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Hemifacial Spasm

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    What is hemifacial spasm?

    Hemifacial spasm (HFS, also known as tic-convulsive) is an uncontrolled twitching of the facial muscles, usually one side of the face.

    What are the mimic muscles and how they are controlled?

    There are about 600 muscles in human body. Most of them are deep and attach to bones. Muscles are capable of contraction and by doing so they move the bones. Facial muscles (AKA mimetic muscles) are different. Firstly they are located shallow, directly under the skin. Secondly, they originate from the skull and terminate in the skin of the face. Their contractions therefore moves the skin of the face. Facial muscles are extremely important for speaking, eating, blinking and facial expressions during communications. All emotions that we experience are reflected in our face by facial muscles. Therefore, they also known as mimetic muscles. Facial muscles are also responsible for wrinkling of the face skin. Technically a wrinkle is basically a skin fold that develops over particular muscle and is always perpendicular to the line of contraction.

    What is the facial nerve and what does it do?

    All mimetic muscles are innervated by the facial nerve (VII cranial nerve). The facial nerve is predominantly a motor nerve and mostly it contains fibers designated to mimetic muscles. Like any other cranial motor nerve, its nucleus located deep in the brainstem. Nucleus is basically a group of neurons designated for a specific function. Facial nucleus neurons are responsible for taking orders from the brain cortex and delivering it to facial muscles. Each neuron has a single, very long protrusion called axon. Axon is the cellular structure connecting the neuron to the muscle. It is basically a communication line between the neuron and the muscle fibers. The communication is one way i.e. neuron generates electric impulses which travel along the axon to the muscle fibers. Once the impulse reach muscle fibers they respond by contracting. If no impulses are received the muscle is not contracting i.e. in the idle mode.

    Axons are wrapped with specific cells that nurture them and speed up impulse velocity. These cells (oligodendrocytes and Schwann cells) contain a very specific substance called myelin. Myelin is very important for accelerated impulse propagation. Also myelin provides insulation for axons. Axons plus supportive cells with myelin are called nerve fibers. Nerve fibers from all facial neurons assemble, leave the brainstem and travel to the facial muscles. This cluster of nerve fibers is known as the facial nerve.

    Basically, the facial nerve is a cable containing thousands of individual nerve fibers. It must be mentioned that every nerve fiber is independent i.e. under normal conditions impulses between different fibers don’t mingle with each other. This is very important feature for orderly contraction of the muscles. The main structure maintaining this independence is myelin. It is a very good insulator and therefore electric impulses from one fiber do not jump to the other.

    After leaving the brainstem the facial nerve enters the temporal bone through internal auditory canal and then passes through the specific canal inside the bone called facial canal. It receives sensory and parasympathetic fiber on its way. After leaving the skull just under the ear facial nerve branches off. These individual branches travel to muscles, enter them off and eventually reach muscle fibers.

    How does hemifacial spasm develop?

    As mentioned previously orderly contraction of the muscles is very important for proper function. One of the most important features of muscle contraction is voluntary control. We can control our facial expressions as well any other muscle in our body.

    There are several diseases when muscles contract involuntary (i.e. out of our control). These contractions are usually non-functional i.e. do not have a purpose. These conditions are collectively called movement disorders. Hemifacial spasm is one of movement disorders. Unlike other diseases in this category it is well-understood and thus can be effectively treated.

    Hemifacial spasm is the disease of the facial nerve. Specifically, the condition develops at the point of nerve’s exit from the brainstem. This area is called root-entry-exit-zone or simply REEZ. One of the interesting points of REEZ, is the transition from central myelin to peripheral. As discussed above myelin is an insulating layer provided by specific cells – either oligodendrocytes in the brain and Schwann cells outside of the brain. At the point of the exit from the brain there is a tiny area where oligodendrocytes meet Schwann cells. There is a very short segment of each axon that is not covered by oligodendrocytes or Schwann cells. This meeting point is the root entry exit zone, and REEZ is a very vulnerable area. The dominant theory regarding origin of hemifacial spasm assumes that there is cross-spread of neural impulses in the REEZ. In other words, normal impulses traveling along some fibers may spread to other, idle fibers and cause their excitation. Normally, this is blocked by myelin. However, if myelin is absent or deficient (like in REEZ area) there might be cross-spread of impulses between the fibers. These newly formed impulses travel down and cause involuntary contraction of muscles that are not supposed to get activated. For example, fibers innervating lip and eyelid muscles are different and should be activated independently. However, if activation of lip fibers spreads to eyelid fibers, then eyelid muscles will contract (patient will blink) each time he talks. This is a usual observation in hemifacial spasm patients. Talking triggers blinking and vice versa. Usually the involuntary activity spreads to the entire facial nerve and causes complete twitching of the half of the face (here is the name hemifacial spasm). Such “lateral” or “ephaptic” transmission is presumed to be at the core of hemifacial spasm disease. However, it should be noted that there are also alternative theories regarding this disease.

    What are the causes of hemifacial spasms?

    It should be noted that everybody has REEZ but hemifacial spasm is quite rare. It is estimated that about 8-10 in 100,000 people develop HFS. REEZ is a vulnerable area where ephaptic transmission or cross-spread may occur. But it usually does not because the transition zone between is very short and nerves are well separated from each other.

    In order to develop hemifacial spasm there must be additional damaging factor in the REEZ area. This damaging factor should result in loss in myelin (which is already scarce in REEZ). This effect is called demyelination. Depending of the causes HFS can be classified as primary or secondary. Primary or idiopathic hem facial spasm develop independently of other diseases and is the most common form. Secondary HFS is a consequence or by-product of other diseases.

    1. Neuro-vascular conflict. This form is the most frequent and is considered as primary or idiopathic. It develops due to compression of the facial nerve by an artery near the brainstem. As noted above, after exiting from the brainstem the facial nerve enters the temporal bone. However there is gap between the brain and skull called subarachnoid space. This space is filled with cerebrospinal fluid and contains nerves and vessels. Some vessels may be in direct contact with facial nerve and compress it. Usually, anterior inferior cerebellar artery (AICA) is implicated. However other vessels like posterior inferior cerebellar artery (PICA), vertebral artery or even veins can cause compression. Usually the loop of offending vessel crosses the nerve. The consistent feature is contact at the brainstem. Sometimes this compression is so severe that the nerve completely flattens. Arterial compression is much severe due to two factors. Firstly, blood pressure in the arteries is much higher than in the veins therefore compressing force is more. Secondly, arteries pulsate which acts like whiplash on the nerve. Hemifacial spasm due to offending vessel is the most common clinical variant. This scenario is called neuro-vascular conflict. One of the most interesting features of neuro-vascular conflict is its progressive nature. With aging vessel loops are getting progressively large due to pulsatile pressure inside and put more pressure on the nerve. In many cases vascular loops literally bury into the nerve.
    2. Familial hemifacial spasm. Most cases of hemifacial spasm are idiopathic and caused by neuro-vascular conflict. However, there have been reports in the literature describing hereditary HFS cases. In this situations close relatives seem to have hemifacial spasms. The exact genetic mechanism underlying familial HFS is not known but it looks like that there is autosomal dominant with low penetrance inheritance pattern. One of the interesting points in familial cases is left sided predominance. Nearly 90% of familial cases occur on the left side while idiopathic cases are roughly distributed between right and left sides.
    3. Bell’s palsy. Facial palsy in some sense is an opposite of hemifacial spasm. Bell’s palsy is the inflammation of the facial nerve and the neural signals cannot pass through the injury site. Therefore, mimetic muscles do not receive impulses and get paralyzed. Most facial palsies recover and the patients resume their normal or near normal facial features. However, during recovery there might be some deviation from normal regeneration. Axons may grow into wrong way. For example, axons that suppose to innervate lip muscles may end up in eyelid muscles. As a result patients develop synkinesis which is involuntary movement associated with voluntary movement. For example, speaking may constantly induce eye blinking on the affected side. Synkinesis are quite common after Bell’s palsy. However in some patients they may progress to hemifacial spasm. In other cases an ectopic focus may develop in the facial nerve and result in hemifacial spasm.
    4. Tumor. Sometimes tumors in close proximity of the facial nerve may trigger HFS. However, tumors do not pulsate (not as much as arteries), grow slowly and allow adaptation. Therefore, hemifacial spasm in the presence of tumor is quite rare though certainly possible.
    5. Arachnoid cyst. These are fluid filled cavities in the subarachnoid space. They are quite common but rarely cause problems. Yet, sometimes they may grow big and compress the cranial nerves. Sometimes the facial nerve may get displaced by arachnoid cyst and pushed toward an artery that it wouldn’t otherwise touch. In these cases the facial nerve is literally smashed between the cyst and artery.
    6. Epidermoid tumor or cyst are benign tumors frequently seen in around facial nerve. They do not have blood vessels, grow slowly and are simply epidermal inclusions in the brain. Sometimes they may be complex and contain hair or teeth. Like tumors they may compress the facial nerve and induce hemifacial spasm.
    7. Multiple sclerosis (MS). MS is a debilitating neurological disease characterized by loss of myelin coating of the nerve fibers in the brain. Typically the patients develop multiple sclerosis plaques scattered across the brain tissue. If there is demyelination in the facial nerve there is a chance of developing hemifacial spasm. Unlike other causes this type of HFS cannot be treated surgically. Only treatment of the main disease i.e. MS itself can provide relief. It should be noted that other demyelinating diseases can cause hemifacial spasm as well. For example AIDS can cause loss of myelin and consequently lead to hemifacial spasm.

    Hemifacial spasm is very similar to trigeminal neuralgia (TN). These diseases have much in common, the only difference is the affected nerve. Trigeminal nerve (V cranial nerve) is implicated in TN and the main symptom is facial pain.

    How is hemifacial spasm (HFS) diagnosed?

    Clinical examination

    The diagnosis of hemifacial spasm is pretty straightforward for an experienced clinician. The half of the face has irregular, uncontrolled contractions that are usually triggered by speech, laughter, or emotional expressions. The appearance of spasms is typical and unforgettable. Twitching is so typical that no other methods are necessary to make the diagnosis. Nevertheless, there are some clinical situations where diagnosis is not so obvious.

    Bilateral hemifacial spasm

    Hemifacial spasm affects one half of the face in the majority of cases. However, both sides can be affected. Usually spasms start on one side and then progressively spread to the other one. Bilateral hemifacial spasms may present significant clinical challenges because the name suggests involvement only half of the face. Therefore many clinicians think that if twitching involves entire face it must be other movement disorder.

    Hemifacial spasm in combination with facial palsy

    In some cases, the facial nerve may be overly compressed, resulting in partial facial palsy. In fact, any demyelination in the facial nerve leads to subtile palsy but it is not noticeable in the majority of cases. But sometimes facial palsy might be quite significant. The combination of facial palsy and spasms may be very confusing for inexperienced clinicians. This mixture of facial palsy and spams is typical for patients who had previous Botox injections.

    Tonic (sustained) hemifacial spasm

    A typical feature of hemifacial spasms is clonic twitching of the facial muscles. However, in some rare case the facial nerve may be significantly compressed and may cause sustained contractions. All mimetic muscles on one half of the face are tonically contracted from time to time and there is no twitching at all. These tonic episodes are perplexing and may cause diagnostic problems. Physicians may misdiagnose tonic hemifacial spasm with Bell’s palsy of the unaffected side and even proceed with steroid treatment.

    Hearing abnormalities

    Hemifacial spasm is the movement disease of the facial muslces. However, there are several auditory abnormalities frequently accompanying HFS. Usually, the patients experience mild hypoacusis (hearing loss), ear clicks or tinnitus. The exact mechanism of these acoustic phenomena is unknown. Three plausible mechanisms have been proposed (but have not been confirmed)

    1. Stapedius muscle spasm. Stapedius muscle is the smallest muscle in human body. It is attached to the stapes bone (which happens to be the smallest bone in human body). Its main function is to stabilize stapes bone and prevent excessive vibrations. For example in Bell’s palsy stapes muscle becomes frequently paralyzed which results in hyperacusis syndrome. In hemifacial spasm the situation is opposite. Abnormal muscle contractions may interfere with sound transmission and result in hypoacusis.
    2. Concomitant vestibular nerve compression. The facial nerve (VII nerve) is anatomically very close to the VIII cranial nerve – vestibulo-cochlear nerve. Vestibular nerve is involved in hearing and cochlear in maintaining normal balance. In fact, the 7th nerve is so close to 8th they are frequently considered together as VII-VIII complex. Therefore, a vessel compressing the facial nerve may compress the vestibular nerve as well and cause hearing abnormalities.
    3. Eustachian tube dysfunction. It was proposed that abnormal facial contractions may interfere with Eustachian tube and cause ear clicks. Though it is not clear how facial muscles may influence Eustachian tube since it is surrounded by muscles that are innervated by other nerves.

    Imaging studies

    Imaging studies are frequently ordered to diagnose the underlying cause of the compression. As mentioned above, the cause of hemifacial spasm is the compression from a nearby vessel, usually anterior inferior cerebellar artery (AICA). This compression is not usually visible on standard MRI, therefore special studies like CISS or FIESTA sequences are required. These specific, high resolution scans at the level of the facial nerve nicely demonstrate the neurovascular conflict. In rare cases MRI may diagnose tumors, arachnoid cysts, epidermoid cysts as a cause of hemifacial spasm.

    Differential diagnosis

    There are many diseases that may mimic hemifacial spasm.

    1. Psychogenic hemifacial spasm
    2. Facial Tics
    3. Facial Myoclonus
    4. Oromandibular Dystonia
    5. Hemimasticatory Spasm
    6. Blepharospasm
    7. Facial myokymia

    How is hemifacial spasm treated?

    Currently, there is no medical treatment available for hemifacial spasms. It is treated either with Botox injections or surgery.

    How do Botox injections help patients with hemifacial spasms?

    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 spasms. First, a single injection is not sufficient since its effect lasts no more than six 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.

    Significant number of patients undergo Botox injection for hemifacial spasm relief (see below). Botox does not treat the disease per se rather paralysis facial muscles. This paralysis is temporary and is usually muscles are back within 2-3 months. Spasms recur with diminishing Botox effect. However, after Botox injections muscles never go back to their original strength. There is always some degree of permanent paralysis. With repeated injections facial palsy becomes progressively worse. Combination

    How does microvascular decompression work for hemifacial spasms?

    Surgery is the only treatment that can provide a permanent cure for hemifacial spasms. 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 way, the brain is minimally disturbed, and no permanent damage is made. The 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 the 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 spasms 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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