The lumbar plexus is formed within the psoas major muscle by the anterior rami of L1-L4 with a variable contribution from T12. Its branches include: the iliohypogastric nerve (L1), ilioinguinal nerve (L1), genitofemoral nerve (L1-L2), lateral femoral cutaneous nerve (L2-L3), obturator nerve (L2-L4), and femoral nerve (L2-L4). The lumbosacral trunk (L4-L5) descends to join the sacral plexus. The plexus lies within the posterior psoas compartment, traversing the muscle belly.
| Origin | Anterior rami of L1-L4, with a contribution from T12 |
|---|---|
| Course | Forms within psoas major anterior to the lumbar transverse processes, its branches emerging from the lateral border, medial border and anterior surface of the muscle |
| Motor |
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| Sensory |
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Lumbar plexus block (psoas compartment block) anaesthetises all major branches by injecting local anaesthetic into the posterior psoas compartment, providing excellent analgesia for hip and anterior thigh surgery. The lumbar plexus is at risk during retroperitoneal surgery and anterior approaches to the lumbar spine where retraction of the psoas is required. Direct psoas haematoma from anticoagulation or trauma can compress the plexus, producing femoral nerve palsy with quadriceps weakness and sensory loss in the anterior thigh.
Spontaneous or anticoagulant-induced haemorrhage into the psoas compartment compresses the lumbar plexus branches, producing femoral neuropathy with quadriceps weakness, anterior thigh numbness, and loss of the knee reflex; managed with anticoagulation reversal and serial imaging for resolution.
Excessive retraction of the psoas muscle during anterior lumbar spine surgery or retroperitoneal dissection stretches the lumbar plexus branches, particularly the genitofemoral and lateral femoral cutaneous nerves, causing anterior thigh and groin numbness that is usually transient.
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