, Ali T. Ghouse2 and Raghav Govindarajan3
(1)
Parkinson’s Clinic of Eastern Toronto and Movement Disorders Centre, Toronto, ON, Canada
(2)
McMaster University Department of Medicine, Hamilton, ON, Canada
(3)
Department of Neurology, University of Missouri, Columbia, MO, USA
Sensory nerve conduction studies are an essential part of the electrodiagnostic examination, for several reasons. Some peripheral nerve lesions involve only sensory nerves or only the sensory fibers of mixed nerves. Sensory conduction studies are typically more sensitive than their motor counterparts to pathophysiological processes occurring along mixed nerves. For example, in these mixed nerves, sensory conduction typically occurs before or is more severe than the conduction in the motor components, such as in the segmental demyelinating lesions of carpal tunnel syndrome. Sensory conduction amplitudes are more sensitive to axon loss than motor nerves.
Sensory conduction amplitudes are not affected by lesions that are proximal to the dorsal root ganglion, such as the intraspinal lesions of myelopathy or radiculopathy. Because the cutaneous nerves are more superficial than the motor nerve fibers, they are more susceptible to trauma, and consequently the sensory responses may be absent or of low amplitude because of coincidental nerve injury.
In the lower extremities, attention should be paid to prior surgical procedures such as vein stripping, phlebotomies, or tendon-lengthening procedures that result in an absence of sensory nerve action potentials (SNAPs). The SNAPs are also affected by lymphedema; gross obesity; and thickened, hyperkeratotic, or calloused skin. Lower extremity sensory conduction studies show abnormalities much sooner than upper extremity studies, such as in patients with generalized peripheral polyneuropathies.
Side-to-side comparison of the sensory responses can be helpful when evaluating unilateral abnormalities. A response of 50 % or lower than the response obtained on the asymptomatic side is considered abnormal in many laboratories. Absent sensory responses can occur as a result of age.
There are several differences in the parameters reported by electromyography laboratories. These include having variable distances between stimulating and recording electrodes, reporting conduction times as latencies versus conduction velocities, and measuring latencies from onset to peak or measuring from peak-to-peak versus baseline-to-peak.
Technical Considerations in Sensory Nerve Conduction Studies
Stimulate nerve and record nerve (no muscle, no synapse).
The requirement in each conduction study is the recording of an evoked potential.
Orthodromic conduction stimulates distally and records proximally.
Antidromic conduction stimulates proximally and records distally; opposite to physiological conduction.
Measure baseline to negative peak or peak-to-peak.
Signal averaging is useful to record low-amplitude potentials.
Use a supramaximal stimulus of more than 25 %, but avoid contamination with compound muscle action potential (CMAP).
The ground is optimally placed between the stimulating and the recording electrodes.
Sural Nerve
The sural nerve is derived from the S1 and S2 roots. It is formed by a medial sural nerve, which is a branch of the tibial nerve given off at the inferior angle of the popliteal fossa. This branch of the tibial nerve, along with the communicating branch of the peroneal nerve, the lateral sural nerve, forms the sural nerve. The sural nerve leaves the popliteal fossa in the groove between the two heads of the gastrocnemius, and it becomes superficial at approximately the junction of the middle and lower third of the leg. It then continues, to pass behind the lateral malleolus and the lateral border of the foot.
Stimulation
Antidromic surface stimulation of the nerve is distal to the lower border of the bellies of the gastrocnemius, at the junction of the middle and the lower third of the leg. This is at a distance of 10–16 cm above the lateral malleolus.
Recording
The active electrode is placed between the lateral malleolus and the Achilles tendon.
Values
Mean latency is 3.8 ms and amplitudes vary between 10 and 75 μV.
Superficial Peroneal Nerve
The superficial peroneal nerve is a branch of the common peroneal nerve that is formed from the L4, L5, and the S1 nerve roots. It provides cutaneous innervation to the distal lateral leg and most of the dorsum of the foot. The nerve becomes superficial in the groove between the peroneus longus and the extensor digitorum longus at the junction of the middle and distal thirds of the leg. The nerve passes in front of the extensor retinaculum at the ankle to reach the dorsum of the foot.
Stimulation
Antidromic surface stimulation is performed 10–15 cm proximal to the lateral malleolus and anterior to the peroneus longus.
Recording
The active surface electrode is placed above the junction of the lateral third of a line connecting the malleoli.
Values
Mean latency is 3.2 ms and amplitudes are 10–70 μV. The amplitude is generally half that of the sural nerve.
Saphenous Nerve
The saphenous nerve is the longest and the largest branch of the femoral nerve. It provides innervation to the anteromedial and posteromedial leg and the medial border of the foot to the base of the first toe. The origin is from the L3-L4 roots. The saphenous nerve branches from the femoral nerve approximately 4 cm distal to the inguinal ligament and travels deep to the sartorius muscle in the adductor canal. It remains anterior to the femoral artery and crosses from the lateral to the medial position in the middle third of the thigh. The nerve then continues along the medial condyle of the femur, where along appears as the fascia and travels between the sartorius and gracilis tendons up to the medial malleolus.
Stimulation
Antidromic surface stimulation is performed over the medial aspect of a slightly flexed knee at a point between the sartorius and gracilis tendons approximately 1 cm above the lower pole of the patella.
Recording
Surface recording is from a line drawn from the stimulation point directly 15 cm to the medial border of the tibia.
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