What causes fasciculations?

Causes of Fasciculations

Fasciculations are primarily caused by hyperexcitability of motor neurons occurring at various points along the motor pathway, with both benign and pathological etiologies possible depending on associated symptoms and neurophysiological findings. 1

Primary Causes of Fasciculations

Benign Causes

• Benign fasciculation syndrome: Occurs without associated weakness or atrophy
• Physiological triggers:
  • Caffeine consumption
  • Stimulant medications
  • Exercise (exercise-induced fasciculations)
  • Stress and anxiety
  • Fatigue
  • Sleep deprivation 1

Metabolic and Electrolyte Disorders

• Electrolyte abnormalities (particularly magnesium, calcium, sodium, potassium)
• Thyroid disorders (hyperthyroidism)
• Dehydration 1

Neurological Disorders

• Motor neuron diseases:

  • Amyotrophic Lateral Sclerosis (ALS) - fasciculations with weakness and atrophy
  • Progressive spinal muscular atrophy
  • Spinal muscular atrophy (SMA)
  • Bulbospinal muscular atrophy (BSMA) 2

• Peripheral nerve hyperexcitability syndromes:

  • Isaac's syndrome (neuromyotonia)
  • Cramp-fasciculation syndrome
  • Morvan syndrome 2

• Autoimmune disorders:

  • Lambert-Eaton myasthenic syndrome (antibodies against voltage-gated calcium channels)
  • Neuromyotonia (antibodies against voltage-gated potassium channels) 1

• Other neurological conditions:

  • Peripheral neuropathies
  • Radiculopathies
  • Spinal cord disorders
  • Hereditary neuropathies 1, 2

Rare Genetic and Hereditary Causes

• GM2-gangliosidosis
• Triple-A syndrome
• Familial amyloidosis
• Spinocerebellar ataxias
• Huntington's disease
• Rett syndrome
• Fabry's disease
• Gerstmann-Sträussler disease
• Mitochondrial disorders
• Muscular dystrophies 2

Pathophysiological Mechanisms

Fasciculations originate from ectopic discharges in the motor system that can occur at different sites:

1. Proximal origin (cell body or proximal axon):

   • More common in early stages of ALS
   • Associated with upper motor neuron influences 3

2. Distal origin (terminal axon):

   • More common in later stages of disease
   • Associated with distal motor axonal sprouting during reinnervation
   • Most common site in both normal subjects and motor neuron disease 4, 3

3. Dendritic spread:

   • Many fasciculations spread to other dendritic spines
   • Can produce antidromic impulses in the main axon 4

Clinical Significance and Evaluation

The clinical significance of fasciculations depends on associated findings:

• Benign fasciculations: No associated weakness, atrophy, or upper motor neuron signs
• Pathological fasciculations: Associated with weakness, atrophy, or other neurological signs 1

Key diagnostic considerations:

• Distribution pattern (focal vs. widespread)
• Progression over time
• Associated symptoms (weakness, atrophy, cramps, sensory changes)
• EMG findings - fasciculation potentials alone suggest benign etiology, while those with neurogenic changes suggest pathology 1

Relationship with Metabolic Function

Interestingly, fasciculations in ALS have been linked to body mass index (BMI) loss, with studies showing:

• Inverse association between fasciculation number and BMI
• Direct association with BMI loss and upper motor neuron impairment
• Suggesting that hyperexcitability may contribute to metabolic dysfunction 5

Common Pitfalls in Diagnosis

• Misinterpreting benign fasciculations as indicative of ALS
• Failing to recognize that fasciculations can occur in numerous hereditary and non-hereditary conditions
• Overlooking the need for temporal monitoring in some cases
• Technical EMG interpretation errors (e.g., misidentifying normal MUAPs as fibrillation potentials) 1, 4

Fasciculations in the tongue deserve special attention as they are more commonly associated with ALS than benign conditions 1.