## 5 Conclusions  
In view of the results of this study, it can be concluded that around 70% of patients with FMS presented an alteration in the perception of VV: 11.1% presented an exclusive alteration in the SVV test, 35.2% an exclusive alteration in the RFT, and 24.1% in the two tests. Therefore, in approximately 60% of these patients, the disorder was detected using the RFT, which suggests that this percentage of patients present visual system dependence to the perception of verticality. Additionally, in approximately 35% of these patients, the disorder was detected using the SVV test, which means the possible alteration in the vestibular contribution to the perception of verticality.  
The MAE of degrees deviation measured with the SVV test showed a moderate correlation with vertigo disability measured with the DHI, with confidence in balance measured with the ABC-16, with catastrophizing measured with the PCS and with the impact of the disease measured with the FIQ. For its part, the MAE of degrees deviation measured during the RFT presented a low correlation with the mean of the anterior-posterior position of the center of pressure with eyes open of the static posturography parameters and a moderate correlation with the dynamic balance measured with the JAEN scale, specifically with the subscales of Instability Support Reduced and Instability Gait Eyes Open.  
These findings show the presence of a poor perception of verticality during the SVV test and RFT in patients with FMS. This could contribute to a better understanding of the pathophysiological mechanisms that are present in the alteration in the balance of these patients.  
Conceptualization, R.L.-V. and M.C.O.-P.; methodology, R.L.-V. and M.C.O.-P.; software, C.J.O.-A. and R.J.S.-S.; validation, A.J.R.-R., C.J.O.-A. and R.J.S.-S.; formal analysis, R.L.-V.; investigation, A.B.P.-R. and A.J.R.-R.; resources, C.J.O.-A., R.L.-V. and M.C.O.-P.; data curation, A.B.P.-R. and M.C.O.-P.; writing--original draft preparation, R.L.-V. and M.C.O.-P.; writing--review and editing, R.J.S.-S.; visualization, C.J.O.-A. and A.J.R.-R.; supervision, A.B.P.-R. and R.L.-V.; project administration, R.L.-V. and M.C.O.-P.; funding acquisition, R.L.-V. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the Project FEDER reference 1263880. The funding source had no involvement in the study.  
The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board (or Ethics Committee) of Ethics Committee for Biomedical Research of Andalusia PEIBA (protocol code: EPSF1).  
Written informed consent has been obtained from the patient(s) to publish this paper.  
Data Availability Statement: Data used to support the findings of this study are available from the corresponding author upon request.  
The authors would like to thank the AFIXA (Asciacion de Fibromialgia de Jaen) association and especially all of the participants in this study for their insightful contributions.  
The authors [PERSON], [PERSON], [PERSON], and [PERSON] own 14% each of the intellectual property rights of the software used to measure verticality. Said software is not commercialized yet.