Upper Motor Neuron (UMN) vs Lower Motor Neuron (LMN) Lesions

Introduction

• Objective: Understanding the differences between upper motor neuron and lower motor neuron lesions.
• Relevance: Key for diagnosing neurological diseases.

Motor Neuron Anatomy

• Upper Motor Neurons (UMN): Two primary tracts
  • Corticospinal Tract: Cortex to spinal cord.
  • Corticobulbar Tract: Cortex to cranial nerve nuclei.
• Lower Motor Neurons (LMN)
  • Correlated with the corticospinal tract: Anterior grey horn of the spinal cord.
  • Destination of corticobulbar tract: Cranial nerve nuclei.

Key Cranial Nerve Nuclei (Corticobulbar Pathway)

• Trigeminal Nerve (CN V): Muscles of mastication.
• Facial Nerve (CN VII): Muscles of facial expression.
• Nucleus Ambiguus (CN IX, X, XI): Muscles of pharynx, larynx, soft palate.
• Hypoglossal Nerve (CN XII): Muscles of the tongue.

Lesion Types

• UMN Lesion
  • Damage anywhere from the cortex to the spinal cord or brainstem.
• LMN Lesion
  • Damage to cranial nerve nuclei, axon, or axon terminals.

Causes of Lesions

• UMN Lesion Causes
  • Stroke (ischemic or hemorrhagic).
  • Demyelinating diseases (e.g., multiple sclerosis, ALS).
• LMN Lesion Causes
  • Viral infections (e.g., polio, West Nile Virus).
  • Genetic disorders (e.g., spinal muscular atrophy).
  • Neuropathies (e.g., diabetic neuropathy, cauda equina syndrome).

Clinical Features

Mass and Atrophy

• UMN Lesion: Disuse atrophy, minimal muscle mass loss (15-20%).
• LMN Lesion: Denervation atrophy, significant muscle mass loss (70-80%).

Fasciculations

• UMN Lesion: Not present.
• LMN Lesion: Present due to acetylcholine receptor upregulation.

Tone and Reflexes

• UMN Lesion
  • Hypertonia: Increased muscle tone.
  • Hyperreflexia: Increased deep tendon reflexes.
• LMN Lesion
  • Hypotonia: Decreased muscle tone.
  • Hyporeflexia: Decreased deep tendon reflexes.

Types of Paralysis

• UMN Lesion: Spastic paralysis.
• LMN Lesion: Flaccid paralysis.

Special Tests

• UMN Lesion
  • Babinski sign, pronator drift, Hoffman's sign are positive.
• LMN Lesion
  • These tests are negative or absent.

Terminology

• Pseudobulbar Palsy: UMN lesion affecting corticobulbar pathways.
• Bulbar Palsy: LMN lesion affecting cranial nerve nuclei.

Summary Chart

• Mass: UMN - minimal decrease; LMN - significant decrease.
• Fasciculations: UMN - absent; LMN - present.
• Tone: UMN - hypertonia; LMN - hypotonia.
• Reflexes: UMN - hyperreflexia; LMN - hyporeflexia.
• Paralysis Type: UMN - spastic; LMN - flaccid.
• Special Tests: UMN - positive; LMN - absent.

Conclusion

• Understanding differences aids in diagnosis and management of neurological disorders.
• Further details to be discussed in follow-up videos and studies.