Upper vs Lower Motor Neuron Facial Palsy

Every facial palsy forces one decision first: is the lesion central (an upper motor neuron lesion, proximal to the facial nerve nucleus in the pons) or peripheral (a lower motor neuron lesion, at or distal to the nucleus)? Everything downstream follows from the answer. A central palsy points at the brain (stroke, tumour) and can be a neurological emergency; a peripheral palsy points at the nerve itself (Bell’s palsy, Ramsay Hunt syndrome, trauma, the ear, the parotid). And you usually settle it at the bedside in seconds, by watching the forehead.

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Start with the forehead

• Forehead moves (spared) → upper motor neuron lesion. Weakness affects only the lower part of the face.
• Forehead does not move (involved) → lower motor neuron lesion. Weakness affects the whole half of the face: forehead, eye, and mouth together.

Almost all bedside localisation of a facial palsy turns on that one sign. The rest of this page explains why it works and what sits on each side of the line.

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Why the forehead is spared — the neuroanatomy

The forehead-sparing rule is not arbitrary; it falls out of how the motor cortex projects to the facial nucleus. Tract-tracing work in primates (Morecraft et al., Brain 2001) mapped these corticofacial projections and found a clear asymmetry:

• The neurons supplying the lower face (perioral muscles) receive their cortical drive mainly from the opposite (contralateral) hemisphere.
• The neurons supplying the upper face (frontalis and orbicularis oculi) receive bilateral cortical input — from both hemispheres.

Put those together and the clinical rule appears. A one-sided upper motor neuron lesion (a stroke in one hemisphere) knocks out the crossed input to the lower face on the opposite side → the lower face droops. But the upper face still has an intact supply coming from the other, undamaged hemisphere → the forehead keeps working. A lower motor neuron lesion sits on the final common pathway (the nucleus or the nerve), downstream of all cortical input, so it takes out the whole hemiface — there is no second hemisphere to rescue the forehead.

The same study noted that projections driving facial movement arise from both classical motor cortex and limbic (emotion-related) cortical areas — an anatomical substrate for the clinical observation that volitional and emotional facial movement can dissociate (see below).

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Upper motor neuron (central) facial palsy

• Distribution: weakness of the lower face only; the forehead is spared (moderate-to-good movement retained).
• Company it keeps: rarely isolated. A central lesion (e.g. stroke) usually brings other signs — limb weakness, dysarthria, or other cranial nerve findings pointing to the brainstem or hemisphere.
• Volitional–emotional dissociation: because volitional and emotional facial movements travel partly separate cortical routes, some central lesions weaken the voluntary smile-to-command while the spontaneous smile at a joke is preserved (or vice versa). Its presence is a strong pointer to a central cause.

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Lower motor neuron (peripheral) facial palsy

• Distribution: the whole half of the face — forehead, eye closure, and mouth — is weak on the side of the lesion.
• Then localise along the nerve: once you know it is peripheral, the associated features place the lesion along the nerve’s course — dry eye, altered taste, hyperacusis, and the topognostic tests (see localising a facial nerve palsy).
• A caveat at the nucleus: a lesion in the pons at the facial nucleus or its emerging fascicle produces a lower-motor-neuron (whole-hemiface) pattern too — but usually with neighbouring brainstem signs (e.g. a sixth-nerve palsy or gaze abnormality), because the nucleus’s neighbours get caught up.

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Causes — which side of the line

	Upper motor neuron (central)	Lower motor neuron (peripheral)
Where	Proximal to the facial nucleus (cortex, internal capsule, corticobulbar tract)	At or distal to the nucleus (nucleus, nerve, ear, parotid)
Forehead	Spared	Involved
Typical causes	Stroke (ischaemic/haemorrhagic); intracranial tumour affecting the corticobulbar pathway	Bell’s palsy; Ramsay Hunt syndrome; cerebellopontine-angle masses (vestibular schwannoma, meningioma); temporal-bone trauma; iatrogenic (otologic/parotid surgery); chronic otitis media / cholesteatoma; congenital (Möbius, Melkersson–Rosenthal)

Note on cerebellopontine-angle masses. A vestibular schwannoma or meningioma at the cerebellopontine angle compresses the facial nerve on the nerve itself — distal to the nucleus — so it produces a lower motor neuron (peripheral) palsy, not a central one, despite sitting inside the skull. Anatomical location, not “inside vs outside the brain,” is what defines UMN vs LMN: the dividing line is the facial nucleus.

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Red flags — when a facial palsy needs imaging

Whichever side of the line, these features argue against a simple idiopathic (Bell’s) palsy and warrant MRI with gadolinium:

• No improvement within 4–6 months.
• Bilateral facial paralysis (fewer than 1% of Bell’s palsy).
• Recurrent paralysis.
• Additional cranial neuropathies.
• Gradual (rather than acute) onset.

A forehead-sparing pattern is itself a red flag: it points at the brain, and an acute central facial palsy is a stroke until proven otherwise.

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Frequently Asked Questions

How do you tell an upper from a lower motor neuron facial palsy at the bedside? Look at the forehead. Ask the patient to raise their eyebrows or wrinkle their forehead. If the forehead moves on the weak side, the lesion is upper motor neuron (central) — only the lower face is affected. If the forehead does not move and the whole half of the face is weak, the lesion is lower motor neuron (peripheral).

Why is the forehead spared in a stroke but not in Bell’s palsy? Because the upper-face muscles receive cortical input from both hemispheres, whereas the lower face is driven mainly by the opposite hemisphere. A one-sided stroke removes only the crossed supply to the lower face; the forehead is rescued by the intact opposite hemisphere. Bell’s palsy damages the nerve itself, downstream of all cortical input, so there is no second supply to spare the forehead — the whole side is weak.

Is a facial palsy from a tumour inside the skull always “upper motor neuron”? No. What matters is whether the lesion is proximal or distal to the facial nucleus, not whether it is inside the skull. A cerebellopontine-angle tumour (like a vestibular schwannoma) is inside the skull but compresses the nerve distal to the nucleus, so it causes a lower motor neuron (whole-hemiface) palsy.

What is volitional–emotional dissociation? Voluntary and emotional facial movements use partly different cortical pathways. Some central lesions weaken the voluntary, on-command smile while sparing the spontaneous, emotional smile — or the reverse. When present, it strongly suggests a central (upper motor neuron) cause.

A patient has a whole-side facial palsy plus a squint — is that still peripheral? A whole-hemiface (lower motor neuron pattern) palsy accompanied by other brainstem signs such as a sixth-nerve palsy suggests the lesion is in the pons at the facial nucleus/fascicle, dragging in its neighbours — still a lower motor neuron pattern, but central in location, and it needs urgent imaging.