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Commentary & Perspective

Commentary & Perspective on
"Results of Preoperative Pulmonary Function Testing of Adolescents with Idiopathic Scoliosis: A Study of Six Hundred and Thirty-one Patients"
by Peter O. Newton, MD, et al.

Commentary & Perspective by
Paul D. Sponseller, MD*,
Johns Hopkins Medical Institutions, Baltimore, Maryland

Patients with adolescent idiopathic scoliosis are frequently concerned that scoliosis may affect their pulmonary function. Prior reports have shown that thoracic curves affect pulmonary function. Early reports, which showed the most dramatic pulmonary effects, included patients with early onset scoliosis¹. Subsequently, virtually all published reports on the topic have shown that pulmonary function is diminished on measured pulmonary function tests in many patients with adolescent idiopathic scoliosis. In this important article, Newton and coauthors analyze the relationship between curve features and pulmonary impairment in the largest reported cohort of adolescents with idiopathic scoliosis, using the most detailed analysis of curve characteristics to date. Factors found to predict decreased pulmonary function were increasing curve magnitude, the number of vertebrae involved in the thoracic curve, and the degree of thoracic hypokyphosis. Thus, they have confirmed what earlier, smaller series had shown, but they also report that these factors explain only a portion of the observed variability in pulmonary function and that the Lenke classification predicts significant differences in the risk of moderate to severe pulmonary impairment, particularly in patients with double thoracic and triple curves. In contrast, there was no relationship between curve flexibility and pulmonary function.

That the expected factors predicted only part of the variability may, in part, be explained by the fact that the pulmonary testing was performed in at least a half-dozen laboratories and by the additional fact that teenagers may vary with regard to comorbidities and physical conditioning. The magnitude of the pulmonary impairment is probably underestimated, as the authors point out, since loss of height due to the scoliosis was not taken into account in calculating the expected reduction in pulmonary function.

The theory that several different genetically determined mechanisms cause adolescent idiopathic scoliosis is becoming increasingly plausible; yet, there seems to be a distinct relationship between adolescent idiopathic scoliosis and pulmonary impairment.

The authors ask thought-provoking questions about the mechanism(s) by which scoliosis causes pulmonary impairment. They could not confirm the theory advanced by others that spinal stiffness contributes to decreased pulmonary function. They note that a few patients with small curves have pulmonary impairment. They raise, but cannot answer, the possibility that curves below the thorax may affect pulmonary function by distorting the diaphragm (or the abdominal cavity).

What remains is to glean clinical significance from these findings. Just as outcomes measures have become popular as a means of giving clinical meaning to technical parameters such as radiographic measurements, the clinical importance of pulmonary function testing needs to be determined. Weinstein et al. have provided much useful information throughout their fifty-year longitudinal analysis of the Iowa idiopathic scoliosis cohort^2,3. Although they summarized that "untreated late-onset idiopathic scoliosis causes little physical impairment other than back pain and cosmetic concerns," they also found that there was a relationship between shortness of breath and the size and location of the curve. They found that a Cobb angle of >50° at skeletal maturity was associated with a significantly increased likelihood of the subsequent development of shortness of breath. The patients who had a thoracic curve of >80° at the time of final follow-up had a significantly increased risk of shortness of breath. Survival probability estimates provided only a very rough estimate of infrequent severe health impairment, since many patients could not be located. They identified three patients of 117 in whom scoliosis potentially contributed to a death from pulmonary causes. Thus, although scoliosis can produce pulmonary impairment, it is only rarely life-threatening.

The degree to which this impairment becomes clinically significant may merit further research⁴. This information, combined with improvements in scoliosis care and the trend toward shared decision-making, may allow us to give patients a more accurate assessment of the possibility of future pulmonary symptoms. From this study, we now know that curve characteristics alone do not completely determine pulmonary impairment. The authors suggest that wider use of preoperative pulmonary function testing may allow us to identify patients with greater pulmonary risk, which may affect a decision about surgery. The findings and questions raised by Newton et al. suggest that further investigation into both the physiology and clinical impact of scoliosis on pulmonary function is appropriate.

*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

1. Nilsonne U, Lundgren KD. Long-term prognosis in idiopathic scoliosis. Acta Orthop Scand. 1968;39:456-65.
2. Weinstein SL, Zavala DC, Ponseti IV. Idiopathic scoliosis: long-term follow-up and prognosis in untreated patients. J Bone Joint Surg Am. 1981;63:702-12.
3. Weinstein SL, Dolan LA, Spratt KF, Peterson KK, Spoonamore MJ, Ponseti IV. Health and function of patients with untreated idiopathic scoliosis: a 50-year natural history study. JAMA. 2003;289:559-67.
4. Lenke LG, White DK, Kemp JS, Bridwell KH, Blanke KM, Engsberg JR. Evaluation of ventilatory efficiency during exercise in patients with idiopathic scoliosis undergoing spinal fusion. Spine. 2002;27:2041-5.