Hemifacial spasm (HFS) is a disorder characterized by repeated, paroxysmal, involuntary twitching of the muscles on one side of the face. It predominantly affects one side, although in rare and severe cases (less than 5%), involvement of bilateral facial muscles may be observed. The prevalence of HFS is approximately 14.5 cases per 100,000 women and 7.4 cases per 100,000 men, with most cases being sporadic in nature. HFS is categorized into two types based on etiology: idiopathic and secondary hemifacial spasm. The cause of idiopathic HFS is unclear, and the term "hemifacial spasm" generally refers to idiopathic hemifacial spasm (idiopathic HFS). Secondary HFS is often caused by compression or irritation of the facial nerve, such as by acoustic neuromas or other lesions. The condition primarily occurs in individuals over the age of 40, but it can also present in younger adults and children.
Etiology and Pathogenesis
Primary Hemifacial Spasm
The pathogenesis of hemifacial spasm involves paroxysmal abnormal excitability of the facial nerve. While no definitive cause has been established, two main hypotheses are microvascular compression theory and nuclei hyperexcitability theory.
Microvascular Compression Theory
This theory is the prevailing explanation, suggesting that microvascular compression of the facial nerve at the cerebellopontine angle by an accompanying artery or vein causes HFS. The primary culpable vessels include the anterior inferior cerebellar artery (AICA), though the posterior inferior cerebellar artery (PICA), the basilar artery, and dilated veins can also be responsible. This area is a transitional zone where oligodendrocytes (forming central myelin) transition to Schwann cells (forming peripheral myelin). Prolonged compression of the facial nerve by the offending vessels can result in myelin degeneration. During nerve impulse conduction, myelin normally acts as an insulating material, preventing abnormal ectopic excitation that spreads laterally and causes facial spasms. Additionally, the pulsation of the blood vessel may directly stimulate the facial nerve, leading to rhythmic spasms of the facial muscles.
Nuclei Hyperexcitability Theory
This theory posits that peripheral facial nerve fibers are subjected to abnormal stimulation, sending aberrant signals to the facial nerve nuclei in the brainstem. This results in abnormal discharges from the nuclei, leading to excessive excitability and involuntary, clonic contractions of the affected facial muscles.
Secondary Hemifacial Spasm
Secondary HFS may occur due to trauma, Bell's palsy, brainstem conditions such as multiple sclerosis or stroke, intracranial vascular abnormalities including aneurysms, arteriovenous fistulas, or hemangiomas, infections of the mastoid or ear such as otitis media or cholesteatomas, parotid gland tumors, posterior fossa abnormalities such as Chiari malformations (cerebellar tonsillar herniation), cerebellopontine angle tumors, or posterior fossa arachnoiditis. These conditions may compress or irritate the facial nerve and provoke abnormal stimulation, resulting in hemifacial spasm.
Clinical Manifestations
Blepharospasm
During the initial phase, the condition often presents as unilateral eyelid spasms, which may subsequently progress to bilateral facial muscle spasms.
Involuntary Facial Muscle Spasms
In mild cases, symptoms may be intermittent and occur without spasms during periods of distraction. In severe cases, spasms may become frequent and uncontrollable by conscious effort.
Association with Other Cranial Nerve Symptoms
Other symptoms, such as trigeminal neuralgia, may occur concurrently in some cases.
Diagnosis and Differential Diagnosis
The diagnosis is primarily based on the patient’s symptoms and clinical signs. Imaging studies, such as cranial CT and MRI, are used to rule out compression by tumors or other structural causes of hemifacial spasm.
Treatment
Pharmacological Therapy
The efficacy of medications is uncertain. Oral medications are an option for patients in the early stages of the disease or with mild symptoms. Commonly used oral drugs include anticonvulsants such as carbamazepine and gabapentin, benzodiazepines (e.g., clonazepam), anticholinergic agents, baclofen, and haloperidol. These medications exhibit varying degrees of effectiveness in alleviating spasms, but their outcomes are inconsistent. The major limitation of most of these drugs is the potential for side effects.
Chemical Facial Nerve Blockade
Botulinum toxin, which is a neurotoxin produced by Clostridium botulinum, acts at the neuromuscular junction by inhibiting the release of acetylcholine from cholinergic nerve terminals, resulting in temporary denervation effects. This nerve blockade is reversible and produces temporary paralysis of the target muscles following injection, lasting for about 3 to 6 months. Botulinum toxin is a commonly used symptomatic treatment for hemifacial spasm.
Surgical Treatment
Surgical intervention may be considered for patients with severe symptoms. The main surgical approaches include:
Microvascular Decompression (MVD)
First described in 1976 by Jannetta, this procedure employs a retrosigmoid approach to expose the cerebellopontine angle and the internal auditory canal. It involves exploring the lower border of the pons to the internal auditory meatus for blood vessels compressing the facial nerve and separating the vessels from the nerve using autogenous muscle grafts. This technique is known as microvascular decompression.
Other Surgical Methods
Techniques such as facial nerve combing, facial nerve wrapping, selective facial nerve sectioning, or facial nerve sectioning combined with facial-hypoglossal nerve anastomosis have also been reported. However, these procedures are less commonly used in current practice.