By Edward W. Pegg III, MD
Nowadays, most people have seen intrauterine pictures of a “soon-to-arrive” niece, nephew, child, or grandchild. These ultrasound pictures are now good enough to show a three-dimensional image of the infant’s face well before the child is born. Ultrasound uses sound waves at the high end of the spectrum that we are unable to hear. This is actually the same principle used by bats. Bats emit high-frequency sound waves that bounce back from objects or prey. While ultrasound scans, also known as sonograms, are a relatively new way to examine internal organs and detect medical problems, bouncing sound waves to get information has been used for many years. Ultrasonic waves were used in World War II to find submerged submarines.
Medical ultrasound produces waves by using a transducer that is run along the skin. The waves, which are harmless to the tissues, penetrate down until encountering a change in tissue density. This density change causes some of the waves to bounce back to the transducer, which picks up these waveforms. The intensity of the returning signals and the elapsed time for their return is fed into a computer, which can then generate an image.
Over the years, continued refinements in imaging have been made. With these improvements in technology, smaller and smaller resolutions have become possible. Resolutions down to 0.006 inch are now possible with ultrasound. This is three times better than the resolution of MRI. As image resolution has improved, we have found additional uses that were only dreamt of a few years ago.
One of the new uses is in the field of neuromuscular disorders. Peripheral nerve disorders are a common problem. Most people have likely heard of carpal tunnel syndrome or a pinched nerve. For a number of years, we have been able to test the integrity of the nerve physiologically through an electromyogram (EMG). This is a test that gives small shocks on the skin’s surface, which activates the underlying nerve and tells us how healthy it is. However, this test does not visualize the nerve, and the absence of a signal cannot determine whether the nerve has died or is damaged but still alive. Also, electrophysiologic studies cannot determine the site of the lesion along the course of the nerve.
Ultrasound, with its improved sensitivity, can now be used to look at the nerves directly in the extremities. The nerves in the arms can now be followed from the neck down to the hand. Tumors or other masses can be easily visualized. Some nerves run close to the bones and can be damaged with bone fracture. EMG can show damage but cannot determine if the nerve has been lacerated by the bone splinters or just badly bruised. Through ultrasound, the length of the nerve can be followed and the integrity of the nerve versus separation of the nerve can be easily seen. These changes can help determine whether surgery is needed to restore the nerve or whether conservative management can be continued. Intra-ganglion cysts, Morton neuroma, degenerative joint disease, and vascular malformation are other causes of nerve entrapment or injury that can be found with ultrasound. Ultrasound can also see changes in muscle, which can give indirect information about the nerve.
Ultrasound is rapidly becoming one other important diagnostic test that the neurologist has in his armamentarium to better diagnose and treat his patients.
For more information on any neurological issue, including sports neurology and concussion management, you may contact Dr. Pegg at 309-661-7344.