How do you tell Bell palsy apart from a stroke?

Look at the forehead. Bell palsy is a lower motor neuron lesion of the facial nerve, so it paralyzes the entire half of the face including the forehead, and the patient cannot raise the eyebrow or close the eye. A stroke is an upper motor neuron lesion that spares the forehead, because the upper face receives input from both cerebral hemispheres, so the patient can still wrinkle the forehead and only the lower face droops. A spared forehead plus other deficits is a stroke until proven otherwise.

Why must the eye be protected in Bell palsy?

The paralyzed eyelid cannot close, so the cornea is left exposed and dries out, which leads to exposure keratitis and corneal ulceration. Patients are given artificial tears during the day and a lubricating ointment with the eye taped or patched at night to keep the cornea covered.

What is the treatment for Bell palsy?

Oral corticosteroids such as prednisone started within 72 hours of onset improve the chance of full recovery by reducing nerve swelling in the bony facial canal. An antiviral may be added in severe cases. Eye protection is essential. Bilateral palsy points to Lyme disease, and ear vesicles with vertigo point to Ramsay Hunt syndrome, which both change the treatment.

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Neurology · Cranial Nerves

Bell Palsy

A facial nerve swells inside a bony tunnel and half the face goes slack. The whole exam turns on one muscle: the forehead. Learn the rule that separates a harmless palsy from a stroke, then the steroids, the eye, and the dangerous look-alikes.

The Setup

A Nerve in a Bony Tunnel

Everything about Bell palsy flows from one fact: the facial nerve runs through a tight bony canal, and a swollen nerve in a rigid tube gets squeezed. Walk the chain once and the forehead rule writes itself.

A 52-year-old woman wakes up and the right side of her face will not move. She cannot smile on the right, her right eye will not close, and when she tries to look surprised the right side of her forehead stays flat and smooth. There is no arm or leg weakness and her speech is clear.

Which single finding tells you this is a peripheral nerve problem, not a stroke?

Start with the wiring. The forehead muscle on each side receives motor commands from both sides of the brain (bilateral cortical input). The lower face gets commands from only the opposite side of the brain. So a stroke in one hemisphere knocks out the lower face on the far side but the forehead keeps moving, because the healthy hemisphere still drives it. The final common wire, the facial nerve (cranial nerve VII) itself, carries everything to that whole half of the face. Damage that wire and the entire half goes down, forehead included.

Now the squeeze. Bell palsy is an idiopathic lower motor neuronA lesion of the final nerve itself (or its motor neuron), as opposed to an upper motor neuron lesion in the brain or cord. Lower motor neuron facial weakness takes the whole half of the face. palsy of cranial nerve VII, strongly linked to reactivation of herpes simplex virus. The virus inflames the nerve → the nerve swells → but it is running through a narrow bony canal with no room to expand → it gets compressed against the bone → the signal stops conducting → that half of the face stops moving. It comes on over hours to a day or two and most people recover over weeks to months.

The forehead rule. Flip each card to lock in the discriminator.

Peripheral (Bell)Tap to flip

Whole half of the faceA lower motor neuron lesion of cranial nerve VII paralyzes the entire half including the forehead. The patient cannot raise the eyebrow and cannot close the eye on that side. This is Bell palsy.

Central (Stroke)Tap to flip

Forehead sparedAn upper motor neuron lesion (a stroke) spares the forehead because the upper face has input from both hemispheres. Only the lower face droops, and the patient can still wrinkle the brow. Look for arm or leg signs and treat it as a stroke.

Why the splitTap to flip

Bilateral input up topThe forehead is the only facial region wired to both cortices, so it survives a one-sided brain lesion. That single anatomic quirk is the whole exam question: forehead moves means central, forehead dead means peripheral.

At the Bedside

More Than a Droopy Face

The facial nerve carries more than movement. A lesion high in the canal knocks out hearing comfort, taste, and tearing too, and those extra clues localize the lesion. Tap each card.

Whole-face weakness

Tap to reveal

Drooping of the brow, eyelid, cheek, and mouth on one side. The patient cannot raise the eyebrow, close the eye, or smile. The whole half is involved, which is the lower motor neuron signature.

Hyperacusis

Tap to reveal

Normal sounds seem painfully loud. Cranial nerve VII powers the stapedius muscle that dampens loud noise in the ear. Lose it and the volume knob is stuck on high.

Lost taste, front of tongue

Tap to reveal

Loss of taste over the anterior two-thirds of the tongue, carried by the chorda tympani branch of cranial nerve VII. Saliva production on that side also drops.

Dry eye

Tap to reveal

Decreased tearing on the affected side, because the nerve drives the lacrimal gland. Combined with an eyelid that will not close, the cornea is doubly exposed.

Bell phenomenon

Tap to reveal

When the patient tries to close the affected eye, the eyeball rolls up and out. It is a normal reflex made visible because the lid no longer covers it, and it offers the cornea a little protection.

From the Attending The hyperacusis and the lost taste are not trivia. They tell you the lesion sits high in the canal, above where the branches to the stapedius and the tongue peel off. But the finding that changes management at the bedside is the eye that will not close. A cornea that cannot blink starts drying within hours. Protect it before the patient leaves the room.

Sorting It Out

Bell, or Something Else?

Bell palsy is a clinical diagnosis of exclusion. Three quick forks separate it from the look-alikes that need a completely different plan. Try each before you reveal it.

A one-sided facial droop. What is the first thing you check at the bedside?

The forehead is the gate. If it is paralyzed too, the lesion is peripheral (Bell). If it is spared and the lower face droops, the lesion is central and you treat it as a stroke. The forehead test costs nothing and decides everything.

The forehead is involved, but the palsy is on BOTH sides of the face. The patient hiked in the woods two weeks ago. What jumps to mind?

Bilateral facial palsy is the red flag. In an endemic area with a tick exposure or a target-shaped rash, think Lyme disease and treat with doxycycline. Other causes of bilateral palsy include Guillain-Barre syndrome and sarcoidosis. Two-sided facial palsy is not plain Bell palsy until you have ruled out Lyme.

One-sided palsy, but there are painful blisters in the ear canal and the patient is dizzy with some hearing loss. What is this?

Vesicles in the ear plus facial palsy and vertigo is Ramsay Hunt syndrome, herpes zoster reactivating in the geniculate ganglion. It is more severe than Bell palsy and demands antivirals plus steroids. Ear vesicles change the diagnosis and the treatment.

Line them up. The table puts the look-alikes side by side so the discriminators are obvious.

Cause	Forehead	The tell	Treatment
Bell palsy	Involved	Idiopathic, one-sided, whole half of the face, often after a viral illness.	Early steroids, eye protection.
Central (stroke)	Spared	Lower face only, plus arm or leg weakness, slurred speech, or facial numbness.	Emergent stroke workup and imaging.
Lyme disease	Involved	Often bilateral, tick or woods exposure, possible target-shaped rash.	Doxycycline (treat the infection).
Ramsay Hunt	Involved	Painful vesicles in the ear, vertigo, hearing loss, more severe.	Antivirals plus steroids.
Tumor	Involved	Slow gradual onset over weeks, twitching, or no recovery at all.	Imaging to find the mass.

The Plan

Steroids Early, Save the Eye

Two moves carry most of the points: dampen the swelling fast, and keep the exposed cornea wet. The trap lives in the one droop you must never call Bell palsy.

Steroids, and the clock. Oral corticosteroids (prednisone) started within 72 hours of onset reduce the swelling crushing the nerve in its canal and improve the odds of full recovery. The earlier the better. An antiviral such as valacyclovir is sometimes added in severe cases, and it is required if this is actually Ramsay Hunt.

Save the eye. The lid will not close, the tear film dries, and the bare cornea is at real risk of exposure keratitis and ulceration. Give artificial tears through the day, then a lubricating ointment with the eye taped or patched shut at night. This single instruction prevents a permanent corneal scar.

What to expect. Most patients recover meaningfully over weeks to months. Failure to improve, a slow creeping onset, recurrence, or other cranial nerves involved should push you toward imaging to hunt for a tumor or another structural cause.

From the Attending Do not get fancy on the workup of a clean, one-sided, whole-half facial palsy with no other findings. That is a clinical diagnosis. Save the MRI for the atypical story: gradual onset, no recovery in weeks, recurrence, or a second cranial nerve in the mix. Steroids early, tears and a patch for the eye, and reassurance that most people come back.

Board Trap The fastest way to fail this question is to call a forehead-sparing droop Bell palsy and send the patient home on prednisone. A spared forehead with a drooping lower face is an upper motor neuron lesion, a stroke until proven otherwise. That patient needs emergent imaging and a stroke pathway, not steroids and tears.

Prove It

Board Walkthrough

Six original clinical vignettes, 5 dealt per round, answer choices shuffled, never repeating within a round. Tap a wrong answer first to see why it almost works, then read the glowing clues.

Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last reviewed June 2026

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