Hemifacial spasm (HFS) is a neurological disorder manifested by
twitching spasms on one side of the face due to involuntary
contractions of the eyelid and other facial muscles. It usually
begins gradually around one eye and may eventually spread to include
the muscles around the mouth and even neck, all on the same side.
These muscle spasms are very brief but occur rapidly and
repetitively. They are generally not painful, but may impact vision
because of forced involuntary eye closure. In contrast to
blepharospasm, a form of focal dystonia, which is associated with
eyelid spasms involving both eyes (bilateral), HFS involves only one
side of the face (unilateral). Very rarely, the other side of the
face may become affected in HFS, but the contractions remain
asymmetric and independent of each other. The facial spasms are also
often noticed by others and can be a source of embarrassment to the
patient; sometimes they are wrongly attributed by others
as "winking". HFS can sometimes be triggered by volitional
contraction of certain facial muscles, especially puckering the lips
or after forcefully closing eyes. Stressful situations or fatigue may
also worsen the spasms. Estimates suggest that one in ten thousand
people have HFS. It may be somewhat more common in women than men and
is more frequently seen in the Asian population.

What causes hemifacial spasm?

HFS usually appears without obvious cause but it is most frequently
attributable to a compression of the facial nerve (seventh of the
twelve cranial nerves) as it exits from the brainstem. This nerve
supplies muscle power to the facial and superficial neck muscles. In
most cases the compression is from a hardened displaced (ectatic)
blood vessel near the base of the brain. Rarely, aneurysms, brain
tumors or trauma may also compress the facial nerve. The compression
then causes the nerve to "short circuit" itself and it begins to fire
independently. This is referred to as "ephaptic transmission". As one
part of the nerve fires, the signal is misdirected to another part of
the nerve causing muscle contractions in different parts, but the
same side, of the face.

In some cases, the condition occurs months or even years after an
episode of unilateral facial weakness (Bell's palsy). The involuntary
contractions involving muscles adjacent to those that contract
voluntarily, termed "synkinesis" and other features usually readily
differentiate typical HFS from spasm that occur following recovery
from Bell's palsy. Bell's palsy is usually caused by a virus which
resides in one part of the facial nerve. The nerve later grows back
imperfectly resulting in short circuits and spontaneous firing. One
complication of Bell's palsy is increased tearing in the affected
eye, termed "crocodile eyes".

How do you test for hemifacial spasm?
There are electrical nerve tests (EMG) that can demonstrate a facial
nerve short circuit. However, an experienced neurologist can usually
diagnose the condition by simply observing it and EMG is rarely
needed. If atypical features are present (e.g. facial numbness or
hearing loss), then neuroimaging tests such as MRI may be useful.

How is hemifacial spasm treated?
Medications used for seizures such as carbamazepine, phenytoin and
clonazepam, and muscle relaxing medi­cations such as diazepam,
baclofen and trihexyphenidyl are only rarely helpful their use is
often associated with adverse side effects. A complicated surgical
procedure, sometimes called the "Jannetta procedure", used in the
past is now rarely needed. While the procedure often relieves the
involuntary facial spasms, it is a complicated neurosurgical
procedure with many risks of complications, including facial
weakness, deafness and stroke. Currently, most physicians consider
botulinum toxin injections to be the best treatment. This protein is
injected directly into the affected muscles. At relatively low doses,
it relaxes the affected muscles enough to prevent the spasms without
causing paralysis. The improvement occurs within 3-4 days and lasts
an average of 4-6 months. Repeat injections are then required at
varying intervals depending on each individual's response. In the
hands of a well-trained practitioner, the procedure is very safe.
Potential side effects include an eyelid droop (ptosis), facial
weakness or increased tearing, all of which resolves over time.