at 28, 30 and 32 weeks did not reveal any changes in the echographic picture of the affected fetus. At 34 weeks, two male infants (1870 and 1740g) and a female infant were delivered by cesarean section. The two male neonates appeared normal. In the female baby, a 20 mm round bony formation with a central depression was observed at the level of the last thoracic vertebra (fig. 5).

Figure 5: Dorsal and lateral views of the neonate show a round formation at the level of the last thoracic vertebra.

A plain X-ray of the spine (fig. 6) evidenced a left hemivertebra at the fourth thoracic vertebra with hypoplasia of the bony process and complete sagittal splitting of the tenth thoracic vertebral body ("butterfly vertebrae"), a dysmorphic bony process at the second lumbar vertebra, partial posterior fusion of the third and fourth right ribs and dorsal convex left scoliosis with rectification of the kyphosis.

 

Figure 6: X-ray of the neonate"s spine reveals butterfly vertebrae at T10 and a dorsal convex left scoliosis.

As the infant did not present neurological disorders, she was discharged after 20 days with a weight of 2050g. At three months of age, MRI evaluation (fig. 7) showed the presence of a thick septum at the level of the 10^th thoracic vertebra that separated the spinal cord into two hemicords.

 

Figure 7: T1 weighted sagittal (left), coronal (middle) and axial (right) images of the thoraco-lumbar spine document the subdivision of the spinal cord into two hemicords that are separated by a thick septum which extends from the posterior profile of the vertebral body to the posterior vertebral cleft at T10. The cords reconnect below the diastematomyelia in the conus medullaris.

There were no other soft tissue anomalies. A subsequent CT scan (fig. 8) and reconstructed 3D/ CT images (fig. 9,10) not only confirmed the type of anomaly, but also the anatomic localization. Radiographic findings consequently demonstrated the presence of a diastematomyelia at the level of the tenth thoracic vertebra.

Figure 8: The CT scan evidences a bony septum within the spinal canal that clearly separates the two hemicords with posterior clefting of the vertebral arch.

Figure 9: Reconstructed 3D/CT images of the thoracic spine show the posterior cleft and the bony septum separating the spinal canal.

Figure 10: A detailed view of the reconstructed 3D/CT images of the thoracic spine demonstrates the posterior cleft and the bony septum.

Discussion

Diastematomyelia means clefting of the spinal cord. The spinal cord and the intradorsal nerve roots are subdivided into two columns by a mass attached anteriorly to the vertebral body and posteriorly to the dura mater. This mass may be a bony, fibrous or cartilaginous septum that partially or completely subdivides the vertebral canal. The neural elements usually present a distinct arachnoid mater but have a common dura mater. Diastematomyelia may be localized to a single vertebra or extend to several vertebral segments. The cleft is most commonly located (70-80%) at the lower thoracic or upper lumbar regions, but it may occur at any level.

Prevalence

The prevalence of diastematomyelia is unknown. Over 100 pediatric cases have been described to date. The first prenatal diagnosis were reported by Williams et al².

Pathogenesis

The nature of this aberration is unknown, but several models have been advanced to explain the phenomenon. Bremer³ suggests that diastematomyelia may result from the retention of the neurenteric canal that transiently connects the yolk sac to the amnion via the primitive knot. This knot migrates distally to the region of the coccyx, where it disappears. If an accessory canal develops, it would split the neural ectoderm with underlying endoderm and result in a midline fistula. The fistula eventually disappears, but not until abnormal vertebral and neural elements have been formed.

Padget⁴ holds that diastematomyelia is secondary to a dorsal and ventral cleft that severs the neural plate near the midline, resulting in separate closure of the two hemicords. Mesenchymal tissue filling the gap results in mesodermal and bony abnormalities.

^Gardner5 maintains that following the formation of the spinal cord, an excessive dilatation of the neural tube could provoke the subdivision of the cord and its internal penetration by mesodermal structures originating from the vertebral body. The vertebral column and the spinal cord have the same length up to the third month of embryonal life, after which growth proceeds with a different speed. The vertebral column lengths more rapidly, and at birth the conus medullaris is at the level of the lower margin of the second lumbar vertebra. This different growth continues up to the age of 5 years, when the conus medullaris reaches the upper margin of the body of the second lumbar vertebra. Diastematomyelia acts as a restraint that slows the normal growth of the spinal cord by impeding the upward migration of the neural elements, with progressive neurologic deficits in the limbs.

Associated anomalies

The anomalies that have been described in association with diastematomyelia include:

g Cutaneous manifestations on the dorsal midline consisting of telangiectasias, atrophic skin, hemangiomas, subcutaneous lipomas and cutaneous nevi. Among the cutaneous nevi, the most characteristic is the nevus pilosus, a large patch of long silky hairs, that is situated over the site of the cleft in the cord in 50-70% of the cases. The location of the cutaneous abnormality, however, is not always indicative of the level of the lesion.

g Vertebral anomalies, such as congenital scoliosis, kiphosis, spina bifida, myelomeningocele and hemivertebrae.

g Orthopedic deformities of the feet, especially clubfoot, are found in approximately half of the patients (Table 1).

Ultrasound diagnosis

The sonographic signs of occult diastematomyelia not associated with myelomeningocele have been illustrated to date in a single case at the 23^rd week of gestation by Willams et al². On coronal section of the thoraco-lumbar region, they evidenced a central, linear high-amplitude echo within a widened spinal canal. In our case, on longitudinal sections of the thoraco-lumbar region we observed a hyperechogenic structure that involved the ossification centers of the last thoracic vertebrae and deformed the morphology of the posterior arches. On a transverse scan, the echoes corresponding to the bony processes of the vertebral bodies appeared bifid, deformed and protruding towards the skin, which was intact.

Differential diagnosis

A prenatal differential diagnosis between occult and open dysraphic forms is possible. The open forms, such as meningocele and myelomeningocele, are routinely identified by sonographic examination, while only in recent years has it been possible to recognize some of the occult forms. However, the differential diagnosis between the various occult forms is possible only after birth by means of radiological investigations (plain X-rays, myelography, CT scan, MRI).

The differential diagnosis between diastematomyelia and other forms of occult spinal dysraphism6 includes:

g tethered spinal cord,

g thickened filum terminale, neurenteric cysts,

g congenital dermal sinus tract

g anterior sacral meningocele

g spinal lipomas.

Moreover, diastematomyelia must be distinguished from true diplomyelia⁸, a very rare anomaly in which there are two complete spinal cords, each having anterior and posterior horns, and its own set of nerve roots.

Prognosis

When diastematomyelia presents as a closed neural tube defect, the prognosis for neurological function may be enhanced by early surgical removal of the septum, dural reconstruction into a single tube, excision of associated developmental masses and division of the tethering filum. If diastematomyelia occurs in association with spina bifida and myelomeningocele, the prognosis is the same as for the spina bifida lesion.

 Table 1: Associated anomalies.

g Cutaneous manifestations Teleangiectasias Hemangiomas Lipomas Atrophic skin Pigmented nevi Hypertrichosis g Vertebral anomalies Congenital scoliosis Kyphosis Spina bifida with myelomeningocele Hemivertebra g Muscular-skeletal anomalies Muscular atrophy Deformity of the foot and calf g Urologic anomalies

Recurrence risk

The risk of recurrence is unknown. The great majority of cases occur in females (M:F = 1:3).

Obstetrical management

The approach to obstetrical management depends on the time of diagnosis and the presence or absence of associated anomalies. If in the second trimester there is the suspect of diastematomyelia associated with spina bifida, congenital scoliosis, kyphosis and hemivertebrae, the option of therapeutic abortion should be offered to the parents. If the diagnosis is formulated in the third trimester, the preferable mode of delivery is cesarean section. There is no reason to modify standard obstetrical management if the diastematomyelia presents as a closed defect without associated anomalies.

References

1. Guth Kelch AN, Jones RA, Zierski J: Diastematomyelia. Dev Med Child Neurol (Suppl) 13:137-138, 1971.

2. Williams AR, Barth AR: In utero sonographic recognition of diastematomyelia. AJR 144:87, 1985.

3. Bremer JL: Dorsal intestinal fistula. Accessory neuroenteric canal: diastematomyelia. Arch Pathol 54:132-138, 1952.

4. Padget DH: Neuroschisis and human embryonic maldevelopment. J Neuropathol Exp Neurol 29:192, 1970.

5. Gardner WS: Diastematomyelia and the Klippel-Feil syndrome: relationship to hydrocephalus, syringomyelia, meningocele, meningomyelocele and miencephalus. Clev Clin A 31:19-44, 1964.

6. Schut L, Sutton NL, Duhaine AC: Congenital neurological disorders of the lumbar spine presenting in the adult. In: The adult spine: Principles and Practice. Frymover W.J., Editor-in-chief., New York. Raven Press, 1991.

7. Rothman RH, Simeone FA: Anomalie congenite del rachide In: Il Rachide. Bologna. Gaggi Ed., 1978.

8. Wolpert SM, Barnes PD: MRI in pediatric neuroradiology. New York, Mosby Year Book, 1992.

9. Carter CO: Spinal dysraphism: genetic relation to neural tube malformations. J Med Genet 13:343-50, 1976.