Upper Motor Neuron vs Lower Motor Neuron Lesions
Upper motor neuron (UMN) and lower motor neuron (LMN) lesions have distinct clinical presentations, with UMN lesions characterized by spasticity, hyperreflexia, and Babinski sign, while LMN lesions present with flaccidity, hyporeflexia, and muscle atrophy.
Anatomical Differences
- UMNs originate in the motor cortex and extend through the corticospinal tracts to synapse with LMNs in the anterior horn of the spinal cord 1
- LMNs are located in the anterior horn of the spinal cord and brainstem nuclei, extending to peripheral muscles 2
- Motor neuron degeneration in conditions like ALS begins focally at both UMN and LMN levels corresponding to the same peripheral body region 3
Clinical Manifestations
UMN Lesions
- Increased muscle tone (spasticity) 2, 4
- Hyperreflexia (brisk deep tendon reflexes) 4, 5
- Positive Babinski sign (extensor plantar response) 5
- Weakness without significant atrophy 4
- Minimal or no fasciculations 4
LMN Lesions
- Decreased muscle tone (flaccidity) 2
- Hyporeflexia or areflexia (diminished or absent deep tendon reflexes) 5
- Muscle atrophy 4, 6
- Fasciculations 4
- Absence of Babinski sign 5
Diagnostic Approach
- Clinical examination is crucial for distinguishing between UMN and LMN lesions, including assessment of muscle tone, deep tendon reflexes, and plantar responses 5
- Electromyography (EMG) is particularly useful for identifying LMN involvement, showing denervation potentials and fasciculation potentials 7
- MRI can help identify UMN pathology, showing abnormal T2/FLAIR signal in the corticospinal tracts 4
- The bulbocavernosus reflex is an important clinical test to differentiate UMN from LMN lesions, especially in spinal cord injuries 5
Clinical Significance
- The distinction between UMN and LMN lesions has important prognostic and therapeutic implications for bowel, bladder, and sexual function, as well as mobility 5
- In conditions affecting both UMN and LMN (like ALS), the degeneration at each level appears to be independent in severity and distribution 7
- Bulbar dysfunction often results from mixed UMN and LMN impairment, while limb dysfunction may derive mostly from LMN loss 8
Common Pitfalls
- One cannot determine the type of lesion (UMN vs LMN) based solely on the neurological level of injury; a detailed clinical examination including sacral reflexes is required 5
- Mixed presentations can occur, particularly in conditions like ALS or in certain spinal cord injury levels (e.g., T10-T12, where 25.3% of lesions show mixed characteristics) 5
- The outward distribution of both UMN and LMN signs appears more directed to caudal body regions than to rostral ones, which can complicate clinical assessment 3