Bell’s palsy is diagnosed primarily through a physical exam, not a specific lab test or scan. There is no single test that confirms it. Instead, your doctor observes the pattern of facial weakness, rules out other causes, and uses the timeline of your symptoms to reach a diagnosis. Most people can be diagnosed in a single office visit.
What Happens During the Physical Exam
The core of a Bell’s palsy diagnosis is watching how your face moves. Your doctor will ask you to perform a series of facial movements: raise your eyebrows, close your eyes tightly, smile, puff out your cheeks, and show your teeth. They’re looking at whether one entire side of the face is affected, including the forehead. This distinction matters because a stroke typically paralyzes only the lower half of the face on one side, while Bell’s palsy affects the upper and lower halves together.
Your doctor will also check for something called Bell’s phenomenon, where the eye on the affected side rolls upward and outward when you try to close it. This is especially noticeable when your eyelid can’t fully shut, a condition called lagophthalmos. It’s a strong clinical sign that the facial nerve itself is involved rather than a problem in the brain.
Beyond facial movement, the exam includes looking inside your ear and mouth, checking your other cranial nerves, and testing sensation. The goal is to confirm that the weakness follows the path of the facial nerve and that nothing else is going on neurologically.
How Symptom Timing Supports the Diagnosis
Bell’s palsy has a characteristic speed. Symptoms tend to come on suddenly and reach peak severity within 48 to 72 hours. If your facial weakness developed gradually over weeks or months, or if it’s still getting worse after three weeks, that pattern doesn’t fit Bell’s palsy and points toward something else entirely. The rapid onset followed by a plateau is one of the most reliable clues your doctor uses.
Mild symptoms sometimes appear before the paralysis sets in: slight fever, pain behind the ear, or neck stiffness. These can precede the facial weakness by a day or two and are consistent with the diagnosis.
Grading the Severity
Once your doctor confirms the diagnosis, they’ll typically grade the severity using the House-Brackmann scale, a six-level system that ranges from normal function (Grade I) to total paralysis (Grade VI). The grades in between capture important distinctions. Grade II means mild weakness with normal symmetry at rest. Grade III describes obvious weakness but with the ability to fully close the eye with effort and good forehead movement. Grade IV means incomplete eye closure and moderate forehead movement with visible asymmetry. Grade V is barely perceptible motion.
This grading isn’t just academic. It helps predict your likely recovery and guides treatment decisions. Someone at Grade III has a very different outlook than someone at Grade VI.
Ruling Out Other Conditions
Because Bell’s palsy is a diagnosis of exclusion, your doctor needs to be confident that nothing else is causing the facial paralysis. Several conditions can look similar, and each has distinguishing features.
Ramsay Hunt syndrome, caused by the same virus responsible for shingles, produces facial paralysis that can closely mimic Bell’s palsy. The key differences are a painful rash with small blisters in or around the ear, dizziness, and hearing loss. Bell’s palsy does not present with a rash, and the paralysis at onset is generally less severe. Your doctor will examine your ear canal and outer ear carefully to check for these blisters.
Lyme disease is another important consideration, particularly if you live in or have traveled to an area where tick-borne illness is common. Lyme disease can cause facial paralysis on one or both sides. If there’s any suspicion, a blood test can check for antibodies.
Stroke is the concern most people have when half their face suddenly stops working. But stroke-related facial weakness almost always comes with other neurological symptoms: arm or leg weakness, speech difficulty, or trouble with balance. The forehead is also usually spared. If these additional symptoms are present, the evaluation shifts immediately to stroke protocols.
When Blood Tests Are Ordered
Lab tests and imaging are not needed to diagnose Bell’s palsy in a straightforward case. However, blood work can identify systemic conditions that cause facial paralysis, particularly diabetes and Lyme disease. If your doctor suspects either, they may order blood glucose or antibody testing. These tests don’t confirm Bell’s palsy itself but help rule out treatable underlying causes that would change your management.
When Imaging or Nerve Testing Is Needed
Most people diagnosed with Bell’s palsy never need an MRI, CT scan, or nerve test. Imaging is not necessary in a typical acute case. But certain red flags change that calculus:
- Other neurological symptoms beyond facial weakness, such as limb weakness or numbness
- Paralysis still progressing after three weeks
- No return of any function by six months
- Recurrent facial paralysis on either side
- Signs of chronic ear disease or a growth near the ear
In these situations, an MRI can reveal whether a tumor, infection, or inflammatory process is compressing the facial nerve. A CT scan may be added if the doctor suspects a structural problem in the bone surrounding the nerve.
Electromyography, or EMG, is sometimes used when the diagnosis is unclear or the paralysis is severe. This test measures the electrical activity of the facial muscles in response to stimulation and tracks how well electrical impulses travel along the nerve. It can confirm that nerve damage is present and give a sense of how extensive it is. EMG is most useful when performed about two weeks after symptom onset, once enough time has passed for the nerve changes to show up on the test. The results can help predict whether recovery will be complete or partial.
What to Expect at Your Appointment
If you’re heading to the doctor with sudden facial weakness, the visit is typically straightforward. Expect a focused neurological exam lasting 10 to 15 minutes, with your doctor watching your face closely as you follow movement instructions. They’ll look in your ears, ask about recent illnesses or tick exposure, and check for weakness elsewhere in your body. In a clear-cut case with rapid onset, one-sided involvement including the forehead, and no other neurological findings, you’ll walk out with a diagnosis and a treatment plan the same day.
If anything about your presentation is atypical, your doctor may order blood work or schedule imaging before confirming the diagnosis. This isn’t cause for alarm. It simply means they want to be thorough before settling on Bell’s palsy as the explanation.