Types of Motor Neuron Lesions

Study Notes

Upper Motor Neuron (UMN) Lesions: Damage to motor neurons in the cerebral cortex, basal ganglia, or brainstem.
- Characterized by:
  - Spasticity (increased muscle tone)
  - Hyperreflexia (exaggerated reflexes)
  - Poor coordination
  - Weakness
Lower Motor Neuron (LMN) Lesions: Damage to motor neurons in the spinal cord or peripheral nervous system.
- Characterized by:
  - Flaccid paralysis (decreased muscle tone)
  - Hyporeflexia (decreased reflexes)
  - Muscle atrophy
  - Weakness

Trauma: Head or spinal cord injuries
Infections: Polio, meningitis, encephalitis
Tumors: Brain or spinal cord tumors
Degenerative diseases: Amyotrophic lateral sclerosis (ALS), primary lateral sclerosis, progressive muscular atrophy
Toxins: Lead, mercury, pesticides
Genetic disorders: Spinal muscular atrophy, Charcot-Marie-Tooth disease

Weakness: Muscle weakness or paralysis
Muscle atrophy: Wasting of muscles
Fasciculations: Visible muscle twitching
Cranial nerve involvement: Weakness or paralysis of facial, tongue, or swallowing muscles
Respiratory involvement: Weakness or paralysis of respiratory muscles

Clinical examination: Assessing muscle strength, tone, and reflexes
Electromyography (EMG): Measuring electrical activity of muscles
Nerve conduction studies (NCS): Measuring electrical activity of nerves
Imaging studies: MRI or CT scans to rule out other conditions
Lab tests: Blood tests to rule out other conditions, such as electrolyte imbalances or infections

Upper Motor Neuron (UMN) Lesions: Result from damage in the cerebral cortex, basal ganglia, or brainstem.
- Symptoms include spasticity, marked by increased muscle tone.
- Hyperreflexia indicates exaggerated reflex responses.
- Poor coordination and notable weakness are also characteristic signs.
Lower Motor Neuron (LMN) Lesions: Caused by damage in the spinal cord or peripheral nervous system.
- Present with flaccid paralysis, characterized by decreased muscle tone.
- Hyporeflexia, or reduced reflex responses, is common.
- Muscle atrophy leads to noticeable wasting, alongside muscle weakness.

Trauma: Injuries to the head or spinal cord can lead to motor neuron lesions.
Infections: Conditions such as polio, meningitis, and encephalitis can affect motor neurons.
Tumors: The presence of brain or spinal cord tumors can damage motor pathways.
Degenerative diseases: Conditions like amyotrophic lateral sclerosis (ALS) contribute to gradual motor neuron loss.
Toxins: Exposure to substances like lead, mercury, and pesticides can result in neuronal damage.
Genetic disorders: Spinal muscular atrophy and Charcot-Marie-Tooth disease are hereditary conditions affecting motor neurons.

Weakness: Patients may experience muscle weakness or paralysis.
Muscle atrophy: Significantly impacts the size and function of muscles, leading to wasting.
Fasciculations: Visible muscle twitching often occurs due to the loss of motor neuron control.
Cranial nerve involvement: Weakness or paralysis may affect muscles controlling facial expressions, tongue movement, or swallowing.
Respiratory involvement: Weakness in respiratory muscles can result in respiratory failure in severe cases.

Clinical examination: Involves evaluating muscle strength, tone, and reflexes to assess motor function.
Electromyography (EMG): An essential test that measures electrical activity in muscles.
Nerve conduction studies (NCS): Assesses the electrical activity of nerves to identify potential damage.
Imaging studies: MRI or CT scans help to exclude other potential causes of symptoms.
Lab tests: Blood tests can rule out conditions like electrolyte imbalances or infections that may mimic motor neuron lesions.

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