Neuromuscular Ultrasound of Polyneuropathy, Motor Neuron Disease, and Myopathy

Polyneuropathy

A variety of polyneuropathies have been studied with neuromuscular ultrasound. It is useful to assess peripheral nerves for their size, shape, fascicular pattern, vascularity, and distribution of abnormalities along the nerve to help determine the etiology of the polyneuropathy. Abnormalities in the size of peripheral nerves and particular distribution of peripheral nerves involved can aid in the differential diagnosis ( Fig. 19.1 ). In addition, peripheral neuropathies often result in denervation of the muscles they supply. Muscles that are chronically denervated show clear changes on ultrasound (see later). Thus, the pattern of muscles that are affected can be used to assess the pattern of a peripheral neuropathy (e.g., distal vs. generalized; symmetric vs. asymmetric, etc.)

Demyelinating Polyneuropathies

Nerve conduction studies are extremely sensitive for detecting conduction velocity slowing as seen in demyelinating polyneuropathies, as myelin is essential for the speed of nerve conduction. However, in some cases of polyneuropathy, nerve conduction studies cannot determine whether the polyneuropathy is due to demyelination or axonal loss for several reasons. In some cases, the polyneuropathy is so severe that most or all of the responses are absent. In other cases, the amount of slowing, although marked, is not in the unequivocal demyelinating range. In these cases, the slowing could have resulted from marked axonal loss, with loss of the fast- and medium-range conducting fibers. Lastly, in some cases of acquired demyelinating neuropathies, the demyelination may be limited to the very proximal nerves, plexuses, and roots, which are difficult to study with nerve conduction studies. It is in these situations that neuromuscular ultrasound can be particularly helpful in detecting a demyelinating polyneuropathy, since neuromuscular ultrasound is most useful in polyneuropathies that result in hypertrophic nerves, most of which are demyelinating.

In chronic demyelinating polyneuropathies, the recurring cycle of demyelination and remyelination leads to structural changes in the nerve that can easily be detected with ultrasound. Histologically, “onion bulbs” are seen that result from concentric layers of Schwann cell proliferation around axons. This proliferation results in thickening of the peripheral nerves and a hypertrophic neuropathy ( Fig. 19.2 ). In inflammatory demyelinating polyneuropathies, endoneurial edema and inflammatory infiltrates are also present, which may contribute to the nerve hypertrophy. Finally, the brachial plexus and major proximal nerves in the upper and lower extremities, which are not easily studied with nerve conduction studies and may be the only place where hypertrophic nerves are found, can be studied with ultrasound.