Late-Onset Idiopathic Scoliosis

Summary of Key Points

Late-onset scoliosis, or adolescent idiopathic scoliosis (AIS), as it has previously been described, is a coronal and rotational deformity that usually presents near puberty.
AIS curves with greater Cobb angles are at higher risk of progression after skeletal maturity, whereas curves with lower Cobb angles tend to stabilize following skeletal maturity.
Curves frequently follow very characteristic patterns, and if an unexpected pattern is discovered, this should prompt a thorough investigation before labeling as AIS.
Despite early publications describing patients with scoliosis as having higher morbidity and mortality rates, AIS patients have a mortality rate similar to that of age-matched peers.
AIS patients report higher rates of back pain as adults but no greater rate of disability.
Patients are prone to concerns regarding body image and mental health. The views of patients are significantly variable, but curve magnitude and rotation (producing rib prominence) are not found to negatively affect body image.
Patients with AIS are gainfully employed, married, and able to successfully bear children at rates similar to unaffected peers.
An appreciation of the natural history and identification of those treatments that alter the natural history in a positive way guides joint decision-making between physicians, patients, and families.

Late-onset scoliosis, otherwise known as adolescent idiopathic scoliosis (AIS), is the most common type of scoliosis, and, despite its frequency, no clear cause exists. There are many proposed hypotheses regarding AIS etiology, including biomechanical abnormalities, genetics, nutrition/bone metabolic aberrations, metabolic defects, and more. Late-onset scoliosis represents a spinal curvature primarily in the coronal plane, with an often-underappreciated rotational component, and is diagnosed after other causes such as neuromuscular disorders, spinal malformations, and syndromic disorders have been excluded. Screening can be and is performed in the community using the Adam forward bend test and or a scoliometer measurement ( Fig. 21.1 ). However, the actual diagnosis relies on evaluation with spine radiographs demonstrating a Cobb angle measurement of greater than 10 degrees. ^, ^, ^, Within the cohort of patients screening positive and those meeting radiographic criteria, few will require active treatment or intervention.

Crucial to the management of AIS is a firm understanding of the natural history of the disease. Numerous authors have studied the natural history of untreated AIS to better understand the condition’s characteristic curvatures, pulmonary effects, mortality, relationship to back pain, and influence on psychosocial concerns. ^, Despite the challenges associated with natural history studies, including loss to follow-up, small sample size, and often retrospective review, a robust body of knowledge has emerged. An appreciation of the natural history and identification of those treatments that alter the natural history in a positive way guides shared decision-making between physicians, patients, and families regarding the management of AIS. Most of the data on natural history come from the Iowa longitudinal study of several hundred untreated patients who were first assessed retrospectively but were then followed prospectively from 1978 on. Although the data can certainly be criticized, a prospective trial of a large number of patients with untreated AIS is unlikely to be forthcoming. The purpose of this chapter is to review the natural history of AIS, including the prevalence, curve patterns and progression, and consequences.

Prevalence

Within the at-risk population of children between 10 and 16 years of age the prevalence of AIS is cited as being between 0.4% and 3%. ^, Even within this at-risk group, there are observed variations based on gender, ethnicity, and even geographic latitude. More northern latitudes are associated with a higher prevalence of AIS when compared with regions closer to the equator. There is a presumed genetic predisposition, with concordance for AIS in monozygotic twins being 73% versus 36% in dizygotic twins. The largest population evaluated for prevalence is in Hong Kong, for which the cited prevalence of curves measuring greater than 10 degrees in adolescents is 2.5%.

As curves become more severe, the prevalence significantly decreases, making accurate quantification more difficult. In a pooled metaanalysis of prevalence, Fong et al. reviewed 36 studies and found that the prevalence of curves with Cobb angles greater than 20 degrees was 0.22%, with far fewer requiring any active treatment. If the known prevalence is applied to the most recent census data for the United States, that would identify over half a million adolescents meeting the radiographic criteria for AIS. Despite the infrequent need for surgical intervention, the cost for surgical treatment is not insignificant, with Vigneswaran et al. estimating the cost at $1.1 billion for the greater than 5000 patients who underwent surgical treatment in 2012. This rate of surgical intervention represents an increasing number of surgeries performed for AIS each year.

Curve Patterns/Progression

AIS patients have four characteristic curve patterns used for research studies and determination of outcomes. ^, ^, Although multiple numerical classification systems exist (e.g., King, Lenke), these are mainly used for surgical decisions. The four characteristic curve patterns in AIS include thoracic curves, lumbar curves, thoracolumbar curves, and double-major curves ( Fig. 21.2 ). Even within these groups there is some ambiguity, resulting in borderline curves falling into thoracolumbar versus lumbar, and so on, depending on the investigator. Regardless, these simple descriptors based on the apex of curvature(s) provide a context for scientific discussion of natural history and curve progression. Most untreated curve progression data comes from the Iowa long-term follow-up of untreated patients. ^, ^, These patients were generally seen at 10-year intervals, so the progression data are based on averages for the group.

Fig. 21.2, Classification of scoliosis on the basis of the location of the spinal curve. Radiographs ( top panels ) and corresponding patient photographs ( bottom panels ) show three different types of scoliosis classified on the basis of the location and the apex of the major spinal curve: right thoracic ( A ), thoracolumbar ( B ), or lumbar ( C ).

Thoracic Curves

Thoracic curvatures are most common, with adolescent females predominating cases with this pattern. Thoracic curves are the most prone to progression. ^, Based on the landmark Iowa natural history study, thoracic curves measuring between 50 and 80 degrees at skeletal maturity continued to progress into adult life. This finding is often used in the literature to inform a recommendation for definitive correction and fusion.

Lumbar Curves

Large lumbar spine curvatures are less common in the natural history literature, and data regarding progression are more heterogeneous. ^, The 1983 Iowa series reported 16 degrees of progression over 29 years for curves measuring greater than 30 degrees at maturity, and 2/5 patients with severe curves (>50 degrees) had no progression, with the remaining 3/5 progressing an average of 20 degrees. These findings were confirmed by Ascani et al., who reported an average progression of 16 degrees over 29 years. Edgar, by contrast, followed 11 patients with lumbar curves that progressed only 3 degrees over 19 years. ^, However, the severity of curvature progression in the Edgar study patients was not stratified according to degree of lumbar curvature at maturity.

Thoracolumbar Curves

Large thoracolumbar curves are problematic as they have the highest likelihood of curve progression with the development of lateral translation (lateral subluxation) within the coronal plane at the lower end of the curvature. These curves had a risk of progression past skeletal maturity on the order of 0.5 degrees per year. The Iowa study demonstrated, at 40-year follow-up, a mean progression of 19.6 degrees for curves measuring greater than 30 degrees at maturity. Ascani et al. had similar findings, with a mean progression of 18.7 degrees at 34-year follow-up for thoracolumbar curves greater than 40 degrees. These findings are further supported by the results published by Edgar et al. (10 degrees of progression at 18-year follow-up). The propensity of large thoracolumbar curves to develop coronal translational deformity, degenerative changes, and continued progression informs the decision for surgical intervention for thoracolumbar curves in the 50- to 60-degree range.

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