Electrodiagnostic Testing
Iliohypogastric, ilioinguinal, and genitofemoral nerves
Unfortunately, there are no reliable electrodiagnostic techniques for fully defining the integrity of the iliohypogastric, ilioinguinal, and genitofemoral nerves. Needle electromyography (EMG) of the lower abdominal musculature may serve as an adjunct in the diagnosis of iliohypogastric nerve injury. Although abdominal needle EMG may help determine the severity of ilioinguinal nerve injury, it is neither sensitive nor specific. A side-to-side sensory comparison study has been described for the genitofemoral nerve, but it is technically difficult to perform. [38]
Femoral nerve
Electrodiagnostic testing typically is performed for diagnosis of femoral nerve entrapment, but it is also important for determining the extent of the injury and the prognosis for recovery. With electrodiagnostic testing, either surface or needle electrodes lateral to the femoral artery in the inguinal region are used for stimulation. The stimulation can be performed above and below the inguinal ligament. Disk electrodes from the vastus medialis are used to record stimulation.
A sensory study of the saphenous nerve (continuation of the sensory portion of the femoral nerve over the medial aspect of the leg and ankle) may also be performed. Needle examination should be completed for the paraspinal muscles, as well as for the iliopsoas (also L2-3) and hip adductors supplied by the obturator nerve, to help distinguish root or plexus injury from peripheral nerve injury. Needle EMG is usually the most revealing portion of the test. The examiner must look not only for denervation potentials but also for any active motor units.
Lateral femoral cutaneous nerve
Electrodiagnostic testing may be performed for diagnosis. With nerve conduction studies, the technique includes using a bar electrode for recording and reference. This can be performed with either antidromic (conduction against the direction of sensory fiber conduction) or orthodromic (conduction in the direction of nerve conduction) methods. The antidromic study is usually easier to perform, though on occasion, response may be absent bilaterally. In obese patients, the response is small and difficult to obtain.
A needle stimulation electrode may be needed. The sensory response is absent in 71% of patients with meralgia paresthetica and is prolonged in 24% of patients with this condition. EMG test results with the needle are normal in patients with this diagnosis, which may help to differentiate it from an upper lumbar radiculopathy.
Technically, the sensory test is a difficult study, and a response must be present on the opposite side to determine entrapment. It may be nearly impossible to obtain a response in an obese patient or a patient with a large abdomen without using a needle for stimulation. Unfortunately, the test may be difficult for the patient to tolerate because of the large amount of current (with respect to more peripheral nerves) required to stimulate a nerve that lies under adipose tissue.
Saphenous nerve
No findings should be present on needle examination of the muscle during EMG. Needle examination should include the quadriceps and the adductor longus to assess for femoral and obturator nerve injury. If findings are present in both of these muscles, then paraspinal muscles definitely should be examined to rule out radiculopathy.
Obturator nerve
No routine conduction studies are available with which to evaluate the integrity of the obturator nerve, and the needle examination is the mainstay of testing with electrodiagnosis. Membrane instability (positive sharp waves and fibrillation potentials) will occur within 3 weeks of the nerve injury, and needle examination should be performed on patients with groin pain of longer than 3 months in whom this neuropathy is suspected. Complete injury results in a lack of active motor unit potentials.
Muscles from the quadriceps (femoral nerve), as well as the paraspinal muscles, must be examined and found to be normal before an obturator nerve injury can be diagnosed. In this manner, one must rule out a radiculopathy and a plexus injury as potential causes of the weakness in adduction during the electrodiagnostic examination.
Posterior tibial nerve
Electrodiagnostic tests are indicated in refractory cases or in cases where the diagnosis of tibial nerve injury is uncertain. A complete EMG and nerve conduction study of the motor and sensory nerves to the foot, with comparison to the other foot, is necessary. It is important for EMG examination to include motor latencies, particularly to the abductor digiti minimi and the abductor hallucis, when tarsal tunnel syndrome is suspected.
Kaplan and Kernahan reported that reduced amplitude and increased duration of the motor response are more sensitive indicators of tarsal tunnel syndrome than distal motor latency is. [39] Sensory action potentials may be affected in earlier stages than motor fibers are; therefore, changes may also be identified before any motor abnormalities. This is because sensory fibers are more susceptible to injury.
In addition, Kaplan and Kernahan believed that the lateral plantar branch of the tibial nerve probably is affected earlier than the nerve’s medial plantar branch is. Sensory studies are, therefore, considered to be the most sensitive studies for tibial nerve entrapment.
Galardi et al reported that after stimulation of the plantar nerves, the accuracy of the sensory-nerve action potential (SNAP) and the mixed-nerve action potential (MNAP) are almost the same. SNAPs are more sensitive and less specific, whereas MNAPs are less sensitive and more specific. Galardi et al concluded that the coexistence of MNAP and SNAP abnormalities, especially if asymmetric, is highly indicative of tarsal tunnel syndrome. The mixed-response test is technically much easier to perform and is better tolerated by many patients.
Approximately 90% of patients with tarsal tunnel syndrome have abnormal findings on EMG and nerve conduction velocity (NCV) studies. However, in the presence of supportive history and physical examination, a normal electrodiagnostic study does not exclude the diagnosis of tarsal tunnel syndrome. Electrodiagnostic tests, however, can be extremely helpful in diagnosing concomitant polyneuropathy, systemic disorders, and lumbosacral radiculopathy.
Positive results on electrodiagnostic tests are an affirmation of the diagnosis of tarsal tunnel syndrome. Golovchinsky reported a high incidence of double crush syndrome with overlapping of peripheral entrapment syndromes and signs of proximal nerve damage of the corresponding nerves (partial muscle denervation or abnormalities of the F wave). [40] In such cases, simultaneous treatment of the two problems may be indicated.
Common peroneal nerve
Electrodiagnostic evaluation is arguably the best method for assessing a potential peroneal nerve insult. It is clinically difficult to isolate and test the short head of the biceps, the evaluation of which is critical in determining whether a lesion is proximal to the knee and whether it involves the sciatic nerve, the lumbosacral plexus, or nerve roots. In patients with exercise-induced symptoms, electrodiagnostic tests should be performed before and after exercise. Such tests include sensory and motor conduction studies, as well as needle EMG.
Sensory conduction studies
A superficial peroneal SNAP is important, and an abnormality of the sensory evoked response implies that the lesion is distal to the dorsal root ganglion, though it does not completely rule out an L5 radiculopathy. A loss in amplitude of this response implies some axonal loss affecting either the common peroneal nerve or its superficial branch.
The particular portion of the nerve that is injured cannot be determined if only a superficial peroneal nerve sensory study is performed. Comparison of the latency and amplitude of the superficial peroneal SNAP with the contralateral limb is required to define an approximate degree of axonal loss.
Motor conduction studies
The most commonly performed test in determining peroneal conduction in the leg and across the fibular head is performed with the active electrode placed on the extensor digitorum brevis. The peroneal nerve usually is stimulated at the ankle, several centimeters below the fibular head and about 10 cm proximal to the fibular head, just medial to the biceps femoris tendon. This allows calculation of the NCV across the fibular head region, with comparison with the distal leg segment.
Comparison with the contralateral limb is often helpful. When significant extensor digitorum brevis atrophy is present (eg, with advanced age or with a polyneuropathy), the active electrode should be placed over the anterior tibial muscle. Generally, lower-extremity motor NCVs less than 40 m/sec are considered abnormal. Generally, proximal-segment NCVs should be greater than distal NCVs, given the greater axonal diameter in the proximal segment of the nerve.
If contralateral limb responses are normal, axonal loss can be estimated by expressing the compound muscle action potential (CMAP) on the affected side as a percentage of that on the unaffected side. This method is independent of the location of the active recording electrode and is valid in both circumstances. A 20-50% change (depending on the source) indicates a conduction block. The degree of conduction slowing and temporal dispersion may also be assessed to determine whether the lesion is mainly demyelinating or axonal.
Needle EMG
Needle EMG helps in confirming axonal loss and in assessing the degree of involvement of the muscles innervated by the superficial peroneal nerve. This portion of the nerve usually is less severely involved than the deep peroneal nerve. It is possible to localize the lesion to either the deep or the superficial peroneal nerve, specifically if appropriate abnormalities are detected in the proper distribution for each nerve.
The most valuable aspect of the needle EMG examination is that it can be used to define the proximal extent of the lesion. If an amplitude drop is lacking across the fibular head but the CMAP for the anterior tibial muscle is lower than that of the unaffected side (suggesting axonal loss), it is difficult to localize the lesion to the fibular head, even though this is the most common site of peroneal nerve injuries.
In any peroneal nerve injury, regardless of the suspected site of nerve compromise, examining the short head of the biceps femoris is important. If this muscle demonstrates membrane instability (positive sharp waves and fibrillations), the lesion is proximal to the fibular head.
Testing muscles innervated by the tibial nerve, particularly the flexor digitorum longus and the posterior tibial muscle, is also important because these muscles contain predominantly L5 neural innervation from the tibial nerve. If a radicular process is present, the muscles innervated by the peroneal and tibial nerves should demonstrate membrane instability.
Superficial peroneal nerve
The value of electrodiagnostic studies for superficial peroneal nerve injury varies in the literature. In many cases, findings from electrodiagnostic tests are normal because these dynamic syndromes frequently improve or resolve at rest; however, in some instances, these tests may reveal an unrecordable evoked response or a prolonged distal latency of a segment of the nerve and thus help better define the zone of compression. Such tests also help in the evaluation of concomitant radiculopathy or peripheral neuropathy.
Deep peroneal nerve
Electrodiagnostic studies of the deep peroneal nerve are helpful in further defining the zone of compression and in evaluating for concomitant radiculopathy or peripheral neuropathy. In deep peroneal nerve injury or entrapment, the results may show a decrease in the amplitude of the response if axonal involvement is present or conduction block occurs from demyelination. The distal latency may be prolonged if entrapment is present in the anterior tarsal tunnel region, and the NCV is decreased across the leg region if the entrapment or injury is more proximal.
An accessory nerve may also be present. The accessory peroneal nerve originates from the superficial peroneal nerve and travels posterior to the lateral malleolus to provide variable innervation to the extensor digitorum brevis. This anomaly is identified when a response is recorded from the extensor digitorum brevis that is larger with proximal stimulation (at the fibular head) than with distal stimulation (at the ankle).
Needle examination may reveal the presence of fibrillations and positive sharp waves in the extensor digitorum brevis only if entrapment is present at the anterior tarsal tunnel. If entrapment is present more proximally, the denervation is present in the anterior tibial muscle as well as in the extensor digitorum brevis.
Denervation may, however, be present with other neurologic conditions. The short head of the biceps femoris, as well as the medial gastrocnemius, the tensor fasciae latae, and the lumbar paraspinal muscles, should be tested if findings in the deep peroneal muscles rule out a more proximal problem (eg, a radiculopathy). The absence of findings in these muscles, as well as in the peroneus longus and peroneus brevis, confirms the presence of a deep peroneal motor-nerve injury.
Some reports stated that there may be a high percentage of denervation in the foot intrinsic muscles in healthy subjects, but subsequent reports found that the actual percentage of abnormal findings in healthy subjects is low for a well-trained electromyographer. In many cases, electrodiagnostic test findings are normal because these dynamic syndromes frequently improve or resolve at rest.
Radiography, CT, MRI, and Ultrasonography
Posterior tibial nerve
Plain radiographs should probably be obtained to exclude extrinsic factors (eg, exostoses, malunions, or osteochondromas) that cause direct compression of the posterior tibial nerve. In patients with posttraumatic symptoms, further investigation (eg, with computed tomography [CT] or magnetic resonance imaging [MRI]) can help identify occult sources of pain, such as medial talar process fractures, medial malleolus stress fractures, and space-occupying lesions.
Common peroneal nerve
Plain radiographs may be helpful in excluding underlying traumatic injuries (eg, proximal fibular head fracture) or osseous tumors or in assessing the severity of angular deformities about the knee. CT and MRI are helpful in finding a compressive lesion along the course of the common peroneal nerve in cases where this is suspected. Metabolic and hematologic studies may be helpful in conditions such as diabetic peripheral polyneuropathy, alcoholic polyneuropathy, polyarteritis nodosa, and hyperthyroidism.
Superficial peroneal nerve
In rare cases, plain radiographs of the leg can reveal bony abnormalities that may contribute to or constitute the cause of superficial peroneal nerve entrapment. In cases of suspected proximal entrapment, knee radiographs may show abnormalities of the proximal fibula (eg, exostoses, osteochondromas, or fracture callus). If necessary, CT can provide more detailed information on the bony anatomy of the area, and ultrasonography (US) can help localize cystic masses that impinge on the nerve. MRI is rarely necessary to obtain additional information.
Deep peroneal nerve
Bony impingement can usually be seen on conventional lateral ankle or foot radiographs. Oblique radiographs taken from different angles are necessary for better definition of smaller osteophytes, exostosis, or other bony masses about the anterior ankle or the dorsomedial midfoot. Knee radiographs are needed for suspected proximal involvement. If necessary, a CT scan will provide more detailed information on the bony anatomy of the area.
US has been useful for diagnosis and localization of cystic masses impinging on the nerve. Occasionally, MRI is necessary to obtain additional information about soft-tissue masses, synovial reaction, adjacent bone, and chondral and soft-tissue involvement.
Plantar and digital nerves
In rare, complex situations, US and MRI may be helpful for defining interdigital neuritis (also known as Morton neuroma, Morton metatarsalgia, interdigital neuroma, or interdigital nerve compression). [41] The accuracy of these studies, however, varies significantly and depends on multiple factors, including the MRI machine, the technician and the technique, and the interpreting radiologist or orthopedic surgeon.
US reveals a hypoechoic, ovoid mass parallel to the long axis of the metatarsal. US can also be used to diagnose other pathologic conditions in the forefoot, such as bursitis and metatarsophalangeal (MTP) joint effusion. Redd et al reported that this study was 95% sensitive in the detection of webspace abnormality but could not clearly distinguish interdigital neuritis from an associated mass (eg, mucoid degeneration in the adjacent loose connective tissue). [42]
In a study by Quinn et al, US revealed the diagnosis in 85% of cases in which it was suspected, [43] though the ability to detect neuromas shorter than 5 mm was limited.
Terk et al reported on MRI with T1- and T2-weighted sequences, along with a combination of fat suppression and the administration of gadopentetate dimeglumine. [44] Williams et al showed that T1-weighted axial and coronal images obtained with an axial, fast spin-echo (FSE), T2-weighted sequence depict neuromata more consistently than other methods do.
Zanetti et al studied 54 feet in which interdigital neuritis was suspected in order to determine the effect of MRI results on diagnostic thinking and the therapeutic decisions made by orthopedic surgeons. [45] The authors noted considerable change in the diagnosis, location, and number of neuromas, as well as in the treatment plans, after MRI.
In another report, Zanetti et al suggested that diagnosis of interdigital neuritis on the basis of MRI results is relevant only when the transverse diameter of the fluid collection in the bursa is 5 mm or more and when the MRI results are correlated with the clinical findings. [46] Fluid collections in the first three metatarsal bursae with a transverse diameter of 3 mm or less are probably physiologic.
In a histomorphologic study of patients and autopsies, Morscher et al concluded that diagnostic MRI or US is unnecessary for making decisions about operative treatment. [33] In addition, Resch et al found that MRI modalities had little or no value in the diagnosis of interdigital neuritis, because of the high rate of false-negative results. [47]
