Congenital Scoliosis

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CHAPTER 22 Congenital Scoliosis

Paul D. Sponseller, MD, Beverlie L. Ting, MD

Congenital scoliosis is a progressive three-dimensional deformity of the spine caused by congenital anomalies of the vertebrae that result in an imbalance of the longitudinal growth of the spine. To understand their natural history and their treatment, it is important to understand the embryologic development of vertebrae.

Embryology

Paraxial mesoderm on either side of the notochord condenses to form somites, in a process known as somitogenesis. Each somite subdivides further into ventral sclerotome and dorsolateral dermomyotome. During the 4th week of development, cells from the sclerotome region of the somite on each side of the body migrate ventrally and surround the notochord and the neural tube. Each vertebra is formed by sclerotome cells from two somite levels (Fig. 22–1). The cranial and caudal parts of adjacent sclerotomes, which are not ossified yet, fuse with each other.¹ The ventral part of each vertebra forms the body around the notochord, and the dorsal part forms costal processes laterally and the vertebra arch dorsally.

FIGURE 22–1 Each vertebra is formed by a part of four somites.

Ossification begins during the 6th week from three primary ossification centers: one in the body (or centrum, formed by early fusion of two centers) and one in each half of the vertebral arch. During the 6th week of development, mesenchymal cells between cranial and caudal parts of the original sclerotome fill the space between two vertebral bodies to contribute to formation of the intervertebral structures.² This stage is called the segmentation stage.

Somitogenesis relies on the Notch signaling pathway and its interactions with FGF and Wnt signaling; however the precise mechanisms remain unclear.³^–⁵ Mutations in downstream components and targets of the Notch signaling pathway, such as Dll3, Mesp2, and Lfng, result in abnormal vertebral development in mouse models and are associated with characteristic vertebral defects seen in patients with spondylocostal dysostosis.³^–⁷ More recent experimental evidence in vertebrate animal models and human stem cells has revealed the oscillatory nature of gene expression in paraxial mesoderm during somitogenesis.^3,^8,⁹ These findings support the concept that a putative segmentation clock triggers the cyclic expression of genes in the Notch, FGF, and Wnt signaling pathways and is essential for normal vertebral development.

Classification

Two types of basic vertebral anomalies can occur: failures of formation and failures of segmentation.¹⁰

Failures of Formation

Failure of vertebral formation (type I deformity) can be partial, causing a wedged vertebra with intact pedicles (Fig. 22–2), or complete, causing a hemivertebra with a unilateral pedicle (Fig. 22–3). Hemivertebrae are classified according to their longitudinal growth potential, which in normal vertebrae is provided by growth apophyses on both vertebral ends.

• Segmented hemivertebra: Both the superior and the inferior ends of the hemivertebra have growth potential. The shape of adjacent vertebrae is normal.

• Semisegmented hemivertebra: Either the superior or the inferior end of the hemivertebra has growth potential. The other end is fused with the adjacent vertebra.

• Incarcerated hemivertebra: Both the superior and the inferior ends of the hemivertebra have growth potential, but the adjacent vertebrae compensate for it. The hemivertebra is “carved into” the adjacent levels.

• Nonsegmented hemivertebra: There is no growth potential. The hemivertebra is completely fused with vertebrae above and below.

FIGURE 22–2 Wedge vertebra is due to a mild form of unilateral vertebral failure. The vertebra height is asymmetrical on the right and left side.

FIGURE 22–3 Hemivertebrae are classified according to their growth potential. A, Segmented hemivertebra. B, Semisegmented hemivertebra. C, Incarcerated hemivertebra. D, Nonsegmented hemivertebra.

Failures of Segmentation

Failure of segmentation (type II deformity) can be partial, causing a bar (Fig. 22–4), or complete, causing a block vertebra. A congenital bar can be anterior, posterior, lateral, or mixed. In many cases, vertebral anomalies owing to failures of formation and failures of segmentation coexist,¹¹ occasionally on several levels, and form a mixed deformity (type III deformity).

FIGURE 22–4 Congenital unilateral bar. Partial fusion between two vertebrae prevents longitudinal growth on its side.

Associated Anomalies

Embryologic development of the spine coincides with the development of many other organ systems. It is not rare to have associated anomalies with vertebral deficiencies. These anomalies occur in 30% to 60% of children with congenital spinal anomalies.¹¹^–¹³ The most common coexistent anomalies involve the spinal cord and the genitourinary tract. Intraspinal anomalies include problems such as tethered cord, diastematomyelia, and syringomyelia. The most common genitourinary defects are renal agenesis, ectopic kidney, duplication, and reflux.

Many of these anomalies are part of the VATER association. The acronym VATER¹⁴ includes the following deficiencies: vertebral defects (V), anal atresia (A), tracheoesophageal fistula (TE), radial limb reduction, and renal defects (R). The acronym VATER was modified in 1975¹⁵ to VACTERL by adding cardiac defect (C) and limb defect (L) (Fig. 22–5).

FIGURE 22–5 Congenital scoliosis with VACTERL association managed by observation since birth. A and B, Right thoracic curve and compensatory lumbar curve remain relatively unchanged from birth at 1 year of age (A) and at 8 years of age (B).

Congenital vertebral anomalies are also found with a high incidence in Klippel-Feil syndrome,^16,¹⁷ which is characterized by the combination of cervical fusion, limited neck range of motion, short neck, and low hairline. More recently, congenital scoliosis has been associated with Sprengel deformity, Mayer-Rokitansky-Küster-Hauser syndrome, Jarcho-Levin syndrome, Goldenhar syndrome, and Genoa syndrome.¹⁸^–²¹

Etiology

Congenital scoliosis is uncommon in the general population. Its true incidence is unknown, but the familial incidence in the congenital scoliosis population is typically 1% to 5%.^13,²²^–²⁴ It is slightly more common in girls than in boys, with a ratio of 3 : 2.

The precise etiology of congenital scoliosis is unclear. Although most cases seem to be sporadic, in contrast to idiopathic scoliosis,²⁵ the role of genetic and environmental factors is often reported.^6,^26,²⁷ The genetic role has been reported in cases of congenital scoliosis in twins,²⁸^–³⁰ but, more recently, several studies have isolated gene mutations.^25,^27,³¹

Environmental factors have also been implicated in the genesis of congenital scoliosis. Maternal acute carbon monoxide exposure during somite formation induces vertebral anomalies in the offspring of mouse and rabbit models.^26,³² The mechanism of carbon monoxide action remains vague, however. Carbon monoxide could act directly on the cartilaginous spine via resulting hypoxia or a gene mutation.²⁶ The etiologic theories are clouded further by the finding of an increased incidence of idiopathic scoliosis in families with congenital scoliosis.

Natural History

As with scoliosis of any etiology, congenital scoliosis progresses in 70% of patients during growth. The potential for increase in curvature is related to imbalances in the number of growth apophyses and the location of vertebral anomalies.³³ Without any treatment, about 85% of patients with congenital scoliosis have a curve greater than 41 degrees by maturity.³³ Curves with segmented hemivertebrae are at risk for progression during growth because segmented hemivertebrae act as enlarging wedges (Fig. 22–6). The most progressive anomaly is a convex segmented hemivertebra associated with a concave unilateral bar because there is absolutely no growth potential on the side of the bar. Conversely, a wedge vertebra has only a slight risk of worsening, whereas a complete block or an incarcerated hemivertebra does not cause any progressive scoliosis.

FIGURE 22–6 Hemivertebra forces spine into a curve. There are two growth apophyses on the hemivertebra side and only one on the other side, leading to worsening during growth.

The location of the anomaly also plays a part in the evolution of scoliosis. The most severe anomalies are those located at the thoracolumbar region, whereas the least severe are located at the upper thoracic spine.

The natural history of congenital scoliosis has to take several factors into account:

• Type of anomaly

• Location

• Number of anomalies

• Initial severity of the scoliosis

• Global growth potential balance between each side of the spine

Analysis of these factors allows one to determine the potential for progressive curvature and the most appropriate treatment.

More recent work has focused on the role of the spine and chest wall in lung development, which predominantly occurs by age 5. Congenital defects in the development of the ribs and vertebrae often occur together. Concurrent scoliosis and rib fusions may constrict the thorax and compromise pulmonary development. The inability of the thorax to support normal respiration is termed thoracic insufficiency syndrome.³⁴ Thoracic insufficiency syndrome can be assessed clinically by respiratory rate and the thumb excursion test and radiographically by plain radiographs and computed tomography (CT) volumetric studies. Early fusion of scoliotic deformity before age 9, especially in patients requiring more than four levels of fusion and patients with proximal fusions, also puts these patients at risk for the development of restrictive pulmonary disease.³⁵ The increasing awareness of the need to preserve pulmonary function has led to a surge in growth-preserving surgical techniques for complex multilevel congenital scoliosis.

Assessment

Physical Examination

The physical examination of a patient with congenital scoliosis is guided by the knowledge of a heightened incidence of other structural and neural anomalies. The examination should begin with an assessment of a patient’s existing balance: sagittal plane balance and coronal balance, shoulder malalignment, and any deviation of head and trunk from the center of the pelvis. In addition, it is crucial to assess and document the neurologic status, including strength, reflexes, and presence of any atrophy. Flexibility of the deformity, gait, and limb-length inequality should be checked. Pain, if present, should be localized and quantified. The presence of a dimple or any cutaneous mark on the back should be noted. The examiner should search for other anomalies of the extremities (particularly radial malformation) and range of neck motion.

Imaging

Radiographs

For patients with congenital scoliosis, early plain radiographs are helpful to determine the type of vertebral abnormality. The best period to categorize the deformity is before 4 years of age. If a patient is seen by the orthopaedic surgeon after this age, valuable information may sometimes be gained by examining prior chest radiographs and abdominal or renal films. The spine surgeon learns to check for subtle clues, such as the presence and spacing of pedicles and fused or absent ribs. Later films make it difficult to assess the type of anomaly present because the vertebrae are too ossified, especially in the area of a fusion or bar (Fig. 22–7).

FIGURE 22–7 A and B, It is easier to analyze segmented hemivertebra (A) or unilateral bar (B) when films are taken before 4 years of age than after. C, Lumbar segmented hemivertebra in a 9-year-old child.

Anteroposterior and lateral films allow one to check the type and the location of deficiency, to measure the spine curvature, and to assess the pedicle width. Measurement of curvature according to the Cobb method ³⁶ can be a challenge, however.³⁷ It has been shown that there is an increase in measurement error³⁸ in congenital scoliosis owing to irregular vertebral landmarks and difficulty in numbering them. It is important always to compare current films against original ones rather than rely on curvature measurement itself. Assessment of curve evolution and compensatory curve development helps to confirm or to refute scoliosis progression. Because compensatory curves involve normally formed vertebrae, they can be more reproducibly measured. If a compensatory curve has not progressed, it is less likely that significant progression has occurred in a congenital curve.