Nerve supply to the pelvis
The nerve supply to the pelvis comes from the lumbar and sacral plexuses with additional autonomic supply from the hypogastric plexuses and pelvic splanchnic nerves.
The lumbar plexus
The lumbar plexus is a web of nerves in the lumbar region that arise from L1-L4 with contributing branches from T12.
This plexus of nerves form the iliohypogastric, ilioinguinal, genitofemoral, femoral, and obturator nerves as well as the lateral femoral cutaneous nerve of the thigh (LFCNT). The ventral rami of the fourth lumbar plexus passes communicating branches to the sacral plexus (lumbar- sacral plexus) (Box 3.5 and Figure 3.7a).The ilioinguinal nerve passes around into the anterior abdominal wall entering 3-4 cm medial and inferior to the ASIS and terminates between the transversus abdominus muscle and the internal oblique about 2-3 cm lateral to the midline and superior to the pubis. The iliohypogastric nerve travels in a similar direction but enters the abdominal wall higher but still below the ASIS and terminates in a more lateral and superior position. The genitofemoral nerve travels inferiorly on the psoas muscle lateral to the external iliac vessels, while the femoral nerve travels within psoas, emerging at its lower and lateral border and passing under the inguinal ligament. The obturator nerve descends through the psoas muscle and emerges from its medial border at the level of the pelvic brim. It then passes behind the common iliac vessel leaving the pelvis through the obturator foramen. The LFCNT emerges from the lateral border of psoas and crosses the iliacus muscle and towards the ASIS, passing underneath the inguinal ligament.
Clinical considerations
Branches of the lumbar plexus are commonly cut during gynaecological surgery. Damage to the ilioinguinal and iliohypogastric nerves can occur during a Pfannenstiel incision or during laparoscopic surgery when ports are placed below the ASIS.
Damage to these nerves can cause paraesthesia and burning pain to the lower abdomen, groin, labia, suprapubic area, and inner thigh.The genitofemoral nerve appears like a piece of white cotton lying on the psoas muscle and can be seen and cut while performing an external iliac lymphadenectomy. Transection of the genitofemoral nerve can cause neuralgia and paraesthesia of the inner thigh and labia majora. The femoral nerve can be damaged by deep retractor blades pressing on the psoas muscle and can cause weakness of the quadriceps muscle, difficulty with ambulation, and severe pain.
The obturator nerve can be seen during an obturator lymphadenectomy. Great care is required to avoid transecting this nerve during this surgery. Transection of this nerve can cause
Box 3.5 Nerves of the lumbar plexus
• T12-L1: iliohypogastric
• L1: ilioinguinal
• L1-L2: genitofemoral
• L2-L3: lateral femoral cutaneous nerve of the thigh (LFCNT)
• L2-L4: femoral
• L2-L4: obturator problems with hip adduction, unstable walking, and paraesthesia of the inner thigh and groin.
Damage to the LFCNT causes numbness of the skin of the outer thigh called meralgia paraesthetica. This can occur in obese patients while the nerve is stretched under the inguinal ligament by the weight of a panniculus hanging down. A similar injury can occur while placing a patient in surgical stirrups causing strain on the nerve as it passes under the inguinal ligament.
The sacral plexus
The sacral plexus derives from S1- S4 with additional nerves from L4 and L5 (Box 3.6 and Figure 3.7b). Fibres run down on the posterior pelvic wall on top of pyriformis.
The superior gluteal nerve (L4-S1) passes through the greater sciatic foramen and innervates the gluteal muscles along with the inferior gluteal nerve (L5-S3). The sciatic nerve (L4-S3) is the largest nerve in the body and also passes through the greater sciatic foramen towards the gluteal area. The nerve to the quadratus femoris (L4-S1) also leaves the greater sciatic foramen and innervates the hip muscles along with the nerve to the obturator internus (L5-S2). The other muscles of the sacral plexus supply the muscles they refer to. The pelvic splanchnic nerves are part of the autonomic nervous supply.