Lumbosacral Plexus and Lower Extremity Nerves

• Anatomical organization: The plexus divides into anterior (flexor) and posterior (extensor) divisions at the level of the psoas major muscle. The anterior division gives rise to the obturator nerve (L2–L4) and contributes to the femoral nerve, while the posterior division forms the superior and inferior gluteal nerves, sciatic nerve (L4–S3), and pudendal nerve. Understanding division-based organization helps predict patterns of injury.

• Nerve compression and stretch injuries: Individual nerves become vulnerable at specific anatomical "pinch points"—the femoral nerve beneath the inguinal ligament, the sciatic nerve at the piriformis muscle, and the common peroneal nerve at the fibular head. Prolonged pressure, hip flexion/internal rotation, or direct trauma causes demyelination and axonal loss depending on severity and duration of compression.

• Metabolic and inflammatory mechanisms: Conditions like diabetes mellitus cause microvascular ischemia of nerve vasa nervorum and accumulation of sorbitol and advanced glycation end products, predisposing to compression neuropathies. Inflammatory conditions (vasculitis, sarcoidosis) and pregnancy-related pelvic changes can compromise plexus structures, as can pelvic masses and radiation therapy causing fibrosis.

• Compartment syndrome and rhabdomyolysis pathways: Massive lower extremity trauma, crush injuries, or prolonged immobilization can lead to increased compartmental pressure causing secondary nerve ischemia and axonal degeneration. This is distinct from direct plexus injury and has different time-sensitive management implications.

• Neuronal degeneration patterns: Wallerian degeneration occurs distal to severe axonal injuries within 3��5 days, while demyelinating injuries may recover within weeks. The distinction determines prognosis and timing of surgical intervention (typically delayed 3–4 weeks to allow demarcation of irreversible injury).

Lumbosacral Plexus Injuries (General)

• Unilateral lower extremity weakness with asymmetric pattern depending on division involved (anterior vs. posterior); may include hip flexion/adduction weakness (anterior) or hip abduction/external rotation weakness (posterior)
• Sensory loss in lower extremity following dermatomal (L1–S4) and/or peripheral nerve distributions; difficult to isolate without careful examination
• Pain and paresthesias, often severe and burning in quality, indicating irritative lesions or acute compression

Femoral Nerve Injury (L2–L4)

• Weakness of knee extension (quadriceps) is the hallmark finding and causes difficulty walking, climbing stairs, or rising from seated position; patient may report knee "giving out"
• Hip flexion weakness (iliopsoas involvement) causing difficulty lifting leg while walking
• Sensory loss over anterior and medial thigh and medial lower leg (saphenous nerve distribution)
• Absent or diminished patellar reflex (L3–L4); preserved ankle reflex helps distinguish from L5 radiculopathy
• Common mechanism: compression under inguinal ligament during pelvic surgery, retroperitoneal hemorrhage, or pelvic masses

Obturator Nerve Injury (L2–L4)

• Hip adduction weakness causing difficulty crossing legs and abnormal gait
• Sensory loss over medial thigh (small, often clinically subtle area)
• Often overlooked clinically; injury may occur during pelvic and gynecologic procedures
• Rarely injured in isolation compared to femoral nerve

Superior Gluteal Nerve Injury (L5–S1)

• Hip abduction weakness (gluteus medius/minimus dysfunction) resulting in Trendelenburg gait—pelvis drops on contralateral side during stance phase
• Hip external rotation weakness
• Sensory loss minimal (small lateral hip area)
• Common cause: injections into upper outer quadrant of buttock (intramuscular injections placed too medially/superiorly) or hip replacement surgery

Inferior Gluteal Nerve Injury (L5–S2)

• Hip extension weakness (gluteus maximus) causing difficulty rising from seated position, climbing stairs, or walking uphill; often described as "weak in the gluteus"
• Patients may have relatively preserved gait initially if quadriceps intact
• Sensory loss: none (purely motor nerve)
• Similar injection risk as superior gluteal nerve

• Foot drop due to common peroneal division injury (weak foot dorsiflexion/eversion) with characteristic high-stepping gait
• Plantarflexion and toe flexion weakness from tibial division injury
• Sensory loss over lateral lower leg, dorsum of foot, and sole
• Absent or diminished Achilles reflex (S1)
• Etiologies: hip dislocation/reduction, hip arthroplasty, prolonged sitting with pressure at gluteal fold, misplaced buttock injections, compartment syndrome from femur fracture
• Nerve is tethered at piriformis muscle (controversial piriformis syndrome entrapment)

Common Peroneal Nerve Injury (L4–S2)

• Foot drop with inability to dorsiflex foot; patient compensates with high-stepping gait to clear toe during swing phase
• Weakness of foot eversion (fibularis muscles)
• Sensory loss over dorsum of foot and lateral lower leg
• Preserved ankle reflex (tibial nerve intact)
• Vulnerable at fibular head just below knee; common etiologies include knee trauma, prolonged leg crossing, tight casts/braces, or fibular head fracture
• Often initially presents as "common peroneal nerve palsy" or "foot drop"

Tibial Nerve Injury (L4–S3)

• Weakness of ankle plantarflexion and toe flexion (difficult walking, weakness in push-off phase of gait)
• Sensory loss on sole of foot and medial heel (via sural and medial plantar nerves)
• Absent Achilles reflex (S1)
• Vulnerable in popliteal fossa (knee surgery, cyst compression) or tarsal tunnel (behind medial malleolus—tarsal tunnel syndrome)
• May be part of posterior tibial nerve syndrome in lower leg

• Sensory loss over lateral foot and heel; patient may report numbness and burning pain
• No motor deficits (important clinical pearl)
• Vulnerable at junction of gastrocnemius and soleus (Baker's cyst), lateral ankle, or during peroneal nerve biopsy
• Often iatrogenic from ankle procedures or minor trauma

Important Clinical Pearls About Presentation

• Nerve injury severity spectrum: Neuropraxia (demyelination with preserved axonal continuity) presents with mild weakness and good prognosis; axonotmesis (axonal loss with preserved connective tissue) causes moderate-to-severe weakness and slower recovery; neurotmesis (complete transection) results in complete paralysis and requires surgery for recovery
• Acute vs. chronic presentations: Acute injuries present with flaccid paralysis and sensory loss; chronic compression neuropathies develop gradually with atrophy and may spare reflexes initially
• Proximal vs. distal patterns: Plexus-level injuries (proximal) affect multiple nerves and territories; distal single-nerve injuries have more limited, focal deficits
• Pain as a localizing feature: Nerve root or plexus irritation (from inflammation or acute stretch) causes severe pain; chronic compression typically

The buzzword-to-nerve map

• Meralgia paresthetica: purely sensory burning/numbness over the anterolateral thigh from lateral femoral cutaneous nerve (L2–L3) compression under the inguinal ligament — classic in obesity, pregnancy, tight belts/low-rise jeans, and tool belts. No weakness and a normal patellar reflex — that is the whole point of the vignette. Management is conservative (weight loss, remove the constricting garment); it is not a surgical emergency.
• Steppage (high-stepping) gait + foot drop + numb dorsum of foot after leg-crossing, a tight cast, prolonged squatting, or lithotomy positioning = common peroneal (common fibular) nerve at the fibular neck — the single most common compressive mononeuropathy of the lower limb.
• Trendelenburg gait (pelvis drops on the contralateral swing side) = superior gluteal nerve / gluteus medius. Examiners love the side confusion — the weak side is the stance side.

The distinctions examiners actually test

• Peroneal palsy vs. L5 radiculopathy: both give foot drop and dorsal foot numbness, but L5 radiculopathy also weakens foot inversion (tibialis posterior, tibial nerve) and hip abduction, and often causes back pain. Preserved inversion localizes the lesion to the fibular neck.
• Deep vs. superficial peroneal branch: deep = dorsiflexion + sensation of the first dorsal web space; superficial = eversion + dorsum sensation. Isolated first-web-space numbness with foot drop is deep peroneal.
• Femoral neuropathy vs. plexopathy: weak knee extension with a lost patellar reflex plus hip flexion weakness and a normal ankle reflex points proximally — in an anticoagulated or post-catheterization patient, think retroperitoneal/iliopsoas hematoma and get cross-sectional imaging (CT) as the next step.

Associations and next steps

• Diabetes: diabetic lumbosacral radiculoplexus neuropathy (Bruns–Garland, "diabetic amyotrophy") — severe unilateral thigh pain then quadriceps wasting and weight loss; the ADA Standards of Care frame management around glycemic control and neuropathic pain therapy rather than surgery.
• Electrodiagnostics (EMG/NCS), per AAN/AANEM practice guidance, are the confirmatory test for localization and to separate demyelinating from axonal injury — but they are low-yield in the first days, so they are deferred a few weeks after acute trauma.
• Posterior hip dislocation and misplaced buttock injections injure the sciatic nerve; the safe intramuscular site is the upper outer quadrant/ventrogluteal region.