Don't worry. You likely have Bell's Palsy.
Bell's palsy is a paralysis of cranial nerve VII resulting in the inability to control the muscles of the affected side of the face. A cause of Bell's palsy has not been defintively identified, but it is believed that a virus (herpes simplex) might be the cause. The nerve gets inflammed and cannot operate properly. As a consequence, you cannot smile, blink or move your facial muscles. Some patients report some pain and numbness.
The treatment of Bell's palsy is usually with antiviral medication and steriods. Studies have shown that the earlier you start these medications, the better off you are. It is a self-limiting disease and most patients will improve without any sequelae, but a small proportion will have some continued weakness (4%). Most get better within 3 weeks.
What should you do? I would make an urgent appointment to be seen by your doctor. In the meantime, I would buy some saline eye drops to put in your eye (on the affected side). This is because the cornea dries out since you cannot close your eye properly. As well, tape your eye closed overnight.
It is important to make an appointment to be seen by your doctor to make sure that you do not have other reasons for facial paralysis, for example- tumor, stroke or Lyme disease.
If you have numbness or weakness of the arm or leg, particularly on the same side of the weakness of the face, you need to be seen in the ER right away..
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