Lower Extremity - Mononeuropathies

Lower Extremity - Mononeuropathies

Key Concepts

Relevant Anatomy

Lower lumbar vertebral column
Sacrum.
The L5/S1 junction is a clinically important level for disc herniation – here the vertebral column angles abruptly posteriorly.
Pelvic bone
Femur
Inguinal ligament

nonspecific causes of mononeuropathies in the lower extremity

Spontaneous causes

See Spinal Cord Compression for imaging findings of abscess and neurovascular compression.
These things are often not entirely spontaneous, as iatrogenic causes can certainly lead to such things as hematoma and abscess or compression or trauma, as well.

Iatrogenic causes

Femoral, Obturator, & Sciatic mononeuropathies

I. Femoral nerve (L2–L4)

Descends between the psoas and iliacus muscles, then underneath the inguinal ligament, and down the anterior thigh to innervate the anterior compartment thigh muscles.
Femoral neuropathy causes hip flexion weakness (from iliopsoas failure), which manifests with difficulty climbing upstairs, and also knee extension weakness from quadriceps failure, which manifests with difficulty walking downstairs: so-called buckling knee.
Notable causes of femoral neuropathy:

II. Obturator nerve (L2 – L4)

Descends medial to the femoral nerve, anterior to the sacrospinous ligament, and exits via the obturator canal, down the medial aspect of the thigh to innervate the medial compartment thigh muscles.
Obturator neuropathy causes hip adduction weakness, which manifests with involuntary hip abduction during walking: gait instability, from adductor failure.
Although obturator neuropathy, itself, is rare, indicate that this is a key muscle to examine because the obturator nerve involvement helps distinguish L2 – L4 radiculopathy from femoral neuropathy.

L2 – L4 radiculopathy produces obturator nerve distribution weakness; whereas, femoral neuropathy do not.

III. Sciatic nerve (L4 – S3)

Exits the pelvis anterior and inferior to the piriformis via the greater sciatic foramen and then descends posterior to the femur.
Sciatic neuropathy causes weakness of hip extension from hamstrings failure, and also total lower leg weakness from peroneal and tibial distribution failure.

Classically, the peroneal deficits are more pronounced than the tibial deficits due, in part, to the more superficial location of the peroneal nerve fibers.

One notable cause of sciatic neuropathy is piriformis syndrome, wherein the piriformis muscle can compress the sciatic nerve.

Peroneal & tibial neuropathies

Relevant anatomy

Femur
Fibula
Tibia
Bones of the foot
The popliteal fossa is the depression behind the knee.
The fibular neck is the continuation of the head of the fibula (the top of the lateral leg bone).
The tarsal tunnel is the medial entry zone of the tibial nerve through the ankle into the foot.

The medial malleolus (medial ankle) and medial calcaneus (the heel bone) form the superior and inferior boundaries of the tarsal tunnel and the flexor retinaculum forms its roof.

Proximal to the popliteal fossa, it unbundles into the common peroneal nerve and tibial nerve.

I. Peroneal nerve (L4 – S3)

The common peroneal nerve passes inferolaterally through the popliteal fossa and wraps around the fibular neck; and then splits into:

The deep peroneal nerve (L4 — S1), which innervates the anterior leg and dorsal foot muscles to provide foot and toe dorsiflexion, so failure causes foot drop (and steppage gate).
The superficial peroneal nerve is supplied by L5, S1, and innervates the lateral leg muscles, so weakness causes failure of foot eversion.

Peroneal nerve compression can occur at the fibular neck.
Involvement of the short head of the biceps femoris localizes the neuropathy to proximal to the fibular head.

II. Tibial nerve (L5 – S3)

Continues straight down the posterior leg to innervate the posterior leg and foot muscles, so weakness causes failure of foot and toe plantar flexion and foot inversion.
The major distal tibial nerve compression site is the tarsal tunnel.
The key proximal compression site is the popliteal fossa (especially from a Baker cyst, which can compression the popliteal neurovascular bundle, and cause vascular compression symptoms: edema and discoloration.

Additional Lower Extremity Mononeuropathies

I. Lateral cutaneous nerve of the thigh) (L2 and L3) (aka lateral femoral cutaneous nerve).

Compression causes meralgia paresthetica: sensory disturbance in the lateral thigh.
Compression occurs where it passes underneath the inguinal ligament, which attaches to the anterior superior iliac spine, the lateral cutaneous nerve of the thigh is often compressed when individuals gain weight such as with obesity or during pregnancy, or simply when tight-waisted pants come in vogue.
Key to diagnosis is sensory disturbance limited to the lateral thigh.

Exits the greater sciatic foramen superior to the piriformis muscle.
Innervates, most notably, gluteus medius, so failure causes weakness of hip abduction.
Notably, this can occur from piriformis compression.

Exits the pelvis along with the sciatic nerve, inferior to the piriformis muscle.
Innervates gluteus maximus, so injury causes weakening of hip extension.
Notably this can occur from gluteal muscle injection with direct inferior gluteal nerve injury.

The integrity of the superior and inferior gluteal nerves helps us distinguish L5, S1 radiculopathies from sciatic neuropathy or (more distal) peroneal or tibial neuropathies.

IV. Pudendal nerve (primarily S4 (also, S2 and S3)

Exits the pelvis via the greater sciatic foramen, passes posterior to the sacrospinous ligament, and then renters via the lesser sciatic foramen.
Pudendal neuralgia affects the external urethral and anal sphincters and external genitalia.
Notably occurs from the traumatic effects of childbirth.