Hemifacial spasm (HFS) is a disorder in which the affected patients experience involuntary twitching or contraction of the facial muscles on one side of the face. In the majority of cases, the spasm starts near the eye and progresses down the face over time. The twitching is usually not painful, but it can be embarrassing and interfere with normal expression and vision. The facial muscles are controlled by the facial nerve (seventh cranial nerve), which originates at the brainstem and exits the skull below the ear where it separates into five main branches (Fig. 1). The facial nerve is primarily a motor nerve, meaning it controls muscles that move the eyebrows, close the eyes, and move the mouth and lips. Hemifacial spasm is a rare condition, affecting only 8 people in 100,000 in the US. The average age of onset is 44 years and occurs slightly more in women.
The most common cause of Hemifacial spasm is compression of the facial nerve by a blood vessel loop, at the point where the nerve exits from the brainstem. Other potential causes of Hemifacial spasm include facial nerve injury, tumor compression or Bell’s palsy. The compression causes the nerve to misfire making your facial muscles contract. This condition is closely related to trigeminal neuralgia—an irritation of the fifth cranial nerve that causes severe facial pain.
The diagnosis is usually made by your physician based on your clinical history and neurologic examination. An MRI scan of the brain is usually ordered to rule out other conditions such as a brain tumor, aneurysm, or AVM that may be causing facial nerve compression. Some physicians also order an electromyogram (EMG) and nerve conduction study (NCV) of the face to evaluate the electrical activity of the facial muscles and nerve.