lumbosacral plexus

**Formation** - Lumbar plexus (L1–L4): forms within the psoas major muscle and emerges from its borders. Lies anterior to the lumbar transverse processes. - Sacral plexus (L4–S4): forms on the anterior surface of the piriformis. The lumbosacral trunk (L4–L5) joins S1–S4. Most branches leave the pelvis via the greater sciatic foramen. - Pudendal plexus (S2–S4) is sometimes treated as a separate part.

**Lumbar plexus branches (mnemonic: "I Ier Get Laid On Fridays" = Iliohypogastric, Ilioinguinal, Genitofemoral, Lateral femoral cutaneous, Obturator, Femoral)** - Iliohypogastric (L1): sensory to suprapubic skin; motor to internal oblique and transversus abdominis. - Ilioinguinal (L1): sensory to the anterior scrotum/labia majora and medial thigh. It travels through the inguinal canal. It is at risk during hernia repair. - Genitofemoral (L1–L2): the genital branch supplies the cremaster muscle (efferent limb of the cremasteric reflex). The femoral branch supplies skin of the upper anterior thigh. - Lateral femoral cutaneous (L2–L3): purely sensory to the anterolateral thigh. Compression under the inguinal ligament causes meralgia paresthetica (obesity, pregnancy, tight belts). - Obturator (L2–L4): passes through the obturator canal. Supplies the medial thigh adductors and the skin of the medial thigh. Injury causes weak adduction and a wide-based gait. It can be injured in pelvic surgery and by a obturator hernia. - Femoral (L2–L4): supplies the iliopsoas, quadriceps, and sartorius, plus the saphenous nerve (sensory to the medial leg). Injury causes weak hip flexion and knee extension, loss of the patellar reflex, and a buckling knee. Causes include diabetic amyotrophy, a retroperitoneal hematoma (anticoagulation), and pelvic surgery.

**Sacral plexus branches** - Superior gluteal (L4–S1): exits above the piriformis. Supplies the gluteus medius, gluteus minimus, and tensor fasciae latae. Injury (often from a misplaced IM injection or hip surgery) causes a positive Trendelenburg sign, with the pelvis dropping on the contralateral side during single-leg stance. The patient leans toward the lesion side to compensate. - Inferior gluteal (L5–S2): exits below the piriformis. Supplies the gluteus maximus. Injury causes difficulty rising from a seated position, climbing stairs, and inclining. - Sciatic (L4–S3): the largest nerve in the body. It exits below the piriformis and divides into the tibial and common fibular (peroneal) nerves. It supplies the hamstrings and the adductor magnus (hamstring part). - Tibial nerve: plantar flexion and inversion. Injury causes loss of plantar flexion and a "can't stand on tiptoes" deficit, plus sensory loss on the sole. - Common fibular nerve: wraps around the fibular neck. Injury (e.g., trauma to the lateral knee or a fibular head fracture) causes foot drop with a steppage gait, loss of dorsiflexion and eversion, and sensory loss over the dorsum of the foot and lateral leg. - Posterior femoral cutaneous (S1–S3): sensory to the posterior thigh and perineum. - Pudendal (S2–S4): exits through the greater sciatic foramen, wraps around the ischial spine, and re-enters through the lesser sciatic foramen into the pudendal (Alcock) canal. Supplies the external urethral and anal sphincters, perineal muscles, and the skin of the perineum and genitalia. Pudendal nerve block is used during childbirth, with the ischial spine as the landmark. - Nerve to the piriformis (S1–S2), nerve to the obturator internus (L5–S1), and nerve to the quadratus femoris (L4–S1). - Pelvic splanchnic nerves (S2–S4): parasympathetic supply to the hindgut (distal to the splenic flexure), bladder, and erectile tissue. Remember "S2, 3, 4 keeps the penis (and the poop) off the floor."

**Clinical correlations** - Lumbosacral radiculopathy (herniated disc): L4–L5 and L5–S1 are the most common levels. - L4: weak knee extension, reduced patellar reflex, sensory loss over the medial leg. - L5: weak great toe dorsiflexion and foot drop, sensory loss over the dorsum of the foot. - S1: weak plantar flexion, reduced Achilles reflex, sensory loss over the lateral foot. - Cauda equina syndrome: compression of the lumbosacral nerve roots below the conus. Presents with saddle anesthesia, urinary retention or overflow incontinence, bowel dysfunction, and bilateral leg weakness with areflexia (a lower motor neuron pattern). It is a surgical emergency requiring urgent MRI and decompression. - Lumbosacral plexopathy: causes include diabetic amyotrophy (diabetic lumbosacral radiculoplexus neuropathy), retroperitoneal hematoma or abscess (psoas), pelvic tumors, radiation, and obstetric injury (the lumbosacral trunk is compressed by the fetal head). Presents with asymmetric weakness and pain in the leg and involves several nerve territories, which separates it from a single peripheral nerve lesion. - Piriformis syndrome: sciatic nerve irritation by the piriformis. - Reflexes to remember: patellar (L2–L4) and Achilles (S1–S2). - Dermatomes: L4 = medial malleolus, L5 = dorsum of the foot/great toe, S1 = lateral foot/small toe. - Psoas abscess (e.g., from Pott disease or Crohn disease) can irritate the lumbar plexus and cause femoral neuropathy.

**High-yield pearls** - Gluteal IM injections go in the upper outer quadrant to avoid the sciatic nerve and the superior gluteal nerve. - Anterior thigh = femoral, medial thigh = obturator, posterior thigh = sciatic (tibial portion), and everything below the knee except the medial leg skin comes from the sciatic branches. The saphenous nerve (femoral) supplies the medial leg skin. - The lumbar plexus lies in the psoas, whereas the sacral plexus lies on the piriformis.