Radiologic Studies in Compression Neuropathy

MONONEUROPATHY: DIAGNOSTIC STUDIES

Sometimes clinical neurologic examination is not precise enough to provide early diagnosis of mononeuropathies. Electrodiagnostic studies are the method of choice for defining the precise anatomic distribution of peripheral nerve damage. This includes nerve conduction studies (NCS) and needle EMG. Thus it is possible to assess the quality of peripheral nerve conduction as well as whether there is damage to muscles specifically innervated by that nerve. NCS allows identification of the site of nerve damage whenever the nerve’s myelin is chronically damaged. Examples include chronic ligamentous thickening over the carpal tunnel (see Plate 5-10), sudden sustained acute pressure over the radial nerve at the humerus (see Plate 5-13), or fibular (peroneal) compression at the knee (see Plate 5-19).

Early signs of a carpal tunnel syndrome are best defined by sensory NCS, and later motor NCS, demonstrating prolongation of the time for nerve conduction across the wrist (distal latency). Motor NCS are especially useful for defining more proximal nerve blocks, that is, at the elbow (ulnar nerve), midhumerus (radial nerve), and knee (fibular [peroneal] nerve). This leads to a diminution of the compound muscle action potential (CMAP) just above the site of nerve block (see Plate 5-3). Conduction slowing provides another means to identify a nerve block. Here there is focal motor NC slowing (by 30%-40%, i.e., 10-20 m/sec) at the site of anatomic compromise, that is, across the fibular head for the fibular (peroneal) nerve.

Needle EMG also may provide a precise means to specifically define the affected muscles. When the nerve’s axon is partially damaged, spontaneous firing of small muscle fibers occurs. These are known as denervation potentials, that is, fibrillation potentials and positive waves. Similarly, when the nerve lesion leads to significant damage, there is a diminution in the number of motor units firing. The healthy remaining motor units (MUPs) attempt to compensate by reinnervation; this results in larger MUPs that are recruited at increased frequency.

Patients with a footdrop secondary to a fibular (peroneal) nerve lesion demonstrate denervation signs confined to fibular-innervated muscles. However, if the footdrop is secondary to an L5 root lesion, signs of denervation are demonstrated not only in fibular (peroneal) muscles but also L5 muscles innervated by both peroneal and posterior tibial nerves. These include the posterior tibial, the gluteus medius, and the lumbosacral paraspinal musculature. Thus by combining NCS and EMG, the electromyographer has the ability to literally map the precise pathoanatomy of the nerve lesion.

Magnetic Resonance Imaging. MRI studies provide an increasingly used means to evaluate for occult tumors or congenital lesions (Plate 5-4). Very rarely, certain congenital or acquired lesions, such as fibrous bands, may entrap the nerve without MRI identification. In this instance, surgical exploration based on the clinical and EMG findings may lead to a diagnosis.

Ultrasound. This modality is gaining an increased presence in some centers for more rapid identification of sites of nerve compression or entrapment.

Skeletal Radiograph. Rarely, bony abnormalities entrap a peripheral nerve. Examples include popliteal fossa bony exostoses entrapping the tibial nerve. The sciatic nerve is rarely entrapped at the pelvic ischium in babies.

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