How should I evaluate and treat an infant with restless‑leg‑type movements that disrupt sleep, including workup for iron deficiency and criteria for specialist referral?

Restless Leg Syndrome in Babies

Diagnosing true RLS in infants is extremely challenging because the standard diagnostic criteria require the child to describe symptoms in their own words, which infants cannot do; however, when an infant presents with sleep-disrupting leg movements, you should immediately check serum ferritin and treat with oral iron supplementation if ferritin is below 50 ng/mL. 1, 2

Diagnostic Challenges in Infants

• No validated diagnostic tools exist for infants or young children with suspected RLS. The International Restless Legs Syndrome Study Group (IRLSSG) diagnostic criteria explicitly require that "the description of these symptoms should be in the child's own words," making diagnosis in pre-verbal infants essentially impossible by standard criteria. 1

• The five essential diagnostic criteria for RLS include: (1) urge to move legs with uncomfortable sensations, (2) symptoms begin/worsen with rest, (3) relief with movement, (4) worse in evening/night, and (5) not explained by another condition—all of which require verbal communication to confirm. 1

Clinical Presentation in Early Childhood

When RLS does occur in very young children, the presentation differs markedly from older children and adults:

• The most characteristic symptom in infants is awakening after 1-3 hours of sleep followed by screaming, crying, kicking, or hitting the legs. 2

• Symptoms can begin as early as the first year of life, with a median onset at 7.5 months of age in one case series. 2

• These infants typically have low ferritin levels and elevated periodic limb movement index during sleep (PLMSI) on polysomnography. 2

Critical Differential Diagnoses to Exclude

Before attributing symptoms to RLS, you must systematically exclude common mimics that can present with similar nighttime leg movements:

• Leg cramps, positional discomfort, habitual foot tapping, and muscle aches can all superficially meet RLS criteria but are distinct conditions. 1

• Periodic limb movement disorder (PLMD) causes repetitive limb movements only during sleep (not before sleep onset), unlike RLS which includes pre-sleep symptoms. 1

• In children with atopic dermatitis, itch-induced leg movements can mimic RLS and may lead to a comorbid diagnosis of restless leg syndrome. 1

• Growing pains, dehydration with electrolyte imbalances (particularly hypocalcemia), and vitamin D deficiency can all cause leg discomfort in young children. 3

Iron Deficiency Workup

Iron deficiency is the single most important treatable cause of RLS-like symptoms in infants and young children:

• Check serum ferritin immediately in any infant with sleep-disrupting leg movements. A ferritin level below 50 ng/mL warrants iron supplementation, even if this is above the threshold for anemia. 4, 5, 2

• The American Academy of Pediatrics recommends universal laboratory screening for iron deficiency anemia at approximately one year of age for all healthy children. 6

• Oral iron supplementation has a positive, ferritin-concentration-dependent clinical effect on RLS symptoms in young children. 2

• The optimal dosage for treating iron deficiency is 3 to 6 mg/kg of elemental iron per day using ferrous preparations. 6

Polysomnography Considerations

• Polysomnography can document elevated PLMSI and correlate this with ferritin levels, but is not required for initial management. 2

• Up to 90% of patients with RLS have associated periodic limb movements during sleep. 4

• Reserve polysomnography for cases where the diagnosis remains unclear after initial evaluation and iron supplementation trial, or when you suspect periodic limb movement disorder. 4

Management Algorithm

Follow this stepwise approach:

1. Obtain serum ferritin level immediately. If <50 ng/mL, start oral iron supplementation at 3-6 mg/kg/day of elemental iron. 5, 2, 6

2. Implement non-pharmacological interventions for all families: maintain consistent bedtime routine, establish healthy eating habits and exercise, avoid caffeine, and incorporate stretching before bedtime. 5

3. Monitor response to iron supplementation. Relief of symptoms should correlate with rising ferritin levels over weeks to months. 2

4. If symptoms persist despite adequate iron repletion (ferritin >50 ng/mL), consider polysomnography to evaluate for periodic limb movement disorder or other sleep disorders. 2

Criteria for Specialist Referral

Refer to pediatric sleep medicine or pediatric neurology when:

• Symptoms persist despite iron repletion to ferritin >50 ng/mL. 5

• Associated neurological symptoms are present (sensory deficits, muscle weakness, gait abnormalities). 3

• Systemic symptoms suggest inflammatory conditions (fever, weight loss, constitutional symptoms). 3

• Daytime symptoms are prominent, which may indicate conditions other than typical RLS. 3

• The diagnosis remains uncertain after initial workup and you need polysomnography interpretation or consideration of pharmacological interventions beyond iron. 5

Common Pitfalls

• Do not assume electrolyte depletion causes the leg movements without evidence—this theory is poorly supported and can delay appropriate iron supplementation. 4

• Do not confuse RLS with simple leg cramps, which are relieved specifically by stretching the affected muscle (not general movement) and present as tightening sensations rather than dysesthesias. 4

• Do not order extensive laboratory panels initially—serum ferritin is the single most important test, and diagnosis is primarily clinical. 4

• Recognize that in infants, you are treating presumptive RLS based on clinical presentation and low ferritin, not confirmed RLS by standard diagnostic criteria. 1, 2