Pelvic vessels and nerves
Pelvic vessels, lymphatics and nerves on imaging
The pelvic side wall carries the iliac vessels, the lymph node chains that follow them, the sacral plexus and the autonomic plexuses. CT angiography, embolisation, nodal staging and nerve-sparing surgery all depend on knowing the branches of the internal iliac artery, the collateral routes when it is tied, which nodes drain which organ, and where the autonomic nerves run.
Common and external iliac arteries
- The aorta divides at L4; each common iliac artery divides into internal and external iliac arteries in front of the sacroiliac joint. The ureter crosses the bifurcation here, a landmark in aneurysm repair and hysterectomy.
- The external iliac gives only two branches, just above the inguinal ligament: inferior epigastric and deep circumflex iliac (the pedicle of an iliac crest bone flap).
- External iliac endofibrosis: exercise-only thigh pain in elite cyclists, ankle-brachial index falling after effort.
- The ovarian artery arises from the aorta at about L2 and runs in the infundibulopelvic ligament; bleeding from it after oophorectomy forms a retroperitoneal haematoma spreading up from the pelvic brim towards the kidney.
Internal iliac artery
- Short (about 4 cm); divides at the upper border of the greater sciatic foramen.
| Division | Branches |
|---|---|
| Posterior | Iliolumbar, lateral sacral, superior gluteal |
| Anterior | Umbilical (superior vesical), obturator, inferior vesical or vaginal, uterine, middle rectal, internal pudendal, inferior gluteal |
- Ligation for postpartum haemorrhage is done 2.5–3 cm beyond the origin (or on the anterior division) to spare the posterior division to the buttock. The ureter lies in front of the artery, stuck to the peritoneum; the internal iliac vein lies behind and lateral, at risk when the clamp passes beneath.
- After ligation, flow returns through lumbar arteries (aorta) to the iliolumbar, median sacral to lateral sacral, superior rectal to middle rectal, and ovarian to uterine arteries. Ligation therefore lowers pulse pressure rather than stopping flow.
- Uterine artery embolisation: the catheter enters the anterior division, then the uterine artery. The uterine artery crosses above the ureter about 1.5–2 cm lateral to the cervix; its descending cervical branches lie at 3 and 9 o'clock.
- Internal iliac occlusion or embolisation: buttock claudication (reduced gluteal flow) and erectile dysfunction.
- Obturator artery runs on the inner surface of the superior pubic ramus; it bleeds with ramus fractures. An abnormal obturator branch from the inferior epigastric (corona mortis) crosses the pubic ramus.
- The posterior division's lateral sacral arteries run vertically on the front of the sacrum lateral to the anterior foramina.
- Persistent sciatic artery: embryonic axial artery from the internal iliac through the greater sciatic foramen; aneurysm gives a pulsatile buttock mass.
- Internal iliac aneurysm: deep in the pelvis and impalpable; compresses ureter, rectum and sacral nerves.
Pelvic veins
- Phleboliths: small round calcifications with a lucent centre, low in the lateral pelvis below the interspinous line; a ureteric stone lies along the ureter and lacks the lucent centre.
- Ovarian vein: the right drains to the IVC, the left to the left renal vein; postpartum thrombosis is commoner on the right.
Lymph node groups
- Chains follow the arteries: external iliac, internal iliac (hypogastric), obturator, common iliac, presacral, then para-aortic.
- Obturator nodes lie below the external iliac vein and above the obturator nerve, the key group cleared in pelvic lymphadenectomy.
- Pedal lymphangiography fills inguinal, external iliac, common iliac and para-aortic nodes but not the internal iliac group.
| Organ | First nodes |
|---|---|
| Cervix | Parametrial, obturator, internal and external iliac |
| Body of uterus | External and internal iliac; para-aortic; cornual region along the round ligament to superficial inguinal |
| Ovary and testis | Para-aortic (L1–L2), following the gonadal vessels |
| Prostate, bladder base | Internal iliac |
| Upper rectum | Along the superior rectal vessels to inferior mesenteric nodes |
| Vulva, scrotum, anal canal below pectinate line | Superficial inguinal |
Somatic nerves
- Lumbosacral trunk (L4–L5) crosses the sacral ala to join the sacral plexus on piriformis.
- Sciatic nerve leaves below piriformis; in a common variant the common fibular division passes through the muscle (piriformis syndrome on MRI).
- Obturator nerve runs on the side wall below the pelvic brim to the obturator canal; ovarian disease can refer pain to the medial thigh.
- Femoral nerve lies in the groove between psoas and iliacus; an iliacus haematoma on anticoagulation causes quadriceps weakness, an absent knee jerk and anterior thigh numbness.
- Pudendal nerve leaves through the greater sciatic foramen, hooks round the ischial spine and re-enters through the lesser foramen into the pudendal (Alcock) canal, a split in the obturator internus fascia. Obturator internus covers the obturator membrane and its tendon turns round the lesser sciatic notch.
- Sacral nerve schwannoma smoothly enlarges and scallops a sacral foramen.
Autonomic plexuses
- Superior hypogastric plexus (presacral nerve): in front of L5 and the sacral promontory, between the common iliac arteries. It carries sympathetic fibres from the lumbar splanchnic nerves and aortic plexus. Damage at aortic surgery gives dry orgasm (no emission) with preserved erection; it is divided in presacral neurectomy for midline dysmenorrhoea.
- It continues as the hypogastric nerves to the inferior hypogastric (pelvic) plexus beside the rectum, cervix, vaginal fornix and bladder base, which is joined by the pelvic splanchnic nerves (S2–S4), parasympathetic.
- Pelvic splanchnics control erection, detrusor contraction and defecation. Injury at radical hysterectomy causes urinary retention with reduced bladder sensation.
- The lumbar sympathetic chain lies on the medial edge of psoas; its injury gives a warm dry leg.
Remember Only three branches go backwards (iliolumbar, lateral sacral, superior gluteal); every branch to a pelvic organ or the perineum comes from the anterior division.
Exam trap Tying the internal iliac artery does not stop pelvic blood flow; collaterals from the aorta and IMA refill its distal branches.
Exam trap Lymphangiography from the feet never shows internal iliac nodes.
Test yourself on this topic
5 of the 38 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
An iliac crest bone flap is planned to reconstruct the mandible. CT angiography is used to map the pedicle artery, which arises from the external iliac artery just above the inguinal ligament, opposite the inferior epigastric artery, and runs laterally along the inner lip of the iliac crest. Which artery is this?
Answer: Deep circumflex iliac artery
The deep circumflex iliac artery arises from the lateral side of the external iliac artery just above the inguinal ligament, roughly opposite the inferior epigastric artery. It runs laterally behind the inguinal ligament to the anterior superior iliac spine and then along the inner lip of the iliac crest, supplying the bone and the abdominal wall muscles. This makes it the pedicle for vascularised iliac crest flaps. The superficial circumflex iliac artery comes from the femoral artery and supplies skin.
Deep circumflex iliac artery
- From external iliac, above inguinal ligament
- Opposite the inferior epigastric
- Along the inner lip of iliac crest
- Pedicle of the iliac crest flap
Remember Inner iliac crest blood comes from the deep circumflex iliac artery.
Exam trap Its ascending branch can be injured by lateral laparoscopic ports, causing abdominal wall haematoma.
Why the other options are wrong
- Superficial circumflex iliac artery A branch of the femoral artery that supplies the skin of the groin.
- Iliolumbar artery Arises from the internal iliac and supplies the iliac fossa from behind.
- Superior gluteal artery Leaves the pelvis through the greater sciatic foramen to the buttock.
- Fourth lumbar artery Runs on the posterior abdominal wall from the aorta.
A 66-year-old man on warfarin with an INR of 6 has right groin and iliac fossa pain and cannot straighten his right knee against gravity. The knee jerk is absent and the front of the thigh is numb. CT shows a large haematoma within the right iliacus beneath the iliac fascia. Which nerve is compressed?
Answer: Femoral nerve in the iliacus groove
The femoral nerve (L2 to L4) emerges from the lateral border of psoas and runs down in the groove between psoas and iliacus, beneath the tough iliac fascia, to pass under the inguinal ligament. A haematoma in the iliacus is confined by this fascia, so pressure rises and compresses the nerve. This causes weak knee extension (quadriceps), a lost knee jerk and numbness over the front of the thigh and medial leg. Anticoagulation and haemophilia are the usual causes.
Iliacus haematoma
- Femoral nerve between psoas and iliacus
- Under the tight iliac fascia
- Weak knee extension, lost knee jerk
- Usually anticoagulation or haemophilia
Remember Anticoagulated patient who cannot extend the knee: iliacus haematoma on the femoral nerve.
Exam trap Hip flexion eases the pain, so the patient lies with the hip flexed, mimicking psoas abscess.
Why the other options are wrong
- Obturator nerve on the pelvic wall Supplies the adductors; would not weaken knee extension.
- Lateral femoral cutaneous nerve Purely sensory to the lateral thigh; no motor loss.
- Genitofemoral nerve on psoas Supplies a small area of the upper thigh and scrotum; no motor loss in the knee.
- Lumbosacral trunk on the sacral ala Contributes to the sciatic nerve; would weaken foot movement, not knee extension.
In some people the sciatic nerve divides inside the pelvis, and one of its divisions passes through the belly of piriformis instead of below it. Which division most commonly takes this course?
Answer: Common fibular division
The sciatic nerve usually leaves the pelvis as a single trunk through the greater sciatic foramen below piriformis. In about 10 to 15% of people its two divisions separate within the pelvis, and the common fibular (peroneal) division then pierces piriformis or passes above it, while the tibial division passes below. These variants may be squeezed by the muscle, one cause of piriformis syndrome with sciatic-type pain. The other nerves listed all pass below piriformis.
Sciatic nerve variants
- Usually below piriformis as one trunk
- High division in about 10-15%
- Common fibular division through or above piriformis
- Can cause piriformis syndrome
Remember The fibular division is the one that goes through piriformis.
Exam trap A split sciatic nerve can be partly missed by a block aimed at a single trunk.
Why the other options are wrong
- Tibial division Almost always passes below piriformis.
- Posterior femoral cutaneous nerve A separate nerve that passes below piriformis.
- Pudendal nerve Leaves below piriformis and hooks round the ischial spine.
- Inferior gluteal nerve Leaves below piriformis to supply gluteus maximus.
At which point does the inferior mesenteric artery change its name to become the superior rectal artery?
Answer: Where it crosses the left common iliac artery
The inferior mesenteric artery arises from the front of the aorta at L3 and runs down and to the left. It gives the left colic and sigmoid branches, then crosses the left common iliac artery, where it becomes the superior rectal artery. This vessel descends in the root of the sigmoid mesocolon and divides at about S3 into right and left branches on the back of the rectum. The crossing of the left common iliac vessels is the standard landmark.
Superior rectal artery
- IMA from aorta at L3
- Gives left colic and sigmoid branches
- Crosses left common iliac artery to become superior rectal
- Divides at about S3 behind the rectum
Remember IMA crosses the left iliac and becomes superior rectal.
Exam trap High ligation of the IMA in cancer surgery is done at its aortic origin, above the left colic artery.
Why the other options are wrong
- At the level of the sacral promontory midline Too far medial and low; the change happens at the iliac crossing.
- At the origin of the left colic artery The IMA continues as the IMA beyond the left colic artery.
- At the rectosigmoid junction at S3 Here the superior rectal artery divides into right and left branches.
- At the anterior peritoneal reflection Lies much lower, on the rectum itself.
Before endovascular repair of an aortoiliac aneurysm, the right internal iliac artery of a 70-year-old man is coil-embolised to prevent a leak. A month later he has cramping pain in the right buttock after walking 100 metres, with normal femoral and foot pulses. What explains his symptom?
Answer: Reduced superior and inferior gluteal flow
The gluteal muscles are supplied by the superior and inferior gluteal arteries, both branches of the internal iliac artery. Occluding the internal iliac artery reduces their inflow, so during exercise the muscles become ischaemic and cause buttock claudication. Femoral and foot pulses stay normal because the external iliac system is untouched. Collaterals from the other internal iliac, the lumbar arteries and the profunda femoris often improve symptoms over months.
Buttock claudication
- Gluteal muscles fed by internal iliac branches
- Superior and inferior gluteal arteries
- Pulses in the leg normal
- Collaterals often improve it with time
Remember Buttock cramp with normal leg pulses: internal iliac inflow.
Exam trap Embolising both internal iliac arteries also risks impotence and colonic or spinal ischaemia.
Why the other options are wrong
- Occlusion of the right femoral artery The femoral and foot pulses are normal.
- Spinal cord ischaemia Causes leg weakness and sensory loss, not exercise-induced buttock cramp.
- Compression of the sciatic nerve by the coils Coils lie inside the vessel and do not press on the nerve.
- Thrombosis of the internal iliac vein Venous thrombosis does not cause exertional claudication.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Pelvis, Pelvic vessels, lymphatics and nerves