A “pinched nerve,” clinically known as radiculopathy, describes the irritation or compression of a nerve root as it exits the spinal column. This compression causes pain, numbness, or weakness in the area the nerve supplies, such as an arm or leg. While a Magnetic Resonance Imaging (MRI) scan cannot capture the instantaneous event of a nerve being actively squeezed, it is the primary tool for visualizing the underlying physical cause. MRI provides detailed images of soft tissues and bony structures, allowing clinicians to see the secondary effects of compression, such as inflammation, and pinpoint the mechanical problem responsible for the symptoms.
What MRI Reveals About Nerve Compression
Magnetic Resonance Imaging uses a powerful magnetic field and radio waves to generate cross-sectional images of the spine and surrounding tissues. This process yields exceptional contrast between soft tissues like nerves, discs, and ligaments, making it superior to X-rays or CT scans for evaluating radiculopathy.
The value of the MRI is its ability to identify the compressing structure and signs of nerve irritation itself. The scan can reveal secondary indications of nerve damage, such as swelling or inflammation, medically termed edema, within the nerve root. Chronic compression can cause the nerve root to thicken or enhance differently on the scan, which correlates with a patient’s reported symptoms. This ability to show both the physical impingement and the tissue’s inflammatory response makes MRI the preferred method for structural assessment.
Common Structural Causes Visible on MRI
MRI excels at detailing anatomical changes in the spine that reduce the space available for a nerve root. The most frequent mechanical problem visualized is a herniated or bulging intervertebral disc, where the soft, gel-like center pushes out against the nerve root. A disc protrusion or extrusion physically occupies the narrow space of the spinal canal or the neural foramen, directly squeezing the adjacent nerve.
Another common cause visible on the scan is spinal stenosis, a general narrowing of the bony spinal canal itself. This narrowing can be caused by degenerative changes, such as the thickening of the ligaments and joint capsules that line the canal. Degeneration often involves the growth of bone spurs, known as osteophytes, which are readily apparent on MRI. These bony growths can project into the small openings where nerves exit the spine, causing foraminal narrowing and subsequent nerve compression.
Functional Testing: Complementing Imaging Results
While MRI is invaluable for mapping the anatomy, it only provides a static, structural image. Therefore, it is often complemented by functional diagnostic tests. Electromyography (EMG) and Nerve Conduction Studies (NCS) measure the electrical activity of the nerve and muscle, offering objective evidence of signal transmission. An NCS assesses the speed and strength of electrical impulses, while an EMG evaluates the electrical activity within the muscles supplied by that nerve.
These functional tests are important because structural findings on an MRI do not always correlate with a patient’s symptoms. It is possible for an MRI to show a significant disc herniation that is not causing pain or nerve dysfunction. Conversely, a patient can experience severe radicular pain with an MRI that appears relatively normal, indicating a functional problem. By measuring the functional integrity of the nerve, EMG and NCS confirm the precise location and severity of the nerve injury, providing a complete picture alongside the structural map.