Clinical Manifestations: Distinguishing Upper vs. Lower Motor Neuron Signs

Diagnosing ALS relies on identifying signs of combined Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) degeneration spreading across neuraxial segments. Notably, objective sensory deficits are incompatible with a diagnosis of ALS and suggest alternative neurological conditions.

Classical Upper vs. Lower Motor Neuron Signs

                  CLINICAL EXAMINATION IN SUSPECTED ALS
                                    │
           ┌────────────────────────┴────────────────────────┐
           ▼                                                 ▼
  Upper Motor Neuron (UMN) Signs                   Lower Motor Neuron (LMN) Signs
  • Muscle Spasticity ( > Weakness )      • Progressive Muscle Weakness
  • Hyperreflexia / Exaggerated DTRs     • Severe Atrophy & Muscle Wasting
  • Pseudobulbar Affect                   • Visible Fasciculations & Cramping
  • Spastic Dysarthria                    • Bulbar Features (Flaccid Dysarthria/Dysphagia)

Regional Disease Progression and Overlap

• Cranial/Bulbar Region: UMN involvement produces spastic dysarthria and pseudobulbar affect (pathological laughing/crying), whereas LMN involvement causes tongue atrophy, fasciculations, and flaccid dysarthria with severe dysphagia.

• Limb/Spinal Regions: UMN disease presents with spasticity, clonus, and hyperreflexia, while LMN dysfunction presents with profound weakness, focal atrophy, and painful muscle cramps.

Key Takeaways

• ALS requires clinical evidence of both UMN and LMN degeneration.

• Objective sensory loss rules out a diagnosis of primary ALS.

• Disease progression is inexorable, with ~50% of patients dying within 3 years of onset without intervention.