The key difference between Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) facial nerve lesions lies in the pattern of facial paralysis, particularly concerning the forehead muscles.
Here's a breakdown of the differences:
UMN vs. LMN Facial Nerve Lesions
| Feature | Upper Motor Neuron (UMN) Lesion | Lower Motor Neuron (LMN) Lesion |
|---|---|---|
| Forehead Involvement | Forehead spared; patient can raise eyebrows. | Forehead involved; patient cannot raise eyebrows. |
| Paralysis Location | Contralateral lower face (opposite side of the lesion). | Ipsilateral entire face (same side as the lesion). |
| Reason | UMNs project to facial nerve nuclei bilaterally for upper face. Thus, unilateral UMN lesion spares upper face function. | LMNs innervate the entire face, thus unilateral LMN lesion paralyzes entire ipsilateral face. |
| Clinical Example | Stroke affecting the motor cortex. | Bell's Palsy (idiopathic facial nerve paralysis). |
In essence, according to the reference, if someone can fully raise their eyebrow on the affected side, the facial palsy is likely an UMN lesion. However, if they cannot raise their eyebrow on the affected side, the lesion is likely an LMN lesion.
