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

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Basics:

Figure 1: Lumbosacral plexus

By Gray822_es.svg Gray822.png Grayderivative work Ninovolador (talk)derivative work Mcstrother (talk) – Gray822_es.svg, CC BY 3.0, httpscommons.wikimedia.orgwindex.phpcurid=12048117.
  • 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: Plantar flexion and eversion of ankle
Peroneus brevis: Plantar flexion and eversion of ankle
Deep peroneal nerve 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: Adduction and flexion of 1st toe
Flexor digitorum brevis: Flexion of 2nd-5th toes
Flexor hallucis brevis: Flexion of 1st toe
Lumbricals I and II: Flexion of MCP joints and extension of interphalangeal joints.
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.
    • EMG will demonstrate acute denervation in involved muscle groups,
    • Pathology will show infarction of proximal nerve trunks and branches.
    • 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


 

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