CHAPTER 290 Congenital Abnormalities of the Thoracic and Lumbar Spine
Epidemiology and Associated Disorders
The overall incidence of congenital thoracolumbar abnormalities is low. Spinal dysraphism is seen in 20 of every 100,000 births in the United States and higher in offspring of affected parents.1 There is a greater risk in pregnancies with maternal diabetes mellitus.2 Congenital scoliosis has an overall incidence of 1 in 1000 to 2000.3 Known genetic abnormalities include linkage to chromosome 18 for multiple defects in segmentation and chromosome 17 for hemivertebrae.4,5
Abnormalities of the spine are often seen in association with disorders in other systems. In one series, up to 61% of patients with congenital spine disease had another system involved.6 The urologic system is the most commonly associated system seen in conjunction with congenital spine abnormalities. Oskovian and associates and Macewen and colleagues found 18% of their congenital scoliosis patients to have urologic abnormalities.7,8 Defects in the cardiac system have been found in 26% of patients with congenital spinal abnormalities as described by Basu and colleagues.9 There is a clear association with pulmonary insufficiency with severe thoracic vertebral deformities related to structural limitations imposed on the thorax by the deformity.10 There is also an association between congenital spine abnormalities and such syndromes as OEIS, VATER, and the Currarino triad (anorectal malformation, partial sacral agenesis, and presacral mass).11–13 The common association between congenital spinal anomalies and others mandates a thorough work-up of a patient with congenital spine abnormalities.
Imaging
Magnetic resonance imaging (MRI) of the entire neuraxis is indicated in the evaluation of congenital spinal abnormalities because 18% to 35% of patients with congenital scoliosis have abnormalities of the brain and spinal cord.14–18 Ultrasound can be useful in neonatal evaluation before the benefit of diagnosis outweighs the risk of the general anesthesia required for MRI.19 There is some debate that MRI should be reserved for patients with lower extremity, bowel, or bladder symptomatology, or signs of closed spinal dysraphism such as hairy patches. However, the utility, safety, and widespread availability of this imaging modality should be considered. The ability to assess cord tethering, asymptomatic mass lesions, and syringomyelia makes MRI pivotal in weighing the benefits, risks, and alternatives of management plans.
Embryology
Embryogenesis
The normal embryo transforms into the blastula after bilaminar induction by the second week of gestation. The epiblast is the origin of future ectoderm and the hypoblast is the origin of future mesoderm. The primitive streak is located on the dorsal embryonic midline and is composed of totipotent epiblastic cells. Human spine development occurs at the site of the primitive streak, which appears on day 15. Cells divide caudally to elongate the primitive streak, ereas the primitive knot or Hensen’s node is the rostral thickening. This elongation defines the rostrocaudal embryonic axis.20
The process of gastrulation involves the induction of the future mesoderm and this happens during week 3. The blastopore, an opening in the blastula, marks the embryo’s entry into gastrulation. Subsequently, the epiblastic cells migrate in a complex motion dictated by the central signaling apparatus, Hensen’s node, or the Spemann organizer, located on the axial side.21 The organizer crucially acts to allow migration of epiblastic cells into the primitive groove, an indentation within the primitive streak. These migrating epiblastic cells interpose in the epiblast-hypoblast transition to form mesoderm, making a trilaminar embryo composed of ectoderm, mesoderm, and endoderm. The midline migratory cells specifically form the basis of the notochord, axial skeleton, and neuroectoderm inductor. Abnormal gastrulation results in failed integration of neural structures into the axial midline and resultant split-cord malformations.22
Multiple genes have been implicated in abnormal gastrulation such as BMP and Pax. The homeobox and zinc finger class transcription factor gene families are necessary for proper induction of mesoderm formation, differentiation of neural crest, and development of the vertebral column and spinal cord.23
Neurogenesis
Cells migrate from the cranial end of the primitive streak from Hensen’s node to form the notochord on day 16. Organogenesis requires an intact notochord and primitive streak. During primary neurulation, the notochord interacts with the overlying ectoderm to form the neuroectoderm and neural plate. Subsequently, on day 18, the neural plate invaginates to form the neural groove in the midline and the neural folds laterally. By day 21, the neural folds fuse to form the precursor of the spinal cord, the neural tube. This portion of the developing spine and neuraxis extends only to the lumbosacral junction. During secondary neurulation, the neural tube distal to the second sacral vertebra forms from the caudal aspect of the primitive streak. These secondary neural elements cavitate with a terminal ventricle and eventually form the cauda equina and distal spinal cord. The conus medullaris contains a dilated central canal that is the remnant of the cavity with the secondary neural tube.24 The neural tube closes in a bidirectional, zipperlike fashion starting around the sixth to seventh cervical somite stage. The cranial neuropore closes around day 24 and the caudal neuropore closes around day 27. Failure of neuropore closure results in the spectrum of disorders known as spinal dysraphism.25
Skeletogenesis
The somite stage of development marks the formation of the spine around the notochord.26 The mesoderm bilaterally thickens and segments into paired cuboidal structures called somites at around 20 days of gestation. They form along a rostrocaudal axis. A total of 42 to 44 pairs of somites form, 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal. During development, there is regression of the first occipital and the last five to seven coccygeal somites, leaving a total of 38. The caudal eminence of the somites then becomes the hindgut, the terminal portion of the sacrum and coccyx and the terminal spinal cord. Failure of proper development of the caudal eminence, can lead to sacral agenesis, imperforate anus, cloacal exstrophy, and malformation/caudal regression.27–29
The ventral portions of the somites are the sclerotomes that eventually become the skeletal system, including the vertebral bodies, the cartilaginous tissue, the disks, and the cells of the spinal meninges.25 The intermediate portions are the myotomes that become the striated musculature. The dorsal portions are the dermatomes that become the skin and subcutaneous tissues. As development progresses, the sclerotomes distinctly divide in two to form the vertebral bodies and the intervertebral disks. Within the disks, the notochord remnants form the nucleus pulposus as control by Pax gene expression.30 Chordomas arise from notochord remnants and occur primarily at the skull base and the sacrum (Fig. 290-1).31,32
The expression of Pax is also important in the overall patterning of the sclerotome and the development of the ventral vertebral body from the neural tube, as demonstrated in a knockout mouse model.30 The dorsal vertebral arch, however, requires the induction from the neural crest and this signaling is dependent on Zic, Gli, and Pax.33 Due to the separate inductive controls and temporal development of the ventral and dorsal elements of the vertebra, abnormalities can be independent of each other.7
Congenital Scoliosis
Congenital scoliosis is defined as an abnormality in the lateral curvature of the spine due to abnormal vertebrae present from birth that induce a coronal plane deformity.34 Specifically, congenital scoliosis differs from infantile idiopathic scoliosis, because the latter is not associated with abnormally formed vertebrae. Although difficult to estimate accurately due to undetected, benign cases, the overall incidence of congenital scoliosis is approximately 1 to 2 per 1000.3,16,34 Clinical presentation varies widely because of the myriad combinations of possible vertebral abnormalities. Approximately, 25% of patients with congenital scoliosis progress rapidly, 50% progress slowly, and the remaining 25% do not progress.7
The congenital scoliotic deformity is caused by some combination of vertebral dysgenesis and/or failure of segmentation.35 The most common vertebral anomaly is the hemivertebra. In contrast, a unilateral unsegmented bar is the most common segmentation failure and involves the facet and disk.16 Both segmentation and formation defects can result in severe progressive scoliosis, especially in the thoracolumbar spine.36 Hemivertebrae can be classified into three major types: fully segmented, semisegmented, and nonsegmented. Fully segmented hemivertebrae allow for full growth on one side and none on the other, leaving a significant potential for deformity based on the anatomic location. The scoliotic curves from fully segmented hemivertebrae may progress at the rate of one to two degrees annually. Semisegmented hemivertebrae are connected to one of the adjacent vertebrae, causing a wedge shape with differential side growths resulting in some scoliosis. Nonsegmented hemivertebrae are connected to both adjacent vertebrae and lack disk spaces, causing deformity but with limited progression due to the absence of growth plates.
The principle type of segmentation failure resulting in congenital scoliosis is the unilateral unsegmented bar. There are no growth plates on the affected side and the unaffected side of the spine continues to grow normally, resulting in a progressive deformity of approximately five degrees annually. This abnormality usually results in a substantial deformity and is usually treated surgically.34,37
From natural history data, the rate of scoliotic deterioration is determined by patient age, anomaly type, and anomalous level and curve pattern.3,34 It is generally agreed that progressive deformity ought to be treated surgically if orthotic therapy fails.7,16 Surgery remains the best treatment for progressive and severe deformity. The many surgical options for the treatment of congenital scoliosis are discussed in detail elsewhere in this text. Common procedures include hemivertebra excision, in situ fusion, instrumentation, and thoracoplasty with vertical expansion prosthetic titanium rib (VEPTR).7 Specific indications are evolving and are beyond the scope of this chapter. However, it is important to note that due to the 18% to 35% risk of associated abnormalities of the neural axis, surgical treatment carries the risk of neurological deterioration, particularly in cases of an untreated tethered spinal cord.15–1738
Congenital Lordosis and Kyphosis
Isolated congenital kyphosis and lordosis, both sagittal plane deformities, are considerably less prevalent than congenital scoliosis. They are both commonly seen in conjunction with coronal plane deformities and referred to as kyphoscoliosis and lordoscoliosis, respectively.7,39 Congenital kyphosis is divided into three classes under the Winter scheme: type I has failed vertebral body formation, type II has failed vertebral body segmentation, and type III has mixed features. There is also associated instability, likely from defects in the pars interarticularis, which predisposes to further neurological injury.39 Early posterior treatment for type I and type III kyphosis can help prevent paraplegia and allow for continued anterior growth to let the spine straighten with time.40 Treatment with posterior arthrodesis is advocated for types I and III kyphosis before it exceeds 60 degrees.41,42 Kyphosis exceeding 60 degrees requires an anterior release and subsequent posterior stabilization.42,43 A combined anterior and posterior approach using costotransversectomy has also been advocated for treatment of kyphosis and kyphoscoliosis.44 Type II congenital kyphosis is less common and not as commonly associated with neurologic injury as types I and III unless associated with kyphoscoliosis.41
Congenital lordosis is the rarest of curvature deformities but can be the most fatal due to pulmonary compromise in the case of thoracic location.45 It results from a failure of posterior segmentation.10 These cases require anterior correction due to excessive anterior growth and may be associated with preexisting diminished pulmonary function or pulmonary artery hypertension.43,45
Congenital Thoracolumbar Stenosis
There are three types of stenosis: congenital, developmental, and degenerative. A distinction is made for congenital stenosis because it results from a prenatal malformation that presents insidiously. Developmental stenosis implies chromosomal or spinal cord malformation such as that associated with a hereditary syndrome such as achondroplasia.46,47 The patients usually become symptomatic earlier in life and involve more spinal segments than their degenerative counterparts and have similar symptoms of claudication or radiculopathy that are relieved by resting or flexion.46,48,49 There are multiple classification schemes for congenital stenosis, including one from Verbiest and another delineated by Singh and colleagues. Neither scheme offers risk stratification for treatment options.7,46,48 Of fundamental importance in consideration of surgical treatment are the presence of neurological deficits, concomitant conditions such as spinal dysraphism, and other nonspinal comorbidities.
