Trigeminal neuralgia (TN) is characterized by sudden, severe, electric shock-like facial pain originating from the fifth cranial nerve, which is responsible for sensation in the face. This intense, episodic pain is often triggered by simple actions like chewing or talking. The most common cause is the compression of the nerve by a blood vessel near where the nerve exits the brainstem. Magnetic Resonance Imaging (MRI) is the standard method for investigating the cause, providing high-resolution images of the nerve and surrounding structures.
Why Imaging is Essential for Diagnosis
The primary function of an MRI for facial pain is to perform a differential diagnosis. While symptoms are the main factor for diagnosing TN, imaging is required to rule out other pathologies that can mimic the condition. The MRI is the most effective way to identify these secondary issues.
The scan must exclude secondary causes, which account for a minority of TN cases but are important to identify for proper treatment. These causes include tumors, such as meningiomas or schwannomas, which may press on the nerve. The MRI also looks for demyelinating diseases, like Multiple Sclerosis (MS), which can create plaques that affect the trigeminal nerve root entry zone.
Specialized MRI Techniques
A standard brain MRI is often insufficient for clearly visualizing the trigeminal nerve, which is a small structure located deep within the skull. Therefore, specialized high-resolution sequences are necessary to provide the required detail and contrast for proper assessment. These techniques, often referred to as heavily T2-weighted steady-state sequences, create a distinct contrast between the fluid, nerve, and vessels.
Techniques like Fast Imaging Employing Steady-state Acquisition (FIESTA) or Constructive Interference in Steady State (CISS) are commonly used. These sequences utilize the high signal intensity of cerebrospinal fluid (CSF) to make the nerve and adjacent structures appear bright against a dark background. This high-contrast resolution allows radiologists to visualize the cisternal segment of the trigeminal nerve, where compression most frequently occurs, in slices often less than one millimeter thick. The use of magnetic resonance angiography (MRA) sequences is often combined with these specialized scans to better delineate the arteries and veins near the nerve.
Identifying the Source of Pain
The most common structural finding the MRI seeks to identify is Neurovascular Compression (NVC), which is believed to cause 80% to 90% of classic TN cases. NVC occurs when an artery or vein is in direct contact with and distorts the trigeminal nerve near the brainstem, at the root entry zone (REZ). The superior cerebellar artery (SCA) is the vessel most frequently implicated, accounting for 60% to 90% of cases.
The MRI allows clinicians to grade the severity of the contact, which can range from simple touching of the nerve to significant indentation and displacement. Evidence of nerve distortion or a change in the nerve’s shape due to the vessel’s pressure is considered a more significant finding than mere contact. The MRI also confirms secondary causes by identifying small tumors, cysts, or demyelination plaques, particularly if NVC is absent.
Guiding Treatment Decisions
The detailed anatomical information provided by the MRI directly influences the choice of treatment. The presence and precise location of NVC is a deciding factor for surgical intervention. When a clear, significant compression is identified, Microvascular Decompression (MVD) surgery is often the preferred option, as it aims to physically move the offending vessel away from the nerve.
The MRI findings guide the neurosurgeon’s approach by mapping the exact vessel and point of contact, improving surgical planning and outcomes. If the scan confirms NVC is absent, or if the patient is not a candidate for surgery, the MRI results support non-invasive options. The information helps guide treatment toward Stereotactic Radiosurgery (SRS) or medication management by ruling out a surgically correctable cause.