Primary Hemifacial Spasm: Research and Treatment

Abstract

Hemifacial spasm (HFS) is a movement disorder that presents with sudden, involuntary contractions of muscles connected to the seventh cranial nerve (facial nerve). It typically takes the form of twitching that progressively expands from the lower eyelid to other ipsilateral facial muscles. The primary form of HFS (pHFS) is caused by the neurovascular compression of the facial nerve, while secondary forms may stem from trauma, tumors, brainstem lesions, or other causes. This report, based on the case of a patient with a characteristic eyelid twitch, examines the epidemiology, pathophysiology, and diagnosis of pHFS, as well as the most common treatments.

Introduction

HFS is a neuromuscular hyperactivity disorder of the facial nerve. It presents as the twitching of the musculature of facial expression, beginning with the periorbital muscles and spreading to incorporate perioral, platysma, and other facial muscles.¹ In the vast majority of cases, this twitching takes place on just one side of the face, although bilateral manifestations occur in fewer than 1% of cases.¹ HFS appears in 11 out of 100,000 people, with a higher prevalence rate in females.¹ The effects of HFS range from social embarrassment to functional blindness and may lead to other complications such as ocular hypertension. The disorder usually develops after the age of 40, with earlier manifestations necessitating an inquiry into possible secondary causes.^1,2 The disease tends to evolve progressively and seldom improves without treatment.³

HFS is commonly separated into primary and secondary forms based on its etiology.¹ pHFS is caused by the benign compression of the facial nerve by a blood vessel in or near its root exit zone. The designation of secondary HFS covers several alternative sources for the same effect, including tumors, lesions, infections, and Bell’s palsy. The primary form of the disorder is considerably more widespread and tends to receive the most scholarly attention.^1,2 The pathophysiology of different forms of HFS is not yet fully understood.² In pHFS, the conflict between the facial nerve and the offending blood vessel appears to cause hyperexcitability, generating an increased amount of action potentials that result in spasms.⁴ This ectopic impulse then propagates to other facial muscles.

Differential Diagnoses

The symptoms presented by the patient suggest multiple differential diagnoses that may affect the same area. In addition to pHFS, those differentials include blepharospasms, tardive dyskinesia, facial motor tics, psychogenic facial movements, focal cortical epilepsy, aberrant regeneration following damage to the facial nerve, facial myokymia, and Meige syndrome.^1,2,3,5 The leading possibilities to be considered in this case are:

• Benign essential blepharospasm;
• Tardive dyskinesia;
• Facial motor tics;
• Psychogenic facial spasms;
• pHFS.

Benign essential blepharospasm is a focal dystonia that manifests in the synchronous and symmetrical bilateral contraction of the orbicularis oculi muscle.² Depressor supercilii, corrugator, and procerus muscles become involved as well, frequently joined by other facial muscles. Blepharospasm is also distinguished by the lowering of the eyebrows beneath the superior orbital rim when eyes are closed (Charcot’s sign).¹ Patient complaints include dry eye and photophobia, while sensory tricks are known to alleviate the symptoms.²

Tardive dyskinesia presents with dystonic or choreic movements of the face or other parts of the body. Facial movements tend to focus on the oral-buccal-lingual area, but can also involve other facial muscles, including the orbicularis oculi muscle. Movements are usually bilateral, irregular, and asynchronous, and often not connected to the facial nerve. This condition emerges as a side effect of past use of neuroleptic or select antiemetic drugs.²

Facial motor tics are abrupt, short-lived, non-sustained stereotyped movements that frequently alternate between the left and right hemiface. Those movements tend to be more complicated than HFS and are non-rhythmic and multi-focal.⁵ Tics can be semi‐voluntary or involuntary and may often be suppressed if the individual focuses on controlling them. They are preceded by premonitory tension and often accompanied by motor tics elsewhere on the body as well as phonic and vocal tics.²

Psychogenic facial spasms are non-patterned movements that vary in intensity, duration, and frequency.¹ They involve different parts and muscles of the face, possibly being unilateral or bilateral, and frequently being limited to lower facial muscles.⁵ The spasms may also be associated with psychogenic movements elsewhere in the body. Compared to pHFS, it is considerably more likely to manifest at earlier ages.⁶ The condition is characterized by acute onset and strong susceptibility to distraction, suggestion, and psychotherapy, which may reduce or remove it entirely.^1,2,5,6

pHFS is characterized by the involuntary contractions of facial muscles innervated by the facial nerve.¹ The condition advances progressively, beginning with the lower eyelid orbicularis oculi muscle and gradually starting to involve more facial muscles. It is usually unilateral; while rare bilateral cases have been encountered, they are asymmetrical and asynchronous. Furthermore, there is a long delay before symptoms spread from one side of the face to the other. With each contraction, the ipsilateral eyebrows are raised (the other Babinski sign).² Due to its higher likelihood, this diagnosis will be considered at length below.

Discussion

Epidemiology

The prevalence of pHFS is hard to estimate due to widespread misdiagnosing, but it appears to be relatively rare. A study based on U.S. Caucasian patient data from 1960 to 1984 indicated a mean prevalence of 11 out of 100,000 people, with subsequent research showing similar results in other Caucasian populations.^1,7 The prevalence rate was 14.5 per 100,000 for females and 7.4 per 100,000 for males.¹ Some studies have shown a higher incidence of pHFS among Asian populations, and a higher prevalence for men among those populations.^1,3 The spread of pHFS among Hispanic populations is as yet poorly studied, but studies conducted so far show a similar prevalence and sex ratio to Caucasians.³ While pHFS can affect a wide range of age categories, it appears typically between the ages of 40 and 60.¹ The disease is not usually hereditary, but rare familial cases are known, suggesting a possible genetic component.² A Hispanic female in her 50s, the case patient appears to fit the typical profile for pHFS.

Etiology

pHFS is caused by the compression of the facial nerve at or near the root exit zone in the posterior cranial fossa by an aberrant or ectatic blood vessel.⁴ This vessel is usually the superior cerebellar, anterior inferior cerebellar, basilar, or vertebral artery.^1,3 More rarely, several blood vessels may be involved at once.¹ An etiologic link has been suggested for arterial hypertension and pHFS. The confirmed increased incidence of hypertension in patients with HFS lends credibility to this suggestion.⁸ The patient’s hypertension is controlled through diet and exercise, but given her continued high-normal blood pressure, its influence on the condition cannot be excluded.

Pathophysiology

The precise mechanics behind pHFS symptoms remains vigorously debated, with two leading hypotheses attempting to explain the conflicting blood vessel’s impact on the facial nerve and muscles. The peripheral hypothesis states that local demyelination caused by neurovascular compression results in the ephaptic transmission of impulses between nearby neurons, resulting in excessive neuronal firing. Conversely, the central hypothesis holds that the irritative feedback from the nerve’s peripheral lesions increases the excitability of the facial motor nucleus.^1,4,7,9 In either event, the facial nerve becomes hyperactive, resulting in the abnormal generation of action potentials that are responsible for the spasms. This ectopic impulse then spreads from one nerve branch to another, leading more facial muscles to become involved.⁴

Clinical Findings

In 90% of cases, pHFS begins with briefly repeated contractions of the orbicularis oculi muscles, resulting in sudden, involuntary eye closure.^5,10 Commonly, a rapid succession of twitches precedes a sustained spasm.¹ Over time, the contractions expand to involve other muscles, first in the upper and then in the lower half of the face.^1,3 Contractions may manifest spontaneously or in response to stress, fatigue, or voluntary movements.^3,5 In contrast to many other movement disorders, contractions associated with pHFS often persist during sleep.⁶ The other Babinski sign – an elevation of the eyebrow during spasms that cannot be reproduced voluntarily – helps distinguish pHFS from conditions like blepharospasms and psychogenic facial spasms.¹⁰ While unilateral contractions are synchronous across all muscles, the rare bilateral form of the disease is asynchronous and asymmetric between hemifaces. There is also a notable latency before the disease spreads to the second hemiface.²

In addition to being a possible cause, hypertension can also be the result of pHFS.^1,8 The patient presents with increased intraocular pressure, but a recent study suggests that intermittent spasms associated with pHFS do not cause this effect.¹¹ However, the irritation due to involuntary eye closure may still be responsible for increased pressure and tearing presented by the patient.¹¹ While the patient’s neuropsychiatric stress might suggest the possibility of a psychogenic movement disorder, psychosomatic factors are known to contribute to pHFS as well.³ The progressively worsening twitches confined to one side of the patient’s face support the diagnosis of pHFS.² The exacerbation of symptoms by fatigue also points in this direction.

Diagnosis

The manifestations of pHFS are usually sufficiently distinctive for doctors to establish a diagnosis based on clinical findings such as the ones listed above. Examining the patient’s condition and medical history remains the primary method of diagnosis.^1,3 Nevertheless, ancillary testing may be required for a more objective and pathognomonic assessment as well as for evaluating more atypical cases.^5,7 Although the patient’s case seems typical, ancillary testing may still prove useful. Magnetic resonance imaging (MRI) can be used to visualize the neurovascular conflict, identifying with some level of precision the offending blood vessel or alternative causes of HFS (tumors, aneurysms, and lesions).^5,7,12,13 If the patient cannot use MRI, a CT scan is recommended instead.⁷ Electroneuromyography makes it possible to conduct electrophysiological tests for abnormalities characteristic of pHFS (lateral spread responses).⁵ Surgical interventions require high-resolution three-dimensional time-of-flight MR angiography to be carried out beforehand to establish all possible vascular contacts to the facial nerve.^7,12 Further electrophysiological tests are required to evaluate nerve function before surgery and confirm the reduction in compression and abnormalities afterward.^1,7

Treatment

pHFS is a chronic condition with progressive onset, usually requiring medical intervention to alleviate or abolish it.³ The most popular form of treatment at present is botulinum toxin therapy, which provides symptomatic relief through the injection of a minimal dose of biological toxin to induce functional reversible paralysis in the targeted muscles.¹ This therapy is effective, exhibiting a 90% success rate, and mostly safe, with minimal lasting side-effects (which may include ptosis, keratitis, and diplopia).^1,2,3 Its main drawback is the requirement for repeated injections at intervals of three to six months, with a steady increase in doses, although this course of treatment can still be cost-effective compared to surgery.^1,7 Also, patients may occasionally fail to respond to treatment or develop a tolerance, necessitating alternative treatment.²

Anticonvulsant medication such as carbamazepine, clonazepam, and gabapentin, as well as other drugs like baclofen and haloperidol, may sometimes improve symptoms. However, their effectiveness is highly inconsistent and often accompanied by adverse side effects, including sedation and fatigue.^1,2 Microvascular decompression (MVD) surgery, which aims to lift the compression by moving the offending blood vessel, is the only way to remove pHFS permanently.¹ MVD has a success rate of 85-90%, but is expensive and bears the general risks of invasive procedures such as the possibility of infection and anesthesia complications.⁷ Risks particular to MVD include disease recurrence, hearing loss, and temporary or permanent paralysis of the facial nerve.^1,7 For reasons that are not yet understood, pHFS may reappear in up to 25% of patients after the surgery.¹³ The choice of treatment in such a relatively mild case should be determined by the patient’s priorities with regard to costs, risks, and outcomes.

New Research

Data and expertise accumulated from surgeries, assessments, and imaging are used to expand the scientific understanding of pHFS.^2,9 Research into the pathophysiological mechanism of pHFS continues, with gathered data being deployed to support or refute the two main hypotheses as well as new alternative explanations. One recent theory suggests that the ectopic generation of action potentials in the facial nerve is triggered by contact with sympathetic nerve endings in blood vessels.⁹ Another article emphasizes the critical role of mechanical factors (either compression or stretching) in raising excitability.⁴ Understanding the mechanism behind pHFS is critical for attempts to improve the efficacy of treatment and prevent a recurrence, as well as make misdiagnosis less likely in the future.^5,13 Other areas of improvement in research include elaborating the use of imaging for studying the disorder’s etiology, investigating possible secondary effects, or refining epidemiology by examining different demographic groups.^3,8,11,12

Conclusion

pHFS is a relatively rare neuromuscular disorder, the precise operation of which remains incompletely understood. It is caused by a neurovascular conflict between one or several of the cranial blood vessels and the facial nerve, which becomes compressed at or near the root exit zone. Whether through local demyelination, irritative feedback, or some other mechanism, this compression results in hyperexcitability of the facial nerve, generating abnormal action potentials which translate into muscle spasms. While those spasms initially only affect the orbicularis oculi muscle, causing involuntary eye closure, the ectopic impulse eventually propagates to other facial muscles on the same or more rarely the other side of the face. This gradual spread defines pHFS’ nature as a progressive onset disease.

Though seemingly benign, pHFS may cause or contribute to several health complications, including hypertension, eye irritation, and functional blindness. MVD surgery and botulinum toxin therapy present viable treatment options with high success rates. While MVD surgery is the only way of permanently removing the disease, it is riskier than botulinum toxin therapy and may still fail to prevent recurrence in up to 25% of cases. Botulinum toxin therapy offers symptomatic relief, but may not work for all patients and requires repeated injections with three- to six-month intervals. Further research is necessary to elucidate the mechanism behind pHFS and improve the ability of eye care providers to remove it. In the meantime, the choice of treatment must be determined by the patient’s condition and preferences.

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