Hemifacial spasm (HFS) is a neurological disorder characterized by involuntary, repetitive contractions of the muscles on one side of the face. Spasms typically begin subtly around the eyelid, often causing brief, intermittent closure of the eye (blepharospasm). Over time, the spasms increase in frequency and intensity, spreading downward to involve the cheek, mouth, and sometimes the neck. The condition is almost always unilateral, affecting only the facial nerve (Cranial Nerve VII).
The Primary Cause: Vascular Compression
The overwhelming majority of hemifacial spasm cases (over 90%) result from a physical interaction between a blood vessel and the facial nerve, known as neurovascular compression. The point of vulnerability is where the facial nerve exits the brainstem, called the root exit zone (REZ). At this site, the nerve transitions from its central nervous system covering to a peripheral sheath. This transitional zone lacks the protective layers found elsewhere, making it highly susceptible to irritation from external pressure.
The vessel responsible is most commonly an artery, such as the Anterior Inferior Cerebellar Artery (AICA) or the Posterior Inferior Cerebellar Artery (PICA), although sometimes a vein can be the culprit. As the heart beats, the adjacent artery pulsates, and this repetitive, mechanical pressure against the unprotected nerve root causes chronic irritation and structural change within the nerve fibers.
The Neurological Mechanism of Involuntary Spasm
The physical compression of the facial nerve at the root exit zone leads to nerve damage. Mechanical irritation causes the protective coating, the myelin sheath, to wear away. Myelin normally acts as an insulator, ensuring electrical signals travel cleanly along a single nerve fiber.
When the myelin is damaged, electrical impulses can aberrantly jump, or “cross-talk,” to adjacent nerve fibers. This short-circuiting is termed ephaptic transmission. An impulse intended for one facial muscle can leak to a neighboring fiber, causing an unintended muscle to contract simultaneously. This malfunction results in the spontaneous, rhythmic firing of the facial nerve, manifesting as visible, uncontrollable twitching.
Less Common Non-Vascular Etiologies
While microvascular compression accounts for the vast majority of cases, a small percentage of hemifacial spasms are caused by other, less common etiologies. These secondary causes typically involve a mass lesion or condition that directly compresses or damages the facial nerve along its course.
Mass Lesions and Structural Anomalies
Mass lesions include benign tumors, like an acoustic schwannoma or meningioma, or a cyst in the cerebellopontine angle. Structural anomalies, such as an aneurysm or an arteriovenous malformation, can also press on the nerve.
Other Causes
The facial nerve can also be damaged by trauma to the head or face. In rare instances, demyelinating diseases of the central nervous system, such as Multiple Sclerosis, can form plaques along the nerve pathway, leading to nerve irritation and spasm.
Identifying the Cause Through Medical Imaging
Confirming the cause of hemifacial spasm requires specialized medical imaging, primarily Magnetic Resonance Imaging (MRI). MRI is the preferred tool because its high-contrast resolution allows for the simultaneous visualization of the cranial nerves and surrounding blood vessels. High-resolution 3D MRI sequences, such as FIESTA or CISS, are used.
These advanced sequences are sensitive enough to detect subtle contact or compression of the facial nerve by a blood vessel, which might be missed on standard scans. Imaging also rules out less common causes, such as a tumor or cyst.
In addition to imaging, Electromyography (EMG) is sometimes used to verify the diagnosis by measuring the electrical activity of the facial muscles. This test reveals the characteristic hyperactivity and abnormal muscle response, known as the lateral spread response, confirming the nerve’s malfunction.