Congenital Abnormalities of the Spine

Embryology and Developmental Anatomy

Spine formation begins early in gestation, commencing at the end of the second gestational week with formation of the Hensen node and continuing into the beginning of the third week with the appearance of the neural plate during gastrulation. The notochordal process forms at day 16 or 17, with transient communication of the amnion through the notochordal canal to the yolk sac and through the neurenteric canal of Kovalevsky. The spine develops in a mostly orderly progression, with the vertebral axis and spinal cord developing synchronously. The rostral spinal cord (to about the level of S2) forms by the process of primary neurulation, whereas the caudal spinal cord (below the S2 level) forms by secondary neurulation, previously referred to as canalization and retrogressive differentiation. Most congenital spinal anomalies can be explained by one or more events going awry during these processes.

The neural tube folds and closes at the end of the third gestational week, during primary neurulation; this leaves temporary cranial and caudal openings called neuropores . Normal neural tube closure by day 25 to 27 signals the end of primary neurulation. Meanwhile, the neural tube separates from the overlying ectoderm during the related process of disjunction. If disjunction occurs prematurely, perineural mesenchyme is permitted access to the neural groove and ependymal lining. This mesenchyme may differentiate into fat and prevent complete neural tube closure, which leads to the lipomatous malformation spectrum. If disjunction fails to occur (nondisjunction), an ectodermal–neuroectodermal tract forms that prevents mesenchymal migration. Nondisjunction results in posterior dysraphism, producing the open neural tube defect spectrum of myelomeningocele (MMC), dorsal dermal sinus, and myelocystocele.

The neuroepithelial cells (neuroblasts) around the inner neural tube form the mantle layer, which produces the spinal cord gray matter. The outermost layer forms the marginal layer, which subsequently myelinates to produce the spinal cord white matter. The central neuroepithelial cells differentiate into ependymal cells along the central canal. Neural crest cells along each side of the neural tube form the dorsal root ganglia, autonomic ganglia, Schwann cells, leptomeninges, and adrenal medulla.

Concurrent with the neural tube folding during primary neurulation, spinal cord development below the caudal neuropore commences within the pluripotent tissue at the caudal eminence in the process of secondary neurulation. The initially solid cell mass canalizes and becomes contiguous with the rostral neural tube that was formed by primary neurulation. By day 48, a transient ventriculus terminalis appears in the future conus. If this persists after birth, it is noted incidentally as a normal variant ventriculus terminalis (“fifth ventricle”), usually of no clinical significance. Failure of proper secondary neurulation leads to caudal spine anomalies in the caudal regression, tethered cord, or sacrococcygeal teratoma (SCT) spectra in addition to terminal myelocystocele and anterior sacral meningocele.

By the third gestational month, the spinal cord extends the entire length of the developing spinal column. In fact, the more rapid elongation of the vertebral column and dura relative to the cord produces the apparent ascent of the cord during the remainder of gestation. Most important, the conus should be at adult level soon after birth, and persistent cord termination below L2–L3 after the first month of life in a full-gestation infant is probably abnormally low-lying.

Occurring simultaneously with spinal cord development is vertebral formation. During neurulation, the notochord induces the surrounding paraxial mesoderm derived from the primitive streak to form paired somite blocks (myotomes and sclerotomes). The myotomes form the paraspinal muscles and skin cover, and the sclerotomes divide into medial and lateral formations to produce the vertebral bodies, intervertebral disks, meninges, spinal ligaments (medial), and posterior spinal elements (lateral). Failure of correct notochordal induction leads to incomplete splitting of the neural plate from the notochord, producing the split notochord syndromes (neurenteric cyst and diastematomyelia).

From day 24 until the fifth week, sclerotomal resegmentation commences, during which a horizontal sclerotomal cleft appears in the vertebra, and the caudal half of one vertebra combines with the rostral half of the vertebra below to form a “new” vertebral body. The notochord within the vertebral body degenerates, and the intervertebral notochord remnant becomes the intervertebral disk nucleus pulposus. Between days 40 and 60, the vertebrae undergo chondrification followed by subsequent ossification at distinct centers within the vertebral body and arches. This process continues past birth and into young adulthood. Ossification begins in the lower thoracic and upper lumbar regions and diverges cranially and caudally. In the cervical region, the vertebral primary ossification centers appear after the neural arch centers, beginning in the lower cervical spine (C6, C7) and proceeding rostrally. Aberrances occurring during the chondrification and ossification process produce myriad segmentation and fusion anomalies (SFAs; hemivertebrae, butterfly vertebrae, block vertebrae).

Spinal Dysraphism

Spinal dysraphism is a broad term that encompasses a variety of disorders that have as a common feature abnormal dorsal spine formation; it is defined as incomplete or absent fusion of midline mesenchymal, osseous, and neural structures. This term refers to large spinal defects, and not to the common spina bifida occulta, in which there is only a small cleft within a spinous process or a minor incomplete fusion of the L5 or S1 laminae. Use of the older term spina bifida occulta is strongly discouraged in favor of the preferred term incomplete posterior element fusion, because this finding is generally incidental and without clinical significance.

The osseous abnormalities associated with true spinal dysraphism may involve multiple vertebrae. Spina bifida (Latin, “cleft into two parts”) is characterized by incomplete neural arch fusion with absence of all or parts of the affected posterior elements (laminae and spinous processes). Associated segmentation anomalies of the vertebral bodies may be present, such as hemivertebrae, butterfly vertebrae, and block vertebrae.

Children with spinal dysraphism may come to medical attention with a back mass, abnormal cutaneous manifestations, gait disturbance, and bowel and bladder incontinence. Classically, spinal dysraphism is classified into two categories, based on the clinical presence or absence of a back mass. Two categories of dysraphism with back mass are described. The first category is spinal dysraphism with back mass that is not covered by skin (e.g., spina bifida aperta or cystica, MMC, myelocele); the second is spinal dysraphism with skin-covered back mass (e.g., lipomyelomeningocele, myelocystocele, dorsal meningocele).

Abnormalities of Primary Neurulation

Primary neurulation abnormalities result from premature disjunction, nondisjunction, or a combination of both.