What is the hemifacial spasm and how does it develop?
Hemifacial spasm (HFS, also known as tic-convulsive), is an uncontrolled twitching of the facial muscles on one side of the face. The facial muscles (AKA mimic muscles) are controlled by the facial nerve (seventh cranial nerve) which originates at the brainstem and exits the skull below the ear, where it divides into several main branches. These branches travel along the face to reach muscles of facial expression. The facial nerve is exclusively motor nerve, meaning it contains fibers designated to muscles only. This nerve provides contraction commands to muscles that move eyebrows, eyelids, mouth, and lips and a platysma muscle in the neck. Irritation of the facial nerve triggers abnormal electric impulses within the nerve which in turn spread along the nerve fibers and transfer to the muscles. Once the muscle receives an abnormal signal it contracts causing visible twitching. The twitching is usually not painful, but can confusing and interfere with normal expression. Usually, the entire half of the face is involved causing embarrassment for patients. Sometimes there is also a small degree of facial palsy, i.e. weakness of the facial musculature due to facial nerve compromise.
What are the causes of hemifacial spasm?
Hemifacial spasm (HFS) can be triggered due to damage to the facial nerve by blood vessel, tumor or a cyst pressing on the nerve. The most frequent cause is the compression of the facial nerve by either posterior inferior cerebellar artery (PICA) or anterior inferior cerebellar artery (AICA) at the brainstem. This condition is usually called neuro-vascular conflict. The pressure causes irritation of the nerve making individual nerve fibers fire signals. Unlike regular, normal signals these impulses develop in all fibers and thus propagate to all facial muscles making them contract. This condition is similar to trigeminal neuralgia (TN) — an irritation of the 5th cranial nerve that causes severe facial pain. Both hemifacial spasm (HFS) and trigeminal neuralgia (TN) are caused by nerve compression, usually from a blood vessel, yet they differ in whether the sensory nerve or motor nerve of the face is affected.
The usual site of the compression is the segment of the facial nerve right after the exit from the brainstem. This area is called REEZ (root entry exit zone) and it is very vulnerable to compression.
How is Hemifacial spasm (HFS) diagnosis made?
The diagnosis of hemifacial spasm is straightforward. The entire half of the face has irregular, uncontrolled contractions that are usually triggered by speech, laughter, or emotional disturbance. The appearance is so typical that no other methods are necessary to make the diagnosis. However, imaging studies are frequently ordered to diagnose the underlying cause of the compression. As mentioned above the usual cause of hemifacial spasm is the anterior inferior cerebellar artery. Since the contact of this artery with the facial nerve is not usually visible on standard MRI, special studies might be requested. CISS of FIESTA thin slice MRI from the brainstem at the level of the facial nerve usually nicely demonstrates the neurovascular conflict.
The physical examination is usually unremarkable except for the spasm itself. However, in rare cases, the facial nerve may be overly compressed resulting in facial palsy. This might present a diagnostic challenge to physicians.
How is the Hemifacial spasm treated?
Currently, there is no medical treatment available for hemifacial spasm. It is treated with either serial Botox injection or surgery.
How do Botox injections help patients with hemifacial spasm?
Botox contains toxins from Clostridium botulinum which interferes with signal transfer from a nerve fiber to the muscle. Therefore, it literally causes muscle paralysis. Abnormal signals generated inside the nerve at the site of compression cannot be transferred to facial muscles. However, Botox does not treat the condition itself, rather it suppresses the manifestation. Additionally, there are several handicaps associated with Botox usage in hemifacial spasm. First, a single injection is not sufficient since its effect lasts no more than 6 months. Therefore, repeat injections are required to provide clinical improvement. Second, Botox leaves some degree of permanent facial palsy. This effect is even more pronounced with repetitive injections. Facial palsy is one of the most undesirable cosmetic side effects of Botox treatment.
How does microvascular decompression work for hemifacial spasm?
Surgery is the only treatment that can provide a permanent cure for hemifacial spasm. The procedure is performed by neurosurgeons and called microvascular decompression (MVD). It is performed from an incision behind the ear. The surgeon makes a small window called craniotomy and reaches the affected facial nerve by working between the skull and the brain.
This was the brain is minimally disturbed and no permanent damage is made. Facial nerve is exposed under the operating microscope and inspected. The impinging vessel is found and separated from the nerve. A tiny piece of Teflon is inserted between the nerve and the vessel to keep them apart and prevent future contact.
Since facial nerve and other nerves passing along it are very fragile, constant monitoring is performed to ensure that no harm is made during the surgery. The recovery from the surgery is usually quite fast. MVD for hemifacial spasm is a highly effective procedure providing a total cure in about 95% of cases. However, symptoms may or may not disappear immediately after the procedure. In some cases, several weeks or months are required to see the improvement.
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