The lumbosacral plexus and lower extremity nerves are “high-yield topics” for the boards and in-service examinations. In this chapter, we will discuss the basic physiology and common syndromes of the lumbosacral plexus and lower extremity nerves. One should take time to recognize how these lesions present on the exam as well as with electrodiagnostic testing.

Author: Brian Hanrahan MD

Chapter Multimedia Content

Flashcards
Table of Contents

Table of Contents

Basics:

Lumbosacral Plexus Diagram

Move the Slider to Practice!

Labeled Lumbosacral Plexus Diagram by NowYouKnowNeuro Unlabeled Lumbosacral Plexus Diagram without labels for practice exam review sheet by NowYouKnowNeuro
Lumbosacral Plexus Diagram by NowYouKnowNeuro
  • The lumbosacral plexus is a network of nerve fibers that innervates muscles and provides sensation from the lower limbs.
  • It is formed by the anterior rami of T12-L4/5 nerve roots.
  • To simplify, the lumbar (T12-L4/5) and sacral plexuses (L4-S4) will be discussed separately.

Table 1: Nerves of lumbar plexus with their associated muscular innervation and function

Nerve (associated nerve root) Muscle(s) and function
Iliohypogastric nerve (T12-L1) Internal oblique: Compresses abdominal cavity
Transversus abdominis: Compresses abdominal cavity
Ilioinguinal nerve (T12-L1) Internal oblique: Compresses abdominal cavity
Transversus abdominis: Compresses abdominal cavity
Genitofemoral nerve (L1-L2) Cremaster (males): Raises the scrotum
Obturator nerve (L2-L4) Obturator: Adduction and lateral rotation of hip
Adductor longus: Adduction and flexion of hip
Adductor brevis: Adduction and flexion of hip
Adductor magnus**: Adduction and flexion of hip
Pectineus*: Adduction and flexion of hip
Gracilis: Adduction of hip and flexion of knee
Femoral nerve (L2-L4) Iliopsoas: Flexion of hip
Pectineus*: Adduction and flexion of hip
Sartorius: Flexion and abduction of hip
Quadriceps femoris
(rectus femoris, vastus lateralis,
vastus intermedius, vastus medialis): Extension of knee.

*Pectineus is innervated by femoral and obturator nerves, **Adductor magnus is innervated by obturator and sciatic nerves.

Table 2: Nerves of sacral plexus with their associated muscular innervation and function

Nerve (associated nerve root) Muscle(s) and function
Superior gluteal nerve (L4-S1) Gluteus medius: Abduction of hip
Gluteus minimus: Abduction of hip
Tensor fascia lata: Abduction and medial rotation of hip
Inferior gluteal nerve (L5-S2) Gluteus maximus: Extension of hip
Sciatic nerve (L4-S3) Hamstrings (semitendinosus, semimembranosus: Extension of the hip, flexion of the knee, and rotation of the knee
Adductor magnus**: Adduction and flexion of hip
Superficial peroneal nerveª,* (L5-S1) Peroneus longus: Eversion of ankle (primarily) and assists in plantar flexion
Peroneus brevis: Eversion of ankle (primarily) and assists in plantar flexion
Deep peroneal nerveª,* (L4-S1) Tibialis anterior: Dorsiflexion and inversion of ankle
Extensor digitorum brevis: Extension of toes
Extensor digitorum longus: Extension of toes
Extensor hollucis longus: Extension of 1st toe and dorsiflexion of ankle
Peroneus tertius: Dorsiflexion and eversion of ankle
Tibial nerve (L4-S3)ª Tibialis posterior: Plantar flexion and inversion of ankle
Biceps femoris (long head): Knee flexion
Gastrocnemius: Plantar flexion of ankle
Soleus: Plantar flexion of ankle
Flexor digitorum longus: Plantar flexion of ankle and flexion of 2nd-5th toes
Flexor hallucis longus: Plantar flexion of ankle and flexion of 1st toe
Abductor hallucis: Abduction and flexion of 1st toe
Flexor digitorum brevis: Flexion of 2nd-5th toes
Flexor hallucis brevis: Flexion of 1st toe
Abductor digiti minimi: Abduction of 5th toe
Flexor digiti minimi: Flexion of 5th toe
Interossei: Abduction and adduction of toes
Adductor hallucis: Adduction of 1st toe
Lumbricals III and IV: Flexion of MCP joints and extension of interphalangeal joints

aTerminal branches of the sciatic nerve, *Terminal branches of the peroneal nerve, **Adductor magnus is innervated by obturator and sciatic nerves, MCP: metacarpophalangeal joint.

Lumbosacral Plexus Syndromes

  • Diabetic lumbosacral plexopathy (diabetic amyotrophy):
    • Presents with acute back, hip, and buttock pain, followed by bilateral lower extremity proximal and distal weakness.

    • It is most commonly observed in patients with well-controlled type 2 diabetes mellitus
      • Can also present less commonly in the brachial plexus.
    • Recovery takes weeks and can be incomplete.
  • Retroperitoneal hematoma
    • Presents secondary to hemorrhage into the psoas or iliacus muscle.
    • CT of the abdomen and pelvis is most useful for imaging acute blood in the retroperitoneum.
    • Small retroperitoneal hematomas may only compress the femoral nerve, leading to weakness of the iliopsoas and quadriceps muscles.
  • Other
    • Radiation Plexopathy
    • Tumor infiltration/neoplastic plexopathy
    • Peripartum plexopathy

Lower Extremity Nerves and Associated Syndromes

Lateral femoral cutaneous nerve (L2-L3)

  • Function
    • Sensory: Provides sensation to the anterior thigh.
  • Associated syndrome(s)
    • Meralgia Paresthetica
      • Presents with pain in the low back, buttock, anterolateral thigh, and lateral knee
      • Often aggravated by standing and relieved by sitting.
      • Associated with obesity or pregnancy, and symptoms disappear with weight loss or delivery.

Obturator Nerve (L2-L4)

  • Function:
    • Motor: Adduction and flexion of the hip.
    • Sensory: Medial thigh.

Femoral nerve (L2-L4)

  • Function:
    • Motor: Flexion of the hip and extension of the knee.
    • Sensory: Anterior and medial thigh (medial cutaneous nerve of the thigh).
  • Most common nerve injured during childbirth secondary to direct compression.

Sciatic Nerve (L4-S2)

  • The largest peripheral nerve in the human body.
  • Provides innervation to the hamstring muscles and adductor magnus before it bifurcates into the common peroneal nerve and tibial nerve.
  • Common peroneal nerve (L4-S1)
    • May also be called the common fibular nerve.
      • Common peroneal mononeuropathy is the most common mononeuropathy of the lower limbs.
    • As it travels deep in the thigh it innervates the short head of the biceps femoris.
    • Leaving the thigh the common peroneal nerve passes laterally around the fibular head, where it divides into the superficial and deep peroneal nerves.
    • Superficial peroneal nerve
      • Function
        • Motor: Plantarflexion and eversion of the ankle.
        • Sensory: Upper lateral leg.
      • Deep peroneal nerve
        • Function
          • Motor: Dorsiflexion and inversion of the ankle, extension of toes.
          • Sensory: Area between 1st and 2nd toes.
    • Associated syndrome(s)
      • Peroneal compression neuropathy at the fibular head
        • The most common site of common peroneal injury.
        • The superficial position as the peroneal nerve passes the fibular head makes it susceptible to compression.
        • Risk factors include significant weight loss and leg crossing.
        • Presents with foot drop and “steppage” gait.

Tibial nerve (L4-S3)

  • The tibial nerve enters the tarsal tunnel behind or inferior to the medial malleolus.
  • Function
    • Motor: Plantarflexion and inversion of the ankle.
    • Sensory: Sole of the foot.
  • Associated syndrome(s)
    • Tarsal Tunnel Syndrome
      • Occurs due to entrapment of the distal branch of the tibial nerve at the medial ankle (the medial plantar nerve).
      • Presents with pain and paresthesias around the medial malleolus
        • Pain is often worse at night and with weight-bearing.
      • Causes include trauma, degenerative bone disease, and rarely, mass lesions.

Sural nerve (S1)

  • Formed by the coalescence of distal branches from the tibial and peroneal nerves.
  • Function:
    • Sensory: Lower posterior/lateral leg, lateral ankle, and lateral foot.

 

Log in to View the Remaining 60-90% of Page Content!

 
 

New here? Get started!

(Or, click here to learn about our institution/group pricing)
   

1 Month Plan

Full Access Subscription
$142.49
$ 94
99
1 Month
  • Access to all chapters
  • Access to all images and cases
  • Access to all flashcards
  • Access to Full Question Bank
Choose Plan

3 Month Plan

Full Access Subscription
$224.98
$ 144
97
3 Months
  • Access to all chapters
  • Access to all images and cases
  • Access to all flashcards
  • Access to Full Question Bank
Choose Plan

1 Year Plan

Full Access Subscription
$538.47
$ 338
98
1 Year
  • Access to all chapters
  • Access to all images and cases
  • Access to all flashcards
  • Access to Full Question Bank
Choose Plan
Popular

Table of Contents

Table of Contents