What are the possible causes of bilateral toe cramping on toe‑curling, especially after morning awakening?

Bilateral Toe Cramping Upon Curling: Differential Diagnosis

The most likely explanations for bilateral toe cramping upon curling, especially after morning awakening, include electrolyte disturbances (particularly hypocalcemia and hypomagnesemia), peripheral neuropathy, foot structural abnormalities (pes planovalgus), and less commonly, movement disorders such as painful legs-moving toes syndrome or restless leg syndrome.

Primary Metabolic and Electrolyte Causes

Hypocalcemia and hypomagnesemia should be ruled out first, as these are common, treatable causes of cramping pain in the lower extremities and feet 1. These electrolyte disturbances can cause muscle cramping that is often worse with voluntary muscle contraction, such as toe curling 1.

• Morning timing may relate to overnight fluid shifts and electrolyte redistribution
• Blood work including calcium, magnesium, and phosphate levels should be obtained 1

Structural and Biomechanical Factors

Pes planovalgus (flat feet) commonly causes nonspecific lower leg and foot pains that may manifest as cramping, particularly with certain movements 1. This condition:

• Is frequently underrepresented in the literature but clinically common 1
• May benefit from orthotic intervention 1
• Can cause symptoms that worsen with specific foot positions, including toe curling

Peripheral Neuropathy

Peripheral neuropathy represents a significant consideration, as it can present with:

• Cramping and abnormal muscle contractions in the feet and toes 2, 3
• Symptoms that may be bilateral 2
• Association with various underlying conditions including diabetes, alcohol use, or nutritional deficiencies 4

The peripheral nerve damage can lead to aberrant sensory input causing "central reorganization" at the spinal cord level, potentially responsible for both pain and involuntary movements 3.

Movement Disorders

Painful Legs-Moving Toes Syndrome (PLMT)

While less common, PLMT should be considered when cramping is accompanied by involuntary toe movements 2, 3:

• Typically presents with pain preceding movements in nearly all cases 2
• Can be bilateral in 58% of cases 2
• Mean age of onset is 58 years, though can occur in younger patients 2
• Associated with peripheral neuropathy in 28% of cases 2
• Electromyography shows irregular 50-millisecond to 1-second bursts of motor unit firing 2

A critical distinguishing feature: In PLMT, the movements are typically writhing or flexion-extension patterns rather than simple cramping, and the pain is usually more distressing than the movements themselves 2, 3.

Restless Leg Syndrome and Periodic Limb Movement Disorder

These conditions can present with:

• Unrelenting urge to move the legs, particularly at night and before sleep onset 1
• Periodic limb movements that occur only during sleep 1
• May be misdiagnosed when itch-induced or cramp-induced movements are present 1

Rare Genetic Causes

Paramyotonia congenita should be considered if:

• Symptoms are provoked by cold exposure 5
• There is a family history of similar symptoms 5
• Cramping develops with exercise and worsens with continued activity 5
• Intrinsic muscle spasm with toe curling occurs 5

This condition responds to phenytoin therapy 5.

Critical Diagnostic Approach

Initial workup should include:

1. Comprehensive metabolic panel focusing on calcium, magnesium, phosphate, and glucose 1
2. Assessment for peripheral neuropathy through history (diabetes, alcohol use, nutritional status) and examination 2, 3
3. Foot structural examination for pes planovalgus and other biomechanical abnormalities 1
4. Observation of the cramping episodes to distinguish simple muscle cramps from involuntary movements characteristic of PLMT 2, 3

If initial workup is unrevealing:

• Consider electromyography to evaluate for myotonia or abnormal motor unit firing patterns 2, 5
• Evaluate for radiculopathy or nerve root compression 2
• Consider genetic testing if family history suggests inherited myotonia 5

Common Pitfalls to Avoid

• Do not overlook simple electrolyte disturbances before pursuing complex neurologic diagnoses 1
• Do not assume all toe movements are voluntary cramping; observe carefully for writhing or involuntary patterns suggesting PLMT 2, 3
• Do not ignore structural foot abnormalities as a source of cramping pain, as these are common and treatable with orthotics 1
• Do not miss peripheral neuropathy as an underlying cause, particularly in patients with diabetes or alcohol use 2, 4

Treatment Considerations Based on Etiology

For electrolyte disturbances: Correct underlying deficiencies 1

For structural abnormalities: Trial of orthotics for pes planovalgus 1

For peripheral neuropathy: Address underlying cause and consider medications for neuropathic pain such as gabapentin 3

For PLMT: Treatment is often unsatisfactory, but gabapentin, spinal blocks, or botulinum toxin injections have been reported anecdotally to help 3. The syndrome persists in 83% of patients, suggesting low likelihood of spontaneous resolution 2

For paramyotonia congenita: Phenytoin can lessen symptoms 5