Bell's Palsy

01Pathophysiology

The facial nerve (cranial nerve VII) exits the brainstem at the cerebellopontine angle and travels through the temporal bone within the narrow fallopian canal before branching to innervate the muscles of facial expression, the stapedius muscle, the lacrimal and submandibular glands, and taste fibers to the anterior two-thirds of the tongue.

Bell's palsy results from inflammation and edema of CN VII within the fallopian canal. The prevailing theory attributes this to reactivation of HSV-1 harbored in the geniculate ganglion. When the virus reactivates, it triggers an inflammatory response. Because the fallopian canal is a rigid bony structure, even minor swelling compresses the nerve, causing demyelination and axonal injury. This is why the onset is acute but not instantaneous: the swelling builds over hours.

The clinical presentation follows directly from this anatomy. Because the entire nerve trunk is affected distal to the nucleus, the result is a lower motor neuron (LMN) pattern of paralysis: both the upper and lower face on the ipsilateral side are involved. This is the single most important concept to understand. The forehead receives bilateral cortical input (both hemispheres project to the portion of the facial nucleus controlling the frontalis muscle). Therefore, in a central or upper motor neuron (UMN) lesion such as stroke, the forehead is spared because the intact contralateral cortex still drives the upper face. In Bell's palsy, the lesion is at the nerve itself, so bilateral cortical input is irrelevant and the forehead is paralyzed.

Additional symptoms are explained by the branches affected at or proximal to the site of compression:

• Hyperacusis occurs because the nerve to the stapedius muscle is disrupted, leaving the tympanic membrane without its normal dampening mechanism.

• Loss of taste on the anterior two-thirds of the tongue results from involvement of the chorda tympani branch.

• Decreased lacrimation follows from disruption of parasympathetic fibers to the lacrimal gland, though paradoxically patients may report tearing ("crocodile tears") because the eye cannot close properly.

02Classification and Clinical Manifestation

House-Brackmann Grading System

Anatomic Branch Involvement

Exam vignettes generally present Grade IV to VI severity and emphasize the entire ipsilateral face being involved.

03Diagnostic Workup

Bell's palsy is fundamentally a diagnosis of exclusion made on clinical grounds. When a patient presents with acute unilateral facial paralysis involving the entire half of the face (forehead included), and no other neurological deficits are found on examination, the diagnosis is Bell's palsy and no further testing is mandatory.

The critical first step is distinguishing LMN from UMN facial weakness at the bedside. Ask the patient to wrinkle their forehead and close their eyes tightly. If the forehead is paralyzed on the same side as the lower face weakness, the lesion is peripheral (LMN). If the forehead is spared, you must consider a central cause such as stroke, and the workup shifts to urgent neuroimaging.

Lyme disease deserves attention because it is one of the most common identifiable causes of facial nerve palsy in endemic regions and is the leading cause of bilateral facial palsy. Any vignette that places the patient in the northeastern United States, mentions a tick bite, or describes erythema migrans should prompt Lyme serology.

MRI is not part of the routine workup but becomes important when the presentation deviates from classic Bell's palsy. Red flags that should trigger imaging include: gradual rather than acute onset (suggests neoplasm), additional cranial nerve deficits, failure to improve after 3 weeks, recurrent episodes, or facial twitching preceding the paralysis.

EMG at 2 to 3 weeks can help predict recovery. If greater than 90% of motor fibers have degenerated on electroneuronography, the prognosis for full spontaneous recovery is significantly worse, and surgical decompression may be discussed (though this remains controversial).

04Management and Treatment

The next best step when a patient presents with classic Bell's palsy is to start corticosteroids immediately, ideally within the first 72 hours. This is the single most evidence-supported intervention. Prednisone reduces inflammation and edema within the fallopian canal, and randomized trials have demonstrated that early steroid use increases the rate of complete recovery from approximately 80% to over 90%.

The role of antivirals is frequently tested. Current evidence shows that antivirals alone are not beneficial. However, the combination of valacyclovir plus prednisone may offer a marginal benefit over prednisone alone in patients with severe paralysis (Grade IV or worse). The key testing point: never choose antivirals as monotherapy; always pair them with corticosteroids.

Eye care is a non-negotiable part of management that vignettes sometimes test indirectly. Because the patient cannot blink or fully close the eyelid, the cornea is at risk for desiccation, ulceration, and potentially vision-threatening keratitis. If a vignette describes a Bell's palsy patient who now has eye pain, redness, or tearing, think exposure keratitis and recognize that inadequate eye protection is the cause.

Contraindications and considerations:

• In pregnancy, corticosteroids can be used but require shared decision-making. Pregnant patients are at higher risk for Bell's palsy and may benefit significantly from treatment.

• In diabetic patients, steroids will transiently worsen glycemic control. Monitor blood glucose closely but do not withhold treatment, as the benefit of preventing permanent facial nerve damage outweighs short-term hyperglycemia.

• Surgical decompression of the facial nerve is rarely tested and remains controversial. It is generally reserved for cases with documented complete degeneration on EMG and no clinical improvement.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on Bell's palsy questions is by failing to distinguish it from a stroke. The vignette will describe acute facial weakness, and the student must immediately determine whether the forehead is involved. If the stem says "the patient cannot raise the eyebrow and cannot close the eye on the same side," that is LMN (Bell's palsy). If it says "the patient has weakness of the lower face but can wrinkle the forehead," that is UMN (think stroke), and the next best step is neuroimaging, not prednisone.

A second common trap involves Ramsay Hunt Syndrome. The vignette will describe what appears to be Bell's palsy, but somewhere in the stem there will be a mention of painful vesicles on the ear or auditory canal. Students who skim past this detail will select Bell's palsy when the correct answer is Ramsay Hunt (caused by VZV reactivation, treated with higher-dose antivirals plus steroids, and carrying a worse prognosis).

A third testing angle is the timing of corticosteroid initiation. Questions may present a patient who has had symptoms for 5 or more days and ask about management. The benefit of steroids diminishes significantly after 72 hours, and some guidelines suggest limited benefit beyond this window. If the vignette emphasizes a delayed presentation, the answer may shift toward supportive care and eye protection rather than aggressive steroid therapy.

Finally, always remember that bilateral facial palsy is not Bell's palsy until proven otherwise. If both sides of the face are affected, the differential shifts to Lyme disease, Guillain-Barre, sarcoidosis, or HIV, and the workup must be broadened accordingly.

The core competency being tested is the ability to localize the lesion (upper vs. lower motor neuron), match the clinical pattern to the correct diagnosis, initiate time-sensitive treatment, and avoid being distracted by close mimics that differ by one or two clinical details.