Pathophysiology

There are two major theories regarding the pathophysiology of hemifacial spasm. “Ephaps” is the ability of two close nervous fibers to let a nerve impulse stimulating one nerve fiber to pass through both nerve fibers. In this theory, the vessel in contact with the nerve induces a chronic compression that leads to focal demyelination through which ephaptic transmissions can occur.

Ephaptic theory1–3 is based on the following electromyographic data: in a healthy subject, the stimulation of a branch of the facial nerve only induces contractions of motor fibers innerved by this branch, whereas in patients suffering from hemifacial spasm, the same stimulation would induce contractions in the innervated expected muscles and also in other muscle territories not innervated by the particular nerve. This abnormal response is called abnormal muscle response. The spraying of contraction to other facial muscle territories is called lateral spread motor responses ( Fig. 17.1 ).

In nuclear theory,4–8 chronic stimulation of the facial nerve by a vessel induces transmission of a nerve impulse toward peripheral muscle in an orthodromic way and also in an antidromic way, toward facial nerve nucleus, which becomes hyperexcitable. This theory explains the delay that is sometimes seen when the patient becomes cured sometime after the surgical neurovascular decompression.

Nowadays, the best hypothesis involves a combination of the two theories:

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  Systolic pulses of the vessel at the site of the compression create ectopic nervous stimulation. This stimulation is due to the compression itself and also to the bioelectrical changes induced by the variations of blood flow in the looped or atheromatous vessel.

  
  
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  Demyelination progressively occurs and allows ephaptic transmissions.

  
  
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  Antidromic conduction of the impulse is responsible for the facial nerve nucleus hyperexcitability, which progressively evolves on its own.

  
  
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  Hyperactivity of the facial nerve explains the occurrence of hemifacial spasms after peripheral stimuli (voluntary or involuntary facial movements) or emotional stimulation via the reticulated formation.

The root entry zone or root exit zone (REZ) corresponds to the emergence of the facial nerve from the pons into the cerebellopontine angle (CPA). It is also called the Obersteiner-Redlich zone. In this zone, intraprotuberential fibers of cranial nerve VII are very superficial, and the nucleus is very close, around 1 mm. Therefore, any compression at this level could affect both the central and peripheral portion of the VII nerve, as well as its nucleus.

Moreover, this zone corresponds to a physiological transition between central nervous sheathing (oligodendrocytes) and peripheral sheathing (Schwann cells). Vascularization is very poor in this area, making the zone very fragile to any insult and representing a kind of Achilles heel along the course of the facial nerve.9,10

Diagnosis

Hemifacial spasm belongs to class of facial movement disorders. Its diagnosis is based on clinical examination. The muscle contractions observed are:

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  Unilateral: The hemiface is wrinkled, the eye is half shut, the corner of the lip is pulled up on the affected side, and the tip of the nose is curved.

  
  
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  Involuntary: Contractions are uncontrollable. They can occur at night, causing sleep impairment. There is no triggering factor but crisis often occurs in periods of stress, overtiredness, and/or anxiety.

  
  
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  Sudden: Contractions are tonic or tonicoclonic. Initial blepharospasm is found in 90% of the cases. After a few weeks, months, or years, spasmodic contractions affect all the hemiface muscles, last for a few seconds, and disappear as quickly as it appeared. Crisis begins with clonical contractions rising from orbicularis oculi, and at its height tonical contractions of the hemiface ( Fig. 17.2 ). As time progresses, the crises become more frequent and more intense.

  
  
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  Isolated: There is no motor trouble or movement disorder between crises. There are never permanent contractions, and above all there is no facial weakness.

Hemifacial spasm has deep repercussion on the patient′s quality of life, and most patients are anxious about the cosmetic appearance as well as the functional problems associated with uncontrolled spasms. Many patients may seek antianxiety therapies.

Clinical Forms

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  Bilateral spasm: It is rare and is a problem of differential diagnosis.

  
  
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  Hemifacial spasm associated with trigeminal neuralgia: It is rare and also called painful tic convulsif.

Differential Diagnosis

The most common cause of hemifacial spasm is due to vascular compression at the REZ. There are other rare but important causes that can cause symptomatic spasms (1–2%) and a thorough evaluation can reveal:

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  Arachnoid cysts

  
  
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  Vascular lesions: Arnold-Chiari, vertebrobasilar aneurysms, ischemia of the pons

  
  
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  Tumors: Brainstem or CPA tumors

  
  
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  Paget disease

  
  
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  Multiple sclerosis

For postparalytic spasms, a period of facial palsy is found in the patient′s background or history. Spasm is then characterized by contracture at rest, clonical contractions at blinking, abnormal voluntary movements with loss of functional synergy, and, above all, facial weakness.

Neuromuscular dystonias are characterized by their central origin and affect both sides of the face. Some etiologies of this type of dystonia include the following:

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  Idiopathic blepharospasm leads to complete or partial closure of the eyes.

  
  
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  Oromandibular dystonia or Meige syndrome corresponds to bilateral blepharospasm associated with periodic contraction of muscles from the lower part of the face.

  
  
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  Cervical dystonia or spasmodic stiff neck affects the shoulders and neck muscles.

  
  
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  Convulsive tics affect mainly children and disappear when sleeping or with voluntary control.

  
  
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  Partial epilepsy.

  
  
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  Myokymia are most of the time very localized, especially on the orbicularis oris or orbicularis oculi, which tremble or spasm. They can be found in anxious patients, and physicians must look for an etiology such as a metabolic disorder or for multiple sclerosis.

Radiological Exploration11–15,25

Radiological evaluation of the CPA is based on magnetic resonance imaging (MRI). MRI only can provide analysis of both the nervous and vascular structures. The objectives of imaging are:

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  To confirm clinical diagnosis by showing a neurovascular conflict.

  
  
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  To eliminate other etiologies in the differential diagnosis.

  
  
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  To identify a precise preoperative vascular lesion for surgical mapping.

Imaging usually begins with a cerebral MRI to eliminate other etiologies in the differential diagnosis and to confirm the diagnosis. The sequences commonly used are as follows:

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  T2 high-resolution: This sequence gives a cisternographic view of the CPA, where cerebrospinal fluid appears hyperintense and vessels hypointense, giving an excellent contrast between cerebrospinal fluid and the neurovascular elements ( Fig. 17.3 ).

  
  
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  Three-dimensional T1 with gadolinium: This sequence offers good delineation of nerves and vessels and can identify venous conflicts.

  
  
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  Magnetic resonance angiography sequences, such as the sequence “time of flight,” highlights vascular structures where the intensity depends on the speed of intravascular blood flow. This sequence can demonstrate either arterial signals and/or venous signals.

  
  
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  Fusion sequences time of flight: T2 high-resolution.

There are three radiological signs of neurovascular compression: (1) a neurovascular contact must be shown at the REZ; (2) the vessel is perpendicular to the nerve; and (3) a deformity of the nerve or the pons is observed.

MRI cannot be interpreted without any data concerning the patient′s clinical background, as up to 3% of asymptomatic patients have a neurovascular contact on MRI and 0.5% of asymptomatic patients present with a neural deformity.

Therapeutic Care of Hemifacial Spasm

Many patients undergo various medical treatments such as antidepressants, anxiolytics, neuroleptics, etc., but none of these are effective on the spasm itself. There are only two treatments that can really treat the hemifacial spasms.

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  Botulinum toxin blocks neuromuscular transmission of the nervous signal and therefore prevents the injected muscles of the face from facial contractions. Botox is commonly used to treat essential blepharospasms. Botox can also be used in selected patients to control the effects of muscle contraction associated with hemifacial spasm. The effects of Botox can be very satisfactory in some patients. However, the effect is nevertheless transitory, lasting from 2 to 4 months depending of the type and the amount of toxin injected. Botox is not a curative treatment of hemifacial spasm; it only stops or slows the muscle spasms until the neuromuscular junction repairs itself and normal nerve conduction occurs and the symptoms return. For a more detailed explanation, see Chapter 26.

  
  
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  Neurovascular decompression: This surgical technique was first described by W.J. Gardner to cure trigeminal neuralgia in 1959. Gardner and Sava applied the same technique to the facial nerve in 1962 to successfully cure hemifacial spasm. The widespread acceptance of this technique was due to Jannetta with the use of operative microscope; he intensively described the microvascular decompression technique.26 Then the procedure was improved in the 1990s by Magnan with the complementary help of endoscope and the development of the minimally invasive retrosigmoid approach. Surgical decision to perform this procedure is made in cases of persistent and severe hemifacial spasm, immediately or after toxin injections that do not modify surgical prognosis and results.