What causes nocturnal muscle cramps?

Causes of Nocturnal Muscle Cramps

Nocturnal leg cramps are primarily caused by altered neuromuscular control and muscle fatigue, with secondary contributions from electrolyte imbalances, medications, and underlying medical conditions.

Primary Mechanism: Neuromuscular Dysfunction

The strongest evidence points to muscle fatigue and nerve dysfunction as the primary cause, not electrolyte abnormalities as commonly believed 1, 2. The pathophysiology involves:

• Disrupted balance between muscle spindle excitatory signals and Golgi tendon organ inhibitory signals, leading to spontaneous muscle contractions 2
• Localized muscle overload and fatigue affecting specific muscle groups, which explains why cramps occur in working muscles rather than systemically 2
• Nerve dysfunction contributing to abnormal firing patterns that trigger involuntary contractions 1

This neuromuscular theory is more compelling than the dehydration/electrolyte theory because cramps are localized phenomena, whereas electrolyte disturbances would cause systemic symptoms 2.

Medication-Induced Causes

Several medications strongly predispose to nocturnal leg cramps and should be reviewed immediately 1:

• Diuretics (particularly common in patients with ascites or heart failure) 3, 4
• Intravenous iron sucrose 1
• Conjugated estrogens and raloxifene 1
• Naproxen and other NSAIDs 1
• Teriparatide 1
• Long-acting beta-agonists 4
• Calcium channel blockers 3
• Lithium 3

Electrolyte Imbalances (Secondary Contributors)

While not the primary cause, electrolyte abnormalities can exacerbate cramps 3:

• Hypokalemia (low potassium) 3
• Hypomagnesemia (low magnesium) 3
• These should be checked and corrected, though correction alone may not resolve cramps 3

Associated Medical Conditions

Multiple underlying diseases increase cramp frequency 1:

• Liver disease with cirrhosis (particularly in patients on diuretics for ascites) 3, 1
• Vascular disease and peripheral arterial disease 1
• Lumbar canal stenosis 1
• Chronic kidney disease and hemodialysis 1
• Pregnancy 1
• Neurological conditions causing lower limb weakness, abnormal gait, or tremor 3

Sleep-Related Movement Disorders (Differential Diagnosis)

Restless legs syndrome (RLS) and periodic limb movements must be distinguished from true cramps 3, 5:

• RLS presents with uncomfortable sensations and urge to move, relieved by movement, worsening in evening 3, 5
• True nocturnal cramps are abrupt, painful muscle contractions without the urge to move 5
• Ask: "Does your bed partner complain that you have twitchy legs or make kicking movements in your sleep?" to screen for periodic limb movements 3

Clinical Pitfalls to Avoid

Do not assume cramps are purely benign or idiopathic without systematic evaluation 6:

• Always check electrolytes, renal function, thyroid function, calcium, and HbA1c to identify metabolic derangements 3, 6
• Review all medications critically, as drug-induced cramps are common and reversible 3, 1
• Assess for peripheral edema suggesting heart failure or fluid retention 3
• Examine for neurological signs including lower limb weakness, abnormal gait, speech disturbances, or tremor 3
• Screen for sleep disorders including obstructive sleep apnea, which may coexist 3, 6

Age-Related Considerations

Up to 60% of adults experience nocturnal leg cramps, with higher prevalence in older adults 1:

• Frail older adults over 75 years are particularly affected, with significant impact on sleep quality and quality of life 4, 7
• Multiple comorbidities and polypharmacy increase risk in this population 3