What are the differences between upper and lower motor neuron lesions in Bell's palsy?

Bell's Palsy is a Lower Motor Neuron Lesion

Bell's palsy is definitively a lower motor neuron (peripheral) facial nerve lesion, which is distinguished from upper motor neuron (central) lesions by the involvement of the forehead muscles. 1

Key Distinguishing Features

Lower Motor Neuron Pattern (Bell's Palsy)

• Complete unilateral facial weakness including the forehead is the hallmark of Bell's palsy and all peripheral facial nerve lesions 1, 2
• Patients cannot wrinkle their forehead, close their eye completely, or raise their eyebrow on the affected side 3, 4
• The entire ipsilateral side of the face is affected because the facial nerve (CN VII) innervates all facial muscles on one side after it exits the brainstem 5
• Additional features may include taste disturbance, hyperacusis, dry eye, and sagging of the mouth corner due to involvement of other facial nerve branches 5

Upper Motor Neuron Pattern (Central Lesion - NOT Bell's Palsy)

• Forehead sparing is the critical distinguishing feature - patients can still wrinkle their forehead and close their eye on the affected side 1
• Only the lower face is weak (mouth drooping, nasolabial fold flattening) while forehead movement remains intact 1
• This occurs because the upper facial muscles receive bilateral cortical innervation, so an upper motor neuron lesion (stroke) cannot completely paralyze the forehead 1
• The presence of forehead sparing immediately excludes Bell's palsy and indicates stroke or other central pathology 1

Clinical Diagnostic Algorithm

When You See Unilateral Facial Weakness:

Step 1: Test Forehead Function

• Ask the patient to raise their eyebrows and wrinkle their forehead 1
• If forehead is weak/paralyzed → peripheral lesion (possible Bell's palsy) 1, 3
• If forehead is normal/strong → central lesion (stroke until proven otherwise) 1

Step 2: If Peripheral Pattern, Confirm Bell's Palsy Criteria

• Onset within 72 hours 5, 1
• No identifiable cause (diagnosis of exclusion) 5, 1
• No other cranial nerve involvement 6
• Unilateral only (bilateral is extremely rare and warrants investigation for Lyme disease, Guillain-Barré, or sarcoidosis) 1, 7

Critical Pitfall to Avoid

Never assume facial weakness with forehead sparing is Bell's palsy - this pattern indicates an upper motor neuron lesion requiring immediate stroke evaluation 1. The forehead test is your most important clinical tool for distinguishing peripheral from central facial nerve pathology 1.

Anatomical Basis

• The facial nerve nucleus in the pons receives bilateral cortical input for upper facial muscles but predominantly contralateral input for lower facial muscles 1
• Bell's palsy affects the facial nerve after it exits the brainstem, typically within the narrow temporal bone canal where inflammation causes compression 5
• This peripheral location means all ipsilateral facial muscles lose innervation, producing the characteristic complete hemifacial weakness 5, 2