JUNTA ADMINISTRATIVA DEL REGISTRO NACIONAL

The approach to peripheral neuropathy should include sensory and motor NCS in at least one arm and one leg. Motor and sensory nerves in each limb should be tested, preferably in at least two segments, a more proximal one as well as a distal one. EMG should be done in at least one proximal and one distal muscle in each limb. This allows an

opportunity to gauge the extent of the axonal involvement and also to look for conduction block and any degree of demyelination. In the western hemisphere the most common cause of neuropathy is diabetes mellitus. Diabetes can cause a wide range of

abnormalities including axonal neuropathy, demyelinating neuropathy and a mixed picture. Electromyography is sensitive and helpful in identifying acute and chronic denervation changes associated with axonal involvement in neuropathy. It should be considered an essential part of a complete electrophysiological assessment.

Generalized neuropathies can be primarily axonal, primarily demyelinating, or a combination of both.

Mononeuropathy is extremely common in clinical practice. While almost any nerve can suffer entrapment or injury at one time or another, some are seen more commonly than others and we will confine our discussion to those most likely to be encountered in the typical electrophysiology laboratory.

Median nerve

Median motor NCS is performed by placing the active recording electrode on the

midbelly of the APB over the thumb. The reference is placed 2 cm distal. The cathode for distal stimulation is 7 cm proximal to the active recording electrode near the wrist crease. The proximal site is over the median nerve proximal to the antecubital fossa. In both instances, the anode is placed 2 cm proximal to the cathode.

Median sensory NCS is performed by recording from one of the fingers, usually the index, with ring electrodes while stimulating 13 cm proximally, above the wrist crease. The stimulating and recording electrodes can be reversed, and the results may differ slightly depending on the direction of conduction.

The most common entrapment neuropathy seen in our laboratory is Carpal Tunnel Syndrome, (CTS) which is a distal median neuropathy within the carpal tunnel at the wrist. Clinical diagnosis is usually straightforward, and the characteristic

electrophysiological findings no less so. The most common finding is slowing of distal sensory NCV and/or prolonged median nerve terminal latency, in the absence of

abnormalities in other tested nerves. It is commonly bilateral and usually worse in the dominant hand. Denervation in the ABP muscle is sometimes seen in severe cases and warrants more aggressive treatment typically consisting of surgical decompression of the nerve in the carpal tunnel.

Ulnar nerve

Ulnar motor NCS is performed by placing the recording electrode on the midbelly of the ADM with the

reference electrode 2 cm distal. Distal stimulation is delivered to the ulnar nerve 7 cm proximal to the active recording electrode, overlying the ulnar nerve in the distal forearm. Proximal stimulation is performed usually in two locations: below the ulnar groove at the elbow, and 10 cm proximal to that point. This provides two complete segments for

assessment of nerve conduction. Ulnar sensory NCS is performed by recording from digit 5 using ring electrodes and stimulating the ulnar nerve in the distal forearm 11 cm proximal to the active recording electrode.

Ulnar neuropathy at the elbow is another commonly seen

entrapment syndrome. Slowing across the elbow in motor NCV and abnormalities in the sensory NCS are the usual findings. Occasionally, the lesion can be primarily axonal in nature and very little slowing is seen but the EMG shows denervation in the ulnar- innervated hand intrinsic muscles. Denervation is not seen in the flexor carpi ulnaris because the nerve branch innervating this muscle leaves the ulnar nerve proximal to the elbow.

Radial nerve

Motor NCS is performed by recording from the extensor indicis. The active recording electrode is placed over the belly of the muscle and the reference is placed 2 cm distal. The distal stimulation site is in the forearm, between the ECU and extensor digitorum

Figure 8-10: Median nerve anatomy.

Anatomy of the median nerve with important muscles shown.

Figure 8-11: Ulnar nerve anatomy.

minimi, 10 cm proximal to the styloid process. Proximal stimulation is just proximal to the antecubital fossa, between the biceps tendon and the brachioradialis. Sensory NCS is performed by recording from the superficial sensory branch on the dorsum of the hand while stimulating from one of the more proximal sites, listed just above.

Radial nerve entrapment or damage in the spiral groove in the humerus is not rare and typically follows a night of

overindulgence. This is the so-called Saturday Night Palsy. Wrist drop and finger drop are seen along with loss of sensation in the distribution of the radial sensory nerve. Abnormalities in radial sensory NCS are seen and in motor NCS. The amount of denervation in radial muscles seen on EMG is variable. This entity is easily distinguished from C7/C8 radiculopathy by demonstrating normal function in median and ulnar-innervated muscles.

Peroneal nerve

Peroneal motor NCV is usually

performed by recording from the EDB. Distal stimulation is in the lower leg, adjacent to the tendon of the TA. Proximal stimulation is at the fibular neck. When the nerve is thought to be injured across the fibular neck, proximal stimulation is then performed in the popliteal fossa. The NCV across the fibular neck is compared to the NCV distal to the neck. A difference of greater than 10 m/sec is abnormal.

Peroneal nerve entrapment at the fibular head producing foot drop is the most common entrapment syndrome of the lower extremity seen in our laboratory. Slowing of peroneal NCV at the fibular head is the classic finding. Denervation in tibialis anterior and peroneus muscles is variably seen. Care must be exercised to exclude the possibility of L5 radiculopathy and sciatic neuropathy. In the case of the former, denervation and weakness in posterior tibialis and hamstring muscles is seen in addition to findings in muscles supplied by the common peroneal nerve. In Sciatic neuropathy, an abnormal EMG in the biceps femoris short head is sufficient to establish a more proximal localization of injury.

Figure 8-12: Radial nerve anatomy.

Anatomy of the radial nerve with important muscles shown.

Figure 8-13: Peroneal nerve anatomy

Anatomy of the peroneal nerve with important muscles shown.

Tibial nerve

The tibial nerve innervates the medial and lateral gastrocnemius and soleus muscles and supplies sensation to those portions of the sole and dorsolateral foot that are not served by sural and superficial peroneal nerves. The tibial nerve also innervates most of the intrinsic muscles of the foot via the medial and lateral plantar nerves. Tibial sensory NCVs are rarely performed and will not be discussed. Motor NCVs are performed by recording from the belly of the abductor hallicus muscle on the medial aspect of the foot. Distal stimulation is delivered to the nerve as it passes behind the medial epicondyle, and proximal stimulation is delivered in the popliteal fossa.

Sural nerve

The sural nerve is the only purely sensory nerve in the leg that is tested routinely. The sural is formed in the midcalf by the joining of branches from both the peroneal and tibial nerves. Sural sensory conduction is recorded from the nerve behind and slightly inferior to the lateral epicondyle. The stimulation site is on the posterior surface of the leg, 14 cm proximal to the recording site. There are no definite landmarks for the site of proximal stimulation, so some hunting may be needed. For most other NCS, the ground is placed proximal to the stimulating electrode to minimize the potential for current to pass through the body,

however for sural NCS, the ground is often placed between the stimulating and recording electrodes to minimize noise.

Sciatic nerve

Sciatic neuropathy is seen less commonly but often can masquerade as peroneal neuropathy. In most sciatic nerve injuries the peroneal division bears the brunt of the injury with relative sparing of the posterior tibial division. The reason for this is unclear. EMG revealing denervation in the distribution of both divisions of the sciatic nerve is helpful in establishing this diagnosis. NCS are not performed on the sciatic nerve, per se, but rather on the individual divisions: peroneal and tibial.

Femoral nerve

Figure 8-14: Tibial nerve anatomy

Anatomy of the tibial nerve with some important muscles shown.

The femoral nerve is supplied by roots from the lumbar plexus, and L2-L4 make up most of the innervation. NCS of the femoral nerve can be technically challenging, so most of the diagnosis rests on EMG. Denervation in the quadriceps suggests a femoral

neuropathy.