Adolescent idiopathic scoliosis (AIS) is a lateral curvature of the spine of at least 10°, measured with Cobb angle, of unknown etiology occurring in children and adolescents aged 10–18 years (1). AIS is the most common form of scoliosis and is distinguished from other types of scoliosis by the absence of underlying congenital or neuromuscular abnormalities (2). In the United States, the estimated prevalence of AIS with a Cobb angle of at least 10° among children and adolescents aged 10–16 years is 1–3% (3). The development of spinal deformity may impact a patients' level of reported quality of life (QOL) (4).

Various theories have hypothesized the causes of AIS, including biomechanical, neuromuscular, genetic, and environmental origins; yet our understanding of scoliosis etiology is still limited. There is moderate-quality evidence for decreased postural stability in AIS measured with the following Center of Pressure (CoP) parameters: sway area, and medio-lateral and antero-posterior range with a positional shift posteriorly in the sagittal plane (5). Another study (6) found that in most patients with AIS, Subjective Postural Vertical (SPV) was shifted to the right, with no alteration of the Subjective Visual Vertical (SVV). Catanzariti et al. (7) proposed a global pathophysiological concept, which involves an issue with orthostatic postural control, with disturbance in the multisensory integration of vestibular, visual, and somesthetic inputs. Thus, AIS could be the consequence of a reorientation of the longitudinal body axis in accordance with an erroneous central representation of verticality (7). Another author proposed that AIS might be the consequence of a reoriented longitudinal body axis aligned with an erroneous vertical reference (6).

Nonetheless, the evidence investigating the effect of different therapies on the most used tests of verticality, such as SVV and postural control in patients with AIS, are scarce. For this reason, the primary aim of this study was to describe these variables in patients with AIS who underwent different types of conservative management, such as observation only, exercise, or brace orthosis. Additionally, we compared the different types of management approaches and their influence on posture perception.

Eighteen patients were recruited and evaluated after applying the selection criteria. Nine patients underwent conservative treatment (four orthotic and five exercises), and nine were under observation only. Sociodemographic characteristics are shown in Table 1.

In terms of SVV measures (Table 2), differences in constant error (with ± sign) were not statistically significant; however, in descriptive analysis, patients who underwent conservative treatment showed good alignment of SVV for the true vertical, but patients who were under observation only showed a deviation of the SVV to the left. Conversely, patients who underwent treatment showed poor constant error in absolute values and in mean absolute error, with statistically significant differences (p < 0.05).

No significant results were obtained for posturographic parameters (Table 3). Only the Romberg Quotient for Sway Area neared the limits of statistical significance, with a lower value for patients under observation only, which could be interpreted as less influence of vision in postural balance.

This study's objective was to describe the sense of verticality measured with SVV test and postural control in patients with AIS, comparing these variables in different types of conservative management: under observation only, therapeutic exercise only, or orthotic brace only. The overall results showed no differences in postural control measured by posturographic parameters. However, there appeared to be differences in verticality perception between different conservative treatments. Concretely, it appears that the conservative treatment applied to the scoliotic curve provoked poor results in verticality perception compared to solely observation. This could be interpreted as the disruptive effect that the treatment produces on the perception of verticality of patients with AIS, who are previously adapted to perceive good verticality while living with the scoliotic curve. At the same time, based on these observations, the results of this study support that conservative treatments for the scoliotic curves influence the posture perception of patients with AIS. These conservative treatments include well-designed and adapted braces providing 50% correction, applied in progressive curves between 20 and 45° (17), and Schroth exercises in those patients with AIS with a Cobb's angle between 10 and 30°. However, evidence does not support this latter as more effective than traditional exercises (18). There are other methods based on respiratory exercise, such as Global Postural Reeducation (GPR), that has proven to be effective in treating spinal pain (19), but its efficacy in treating scoliosis is at an early stage (20), and the evidence is still scant.

Available evidence is scarce concerning the treatment of verticality perception and postural control in AIS. As the results of this study showed worse perception of verticality in patients receiving some type of conservative treatment, our hypothesis is that with the progression of the scoliotic curve, a compensatory mechanism for the altered SVV occurs, appearing as a mismatch in the SVV while the scoliotic curve is being corrected. In a clinical trial conducted by Yagci et al. (21), subjective vertical visual perception only improved significantly (p < 0.05) in patients with AIS who performed body awareness exercises; however, this did not happen in those patients with AIS who performed Core Stabilization Exercises or Traditional Exercises. In contrast, there was a significant improvement (p < 0.05) in subjective visual horizontal perception, postural vertical perception, total postural perception scores, total haptic perception scores, and haptic perception 45° to the right by the use of the stabilization and body awareness exercises. This indicates that body awareness exercises significantly improve SVV and that perception of verticality and postural perception play an important role in AIS. Another study found that wearing a spinal brace improved postural stability in terms of increased proprioception, equilibrium performance, and rhythmic movement ability in patients with AIS (22), during the time of wearing the orthosis. However, both studies had small samples, and their results can be considered preliminary.

Additionally, in this study, only the Romberg Quotient for Sway Area of posturographic parameters neared the limits of statistical significance with a lower value for the patients being followed-up, which could be interpreted as less influence of vision in postural balance. Overall, in our study, no differences were found in postural control according to the type of treatment the participants received. However, significant differences appeared in the variables related to SVV. This fact is very significant because it indicates that subjects with scoliosis can establish different strategies to maintain adequate control of posture while living with the curve. While untreated patients have good postural control, good perception of verticality, and scoliotic curves, treated subjects appear to enter a different state in which they obtain good postural control by distorting their perception of verticality to fit the curves.

If the data observed by this study are confirmed, it could mean that the treatment of scoliosis produces a distortion of the perception of verticality of the subjects, whose postural control system would tend to recover while maintaining the scoliotic curves, producing resistance to the improvement of the deformity. If this hypothesis is confirmed, the effect of the readjustment of the perception of verticality should be explored to progressively adapt it to the curve's improvement, thus consolidating the results of the treatment. Treatments linking the correction of the scoliotic curve and the management of postural control and SVV could be the key to obtain success in this pathology.

Some limitations of this study must be considered when interpreting the results. The sample recruited in the present study was limited, and the treatment was carried out heterogeneously (brace orthosis and GPR). Furthermore, our research was performed with adolescents from a specific geographic area, and generalization of its results may be limited to individuals with characteristics similar to those of our sample population. Moreover, the cross-sectional nature of our design does not allow causal relationships to be established between postural control. At our best knowledge, the sitting measurement of perception of verticality is not the only way for assessment of the SVV in scoliosis, so it cannot be assured that it is the best way. In this line, future studies should compare different measuring positions, such as standing or standing on foam ground. Forthcoming research should explore prospective designs with a larger and more diverse population, using objective measures to evaluate these variables.

In conclusion, the results of this study showed a worse perception of verticality in patients receiving some type of conservative treatment compared to patients only under observation. Furthermore, only the Romberg Quotient for Sway Area of posturographic parameters showed lower values for the patients under observation, which could be interpreted as a lower contribution of vision in postural balance; however, this result was not significant in our study.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Ethics committee of the Jaen Hospital (Code VEFRE-30/05/19). Written informed consent to participate in this study was provided by the participants' legal guardian/next of kin. Written informed consent was obtained from the minor(s)' legal guardian/next of kin for the publication of any potentially identifiable images or data included in this article.

NZ-A, JA-O, and SO-S: conceptualization. NZ-A, JA-O, and RL-V: methodology. RL-V: software, formal analysis, and writing—review and editing. AI-V: investigation. FL-M, AI-V, and JA-O: resources. FL-M and AI-V: data curation. NZ-A: writing—original draft preparation. SO-S: supervision. NZ-A and FL-M: project administration. All authors have read and agreed to the published version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.