Causes of Lower Limb Jerking Movements During Sleep
Lower limb jerking movements during sleep are most commonly caused by Periodic Limb Movements of Sleep (PLMS), which consist of rhythmic extensions of the big toe and dorsiflexions of the ankle with occasional knee and hip flexions, occurring approximately every 20-40 seconds throughout the night. 1
Primary Causes
Periodic Limb Movement Disorder (PLMD)
- PLMD is diagnosed when polysomnography demonstrates repetitive, stereotyped limb movements with a PLMS Index exceeding 15 per hour in adults, accompanied by clinical sleep disturbance or daytime fatigue, and not better explained by another disorder. 1
- Each movement lasts approximately 2-4 seconds and may cause brief awakenings or arousals that patients may not recognize. 1, 2
- PLMS become more common with aging and occur predominantly during the first part of the night. 1
Restless Legs Syndrome (RLS)
- Up to 90% of individuals with RLS have associated PLMS, making RLS the most notable association with periodic limb movements. 1, 3
- RLS is characterized by an uncomfortable urge to move the legs with dysesthesias that worsen during rest/inactivity, are relieved by movement, and worsen in the evening or at night. 1, 4
- The rate of PLMS correlates with subjective RLS severity, though limb movements are neither necessary nor sufficient to diagnose RLS. 1
Secondary Causes
Medication-Induced
- Antidepressants (particularly SSRIs, tricyclic antidepressants, venlafaxine, and mirtazapine) commonly cause or exacerbate PLMS. 1, 5
- Dopamine antagonists (antipsychotics and antinausea medications) can trigger or worsen symptoms. 1, 5
- Lithium and antihistamines are also associated with increased PLMS. 1, 5
Medical Conditions
- Iron deficiency (serum ferritin <50 ng/mL) is strongly associated with both RLS and secondary PLMS. 1, 3
- Chronic renal failure can cause secondary PLMS. 5
- Peripheral neuropathy is associated with increased limb movements during sleep. 1, 3
- Sleep-disordered breathing is commonly associated with PLMS. 1
Critical Diagnostic Distinctions
Differentiating from Nocturnal Leg Cramps
- Nocturnal leg cramps present as painful, involuntary muscle contractions with tightening sensation, relieved specifically by stretching the affected muscle, not general movement. 2, 3
- Unlike PLMS, cramps do not involve an urge to move the legs and lack the rhythmic, stereotyped pattern. 2, 3
Pathophysiology
- PLMS likely originate from abnormal spinal cord hyperexcitability triggered by sleep-related factors, with different, independent, and unsynchronized generators rather than direct cortical participation. 6
- Impaired dopamine transport in the substantia nigra due to reduced intracellular iron appears critical in most RLS patients with associated PLMS. 1
Essential Clinical Evaluation
Ask these specific questions to establish the diagnosis: 2, 3
- "What does it feel like?" (urge to move vs. painful tightening)
- "Is it relieved by movement?" (any movement for RLS vs. specific stretching for cramps)
- "When does it occur?" (evening/night worsening suggests RLS)
Perform a thorough neurological examination to identify peripheral neuropathy or radiculopathy. 1, 3
Obtain serum ferritin levels—values less than 50 ng/mL warrant iron supplementation. 1, 3
Review all medications for agents that may exacerbate PLMS, particularly antidepressants and dopamine antagonists. 1
Common Pitfalls to Avoid
- Do not order polysomnography for simple nocturnal leg cramps—reserve this for suspected PLMD or RLS when diagnosis is unclear. 3
- Do not assume PLMS automatically indicate PLMD—PLMS are nonspecific and occur with other sleep disorders and in normal individuals; diagnosis requires clinical sleep disturbance and exclusion of other causes. 1, 7
- Do not diagnose RLS based solely on the presence of PLMS—RLS is a clinical diagnosis based on history, not polysomnography findings. 4, 7
- Avoid confusing cramps with RLS, which can lead to inappropriate dopaminergic therapy when stretching would suffice. 3