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
Basics:
Lumbosacral Plexus Diagram
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- 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 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: Abduction 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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Brian Hanrahan, MD & Steven Gangloff, MD
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* RITE® is a registered trademark of the American Academy of Neurology (AAN) Institute. NowYouKnow Neuro™ (NYKN) is a preparatory resource for this and other neurology exams but is in no way affiliated with the RITE® exam, the AAN Institute, or the AAN.
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