Hemifacial Spasm
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The Neurosurgical Atlas by Aaron Cohen-Gadol, M.D. Hemifacial spasm Hemifacial spasm (HFS) is a cranial nerve hyperactivity disorder most likely caused by neurovascular conflict (compression) as one of its underlying etiological phenomenon. It is typically characterized by unilateral involuntary intermittent twitching of the muscles of the face. The spasms usually start around the eye, involving the orbicularis oculi muscle, and later spread to other muscles of the face that are innervated by the facial nerve, including the platysma. The spasms are bilateral in about 2% of the patients with this disorder. Hemifacial spasm has an estimated prevalence of 11 cases per 100000 individuals and is twice as common in females as males. Onset is mostly during the 4th and 5th decades of life. On average, patients suffer from HFS for about 8 years before definitive treatment is found. Familial clustering is rare. Clinically, HFS presents as progressive, involuntary, irregular clonic or tonic movements of the muscles innervated by the facial nerve. The symptoms usually persist during sleep. Some patients complain of a “ticking” sound on the affected side, which is caused by contractions of the stapedius muscle. Although HFS is not life threatening, patients may suffer severe psychological stress because of cosmetic concerns, and their binocular vision may be compromised by prolonged tonic spasms of the orbicularis oculi. The symptoms are often exacerbated by psychological stress and speaking. Differentiating HFS from other movement disorders involving the face can sometimes be challenging. Some common mimickers of HFS are blepharospasm, facial nerve tics, and synkinesis after facial nerve paralysis. A careful history and physical examination can greatly help the clinician reach the correct diagnosis. In most patients with HFS, the underlying cause is usually a neurovascular conflict caused by an ectatic or aberrant vessel loop of the posterior inferior cerebellar artery (PICA), the anterior inferior cerebellar artery (AICA), or the vertebral artery (VA), causing compression at the root exit zone of the facial nerve near the brainstem. The root exit zone is particularly sensitive to compression because the nerve is covered only by arachnoid in this location and epineurium is absent. Also, no connective tissue septa traverse the individual fascicles in this region that is the transition zone between central and peripheral myelin. Two theories have been proposed to explain the pathogenesis of HFS. According to the peripheral hypothesis, ephaptic excitations occur at the root exit zone altered by the offending vascular loop. In contrast, the central hypothesis assumes that hyperexcitability of the facial motor nucleus within the brainstem is the underlying cause. I believe a combination of these two hypotheses most likely accounts for the pathogenesis of HFS. Vascular compression is a contributory and not necessarily the primary cause of this disorder. Figure 1: One of Cushing’s patients (Circa 1920). Cushing is triggering the spasms by pinching the right side of the patient’s face. Diagnosis and Evaluation The clinical features at presentation are crucial for making the correct diagnosis of HFS. No imaging or testing modality has been found to reliably reach the diagnosis. Patients usually present with a fairly long history of involuntary spasms of the facial muscles innervated by CN VII, mostly originating in the periorbital region, involving the orbicularis oculi, and spreading to other facial muscles as the disease progresses. They may complain of a “ticking” sound on the symptomatic side caused by stapedius muscle contractions. Patients are generally concerned about cosmesis, and some are significantly burdened by visual impairment (caused by prolonged tonic spasms of the orbicularis oculi) hindering their abilities to read and drive. A detailed history and physical examination are important to reach the correct diagnosis. The patient should be asked about the onset of spasms, location, progression, exacerbating and relieving factors, and previous chemodenervating treatments with botulinum toxin injections. The patient should also be asked about a recent occurrence of Bell’s palsy because postparalytic facial synkinesis is an alternative diagnosis, although rare. It is not unusual to find mild facial weakness caused by prior botulinum toxin injections, facial neuropathy related to vascular conflict, or facial muscle weakness caused by ongoing repetitive spasms. Any additional findings on exam, such as hearing loss, should raise concerns for an underlying structural lesion within the cerebellopontine angle such as schwannoma, meningioma, epidermoid tumor, or arachnoidal cyst. Certain disorders intrinsic to the brainstem, such as gliomas, multiple sclerosis, and brainstem stroke, can also give rise to similar symptoms and should be ruled out. Some movement disorders that can mimic hemifacial spasm include blepharospasm (bilateral and synchronous symmetrical contractions of the eyelids), oromandibular dystonia (involuntary and sustained muscle spasms of the mouth and lower face), facial nerve tic (complex, coordinated, multifocal movement patterns, and switches between the right and the left sides of the face), hemimasticatory spasm, focal seizures, and synkinesis after facial nerve paralysis following acoustic neuroma surgery. Before considering a microvascular decompression (MVD) procedure, the patient should undergo a CT scan or preferably a high-resolution dedicated posterior fossa MRI to rule out any structural pathology. The MRI will determine the extent of the tumor (epidermoid, meningioma, etc) and necessary preoperative considerations to resect the tumor. A high resolution T2–weighted sequence may display an aberrant vascular loop at the root exit zone of the nerve. Even if a high-resolution MRI does not identify an offending vascular loop, I offer posterior fossa exploration to the patient if I am relatively certain of the diagnosis. The MR sequences can also evaluate the presence of a large tortuous vertebral artery that may not be safely or effectively mobilized intraoperatively. In this situation, the surgeon should discuss the lower rate of spasm freedom after surgery with the patient. Figure 2: Axial T2-weighted MRI demonstrates a vascular loop (arrow) around the root exit zone of CN VII. Figure 3: The only presentation of this large left-sided petroclival epidermoid tumor (note the CP angle extension around the facial nerve *) was hemifacial spasms. This tumor was resected through staged retrosigmoid and pterional craniotomies. Medical Therapy The drugs used to treat hemifacial spasm include carbamazepine, clonazepam, baclofen, and gabapentin, but success rates with these medications have been disappointing. The adverse effects of these medications notably affect some patients: fatigue, exhaustion, and poor performance. Botulinum toxin (Botox) targeted chemical denervation injections may help reduce spasms, but this is merely symptomatic treatment. Since these injections do not treat the cause of the problem, spasms gradually return at the end of each 3 to 6-month Botox cycle, necessitating repeat treatment. Botox injections may also injure some of the motor nerve terminals and partially account for some residual facial weakness after MVD surgery, despite successful relief of spams. Most importantly, the gradual return of spasms at the end of each Botox cycle often leads the patient to seek a more lasting definitive treatment. Microvascular decompression surgery has a reported lasting success rate of 80-90% in experienced hands and appropriately selected patients, and is the only durable therapeutic option. Indications for Surgery Most patients elect to proceed with surgery because of the disfiguring nature of HFS spasms and a desire for an improved cosmetic outcome. Although Botox injections can provide considerable albeit short-lasting relief, the need to undergo these injections every 3 to 6 months and the cosmetic deformity associated with HFS and Botox injections are the main reasons to consider MVD surgery. Many surgeons prefer to see the patient in his or her “off” period to assess the validity of the spasms before offering surgery, but this has not been the routine in my practice. Patients suffering from HFS are generally reliable and know much about their disorder. They frequently have videotaped their spasms during their “off” period. Since HFS is usually not a disabling disease, surgical intervention must be conducted with very minimal risk to justify the risk-to-benefit ratio. As with MVD for trigeminal neuralgia, the operator’s experience is especially important in achieving a favorable and safe outcome. The patient’s history and severity of spasms are very important because spasms are sometimes self-limiting and resolve spontaneously. For this reason, I offer MVD surgery only if the patient has experienced severe HFS symptoms for longer than 1-2 years. Contralateral hearing loss is also a contraindication to MVD surgery for HFS. Functional hearing loss is a real complication from this procedure (<5%) and should be candidly discussed with patients preoperatively. Preoperative Considerations I advocate using brainstem auditory evoked response (BAER) monitoring during MVD to treat HFS (in contrast with MVD for trigeminal neuralgia, when I do not use BAER monitoring). Latency of Peak V is considered the best electrophysiologic indicator for signaling cochlear nerve damage. I also look for intraoperative disappearance