Lower Motor Neuron Lesion

Aka: Lower Motor Neuron Lesion, Lower Motor Neuron Deficit, Lower Motor Neuron, Skeletal Muscle Fasciculation, Fasciculation, Muscle Twitch, Skeletal Muscle Fibrillation

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II. Physiology

  1. Corticospinal tract (motor, lateral cord) transmits motor signals from the cerebral cortex
    1. Fibers cross the midline at the Brain Stem and course to the contralateral Muscles
  2. Motor fibers Synapse in the anterior horn (grey matter of the spinal cord)
    1. Upper Motor Neurons (UMN) are from the cerebral cortex to the Synapse
      1. Affect the cerebral cortex, Brainstem and Corticospinal tracts
    2. Lower Motor Neurons (LMN) are from the Synapse to the Muscle
      1. Affect the anterior horn (spinal cord), nerve roots, and Peripheral Nerves

III. Causes: Examples

  1. See Muscle Weakness Causes
  2. Myasthenia Gravis
  3. Lambert-Eaton Syndrome
  4. Guillain Barre Syndrome (other autoimmune polyneuropathies)
  5. Neuropathy
    1. See Peripheral Neuropathy
    2. See Symmetric Peripheral Neuropathy
    3. See Asymmetric Peripheral Neuropathy
    4. Diabetic Neuropathy
    5. Peripheral Nerve Injury or Compression Neuropathy (e.g. Carpal Tunnel Syndrome, Lumbar Radiculopathy)

IV. Signs: Lower Motor Neuron Lesion findings

  1. Flaccid Paralysis
  2. Muscular atrophy
  3. Muscle Fasciculation (Muscle Twitch)
    1. Involuntary fine Muscle Contractions and relaxations of an entire motor unit synchronously
    2. May be a benign finding, in the absence of a lower motor lesion
  4. Muscle fibrillations
    1. Small, local Muscle Contractions of single Muscle fibers that contract independently of surrounding fibers
    2. Contractions are typically rapid, irregular and unsynchronized
  5. Hyporeflexia
  6. Absent Babinski Reflex

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