A prospective cohort study. We conducted a prospective cohort study to identify the association between steroids and clinical worsening and compare outcomes between patients with and without preoperative steroid administration. Patients with SDAVFs often were misdiagnosed and treated with steroids which led to acute worsening. Patients with angiographically-confirmed SDAVFs were recruited consecutively between March 2013 and December 2014 in two referral centers. We reviewed the history of all the patients to identify those patients who were treated with steroids prior to exclusion of the fistulas. Modified Aminoff & Logue's scale (mALS) was used to evaluate the spinal cord function at different time points: before and after steroid administration, before operation and at 1-year follow-up. Paired t tests were used to assess the mALS of patients with steroid administration at different time points. Unpaired t tests and Pearson chi-square test were used to assess differences between patients with and without steroid administration. 18 patients with (18.2%) and 81 patients without (81.8%) steroid administration were included in this study. At baseline, there were no difference between both patient groups, in regards to age, gender, duration, location of fistula, treatment and preoperative mALS. However, patients without steroid administration had statistically significant better outcome according to their mALS at 1-year follow-up (P < 0.05). Steroid administration can induce acute clinical worsening in patients with SDAVFs that may persist despite successful obliteration of the fistula and should thus be avoided. 3.