Distinguishing Upper vs Lower Motor Neuron Lesions

Introduction

Motor neuron lesions can be broadly categorized into upper motor neuron (UMN) and lower motor neuron (LMN) lesions. Understanding their distinction based on side, site, and probable nature is crucial for diagnosis and treatment.

Upper Motor Neuron (UMN) Lesions

Characteristics

• Spastic paralysis
• Hyperreflexia
• Babinski sign positive
• No significant muscle atrophy

Common Causes

• Stroke (Cerebrovascular accident)
• Multiple sclerosis
• Traumatic brain injury
• Spinal cord injury affecting descending pathways

Pathophysiology

UMN lesions disrupt signals from the cerebral cortex to the spinal cord, leading to increased muscle tone and exaggerated reflexes.

Lower Motor Neuron (LMN) Lesions

Characteristics

• Flaccid paralysis
• Hyporeflexia or areflexia
• Muscle atrophy
• Fasciculations

Common Causes

• Peripheral nerve injury
• Polio (Poliomyelitis)
• Guillain-Barré syndrome
• Amyotrophic lateral sclerosis (ALS, affecting LMNs)

Pathophysiology

LMN lesions result in the loss of direct innervation to muscles, leading to muscle atrophy and diminished reflexes.

Diagnostic Approach

Clinicians use neurological examinations and imaging techniques like MRI and EMG to differentiate UMN vs. LMN lesions.

Treatment Considerations

• UMN lesions may require rehabilitation and spasticity management.
• LMN lesions might need supportive care and neuromuscular therapy.