AIS can significantly affect pulmonary function [5, 10,11,12], which is recognized by both clinicians  and patients . Currently, no consensus exists on how to measure pulmonary functioning and symptoms in this group of patients, and a plethora of measurement instruments are being used . This systematic review identified a total of seven clinical measurement instruments and five patient-reported outcome measures (PROMs) that have been used in studies of AIS patients from 2000 to 2020. No studies were identified on concomitant measurement properties to determine the adequacy of the identified measurement instruments. As such, floor-ceiling effects, validity, reliability, responsivity and interpretability of identified PROMs could not be evaluated in an AIS population. This study has not been able to identify any currently available adequate patient centric instrument to measure pulmonary outcomes following treatment for AIS in routine daily practice.
Clinical measurement instruments
A total of seven clinical measurement instruments were identified, measuring 50 pulmonary parameters such as FVC, FEV1, FEV1/FVC and TLC (Table 2 and Appendix 4). Spirometry and plethysmography are the most frequently used clinical-based measurement instruments and generally provide an adequate assessment of the volume and flow functions of the lungs . Although both are reliable measurement instruments for the diagnosis of restrictive lung defects [9, 30, 31], as yet, no evidence exists for the adequacy of these instruments as outcomes measurement instruments in patients with AIS.
Even though many clinical measurement instruments are frequently used in the literature, they are not suited for routine outcome measurement for patient-centered care reporting in an AIS population. They lack clinical relevance as they do not cover the patients’ perspective, are time-consuming and are expensive to obtain in routine clinical daily practice .
Patient-reported outcome measurements (PROMs)
Five PROMs were identified that have been used to assess pulmonary symptoms in AIS (Table 3). As yet, the quality of these PROMs, in terms of measurement properties as described by Terwee et al. , has not been evaluated in the AIS population. The Borg dyspnea scale, Borg RPE scale, MRC breathlessness scale and the breathing effort scale evaluate the amount of breathlessness/dyspnea in a single point in time, scoring it from 6–20 (Borg RPE), 0–10 (Borg dyspnea), 1–5 (MRC) or 1–9 (breathing effort). The 24 item UCSD SOBQ is the only scale that includes the experienced limitations during daily activities. It consists of 21 items covering the amount of breathlessness (0–5) during daily activities and 3 items concerning shortness of breath, fear of hurting themselves by overexerting and fear of shortness of breath limiting daily lives.
No evidence was found regarding the measurement properties of any of the identified PROMs in an AIS population. This does not mean that the evidence is absent. To evaluate which PROM might be eligible for future use, a post hoc literature search was performed to find studies that assessed the measurement properties of the identified PROMs in populations other than AIS (Appendix 5). Twenty studies were found and the quality these studies was good [“very good” (13/20; 65%), “adequate” (6/20; 30%), “doubtful” (1/20; 5%) (Appendix 6 and 7)]. For substantiation of the COSMIN checklist, see appendix 8. None of these studies evaluated all measurement properties as described by Terwee et al.  Overall, the UCSD SOBQ seems adequate: 6/9 measurement properties, being content validity, internal consistency, criterion validity, construct validity, agreement and responsiveness were evaluated and were scored “positive” (Appendix 7). The UCSD SOBQ has been studied in populations with lung disease; obstructive lung disease (OLD) ; chronic obstructive pulmonary disease (COPD) [32, 33] or asthma and idiopathic pulmonary fibrosis (IPF) [28, 34]. Although the UCSD SOBQ has good measurement properties and seems promising, patients with lung diseases cannot be directly compared to patients with AIS as patients with AIS have restricted, but overall healthy lungs. Research regarding the measurement properties in an AIS population is needed to demonstrate the adequacy and the clinical usefulness of this PROM.
Besides the UCSD SOBQ, the only other PROM with promising measurement properties was the Borg RPE scale (3/9 properties evaluated), and it appeared to have a good criterion validity, meaning that it relates to the gold standard . The populations studied have been more variable, including patients without primary pulmonary disease, ranging from children to healthy adults and Parkinson patients to patients recovering from a stroke.
Overall, the UCSD SOBQ seems promising as it has good measurement properties and includes the limitations of daily activities, which are important in an AIS population . However, these measurement properties have not been assessed in an AIS population and the questionnaire seems too comprehensive for routine outcome assessment when, for example, it is compared to the frequently used SRS 22 questionnaire. This questionnaire comprises 22 questions for five different outcome domains, versus 24 questions for one outcome domain in the UCSD SOBQ. Where the UCSD SOBQ seems too comprehensive for routine use, the Borg RPE scale seems too concise for assessing the pulmonary problems in AIS, even though it has good criterion validity (also not evaluated in an AIS population). The Borg RPE scale only assesses the amount of breathlessness in a single point in time, mostly used during or directly after exercise. It does not include any other information on pulmonary symptoms such as an increased fatigue, which is a regularly reported symptom in AIS patients .
Patients experience a large variety of pulmonary signs and symptoms such as shortness of breath, reduced exercise tolerance, respiratory fatigue and perceive limited daily functioning due to these pulmonary symptoms . However, as yet, the underlying cause and theoretical construct is not understood. Are the experienced limitations based on the dysfunction of the lungs themselves (e.g., limited inflation of the lungs) or is the fatigue due to increased energy consumption by the musculoskeletal system the main problem? Could the symptoms have a cardiovascular origin? A recent study showed that a proportion of AIS patients seem to have impaired right cardiac function, with pulmonary hypertension. This dysfunction normalized after scoliosis surgery, indicating the benefits of spine surgery on cardiac function, possible due to re-alignment of the spine, cardiovascular structures or rib cage . Insight in the pulmonary symptoms that are experienced by patients with AIS will support the process of the development of an adequate PROM that might be implemented for outcome measurement in routine daily use. This will also aid the evaluation of aging, progression of the curve or different treatment strategies on the pulmonary function and symptoms in patients with scoliosis. We therefore recommend to explore further with patients what limitations they experience, and with which questions/items this might be measured. This will help create a theoretical construct for the pulmonary problems experienced by patients with AIS, ultimately leading to the identification and validation of an existing PROM, such as the UCSD SOBQ or the development of a new disease specific PROM.
Several limitations of this study should be mentioned. First, selection bias might have occurred, as articles prior to 01.01.2000 and non-English articles were excluded to acquire the most relevant literature. Second, measurement properties of only the four most used clinical measurement properties (FVC, FEV1, FEV1/FVC and TLC) were assessed. However unlikely, it is possible that measurement properties of other clinical measurement instruments have been missed. Third, common language is lacking. A clear definition of the pulmonary problems in AIS does not (yet) exist, which subsequently makes it challenging performing research in this subject. Pulmonologists and physicians treating scoliosis patients have different perspectives on the matter. Pulmonary fatigue, for instance, is a definition unheard of in the pulmonary department but has been included in a core outcome set for adults and young adolescents with spinal deformity .