Stroke Spares the Nasolabial Fold
In stroke (central facial palsy), the nasolabial fold is preserved, while in Bell's palsy (peripheral facial palsy), the nasolabial fold is lost or flattened on the affected side.
Key Distinguishing Features
Bell's Palsy (Peripheral/Lower Motor Neuron Lesion)
- Complete ipsilateral facial paralysis affecting both upper and lower face
- Loss of nasolabial fold on the affected side 1, 2, 3
- Inability to wrinkle forehead on affected side
- Inability to close the eye completely (risk of corneal injury) 4
- Loss of forehead muscle movement (24.4% of cases) 3
- Affects all facial muscles on one side because the facial nerve (CN VII) is damaged peripherally 4
Stroke (Central/Upper Motor Neuron Lesion)
- Nasolabial fold remains intact (spared)
- Forehead movement preserved on affected side due to bilateral cortical innervation of upper facial muscles
- Only lower face affected (mouth droops, but can still wrinkle forehead and close eye)
- Often accompanied by other neurological deficits (limb weakness, speech problems, sensory changes) 5, 6
Anatomical Explanation
The critical difference lies in the innervation pattern:
- Upper facial muscles (forehead, eye closure) receive bilateral cortical input from both cerebral hemispheres
- Lower facial muscles (mouth, nasolabial fold) receive contralateral cortical input only
In stroke, the supranuclear lesion affects only the contralateral lower face, sparing the forehead and preserving the nasolabial fold's baseline appearance. In Bell's palsy, the peripheral facial nerve damage affects all ipsilateral facial muscles equally, causing the nasolabial fold to disappear as the muscles of facial expression lose function 1, 2.
Clinical Pitfalls
Critical warning: Some pontine strokes can mimic Bell's palsy by causing isolated peripheral-pattern facial weakness 7, 8. These cases require neuroimaging, particularly when:
- Patient has vascular risk factors (hypertension, diabetes, smoking) 7, 8
- Atypical features present (gradual onset over days, other subtle neurological symptoms) 8
- Symptoms are progressive or recurrent 4, 9
The nasolabial fold dynamics during facial animation are as significant as oral commissure movement—an immobile nasolabial fold during smiling is distinctly unnatural and characteristic of peripheral facial nerve dysfunction 2.
For Bell's palsy diagnosis: The condition must have acute onset (<72 hours), be unilateral, and have no other identifiable cause after clinical evaluation 4. Routine imaging is not recommended for typical Bell's palsy presentations, but selective imaging should be pursued for atypical features 4, 9.