Causes of Hemifacial Spasm
Hemifacial spasm is most commonly caused by vascular compression of the facial nerve, typically by an ectatic vessel at or near the root exit zone from the brainstem. 1
Primary Cause: Neurovascular Compression
The overwhelming majority of hemifacial spasm cases result from pulsatile vascular compression of the facial nerve (CN VII), specifically within the centrally (oligodendrocyte) myelinated portion of the nerve. 1 This compression typically occurs at specific anatomical locations:
Location of Compression
- Attached segment (64%): The most common site, where the nerve courses along the pontine surface 2
- Root detachment point/transition zone (22%): The Obersteiner-Redlich zone where central and peripheral myelination meet 2
- Root exit point (10%): Where the nerve emerges from the pontomedullary sulcus 2
- Distal cisternal portion (3%): Rarely the primary site of compression 2
Offending Vessels
The culprit vessels identified during surgical decompression include:
- Anterior inferior cerebellar artery (43%) 2
- Posterior inferior cerebellar artery (31%) 2
- Vertebral artery (23%) 2
- Large veins (3%) 2
- Multiple vessels may be involved in 38% of cases 2
Secondary Causes
While vascular compression accounts for the vast majority of cases, hemifacial spasm can also be secondary to:
Brainstem Pathology
- Infarction affecting the facial nerve nucleus or fascicles 1
- Vascular malformations within the pons 1
- Tumors affecting the brainstem 1
- Multiple sclerosis involving the facial nerve nucleus 1
Cerebellopontine Angle Lesions
As the facial nerve exits the brainstem and traverses the cerebellopontine angle, it may be affected by:
Temporal Bone Pathology
- Cholesteatomas 1
- Intrinsic bone tumors 1
- Trauma affecting the facial nerve course 1
- Inflammatory middle ear disease 1
Extracranial Causes
- Parotid tumors affecting the extracranial facial nerve segment 1
- Skull base carcinomas and sarcomas 1
- Inflammatory disease of the skull base 1
Pathophysiological Mechanism
The spasm-related electromyographic activity is generated by ephaptic transmission due to local demyelination at the entry zone of the facial nerve root, likely caused by chronic nerve damage from the compressing vessel. 3 This supports the "nerve origin hypothesis" as the primary pathophysiological mechanism. 3
Clinical Distinction: Primary vs. Secondary
Primary hemifacial spasm (vascular compression) typically begins with involvement of the orbicularis oculi muscle and gradually spreads to lower facial muscles over time. 3
Secondary hemifacial spasm (from structural lesions) may present with simultaneous involvement of upper and lower facial muscles from onset, which is an important distinguishing feature. 3
Diagnostic Imaging Considerations
When evaluating for the cause of hemifacial spasm, MRI with 3D heavily T2-weighted sequences and MRA is recommended to characterize vascular loops potentially compressing the facial nerve. 4 Neurovascular contact is identified in 83-100% of cases on high-quality imaging, though findings should be considered supportive rather than diagnostic. 1