Craniofacial Muscles

16 Spastic Facial Muscle Disorders
311
16.9.3 Pathophysiologic Features
The pathogenesis of hemifacial spasm is not yet fully understood. Any compressive
lesions along CNVII may cause axonal damage and stimulate ephaptic impulses,
which cause the involuntary muscle contractions (Ishikawa et al. 1997 ) . The most
frequent location is the exit of CNVII from the brainstem, within the cerebellopontine
(CP) angle, and less commonly at its entry into the internal auditory meatus.
The most common fi nding is vascular pulsatile compression of the nerve by a
dolichoectatic artery; typically the offending vessel is the anterior inferior cerebellar
artery, the posterior inferior cerebellar artery, or the internal auditory artery. A dilated,
tortuous, atherosclerotic basilar artery may cause similar compression as well as
compressing additional cranial nerves simultaneously. Rarer causes include aneurysms
and CP angle tumors (Rahman et al. 2002 ) .
The post-paralytic form of hemifacial spasm may be caused by aberrant reinnervation
of the facial musculature from branches of the functioning facial nerve that
are proximal to the site of nerve injury. Other theories include ephaptic transmission
at the site of injury and spontaneous discharge from the deafferented facial nerve.
16.9.4 Diagnosis and Differential Diagnosis
Patient interview and examination help differentiate hemifacial spasm from blepharospasm
or cranial dystonia, facial myokymia, tic disorders, focal seizures and, rarely,
hysterical conversion reaction. The typical and post-paralytic forms of hemifacial
spasm are differentiated by a history of facial nerve palsy or injury and clinical
examination. In the post-paralytic form, there is residual facial weakness on the
affected side. Spasms in the typical form of hemifacial spasm are brief, stereotyped,
but not synkinetic, unlike post-paralytic facial spasm.
The identi fi cation of associated neurological signs helps differentiate secondary
hemifacial spasm from a primary (idiopathic) hemifacial spasm. Any additional
cranial nerve dysfunction (e.g., in hearing or facial sensation) should prompt a
detailed search for a de fi nable cause. However, even if all fi ndings point towards a
primary hemifacial spasm, imaging studies of the brain must be performed systematically.
Magnetic resonance angiography (MRA) is necessary to demonstrate vascular
compression of facial nerve, which is present in 88% of patients and also
sometimes present on the asymptomatic side. MRI is helpful to rule out any other
intracranial pathology, including a cerebellopontine angle tumor (e.g., pontine
glioma) and tumor or swelling around the temporal bone or stylomastoid foramen,
which may be the cause of 1% of cases (Adler et al. 1992 ; Ho et al. 1999 ) .