What tests are used to rule out Amyotrophic Lateral Sclerosis (ALS)?

Diagnostic Testing to Rule Out Amyotrophic Lateral Sclerosis (ALS)

Electromyography (EMG) and nerve conduction studies (NCS) are the cornerstone diagnostic tests for ruling out ALS, with imaging studies primarily used to exclude alternative diagnoses rather than confirm ALS itself. 1

Primary Diagnostic Tests

Electrophysiological Studies

• Electromyography (EMG) and Nerve Conduction Studies (NCS):

  • Most important diagnostic tests for ruling out ALS
  • EMG evaluates for:
    • Fasciculation potentials (highly specific for ALS)
    • Fibrillation potentials
    • Positive sharp waves
    • Evidence of chronic denervation/reinnervation
  • NCS evaluates:
    • Motor and sensory nerve function
    • In ALS, motor conduction velocity rarely falls below 80% of lower limit of normal
    • Distal latency and F-wave latency rarely exceed 1.25 times upper limit of normal 2
    • Sensory nerve action potentials typically remain normal in ALS

• Key diagnostic value:

  • Helps differentiate ALS from conditions that mimic it
  • Demonstrates lower motor neuron dysfunction in clinically unaffected regions
  • Particularly useful for examining bulbar muscles like the tongue 3

Imaging Studies

MRI

• Brain and Spinal Cord MRI without IV contrast:
  • Recommended as the primary imaging modality for suspected ALS 1
  • Purpose: to exclude other conditions with similar presentations
  • May show:
    • Abnormal T2/FLAIR signal in corticospinal tracts
    • "Snake eyes" appearance in anterior horns of spinal cord (later in disease) 1

CT

• Not preferred for ALS diagnosis due to limited soft-tissue characterization
• Only useful for excluding other etiologies when MRI is contraindicated 1

Additional Testing

• Lumbar puncture: To rule out inflammatory or infectious conditions
• Blood tests:
  • Complete blood count
  • Comprehensive metabolic panel
  • Thyroid function tests
  • Vitamin B12 and folate levels
  • Inflammatory markers (ESR, CRP)
  • Heavy metal screening if exposure suspected

Clinical Assessment Tools

• ALS Functional Rating Scale-Revised (ALSFRS-R): Evaluates disease progression
• ALS Swallowing Severity Scale (ALSSS): Assesses dysphagia 1
• Structured dysphagia questionnaires (e.g., EAT-10):
  • High sensitivity (86%) and specificity (76%) for identifying aspiration risk 1
• Volume-Viscosity Swallowing Test (V-VST):
  • 92% sensitivity and 80% specificity compared to videofluoroscopy 1

Diagnostic Algorithm

1. Initial clinical suspicion based on progressive weakness pattern
2. EMG/NCS to detect:
   • Lower motor neuron signs in multiple body regions
   • Normal sensory responses
   • Conduction parameters within acceptable limits for ALS
3. MRI brain and spinal cord to exclude alternative diagnoses
4. Laboratory studies to rule out metabolic, toxic, or inflammatory causes
5. Swallowing assessment if bulbar symptoms present

Common Pitfalls and Caveats

• Incomplete EMG sampling: Ensure multiple body regions are examined, including bulbar muscles when appropriate
• Overinterpretation of minor NCS abnormalities: Some ALS patients may have mild sensory abnormalities or conduction slowing that doesn't exclude the diagnosis 4
• Relying solely on imaging: MRI findings in ALS are often subtle or absent, especially early in disease
• Missing concurrent conditions: Some patients may have both ALS and peripheral neuropathy 4
• Delayed diagnosis: Initial EMG may not show widespread denervation in early disease; follow-up studies may be necessary 4

Remember that no single test can definitively rule out ALS. The diagnosis is primarily clinical, supported by electrodiagnostic findings, and involves excluding alternative diagnoses through appropriate imaging and laboratory studies.