Bell’s palsy is diagnosed primarily through a physical exam, not a specific lab test or scan. A clinician evaluates facial muscle weakness on one side of your face, checks that the pattern of paralysis points to a nerve problem rather than a brain problem like stroke, and rules out other conditions that can mimic it. The whole process often takes a single office visit, though some cases require imaging or electrical nerve testing afterward.
The Physical Exam
The core of a Bell’s palsy diagnosis is watching how your face moves. You’ll be asked to close your eyes, raise your eyebrows, show your teeth, puff out your cheeks, and frown. The clinician is looking for weakness in both the upper and lower parts of one side of your face, including the forehead, eyelid, and mouth. This full-side involvement is the hallmark of Bell’s palsy, and it’s what separates it from most other causes of facial weakness.
Timing matters too. Bell’s palsy comes on suddenly, with symptoms reaching their worst point within 48 to 72 hours. If weakness develops slowly over more than three days, or if it gets progressively worse over weeks, the diagnosis shifts away from Bell’s palsy and toward something that needs further investigation.
How Clinicians Rule Out Stroke
The single most important distinction in the exam is whether the forehead is affected. In Bell’s palsy, the entire half of the face is weak because the facial nerve itself is damaged. In a stroke, the brain’s signals to the face are disrupted higher up in the pathway, and because of the way the brain’s wiring is arranged, the forehead is typically spared. So a person having a stroke can still wrinkle their forehead on the affected side, while someone with Bell’s palsy cannot.
The clinician also checks for other neurological signs: arm or leg weakness, slurred speech, vision changes, or difficulty with balance. Bell’s palsy affects the facial nerve alone. If any of these additional problems are present, the diagnosis is likely something else entirely, and the evaluation becomes more urgent.
Conditions That Look Like Bell’s Palsy
Several conditions can cause one-sided facial paralysis and need to be considered before settling on a Bell’s palsy diagnosis.
Ramsay Hunt Syndrome
This is caused by the same virus responsible for chickenpox and shingles. The classic presentation is facial paralysis plus severe ear pain plus small, painful blisters on or around the ear and ear canal. When all three are present, distinguishing it from Bell’s palsy is straightforward. The challenge is that up to 30% of Ramsay Hunt cases present without a visible rash, a variant called zoster sine herpete. In those cases, the main clue is that the ear pain tends to be significantly more severe than the mild ear discomfort sometimes seen with Bell’s palsy.
Lyme Disease
In areas where Lyme disease is common, clinicians may order blood tests to check for it, especially if you’ve had a tick bite, a characteristic rash, or joint pain. Lyme disease can cause facial paralysis that looks identical to Bell’s palsy, and it occasionally affects both sides of the face.
Tumors and Other Structural Causes
A slow-onset facial weakness, a lump near the ear, or unexplained weight loss and fatigue all raise concern for a tumor or other structural problem pressing on the facial nerve. These cases require imaging and typically an immediate referral to a specialist.
When Imaging Is Needed
Most people with a straightforward Bell’s palsy presentation do not need an MRI or CT scan. Imaging becomes appropriate when the clinical picture doesn’t fit the typical pattern. The American College of Radiology considers MRI with contrast the most appropriate imaging choice when it’s needed.
Red flags that prompt imaging or urgent specialist referral include: facial weakness that develops gradually rather than suddenly, weakness accompanied by other neurological symptoms, a palpable lump near the ear, symptoms affecting both sides of the face, or constitutional symptoms like fever and weight loss. If your paralysis follows the typical sudden-onset, one-sided pattern and the exam is otherwise normal, your clinician can generally make the diagnosis without a scan.
Grading Severity
Once Bell’s palsy is diagnosed, clinicians grade its severity using the House-Brackmann scale, a six-point system that ranges from normal function (Grade I) to complete paralysis (Grade VI). This grading isn’t just academic. It helps predict recovery and guides treatment decisions.
- Grade II (mild): Your face looks roughly symmetrical at rest, with only slight asymmetry when you move. You can close your eye with gentle effort.
- Grade III (mild-moderate): Noticeable asymmetry when moving. You can still close your eye, but only with full effort. Brow and smile movement are moderately reduced.
- Grade IV (moderate): You cannot fully close your eye. Brow and smile movement are severely limited.
- Grade V (severe): Your face is visibly asymmetric even at rest, with drooping of the brow, lower eyelid, and corner of the mouth. Only trace movement is possible.
- Grade VI (complete): No movement at all on the affected side.
Your grade at the initial visit serves as a baseline. Clinicians compare it to later exams to track whether you’re recovering, staying the same, or getting worse.
Nerve Testing for Difficult Cases
Electrodiagnostic tests measure how well the facial nerve is conducting electrical signals and whether the muscles it controls are degenerating. These aren’t part of a routine Bell’s palsy workup, but they become valuable when paralysis is severe or recovery stalls.
Timing is critical. Signs of muscle degeneration don’t show up on these tests until 10 to 14 days after symptoms begin, so testing too early produces misleading results. Needle electromyography is most useful in the window from two to three weeks after onset up to about three months. During this period, it can detect whether the nerve fibers are breaking down or whether early signs of regrowth (called reinnervation potentials) are appearing. If a first test is done early, a repeat measurement at least 14 days later gives a more accurate picture.
These results help clinicians estimate your likelihood of full recovery and decide whether additional interventions are worth considering.
How Common Bell’s Palsy Is
Bell’s palsy affects roughly 25 out of every 100,000 adults per year in the United States, based on data spanning 2007 to 2022. Notably, the diagnosed incidence has been rising: from about 15 per 100,000 in 2007 to over 35 per 100,000 by 2022. Whether this reflects a true increase in cases or better recognition and diagnosis isn’t fully clear. Either way, it remains the most common cause of sudden one-sided facial paralysis, which is why clinicians evaluate for it first and look for other causes only when the presentation doesn’t fit.