How can restless legs syndrome present on only one side?

Unilateral Restless Legs Syndrome: A Recognized Clinical Variant

Unilateral RLS is a well-documented variant that occurs in approximately 17% of RLS cases and represents a legitimate presentation of the disorder, though it more commonly indicates secondary RLS rather than primary disease.

Clinical Recognition and Prevalence

Unilateral RLS is not rare and has distinct epidemiological characteristics:

• Approximately 17% of RLS patients present with strictly unilateral symptoms, with an additional subset showing unilateral onset that later becomes bilateral but maintains persistent asymmetry 1
• Symptoms often begin in one leg only, even in cases that eventually become bilateral—this unilateral onset pattern is common in the natural history of RLS 2
• The 2012 International RLS Study Group diagnostic criteria explicitly accommodate unilateral presentations, as criterion #1 states "an urge to move the legs" without requiring bilateral involvement 3

Key Distinguishing Features of Unilateral RLS

When evaluating unilateral RLS, specific clinical patterns emerge that differentiate it from bilateral disease:

Higher Likelihood of Secondary Causes

• Unilateral RLS is significantly more likely to be secondary rather than primary (44% secondary in unilateral vs. 23% secondary in bilateral; odds ratio 4.2) 1
• Secondary causes to investigate include: peripheral neuropathy, radiculopathy, vascular insufficiency, structural lesions, or localized iron deficiency 1

Family History Patterns

• Positive family history is less common in unilateral RLS (11% in unilateral vs. 36% in bilateral; odds ratio 2.6), suggesting reduced genetic contribution 1
• This lower familial clustering further supports the hypothesis that unilateral presentations more often reflect acquired or secondary pathology 1

Polysomnographic Characteristics

• Patients with unilateral RLS demonstrate predominantly ipsilateral periodic limb movements in sleep (PLMS), matching the side of symptomatic complaints 4
• This ipsilateral PLMS pattern provides objective confirmation of the unilateral nature and distinguishes it from bilateral disease 4

Diagnostic Approach

When encountering suspected unilateral RLS, the evaluation should focus on:

Confirm RLS Diagnosis

• Verify all five essential diagnostic criteria are met, with particular attention to criterion #5: excluding mimics such as unilateral leg cramps, venous stasis, radiculopathy, arthritis, or positional discomfort 3
• The unilateral presentation itself does not invalidate the diagnosis if all other criteria are satisfied 3

Investigate Secondary Causes

• Obtain serum ferritin and transferrin saturation to assess iron status, as iron deficiency can manifest with asymmetric symptoms 5
• Perform neurological examination and consider nerve conduction studies to exclude peripheral neuropathy or radiculopathy 1
• Review neuroimaging if focal neurological deficits are present or if symptoms suggest structural pathology 4
• Assess for comorbid conditions: diabetes, kidney disease, multiple sclerosis, or Parkinson's disease 5

Document Clinical Course

• Determine whether symptoms remain strictly unilateral or show progression to bilateral involvement with persistent asymmetry 1
• Evaluate family history, as absence of familial RLS supports secondary etiology 1

Treatment Considerations

Management follows standard RLS treatment algorithms, though response patterns may differ:

First-Line Pharmacotherapy

• Gabapentin enacarbil, gabapentin, or pregabalin remain first-line treatments per 2025 AASM guidelines, regardless of unilateral presentation 6
• However, unilateral RLS may show variable response to dopaminergic drugs, with some patients demonstrating poor efficacy compared to bilateral disease 4

Iron Supplementation

• Initiate IV ferric carboxymaltose for serum ferritin ≤100 ng/mL or transferrin saturation <20% (strong recommendation) 6
• Alternatively, use ferrous sulfate 325-650 mg daily or every other day for appropriate iron parameters 5

Address Secondary Causes

• Treat underlying conditions such as peripheral neuropathy, radiculopathy, or vascular disease that may be driving unilateral symptoms 1
• Discontinue exacerbating medications (serotonergic antidepressants, dopamine antagonists, antihistamines) if present 5

Important Clinical Pitfalls

• Do not dismiss unilateral RLS as "not real RLS"—it represents a legitimate variant that fulfills diagnostic criteria 3, 1
• Do not assume unilateral RLS is always primary disease—maintain high suspicion for secondary causes and investigate accordingly 1
• Do not expect uniform response to dopaminergic therapy—unilateral cases may require alternative approaches with gabapentinoids or opioids 4
• Be aware that unilateral RLS can occur in unusual contexts, such as following deep brain stimulation in Parkinson's disease patients, where it may affect the dominant PD side 7