A drooping face: is it a stroke, facial palsy, or something to do with dental treatment?

The command that works the muscles of the face is passed on in two stages. The command arises in the brain, and the facial nerve, which leaves the brainstem, carries it through the inside of the ear to the facial muscles. A problem anywhere along this route can make the face droop. The commonest and most dangerous form of a problem in the brain is a stroke, meaning the blood supply to the brain being cut off by a blocked vessel or a bleed. The commonest form of a problem in the nerve is Bell's palsy. The NHS sums up the main symptoms of a stroke with the acronym FAST: one side of the face dropping and smiling becoming difficult, not being able to lift both arms and keep them there because of weakness or numbness in one arm, speech becoming slurred or garbled, and calling for help straight away if any of these is present.

The first place doctors look when telling the two apart is the forehead. MSD Manuals explains why: the forehead muscles receive commands from both halves of the brain, while the lower half of the face receives them only from the opposite side. That is why a stroke in the brain mostly makes the lower part of the face droop; the person can wrinkle their forehead, frown and close their eyes tightly. When the facial nerve itself is damaged, however, that half of the face is involved completely: the forehead does not wrinkle, the eye cannot blink and the mouth is pulled to one side.

This rule should not be used to rule out a stroke at home. A small stroke in a particular part of the brainstem, because it lies close to where the nerve leaves, can paralyse the whole half of the face, forehead included, just like Bell's palsy. In a case published in BMJ Case Reports, a 66-year-old woman with diabetes and high blood pressure came in with 3 days of headache, dizziness, nausea, vomiting and complete paralysis of half of her face; although the picture resembled Bell's palsy, an MRI scan found a small brainstem stroke. The authors stress that facial palsy seen on its own should not always be put down to Bell's palsy.

In emergency departments this distinction is usually made correctly, but the margin of error is not zero. In a study that followed 43,979 adults discharged from emergency departments in California with a diagnosis of Bell's palsy between 2005 and 2011, 0.8% of patients were given a different diagnosis within 90 days, and 39.9% of these came to light in the first 7 days. Stroke, shingles, a nerve disease called Guillain-Barré syndrome and middle ear infection made up 85.4% of the later diagnoses. This likelihood rose slightly with age and in people with diabetes. That is why it matters that someone diagnosed with Bell's palsy goes back if a new symptom appears.

Bell's palsy has a course of its own. MSD Manuals writes that pain behind the ear often comes before the paralysis, that the weakness develops over hours and that it usually reaches its worst at 48–72 hours. The person may feel numbness or heaviness in the face, but the sense of touch turns out normal on examination. The NHS adds drooling from the mouth, a dry mouth, loss of taste, a dry or watering eye and not being able to close the eye on the affected side. MSD reports evidence suggesting that behind this picture, once counted as paralysis of unknown cause, there is mostly swelling in the nerve caused by the cold sore virus, and in second place by the shingles virus. It adds that only around half of facial nerve palsies are paralysis with no cause found.

Time matters in Bell's palsy too. The NHS writes that treatment is more effective when it starts within the first 72 hours, so a person who suspects Bell's palsy should ask for an urgent appointment. Treatment consists of a 10-day course of steroids (corticosteroids), sometimes with an antiviral medicine added, and eye drops, ointment and tape that keeps the eye closed at night to protect the eye. An evidence review in the journal of the American Academy of Family Physicians states that steroids are the first-line treatment, that an antiviral is not recommended on its own because it is ineffective alone, and that in typical Bell's palsy more than two-thirds of patients recover completely on their own.

The link with dental treatment appears in two separate forms. The first is the medicine from a nerve block injection, given to numb the lower teeth, accidentally reaching the facial nerve that passes through the salivary gland in front of the ear. In this case the face droops on the side where the injection was given, during the same period as the numbness, and the eye cannot blink. A case report in BMJ Case Reports writes that this is a rare but known side effect, that it is temporary, and that the eye needs protecting until the blink reflex returns. In a case reported from Italy, the palsy began 1 hour 40 minutes after the injection and lasted a short time. The second form is a palsy resembling Bell's palsy that starts days after dental treatment.

There are two studies that have looked at this second form. In a retrospective review by researchers from Harvard and Hamburg, in 16 of 2,471 facial palsy patients (0.7%) the palsy began within 30 days of a dental procedure on the same side; on average it began 3.9 days after the procedure, 14 of the patients had the picture of Bell's palsy and 2 that of Ramsay Hunt syndrome, the form of the shingles virus that involves the area around the ear, and in half of them partial paralysis remained permanent. In a study in Korea, in 11 of 924 patients (1.2%) the palsy began on average 3.2 days (between 1 and 11 days) after a dental procedure, and the MRI findings resembled Bell's palsy. The authors suspect reactivation of the virus. Both studies are retrospective and involve small groups; they do not prove that the dental procedure causes the palsy, they only show this link in the timing.

There is also a mistake in the opposite direction: facial palsy can be taken for swelling caused by a tooth. In a case reported in Dental Update, the one-sided facial weakness of a 6-year-old girl was at first taken for facial swelling caused by her decayed teeth, and Bell's palsy was diagnosed late. Swelling makes the face fuller and can pull the corner of the mouth, but it does not take away the movement of the facial muscles. Swelling that spreads to the eyelid and closes the eye, on the other hand, is a separate emergency sign. For the emergency signs of swelling caused by a tooth, you can look at our separate page; the subject here is the loss of muscle movement.

Neither list is for making a diagnosis at home; they are for getting clear what to tell the doctor. With any drooping that starts suddenly, 112 is called first. The first list gives the signs that, once a stroke has been ruled out, suggest the picture belongs to the facial nerve; if any item from the second list applies, do not wait.

The sequence deliberately starts with the most dangerous possibility. Once a stroke has been ruled out, the cause and treatment of the nerve palsy are discussed.

1. 1

   The FAST check and 112

   Look in the mirror and smile, or ask the person with you to smile. Try lifting both arms forwards and keeping them up. Say a simple sentence. If there is any drop in the face, drop in an arm or problem with speech, call 112 straight away. Note the time the symptoms started; if you noticed them on waking, give the time you were last normal. Do not drive yourself.

2. 2

   The same route even if the symptoms have passed

   Even if the drooping and other symptoms passed quickly, the NHS recommends that anyone who has had stroke symptoms within the last 24 hours asks for help straight away. Relaxing and waiting for the morning is the wrong decision in this case.

3. 3

   Examination at A&E and imaging if needed

   The doctor checks the forehead, eyelid, mouth, the strength of the arms and legs, speech, vision and balance. If a stroke is possible, a CT or MRI scan of the brain is done. MSD writes that an MRI scan is recommended if there is another nervous system finding or if the palsy started slowly.

4. 4

   The diagnosis of Bell's palsy and the search for a cause

   Typical Bell's palsy is diagnosed by examination; the American Academy of Family Physicians writes that no laboratory tests or imaging are needed for the diagnosis. When another cause is considered, tests for diabetes, Lyme disease or sarcoidosis are requested; an ear examination is done to rule out middle ear infection and shingles.

5. 5

   Medicine within the first 72 hours

   In Bell's palsy the doctor usually starts steroids, and in some cases adds an antiviral medicine. The NHS and MSD write that treatment is more effective when it is started within the first 3 days. Your doctor decides the medicine and how long to take it; do not start on your own with old medicines you have at home.

6. 6

   Protecting the eye

   If the eye does not close, its clear front layer can dry out and be damaged. The NHS recommends eye drops and ointment during the day and surgical tape at night to keep the eye closed. If you have pain or redness in the eye or blurred vision, see an eye doctor the same day.

7. 7

   What to tell the doctor if it started after dental treatment

   Which procedure was done on which tooth, on which day, and on which side the injection was given; whether the drooping started straight after the injection or days later; whether the numbness has worn off; whether there are blisters in the ear or mouth, a high temperature or facial swelling. This information makes it easier to tell a temporary injection effect, Bell's palsy and an infection apart.

8. 8

   Follow-up

   The NHS recommends going back to a doctor if the symptoms do not start to improve within 3 weeks. If recovery is slow, the doctor may request a nerve conduction test and electromyography; MSD writes that these tests help predict the likelihood of recovery.

Stroke and Bell's palsy are the two commonest causes, but not the only ones. MSD Manuals writes that facial nerve palsy is now regarded as a picture with a differential diagnosis of its own.

The following are the wrong decisions commonly seen with this picture, and what they cost.

The course below is for a person in whom a stroke has been ruled out and Bell's palsy has been diagnosed. The times vary from person to person; the follow-up plan your doctor gives you takes priority.

1. The first hours and the first 3 days

   The palsy increases over hours and, according to MSD, usually reaches its worst at 48–72 hours. Medicine is started during this period; eye care is needed from the first day.

2. The first weeks

   Food staying in the cheek while you eat, liquid escaping from the corner of the mouth and some sounds becoming difficult when you speak are common. Soft foods, small mouthfuls and cleaning between the cheek and the gum after meals make things easier.

3. Week 3

   The NHS recommends going back to a doctor if improvement has not started within this time. The doctor may request nerve tests if needed.

4. 6 months

   The NHS writes that most people recover within 6 months, and that for some it can take longer. MSD states that if some function has remained in the nerve, complete recovery usually happens within a few months.

5. Lasting effects

   In some people, permanent facial weakness, a constantly watering eye or a change in taste can remain. MSD writes that nerve fibres growing back in the wrong direction can lead to involuntary linked movements, such as the corner of the mouth moving when you blink, or to the eye watering when you eat. The American Academy of Family Physicians states that physiotherapy can be helpful in severe palsy.

We do not give figures here, because the scope varies a great deal with the cause of the drooping and how severe the palsy is. The items that determine the scope:

Emergency assessment and imaging

If a stroke is possible, a CT or MRI scan of the brain and observation at A&E are needed.

Looking for the cause

While typical Bell's palsy needs no tests, blood tests, chest imaging and an ear examination are added when another cause is considered.

Eye follow-up

If the eye does not close, drops, ointment and, where needed, check-ups with an eye doctor become part of the process.

The speed of recovery

With slow recovery or severe palsy, nerve tests, physiotherapy and long-term follow-up may be needed.