Hemifacial spasm (HFS) is a disorder characterized by paroxysmal, involuntary twitching of facial muscles of one side of face innervated by ipsilateral facial nerve (seventh cranial nerve). It is considered a subtype of peripheral (neuromuscular) movement disorder. As the name suggests, the disorder is almost always unilateral except in some rare cases of severe hemifacial spasm (less than 5%) during which one can see the bilateral involvement of facial muscles.
Causes of Hemifacial Spasm
Primary Hemifacial Spasm
Most common arterial anomalies leading to facial nerve root compression involve anterior inferior cerebellar artery (AICA), posterior inferior cerebellar artery, vestibular artery), among others. The most common cause is aberrant/ectatic blood vessels in the posterior cranial fossa causing facial nerve compression.
Secondary Hemifacial Spasm
A hemifacial spasm can occur secondary to following etiologies:
- Trauma
- Bell’s palsy
- Brainstem lesions such as demyelinating plaques of multiple sclerosis, stroke
- Arteriovenous anomalies, intracranial arterial aneurysm, arterio-venous fistula, angiomas
- Mastoid and ear infections (otitis media, cholesteatoma)
- Parotid gland tumors
- Chiari malformation and other structural anomalies of the posterior cranial fossa
Pathophysiology
Chronic irritation of the facial nerve nucleus and proximal nerve segment (root) due to several underlying etiological factors is the main pathophysiologic mechanism of hemifacial spasm. Compression of the facial nerve root at the junction of central (point of exit from brainstem) and peripheral segment (root exit/entry zone), by aberrant/ectatic blood vessels, is the most common cause of hemifacial spasm as reported in the literature.[rx] The root exit/entry zone is the area of transition of oligodendroglial (central myelination) to Schwann cells (peripheral myelination). The compression of this zone leads to demyelination of axons in the region. Several theories have been proposed to explain the pathophysiological mechanism by which facial nerve compression leads to hemifacial spasm.
Nerve Origin Hypothesis/Peripheral Theory
As proposed in this theory, the ephaptic transmission of impulses, or the lateral spread of excitation to adjacent nerve fibers leads to abnormally excessive firing of the facial nerve. This is due to demyelination of the facial nerve at the site of compression.[rx] Myelin is an insulating material and plays an important role during nerve impulse conduction by preventing such abnormal lateral spread resulting in the ectopic transmission of impulses that cause hemifacial spasm.
Nuclear Origin Hypothesis/Central Theory
Irritation of peripheral afferent facial nerve fibers leads to aberrant signaling to the central facial nerve nucleus leading to the abnormal firing of the nucleus. This facial nucleus hyperexcitability results in involuntary myoclonic contraction of facial muscle on the affected side.[rx]
Diagnosis of Hemifacial Spasm
Clinical Features of Hemifacial Spasm
Classically the involuntary tonic/clonic contractions of one side of the face begin with the involvement of orbicularis oculi leading to brief, intermittent, painless, involuntary closure of the ipsilateral eye. This is associated with infrequent elevation of the eyebrow with eye twitching also known as “other Babinski sign“ named after Joseph Babinski who first described it in 1905. “Other Babinski sign” is pathognomic of hemifacial spasm and helps to differentiate from eye twitching/closure due to blepharospasm, in which it is characteristically absent.
In primary hemifacial spasm, gradually, over several months to years, the irregular tonic/clonic contractions progress to involve lower facial muscles such as perioral muscles (orbicularis oris, mentalis, zygomaticus major, platysma).
In secondary hemifacial spasm, both upper and lower face develop synchronous, intermittent contractions eventually leading to sustained spasms as the disease progresses. The persistence of contractions during sleep is one of the characteristics of hemifacial spasm distinguishing it from most of the movement disorders. This may predispose the individual to disturbed sleep and insomnia. In advanced cases, facial deviation/asymmetry and grimacing is evident due to pull of contracted muscles on one side. There are unusual symptoms such as clicking sound in the ear (due to stapedius involvement), ear pain, and hearing loss reported in some cases of secondary hemifacial spasm. Symptoms are typically worsened by stress, fatigue, eating, and anxiety. Relaxation techniques or simply touching the face subsides the spasm/twitching. Although hemifacial spasm is known for a chronically progressive course, spontaneous resolution can be seen in 10% of cases.[7]
Association of HFS and Hypertension
Researchers have reported that 40% of patients with hemifacial spasm have coexisting hypertension disorder.
When a patient younger than 40 years presents with hemifacial spasm, one must evaluate the possibility of multiple sclerosis, although it is very rarely reported to be presented as hemifacial spasm. Demyelinating plaques in the brain stem region can involve the facial nerve roots leading to aberrant signaling that result in hemifacial spasm.
Evaluation
Diagnosis of hemifacial spasm is made clinically based on the detailed clinical history and a neurological and local physical examination. Electromyogram (EMG) can be used in the early stages of the disease when it is difficult to distinguish it clinically from facial myokymia, blepharospasm, complex partial motor seizures, or motor tics. The diagnostic finding on electrophysiological testing is lateral spread and variable synkinesis on blink reflex testing. Stimulation of one branch of the facial nerve leads to the contraction of facial muscles supplied by a different branch. Needle EMG characteristically shows irregular, brief motor unit potentials with high frequency (150 to 400 Hz) and facial twitching can be clinically correlated with this finding.
In cases with suspected facial nerve compression, it is necessary to rule out the vascular malformation or tumor at the brainstem exit site of the nerve. Advanced MRI scanning techniques such as high-resolution T1- and T2-weighted spin-echo or gradient-echo imaging with gadolinium assist in better visualization of the same. MR angiography is reserved for those planned for the microvascular decompression surgeries.
Treatment of Hemifacial Spasm
The primary goal of therapy is to reduce the abnormal muscle contractions resulting from abnormal impulse transmissions to adjacent neurons (ephaptic). the hemifacial spasm can be either treated medically or surgically based on the underlying etiology and severity of the disorder.
Medical Management
Botulinum Neurotoxin (BoNT) Injections in Hemifacial Spasm
The advent of botulinum toxin has revolutionized the management of hemifacial spasm. Injecting botulinum toxin has become the first choice among treating physicians as well as the patients due to its efficacy and fewer side-effects. The most commonly used BoNT in treating hemifacial spasm is Onabotulinum toxin A. Other commercially available BoNT preparations include rimabotulinumtoxinB and Abobotulinum toxin A. A thorough evaluation excluding the secondary causes such as tumors or vascular malformations is essential before the treatment with botulinum toxin to achieve maximum benefit.
Mechanism of Action of Botulinum Toxin
BoNT acts on the presynaptic terminal at the neuromuscular junction and blocks the calcium-mediated release of presynaptic neurotransmitter-acetylcholine (Ach) leading to non-conduction of a nerve impulse across the NM junction leading to temporary paralysis of muscles supplied. This chemodenervation effect usually lasts for 3 to 6 months and subsequently reverses back to baseline.
Dosing and Scheduling of Botox Injection
The usual dose of onabotulinum toxin A is 10 to 36 U. There is a need for repetitive injections scheduled at intervals of 3 to 6 months. Some physicians prefer flexible dosing interval ranging from 6 to 20 weeks as lasting effects vary. The symptomatic improvement after botox injection is evident 3 to 6 days after the injection and reaches to peak after 2 weeks of injection.
BoNT is injected in affected muscles, most commonly the orbicularis oculi (upper and lower eyelids), corrugator, frontalis, zygomaticus major, buccinators, and masseter.
Injecting Botulinum toxin is a simple and non-invasive procedure. However, the patient should be forewarned about the side effects such as ptosis, temporary bruising, swelling, facial asymmetry, asymmetric eyebrows, facial weakness, among others which are mild and transient. The frequent repetition of procedures and high cost are the two major drawbacks of this therapy.
Botulinum toxin treatment has reported achieving notable symptomatic improvements in 85% to 95% of patients in various clinical trials.[rx][rx]
Botulinum toxin has a dual benefit as it reportedly reduces depression among patients with hemifacial spasm. The theory postulated includes 2 ways by which botulinum toxin alleviate depression. The first way is by providing symptomatic relief leading to less social embarrassment. The other possible mechanism is reducing the hyperactivity of the amygdala causing depression and anxiety. Botulinum toxin causes chemical denervation in adjacent muscles of the face causing interruption of afferent sensory information to the trigeminal tract leading to reduced trigeminal sensory input to the brainstem and amygdala.
Oral medications
Traditionally, the treatment of hemifacial spasm, especially in the mild form and early stages, was oral medications. Commonly prescribed oral medications include anticonvulsants such as carbamazepine, gabapentin, benzodiazepines (clonazepam), anticholinergics, baclofen, haloperidol, among others. These drugs have some efficacy in relieving the spasm but the results are inconsistent. The major drawback of most of these medications is side effects such as excessive sedation and fatigue associated with their long-term use. Oral medications can be considered as first-line therapy in those who are unwilling for botulinum toxin therapy and not good candidates for surgery.
Surgical Management
Surgical treatment is the only permanent treatment solution that addresses the underlying cause. It is preferred in most severe cases of hemifacial spasm and those refractory to botulinum toxin therapy. The procedure of choice is microvascular decompression that entails releasing the facial nerve from the aberrant vessel causing compression at the brainstem. As discussed earlier in electrophysiological findings, the lateral spreading response results from ephaptic transmission of nerve signals to neighboring neurons due to compression related demyelination of facial nerve root at the exit/entry zone. When such compression is relieved, the lateral spreading response on nerve stimulation will disappear. Thus EMG can be used to assess the achievement of successful decompression post-operatively. Following the first year after surgery, the average success rate of microvascular decompression surgery is reported to be 80% to 88%.[rx]
The complications due to an invasive procedure, for example, anesthetic complications, transient or permanent facial nerve palsy, hearing loss, cerebrospinal (CSF) leakage, recurrence of symptoms are some of the disadvantages of this surgical modality. Recurrent hemifacial spasm is rarely seen to develop (1%) when patients remain asymptomatic for at least 2 years after surgery.[rx]
References
