A 52-year-old man presents with sudden-onset, severe, lancinating pain in the right side of his face that lasts for a few seconds to 2 minutes and is triggered by touching his cheek while shaving. Neurological examination is otherwise normal. Which is the most common site of compression of the trigeminal nerve causing this clinical presentation?

Explanation

## Trigeminal Neuralgia: Pathophysiology and Compression Sites **Key Point:** Trigeminal neuralgia (TN) is characterized by sudden, brief, severe, lancinating facial pain triggered by light touch (allodynia) in the distribution of one or more divisions of the trigeminal nerve. ### Most Common Cause: Vascular Compression The most common cause of trigeminal neuralgia is **neurovascular compression** at the root entry zone (REZ) of the trigeminal nerve as it exits the brainstem. **High-Yield:** The **superior cerebellar artery (SCA)** is the most frequent offending vessel, followed by the anterior inferior cerebellar artery (AICA) and vertebral artery. The SCA typically compresses the trigeminal nerve at the REZ in the cerebellopontine angle region. ### Pathophysiology of Vascular Compression 1. Chronic compression of the trigeminal nerve root by an artery causes demyelination at the REZ 2. Loss of myelin allows ectopic impulse generation and ephaptic transmission (cross-talk between adjacent nerve fibers) 3. This results in the characteristic sudden, brief, severe pain with trigger zones 4. The compression is typically ipsilateral to the pain ### Sites of Trigeminal Nerve Compression | Site | Vessel | Frequency | Clinical Notes | | --- | --- | --- | --- | | Root entry zone (REZ), cerebellopontine angle | SCA (most common), AICA, VA | ~80–90% of TN cases | Demyelination leads to ectopic firing | | Brainstem (intramedullary) | AICA | Rare | Usually associated with other brainstem signs | | Cerebellopontine angle (tumor) | N/A | ~5% of TN cases | Acoustic neuroma, meningioma, epidermoid | | Foramen ovale / Meckel's cave | Tumor, arteriovenous malformation | Rare | Secondary TN | **Clinical Pearl:** The REZ is the transition zone between the central nervous system (CNS) myelin—produced by oligodendrocytes—and the peripheral nervous system (PNS) myelin—produced by Schwann cells. This zone is particularly vulnerable to demyelination from chronic compression. ### Diagnostic Confirmation - **MRI with constructive interference in steady state (CISS)** or **3D-FIESTA sequences**: Can visualize the offending vessel compressing the trigeminal nerve - **Microvascular decompression (MVD)**: Gold standard surgical treatment, with cure rates of 70–90% in idiopathic TN **Mnemonic: SCA-AICA-VA** — the three vessels most commonly compress the trigeminal nerve root entry zone, in order of frequency.