Common Peroneal Motor NCS

Common Peronal Motor NCS

  1. Active recording electrode – on EDB
  2. Reference Electrode – distal to recording electrode
  3. Stimulating electrode
    1. Anterior ankle 10 cm from active recording electrode
    2. Immediately inferior to fibular head
    3. Posterolateral portion of popliteal fossa, 10cm above previous site
  4. Ground electrode – placed in between the active and stimulating electrodes

Common Peroneal Motor NCS Measurements

Common Peroneal Motor NCSCommon Peroneal Motor NCS2

Superficial Peroneal NCS

Superficial Peroneal NCS

  1. Active recording electrode – immediately behind the lateral malleolus
  2. Reference Electrode – distal to recording electrode
  3. Stimulating electrode –  12 cm from the recording electrode on the posterolateral portion of the calf (along the lateral border of the gastrocnemius and soleus tendons)
  4. Ground electrode – placed in between the active and stimulating electrodes

Measure the following parameters

  • Latency       ___ ms
  • Amplitude  ___µV
  • Distance      ___ cm (i.e., 12 cm)

Velocity = Distance/Latency (cm/ms)

Superficial Peroneal Sensory NCS

Sural NCS

Sural NCS

  1. Active recording electrode – immediately behind the lateral malleolus
  2. Reference Electrode – distal to recording electrode
  3. Stimulating electrode –  14 cm from the recording electrode on the posterolateral portion of the calf (along the lateral border of the gastrocnemius and soleus tendons)
  4. Ground electrode – placed in between the active and stimulating electrodes

Measure the following parameters

  • Latency       ___ ms
  • Amplitude  ___µV
  • Distance      ___ cm (i.e., 14cm)

Velocity = Distance/Latency (cm/ms)

Sural Nerve NCS2

Superior Gluteal Nerve (L4 – S1) and Inferior Gluteal Nerve (L5 – S2)

Superior Gluteal Nerve (L4-S1)

Innervates the gluteus medius and minimus muscles which act to stabilize the pelvis, abduct the thigh, and medially rotate the hip.

Clinical Pearl: Weakness of the gluteus medius muscle can be assessed on exam by the Trendelenberg sign in which the patient is asked to stand on one leg. If the patient is asked (for example) to stand on their right leg and their left hip falls down this would indicate weakness of the right gluteus medius muscle.

Inferior Gluteal Nerve (L5-S2)

Innervates the gluteas maximus and tensor fascia lata muscles which acts to extend and laterally rotate the hip. These muscles also acts to stabilize the knee in the extended position via the iliotibial tract.

Clinical Pearl: The iliotibial band can become inflamed and cause lateral knee pain – a common condition among runners.

Gluteal Muscles Gluteal Muscles2Trendelenburg sign

H-reflex and F-wave

H-reflex

The H reflex is the EMG counterpart of the ankle jerk reflex. It is a EMG test used only to evaluate patients with suspected S1 nerve root pathology. By stimulating the tibial nerve at the popliteal fossa, an afferent impulse is generated which travels toward the spinal cord in the Ia sensory fibers. Coincidentally, an impulse also travels orthodromically down the tibial nerve to stimulate the soleus muscle from which are recording. This generates the early response, the CMAP. The proximal traveling impulse travels to the spinal cord and activates the alpha motor neuron of the S1 reflex arc. A action potential is generated which travels down the tibial nerve to the soleus muscle, at which point a late action potential is generated, the H response.

H reflex+

The F-wave

The F wave study tests the conduction of motor axons proximal to the stimulation site. When motor nerves are stimulated for routine NCV studies, the stimulation creates action potentials which travel not only orthodromically toward the muscle, but also antidromically toward the motor neuron. Depolarization is conducted to the axon hillock, which is now repolarized, sending a new action potential back to the muscle, causing action potentials in muscle fibers. This late response is the F wave. The recording electrodes for the F wave study are in the same location as for the motor NCV study, but, the stimulating electrodes are flipped so that the cathode is directed towards the spine. 

F wave

Comparison of H-reflex and F-wave

Comparison of F wave and H reflexComparison of F wave and H reflex2

 

Myotomes

Myotomes

Lumbosacral Plexus

The lumbosacral plexus is divided anatomically into 3 main components

  1. Lumbar plexus (L 1,2,3,4)
  2. Lumbosacral trunk (L4,5)
  3. Sacral plexus (LS trunk + S 1,2,3,4)
  • The lumbar plexus is formed by the ventral rami of L1,2,3, and most of the L4 roots). These rami divide near the vertebral column into ventral and dorsal branches:
    • Ventral branches from the obturator nerve
    • Dorsal branches form the femoral nerve
  • The lumbosacral trunk is formed primarily by the L5 root, with a contributing branch from the L4 root. It then travels a relatively long distance before at the pelvic rim it where it is joined first by the S1 root
  • The sacral plexus is formed by the fusion of the LS trunk with the ventral rami of S1, S2, S3, and S4. Its main branches are:
    • Sciatic Nerve (L4-S3)
    • Superior Gluteal Nerve (L4-S1)
    • Inferior Gluteal Nerve (L5-S2)

Lumbosacral plexus image

LS Plexus 2

Note:

The lumbar plexus gives rise to direct motor branches to the psoas and iliacus,  ilioinguinal nerve (L1 – sensory) , iliohypogastric nerve (L1 – sensory) , genitofemoral nerve (L1. L2   – sensory/motor cremasteric reflex), and the lateral cutaneous nerve of the thigh L2, L3).

The main branches of the sacral plexus include the sciatic nerve (L4 – S3), the superior gluteal nerve (which innervates the gluteus medius and minimus) and the inferior gluteal nerve (which innervates the gluteus maximus). Other nerves that arise from the sacral plexus are the posterior cutaneous nerve of the thigh and the pudendal nerves.

Tibial Nerve (L5 – S2)

  • Function: Flex and invert the foot and provide sensation to the lateral leg and the sole of the foot.
  • Anatomy:
    • In the popliteal fossa the tibial nerve gives off the sensory sural nerve (provides sensation to calf and lateral aspect of foot).
    • It then descends in the calf and innervates the gastrocnemius, plantaris, soleus, tibialis posterior, flexor digitorum longus, and flexor hallicus longus muscles.
    • Finally, it enters the ankle posterior to the medial malleolus and passes through the tarsal tunnel to terminate as the medial and lateral plantar nerves which have motor and sensory branches
      •  Motor innervation to the small muscles of the foot (interossei, toe abductors/adductors, and flexors)
      •  Sensory innervation to the sole of the foot via plantar and calcaneal nerves

Tibial Nerve1

Sole of Foot Sensory Map

Tibial Muscles 1

Tibial Muscle Actions

“Tom, Dick And Very Nervous Harry” is a mnemonic for remembering the structures that pass posterior to the medial malleolus at the ankle.

T = tibialis posterior tendon
D = flexor digitorum tendon
A = posterior tibial artery
V = posterior tibial veins (usually a pair on either side of the artery)
N = tibial nerve
H = flexor hallucis longus tendon

Medial Mallelous Anatomy

Common Peroneal Nerve (L4 – S1)

  • Function: Extend and everts the foot and provide sensation to the lateral leg and dorsum of foot.
  • Anatomy:
    • After separating from the tibial nerve in the popliteal fossa, the CPN gives off the lateral cutaneous nerve of the calf, which supplies the lateral aspect of the leg.
    • Then it winds around the fibula and divides into superficial & deep branches.
    • The superficial peroneal nerve innervates the peronei mucles and the skin of the lateral leg and dorsum of the foot.
    • The deep peroneal nerve innervates all the ankle and toe extensors (TA, EHL, EDB, EDL) and supplies sensation to the web space between the first and second toes.

Common Peroneal Nerve 1Common Peroneal Nerve 2

Peroneus Muscles

Common Peroneal Nerve Muscles

Sciatic Nerve (L4 – S2)

  • Function: The sciatic nerve supplies the hamstrings (semimembranous, semitendinous, biceps femoris) and the adductor magnus muscles which act to flex the knee, extend the hip, and adduct the leg. The sciatic nerve then divides into the tibial and common peroneal nerves in the popliteal fossa.
  • Anatomy:
    • Arises from posterior rami of L4-S2 segments
    • It emerges from the pelvis via the greater sciatic foramen below the piriformis muscle
    • It then descends laterally down the leg to supply the hamstrings and the adductor magnus. (which is also supplied by the obturator nerve)
    • At the popliteal fossa it divides into its two terminal branches, the tibial (medially) and the common peroneal (laterally) nerves

Sciatic Nerve

Hamstrings3

Sciatic Nerve Proximal Leg Muscle Actions