An ultrasound is a non-invasive imaging technique that uses high-frequency sound waves to create real-time pictures of the body’s internal structures. This technology allows medical professionals to visualize soft tissues like muscles, tendons, and organs without using radiation. One specific application is for examining nerves, including the median nerve. This nerve travels from the forearm through the wrist and into the hand, controlling sensation in the thumb and most fingers, as well as movement in some muscles at the base of the thumb.
Purpose of a Median Nerve Ultrasound
The primary reason a healthcare provider requests a median nerve ultrasound is to investigate symptoms associated with carpal tunnel syndrome. This condition arises from compression of the median nerve as it passes through a narrow passageway in the wrist called the carpal tunnel. An ultrasound can visually confirm if the nerve is being squeezed, helping to pinpoint the cause of symptoms like numbness, tingling, and pain in the hand and fingers.
Beyond carpal tunnel syndrome, this examination serves to identify other potential issues affecting the nerve. It can detect different nerve entrapment points, such as pronator teres syndrome, where the nerve is compressed in the elbow or forearm. The ultrasound is also effective in assessing physical trauma, showing if the nerve has been bruised, stretched, or even severed due to an injury.
The imaging is also capable of revealing structural abnormalities that might be the source of nerve compression. A sonographer can identify growths such as ganglion cysts, benign tumors, or other masses that are pressing on the median nerve. Visualizing these structures helps a physician understand the precise anatomical reason for a patient’s symptoms, which informs the appropriate course of treatment.
The Examination Process
Patients undergoing a median nerve ultrasound do not need any special preparation. The process begins with the patient seated or lying down with their arm resting on a table, palm facing up. A sonographer applies a small amount of warm, water-based gel to the skin over the wrist and forearm.
This gel helps eliminate air pockets between the skin and the ultrasound transducer, ensuring the sound waves are transmitted effectively into the body. The sonographer then gently presses a small, handheld device called a transducer against the skin. This device is moved along the path of the median nerve, from the forearm down to the palm, capturing live images that are displayed on a monitor.
The entire examination is painless. Patients may feel light pressure from the transducer, but there is no discomfort associated with the sound waves themselves. The process is also relatively quick, usually lasting between 15 and 30 minutes. Once the sonographer has captured all the necessary images, the gel is wiped off, and the patient can immediately return to their normal activities.
Interpreting the Images
When a radiologist or physician reviews the ultrasound images, they are looking for specific anatomical indicators of nerve health. A primary measurement is the nerve’s cross-sectional area, which is calculated at the entrance to the carpal tunnel. An enlarged area, often considered greater than 10 square millimeters, suggests swelling due to compression, a common indicator in carpal tunnel syndrome.
Changes in the nerve’s shape and appearance are also evaluated. A healthy nerve usually appears round or oval, but under pressure, it can become flattened at the site of compression. The imager will also assess the nerve’s echotexture, the pattern of bright and dark areas on the screen. A compressed nerve may appear darker than normal, a condition known as hypoechogenicity, indicating fluid buildup or changes in the nerve fibers.
The surrounding structures within the carpal tunnel are examined to identify any external causes of compression. The sonographer will look at the flexor tendons that share the tunnel with the nerve to check for inflammation or swelling. They will also look for any abnormal bowing of the flexor retinaculum, the ligament that forms the roof of the carpal tunnel, as this can indicate increased pressure within the space.
Comparison with Nerve Conduction Studies
A median nerve ultrasound is often used alongside another diagnostic tool, the nerve conduction study (NCS). The two tests provide different but complementary information about the nerve’s health. An ultrasound shows the nerve’s anatomy—its size, shape, and physical relationship to surrounding structures.
In contrast, an NCS measures the nerve’s function. During this test, small electrodes are placed on the skin over the median nerve, and a mild electrical impulse is sent through it. The study measures the speed and strength of this signal as it travels along the nerve. A slowing of the electrical impulse as it passes through the carpal tunnel indicates that the nerve is not functioning properly, which can confirm a diagnosis of carpal tunnel syndrome.
A physician may order one or both tests depending on the clinical situation. An ultrasound can identify a structural cause of compression, like a cyst, which an NCS would not detect. An NCS, on the other hand, can quantify the degree of functional impairment, which helps in grading the severity of the condition. The patient experience also differs; an ultrasound is painless, while an NCS involves small electrical shocks that some may find uncomfortable.