The Lumbar Plexus: Anatomy, Function, and Clinical Significance

STUDY MATERIAL

7/26/2025

Introduction

The lumbar plexus is a complex network of nerves that provides motor and sensory innervation to the lower abdominal wall, pelvis, and lower extremity. For medical students, understanding the anatomy and clinical relevance of this plexus is essential not only for anatomy exams, but also for fields such as surgery, anesthesiology, neurology, and orthopedics.

Anatomical Formation

The lumbar plexus is formed by the ventral rami of the L1 to L4 spinal nerves, with occasional contribution from T12 (subcostal nerve). The plexus forms within the substance of the psoas major muscle, anterior to the transverse processes of the lumbar vertebrae.

L1 often bifurcates into iliohypogastric and ilioinguinal nerves.
L2 to L4 contribute to the formation of larger mixed nerves such as the femoral and obturator nerves.

Mnemonic to Remember the Branches (from superior to inferior):

I (Iliohypogastric)
I (Ilioinguinal)
Got (Genitofemoral)
Lazy (Lateral femoral cutaneous)
Over (Obturator)
Flat (Femoral)

Major Branches and Their Functions

1. Iliohypogastric Nerve (L1)

Motor: Internal oblique and transversus abdominis.
Sensory: Suprapubic region and lateral gluteal skin.

2. Ilioinguinal Nerve (L1)

Motor: Same as iliohypogastric.
Sensory: Skin of the medial thigh, root of penis and scrotum (male), or mons pubis and labia majora (female).

3. Genitofemoral Nerve (L1–L2)

Pierces the psoas major.
Motor: Cremaster muscle (via genital branch).
Sensory: Skin of scrotum/labia majora and upper anterior thigh.

4. Lateral Femoral Cutaneous Nerve (L2–L3)

Passes under inguinal ligament near ASIS.
Sensory: Anterolateral thigh.
Clinical Note: Entrapment = Meralgia paresthetica.

5. Obturator Nerve (L2–L4)

Passes through the obturator foramen.
Motor: Adductor muscles of the thigh.
Sensory: Medial thigh.
Clinical Note: Obturator neuropathy can impair thigh adduction.

6. Femoral Nerve (L2–L4)

Largest branch of the plexus.
Motor: Quadriceps femoris, iliacus, sartorius, and pectineus.
Sensory: Anterior thigh, medial leg (via saphenous nerve).
Clinical Note: Femoral nerve palsy leads to knee extension weakness and loss of patellar reflex.

Relations and Course

The lumbar plexus lies within the psoas major, and its branches emerge either:

Lateral to the psoas: iliohypogastric, ilioinguinal, lateral femoral cutaneous, femoral.
Anterior to the psoas: genitofemoral.
Medial to the psoas: obturator.

Clinical Correlations

1. Nerve Blocks

Lumbar plexus block (psoas compartment block) is used for lower limb surgeries.

Femoral nerve block offers analgesia for anterior thigh and knee.

2. Surgical Risk

Pelvic or retroperitoneal surgeries may damage the plexus (e.g., during lymph node dissection).

3. Psoas Abscess

May compress the lumbar plexus, presenting with pain, weakness, or paresthesia in innervated areas.

4. Diabetic Amyotrophy (Lumbosacral Radiculoplexus Neuropathy)

Presents with asymmetric proximal leg weakness and severe pain.

Involves inflammation of the lumbar plexus in diabetic patients.