Restless legs · Iron
When dopamine agonists stop working: iron and beyond
Restless legs syndrome (RLS) affects a meaningful proportion of the adult population, and periodic limb movement disorder (PLMD) is one of the most common but under-recognised causes of disturbed sleep in both patients and bed partners. Standard first-line treatment with dopamine agonists (pramipexole, ropinirole) is effective in the short term, but a significant proportion of patients on long-term therapy develop augmentation — a paradoxical worsening of symptoms that is often misidentified as inadequate dosing and treated by dose escalation, accelerating the problem.
Augmentation: what it is
Augmentation manifests as earlier onset of symptoms during the day, spread to previously unaffected body parts, increased intensity, and reduced response to medication. It affects a substantial proportion of patients on long-term dopamine agonist therapy and is now the main reason these agents are being replaced by alternatives in specialist practice (Allen RP et al., Sleep Medicine 2018; Trenkwalder C et al., Lancet Neurology 2021).
The role of iron
Iron is an essential cofactor for tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. Central iron deficiency — even in the absence of systemic anaemia — is a well-established contributor to RLS severity. The relevant measure is serum ferritin, and the therapeutic target in RLS is considerably higher than the anaemia threshold: most specialist guidelines suggest aiming for ferritin above 75–100 µg/L. Intravenous iron (for example ferric carboxymaltose) has demonstrated efficacy in randomised trials in RLS, particularly where oral iron is insufficient or poorly tolerated.
Key references
- Allen RP et al. Evidence-based and consensus clinical practice guidelines for the iron treatment of restless legs syndrome/Willis-Ekbom disease in adults and children. Sleep Med 2018;41:27–44.
- Trenkwalder C et al. Augmentation in restless legs syndrome is common, bothersome and difficult to manage. Lancet Neurol 2021;20(12):1071.
- Earley CJ, Connor J. Ferrous sulfate and restoring iron status in RLS. Curr Opin Neurol 2011;24(4):376–381.
- Winkelmann J et al. Treatment of restless legs syndrome: evidence-based review and implications for clinical practice. Lancet 2007;369(9563):730–733.
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