Spinal Deformity in Children With Achondroplasia
Introduction
Achondroplasia is the most common inherited form of bone dysplasia, with an incidence of 0.36–0.60 affected babies per 10,000 live births. It is inherited as an autosomal dominant condition with 100% penetrance, and results from a gain in function point mutation in the fibroblast growth factor receptor-3 gene, which causes a disturbance in chondrocyte proliferation and therefore endochondral bone growth. Up to 90% of patients with achondroplasia have the condition as a result of a spontaneous mutation, whereas 10% have an affected parent. Achondroplastic children are readily diagnosed at birth from their characteristic phenotype, including dwarfism (with shortening occurring in a rhizomelic pattern), frontal bossing, midface hypoplasia, and normal trunk length. Intelligence is normal, but development may be somewhat delayed because the limbs are short relative to the normal head and trunk [1].
Spinal involvement in achondroplasia is universal, although symptoms vary among affected children. The disturbance in endochondral growth results in short but wide pedicles. In the normal individual, the interpediculate distance widens progressively from the lower thoracic spine to the sacrum. The interpediculate distance narrows in the patient with achondroplasia, resulting in spinal stenosis.
Although many forms of bone dysplasia result in cervical spine instability (ie, the mucopolysaccharidoses, pseudoachondroplasia, metatropic dysplasia), the patient with achondroplasia is not prone to instability. Similarly, scoliosis is seen frequently in such bone dysplasias as diastrophic dysplasia and chondrodysplasia punctata, but is uncommon in patients with achondroplasia.
The child with achondroplasia may present with symptomatic or asymptomatic foramen magnum stenosis (cervicomedullary stenosis), lumbar spinal stenosis, or thoracolumbar kyphosis. The following sections will further detail these conditions in the achondroplastic pediatric patient.
Section snippets
Cervicomedullary Stenosis
Cervicomedullary stenosis results from narrowing of the foramen magnum, producing compression at the cervicomedullary junction. Not only is the foramen magnum narrow, it does not expand during growth in children with achondroplasia, as a result of abnormal skull growth including premature fusion of the posterior synchondroses of the foramen [2]. The posterior rim of the foramen is thickened more horizontally than normal, and indents the posterior aspect of the spinal canal (Fig. 1).
Presenting
Thoracolumbar Kyphosis
Thoracolumbar kyphosis is chronologically the next spinal concern in the child with achondroplasia. Presenting in the latter half of the first year of life, the parents note a bump at the thoracolumbar junction, seen best when they prop the infant in the sitting position. Thoracolumbar kyphosis is present in up to 97% of infants with achondroplasia at <1 year of age [10].
Thoracolumbar kyphosis results from relative hypoplasia of the anterior aspect of the L1 vertebral body. As the infant is
Spinal Stenosis
Lumbar spinal stenosis is present anatomically in all patients with achondroplasia. Several anatomic studies have shown that not only does the interpediculate distance narrow in a craniocaudal direction in the lumbar spine, but the actual thickness of the pedicles increases as one progresses distally in the lumbar spine (Fig. 5). This results in a significant decrease in the spinal canal area in the lumbar spine. In addition, the ligamentum flavum may be thickened, the facet joints are
Conclusions
The child with achondroplasia may present with significant spinal involvement requiring prompt diagnosis and careful treatment. Cervicomedullary stenosis can be life-threatening in the young baby, and it may be left to the orthopedic surgeon to determine whether an infant is at risk for sudden death as a result of central apnea. Thoracolumbar kyphosis is nearly universal in the young child, but typically resolves. Bracing may be indicated in ambulatory children, and surgical correction is
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