A pinched nerve, also called nerve compression syndrome or compressive neuropathy, occurs when increased pressure on a nerve results in decreased nerve function. A common cause of a pinched nerve is a herniated disk in the spine pressing on an adjacent nerve. Other causes include bone spurs, swelling, scar tissue, and tumors. A pinched nerve can occur at various locations in the body, such as in the spine, wrist, or elbow.
Symptoms of a Pinched Nerve
Signs and symptoms of a pinched nerve depend on the location and severity of the nerve compression. Common symptoms include:
- Numbness or tingling in the distribution of the affected nerve
- Sharp, shooting, or burning pain in the distribution of the affected nerve
- Muscle weakness in the region supplied by the affected nerve
- Reflex changes (enhanced or diminished) in the area corresponding to the compressed nerve
For example, a pinched nerve in the neck can cause numbness, tingling, or pain in the shoulder, arm, or hand. A pinched nerve in the lower back can radiate pain down the leg. The specific signs and symptoms help identify the location of the pinched nerve.
Diagnosing a Pinched Nerve
To diagnose a pinched nerve, physicians obtain a medical history and perform a physical exam to evaluate the symptoms. They check for sensory deficits, muscle weakness, and changes in reflexes that may indicate nerve compression.
Some common diagnostic tests for a pinched nerve include:
- X-ray: Can identify bone spurs or arthritis that may be contributing to nerve compression.
- MRI: Provides detailed images of soft tissues like herniated disks, tumors, or nerves.
- CT myelogram: CT scan done after injecting contrast dye around the spinal cord to outline the spinal cord and nerves.
- Electromyography (EMG): Records electrical activity of muscles and can determine if there is nerve damage.
- Nerve conduction study: Measures how fast electrical signals move through a nerve. Slowed conduction indicates nerve compression.
Using MRI to Diagnose a Pinched Nerve
An MRI scan is often done to diagnose a pinched nerve. It provides clear images of soft tissues like the spinal discs, spinal cord, nerves, muscles, and ligaments, making it an excellent test to visualize a compressed nerve.
MRI can accurately show the cause and location of a pinched nerve, such as:
- Herniated disc in the spine
- Enlarged facet joints causing spinal stenosis
- Tumor or cyst
- Nerve entrapment in the wrist or elbow
MRI does not use ionizing radiation like X-rays or CT scans, making it a safe imaging test. It may require the injection of a contrast dye called gadolinium to improve visualization of certain tissues. This is called an MRI with contrast.
Can an MRI without contrast detect a pinched nerve?
Yes, an MRI scan without contrast dye can effectively show a pinched nerve in most cases. The high resolution and excellent soft tissue delineation of MRI allows clear visualization of nerves and surrounding structures without the need for contrast enhancement.
MRI without contrast is generally the first line imaging test ordered for suspected nerve compression. The unenhanced scan acquires multiple sequences that can adequately assess the nerves, spinal cord, intervertebral discs, vertebrae, and extra-spinal space.
The MRI sequences most useful for visualizing a pinched nerve without contrast include:
- T1-weighted images to evaluate anatomy and distinguish tissues based on water content
- T2-weighted images to visualize spinal cord and nerves surrounded by cerebrospinal fluid
- Short tau inversion recovery (STIR) images to suppress fat and fluid signals and highlight areas of inflammation or edema
These standard MRI sequences can pinpoint the site of nerve compression from a herniated disc, bone spur, thickened ligament or other mass lesion. A radiologist looks for evidence of the nerve being displaced, distorted or flattened which indicates pressure on the nerve.
Benefits of MRI without contrast for pinched nerves
Some benefits of using MRI scanning without contrast dye to diagnose a pinched nerve include:
- Avoids risks associated with contrast agents, such as allergic reactions or kidney problems
- Non-invasive and does not use ionizing radiation
- Usually sufficient for visualizing nerve compression without the need for contrast enhancement
- Typically lower cost than an MRI with contrast
Limitations of MRI without contrast
There are a few limitations to MRI scanning without contrast for detecting a pinched nerve:
- Cannot see enhancement or inflammation of involved tissues
- Less effective for evaluating nerve roots exiting the spinal cord
- Lower sensitivity if compression is subtle or from smaller masses
- Less useful in patients with metal implants or claustrophobia using lower strength MRI
In some cases, the radiologist may recommend an MRI with contrast if the initial non-enhanced scan is inconclusive or if more detail is needed. The addition of contrast dye can better delineate the borders of nerve roots, highlight subtle areas of inflammation or edema, and detect smaller masses.
When is contrast needed for pinched nerves?
While an MRI without contrast is usually adequate, there are some situations where using intravenous contrast dye for a pinched nerve is helpful:
- Small disc herniation or masses – Contrast enhancement can better detect subtle or small herniated discs, tumors, scar tissue, ganglion cysts, or blood vessels putting pressure on a nerve.
- Nerve root disorders – An MRI with contrast is more sensitive for disorders involving the nerve roots as they exit the spinal cord.
- Inconclusive non-enhanced MRI – If the initial MRI is non-diagnostic but symptoms strongly suggest a pinched nerve, a follow-up study with contrast may be warranted.
- Surgical planning – Contrast MRI provides precise detail before surgery to decompress a nerve.
- Checking treatment – Can verify if enhancing lesions causing nerve compression have responded to treatment.
Some of the contrast-enhanced MRI sequences used to further evaluate a pinched nerve include T1-fat saturation images and T1 post-contrast images.
Risks of MRI contrast agents
Intravenous MRI contrast dyes can rarely cause side effects such as:
- Headache, nausea, feeling of coldness at injection site
- Hives, itching
- Anaphylactic reaction with severe allergy symptoms
- Nephrogenic systemic fibrosis in patients with kidney failure
Patients are screened ahead of time to avoid giving contrast to those at risk. The majority of people tolerate contrast without problems. Using a very low dose contrast protocol may reduce risks further.
In most circumstances, an MRI scan without contrast is adequate to diagnose a pinched nerve accurately. It can reliably show compression or distortion of nerves from common causes like disc herniations or bony enlargement. Contrast enhancement provides supplemental information in select cases but is not essential in routine evaluation.
MRI without contrast offers a sensitive, non-invasive way to evaluate nerve compression while avoiding risks of contrast agents. This makes it an appropriate first-line study in suspected pinched nerve pathology for most patients. If clinical suspicion is high despite a negative initial MRI, a follow-up scan with contrast can add further diagnostic details to reach a definitive diagnosis.