Hemifacial spasm (HFS) is a neurological condition characterized by involuntary twitching or contraction of the muscles on one side of the face. These spasms typically begin subtly, often starting with the eyelid, before gradually spreading to involve the cheek, mouth, and neck muscles on the same side. The disorder affects the seventh cranial nerve (the facial nerve), leading to uncontrollable facial movements that can be brief and repetitive or nearly continuous. HFS is considered a peripheral movement disorder because the source of the problem lies outside of the brain and spinal cord.
The Primary Mechanism of Hemifacial Spasm
The most frequent cause of hemifacial spasm is neurovascular conflict (NVC). This occurs when a blood vessel, typically an artery, presses directly against the facial nerve at a vulnerable point near the brainstem. The primary site of this compression is the root exit zone (REZ), the area where the facial nerve leaves the brainstem.
The facial nerve at the REZ is particularly susceptible to external pressure because it lacks the protective outer layer (epineurium) found on the rest of the nerve. Chronic, pulsatile compression from the adjacent blood vessel causes the protective sheath, the myelin, to break down. This irritation and demyelination lead to the spontaneous generation of electrical signals within the nerve.
This abnormal electrical activity results in ephaptic transmission, or “cross-talk,” between adjacent nerve fibers. The electrical impulse jumps from one damaged fiber to another, leading to the simultaneous, involuntary firing of multiple muscle groups. The blood vessel most commonly implicated is the anterior inferior cerebellar artery (AICA), followed by the posterior inferior cerebellar artery (PICA).
The constant rhythmic pressure from the beating artery keeps the nerve irritated and hyper-excitable, leading to the characteristic spasms. Factors like aging and hypertension can contribute to HFS by causing arteries to become elongated and tortuous (dolichoectasia), making them more likely to compress the nerve. Approximately 40% of individuals with HFS also have concurrent hypertension.
Atypical and Secondary Causes
While neurovascular conflict accounts for the vast majority of cases, a smaller percentage of hemifacial spasms, referred to as secondary HFS, arise from other underlying conditions. These causes involve structural lesions or previous damage that irritate the facial nerve along its path. Tumors, such as acoustic neuromas or meningiomas, are common structural causes.
These masses can compress the nerve in the same region as an aberrant blood vessel, leading to similar symptoms. Other structural abnormalities, including cysts, arteriovenous malformations (AVMs), or aneurysms, can also cause compression. In these cases, the pressure on the nerve is constant rather than pulsatile, which can lead to different clinical features.
Secondary HFS can also occur as a late consequence of prior facial nerve injury, such as that caused by severe Bell’s Palsy or physical trauma. The nerve’s attempt to regenerate after such damage may lead to “miswiring” of the fibers, resulting in abnormal signals and spasms. Conditions like multiple sclerosis, which causes demyelinating plaques in the brainstem, are rare causes of secondary HFS.
Pinpointing the Source Through Diagnosis
Determining the specific cause of a hemifacial spasm relies heavily on advanced medical imaging. Magnetic Resonance Imaging (MRI) is the standard method used to visualize the facial nerve and the surrounding structures. The primary goal of imaging is to rule out a secondary cause, such as a tumor or demyelination, before confirming neurovascular conflict.
Specialized 3D high-resolution MRI sequences are employed for detailed visualization of the nerve and vessels. These sequences, often called Constructive Interference in Steady-State (CISS) or Fast Imaging Employing Steady-state Acquisition (FIESTA), provide clear images of the cranial nerves and surrounding cerebrospinal fluid. They highlight the precise anatomical relationship between the facial nerve and any adjacent blood vessel.
Clinicians may also use Magnetic Resonance Angiography (MRA) to better visualize the arteries and confirm which specific vessel is causing the compression. Electromyography (EMG) is sometimes used as a supplemental test to confirm the hyper-excitability of the facial nerve. These advanced diagnostic techniques allow doctors to accurately pinpoint the cause.