Patient characteristics predicting responses to intravenous ferric carboxymaltose treatment of restless legs syndrome

Background: Significant benefit of intravenous ferric carboxymaltose (FCM) treatment for restless legs syndrome (RLS) has been well-established. However, no clinical indicators predicting treatment response of RLS have been established. This study aimed to determine factors predicting outcome of clinical FCM treatment of RLS patients. Methods: Data were retrospectively reviewed from all patients who received FCM treatment for RLS from April 2016 to April 2019. These data included: detailed history, international RLS scale score (IRLS), questionnaires, comorbidity, and previous RLS medication use. Morning fasting serum iron, ferritin, and total iron-binding capacity were measured before and at four weeks after treatment. RLS patients with possible secondary RLS were identified by reviewing the medical histories. This included patients with iron deficiency anemia, lumbosacral radiculopathy, and gastrectomy. Primary RLS included those with no indication of secondary medical factors contributing to RLS. Treatment response was assessed using the IRLS and clinical ratings at four weeks after FCM administration. Patients with a greater than 40% decrease in IRLS were classified as responders. Results: The study comprised 164 patients with IRLS and clinical ratings obtained before and at four weeks after intravenous (IV) iron. Treatment responses differed considerably between diagnostic groups of RLS. Percentage responding was: 64.7% (66 of 102) for patients with primary RLS, 90.9% (10 of 11) with gastrectomy, 91.3% (21 of 23) with iron deficiency anemia and 39.3% (11 of 28) with lumbosacral radiculopathy. When responders were compared to non-responders in primary RLS patients, responders had significantly lower serum iron (80.5 ± 26.7 vs. 95.8 ± 30.5 μg/dL, p = 0.022) and percentage transferrin saturation (%TSAT) (25.4 ± 9.6 vs. 30.5 ± 10.5%, p = 0.026) in females, but not males. Logistic regression controlling for major subject variables showed that %TSAT significantly predicted response. (odds ratio [OR]: 0.955, confidence interval: 0.913–0.998, p = 0.040). Conclusion: Intravenous FCM in moderate to severe RLS patients is beneficial as a first-line or add-on treatment, particularly for patients with compromised peripheral iron state. Overall, lower %TSAT predicted better chance of responding to the IV iron treatment especially for females.