Routine Lower Extremity Nerve Conduction Techniques

Abductor hallucis brevis (AHB) muscle:

• G1 placed 1 cm proximal and 1 cm inferior to the navicular prominence

• G2 placed over the metatarsal-phalangeal joint of the great toe

  

Medial ankle: Slightly proximal and posterior to the medial malleolus

Popliteal fossa: Mid-posterior knee over the popliteal pulse

9 cm

The tibial compound muscle action potential (CMAP) often has an initial positive deflection, indicating that G1 is not over the motor endplate. If this occurs, the position of G1 should be changed slightly.

CMAP amplitude at the popliteal fossa stimulation site often is lower than at the medial ankle stimulation site (normal controls may drop up to 50%). Thus caution must be used whenever interpreting a drop in amplitude between the ankle and popliteal fossa as a conduction block on tibial motor studies. Side-to-side comparisons often are useful in this situation.

High stimulation intensities often are required at the popliteal fossa to ensure supramaximal stimulation.

Recording also can be done to the flexor hallucis brevis muscle.

Extensor digitorum brevis (EDB) muscle:

• Dorsal lateral foot with G1 placed over the muscle belly

• G2 placed distally over the metatarsal-phalangeal joint of the little toe

  

Ankle: Anterior ankle, slightly lateral to tibialis anterior (TA) tendon

Below fibular head: Lateral calf, one to two fingerbreadths inferior to the fibular head (one can straddle the fibular neck with the stimulator)

Lateral popliteal fossa (above fibular neck): Lateral knee, adjacent to lateral hamstring tendons, at a distance of 10–12 cm from the below–fibular head site

9 cm

Higher stimulation currents are needed at the below–fibular head site because the nerve lies deep at that location.

Always perform the ankle, below–fibular neck, and above–fibular neck stimulations. If only the ankle and above–fibular neck stimulations are done, one can miss peroneal slowing across the fibular neck.

Avoid excessive stimulation at the lateral popliteal fossa site to prevent co-stimulation of the tibial nerve.

If there is a higher CMAP amplitude at the below–fibular head and popliteal fossa sites than at the ankle, consider an accessory peroneal nerve.

CMAP amplitude at the below–fibular head stimulation site may sometimes be lower than at the ankle stimulation site in normal individuals. Although this is not as marked as the tibial nerve, an average drop in amplitude of 14% occurs, with rare individuals dropping as much as 20%–30%. Thus, caution should be exercised whenever interpreting a drop in amplitude between the ankle and below–fibular neck stimulation sites as partial conduction block.

TA muscle:

• Proximal to mid-anterior lateral calf with G1 placed over the muscle belly

• G2 placed distally over the anterior ankle

  

Below fibular head: Lateral calf, one to two fingerbreadths inferior to the fibular head (one can straddle the fibular neck with the stimulator)

Lateral popliteal fossa (above fibular neck): Lateral knee, adjacent to external hamstring tendons, at a distance of 10–12 cm from the below–fibular head site

Variable (5–10 cm)

Recording the TA is especially valuable in patients with suspected peroneal neuropathy at the fibular neck. Demonstrating a conduction block, focal slowing across the fibular neck, or both may be easier when recording the TA than the EDB.

Higher stimulation currents are needed at the below–fibular head site because the nerve lies deep at that location.

Avoid excessive stimulation at the lateral popliteal fossa site to prevent co-stimulation of the tibial nerve.

Rectus femoris muscle:

• G1 placed over the anterior thigh, halfway between the inguinal crease and knee

• G2 placed over a bony prominence at the knee

  

Middle of the inguinal area: Slightly lateral to the femoral pulse, below the inguinal ligament

Variable

Firm pressure is needed when holding the stimulator.

Difficult study to perform in obese individuals; high currents are typically needed (e.g., >50 mA).

Limited indications; this study usually is used to compare motor amplitudes from side to side to quantitate the degree of axonal loss in femoral neuropathies, lumbar plexopathies, and severe L4 radiculopathies.

Normal amplitude is >3mV; however, side-to-side comparisons are most useful when symptoms are unilateral.