The sciatic nerve is the largest single nerve in the human body, formed by nerve roots from the lower spine. It travels from the lower back through the hips and buttocks, and then branches down the back of each leg to the foot. Sciatica is pain that radiates along the path of this nerve, typically caused by the compression or irritation of one of the spinal nerve roots. Testing for sciatic nerve problems determines the source of the compression, the severity of the nerve involvement, and the most effective course of treatment.
Initial Patient History and Neurological Screening
The diagnostic process begins with gathering the patient’s detailed history of the pain. The physician asks specific questions about the pain’s exact location, intensity, and whether it follows a distinct path, particularly radiating below the knee into the calf or foot. Patients are also asked about factors that worsen or alleviate the symptoms, such as sitting, standing, or walking, to provide clues about the underlying cause.
History-taking is followed by a basic neurological screening to check for objective signs of nerve compromise. The clinician tests deep tendon reflexes (Achilles, S1; patellar, L4) to assess nerve signal integrity. Muscle strength is evaluated through specific movements, like lifting the big toe (L5) or walking on toes and heels, which helps localize the affected spinal nerve root. Sensation is checked across different skin areas (dermatomes) of the leg and foot to identify areas of numbness or tingling corresponding to a specific nerve root distribution.
Standard Physical Examination Maneuvers
Specific physical movements are used to gently stretch the nerve and reproduce the patient’s characteristic pain. These provocative tests help confirm if the symptoms are truly nerve-related and not simply a result of muscle or joint pain. The Straight Leg Raise (SLR) test, also known as Lasegue’s sign, is the most common maneuver used.
For the SLR test, the patient lies on their back while the clinician slowly raises the affected leg with the knee kept straight. A positive result is indicated if the classic radiating leg pain is reproduced when the leg is elevated between approximately 30 and 70 degrees. Pain reproduced in this range suggests tension on the L4, L5, or S1 nerve roots, typically involved in sciatica, often due to a herniated disc.
Another common assessment is the Slump Test, which sequentially positions the patient to place maximum tension on the entire spinal nerve system. The patient sits with hands behind their back, slumps the trunk forward, and brings the chin to the chest. The clinician then slowly straightens the knee and dorsiflexes the ankle of the affected leg. If this sequence of movements reproduces the patient’s radiating pain, it strongly suggests tension or compression of the sciatic nerve pathway.
Assessing Structural Causes Through Diagnostic Imaging
If the patient’s symptoms are severe, persist beyond six to eight weeks of conservative care, or if certain warning signs are present, the physician may order diagnostic imaging. This technology visualizes the underlying physical cause of the nerve compression, rather than diagnosing sciatica itself. Conventional X-rays identify bony abnormalities, such as bone spurs, spinal misalignment, or spondylolisthesis.
However, X-rays cannot visualize soft tissues like intervertebral discs or the nerve roots themselves. Magnetic Resonance Imaging (MRI) is often the preferred imaging method when a structural cause is suspected. The MRI uses magnetic fields and radio waves to create detailed cross-sectional images of the spine, providing a clear picture of the spinal cord, nerve roots, and soft discs. An MRI can reveal common causes of sciatica, such as a disc herniation or spinal stenosis, by showing the precise point where the nerve root is being compressed.
Measuring Nerve Function with Electrodiagnostic Tests
Specialized electrodiagnostic tests objectively measure how well the nerves are functioning and differentiate between a nerve root problem and a peripheral nerve disorder. These tests are typically reserved for cases where the diagnosis remains unclear or when the extent of nerve damage needs to be quantified. They primarily consist of the Nerve Conduction Study (NCS) and Electromyography (EMG).
The Nerve Conduction Study measures the speed and strength of electrical signals traveling along the nerve using electrodes placed on the skin. A slowed or weak signal can indicate damage to the protective coating of the nerve. Electromyography (EMG) involves inserting fine needle electrodes into specific muscles to record their electrical activity at rest and during contraction. EMG helps determine if the muscle is receiving adequate electrical signals from the nerve and can pinpoint the exact spinal nerve root level (L4, L5, or S1) that is injured. NCS and EMG together provide an objective assessment of the severity and location of nerve root dysfunction.