D2I2.
neurological

Hemifacial spasm

Hemifacial spasm (HFS) is a rare disease characterized by irregular, involuntary muscle contractions (spasms) on one side (hemi-) of the face (-facial). The facial muscles are controlled by the facial nerve, which originates at the brainstem and exits the skull below the ear where it separates into five main branches.

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Plain-language summary adapted from Wikipedia. Not medical advice.