Left Iliac Fossa (LIF) — Differential Diagnosis

I. Gastrointestinal Causes

1. Sigmoid Diverticulitis ⭐ (Most common cause)

• Clinical features: LIF pain (often described as "left-sided appendicitis"), fever, nausea/vomiting, altered bowel habit, dysuria (if bladder irritated), and urinary frequency
• Examination: LIF tenderness ± guarding, low-grade fever, palpable mass if abscess has formed
• Complications: Pericolic abscess, pelvic abscess, free perforation → peritonitis, fistula formation (colovesical — pneumaturia, fecaluria), bowel obstruction
• Investigations: CT abdomen/pelvis (gold standard — shows bowel wall thickening, pericolic fat stranding, diverticula, abscess/fluid), CRP/WBC raised
• Classification (Hinchey): I–IV ranging from pericolic abscess to generalized fecal peritonitis
• Management: Antibiotics ± percutaneous drainage; surgery (Hartmann's) for perforation/peritonitis

Gray's Anatomy for Students — The sigmoid colon in the LIF can form a pelvic abscess, colovesical fistula, and ureteric obstruction when diverticulitis is complicated.

2. Colorectal Carcinoma (Sigmoid/Descending Colon)

• Clinical features: Change in bowel habit (usually looser stools or alternating), rectal bleeding, dark blood mixed in stool, weight loss, anorexia, mucus PR
• Examination: May feel a firm, irregular mass in LIF; anaemia on bloods
• Presentation: Can present acutely with bowel obstruction (absolute constipation, distension, vomiting, colicky pain) or perforation
• Investigations: CT colonography, colonoscopy with biopsy, CEA; CT staging (chest/abdomen/pelvis)
• Red flags: Age >50, rectal bleeding + change in bowel habit, unexplained iron-deficiency anaemia → 2-week wait referral

3. Irritable Bowel Syndrome (IBS)

• Clinical features: Chronic/recurrent lower abdominal pain (often left-sided or suprapubic), bloating, alternating diarrhoea and constipation, relieved by defaecation, no systemic symptoms
• Examination: Loaded/tender colon; no mass, no fever, no weight loss
• Diagnosis: Diagnosis of exclusion (Rome IV criteria); exclude organic pathology first
• Important: IBS is the differential after ruling out IBD, diverticular disease, and carcinoma

4. Inflammatory Bowel Disease (IBD)

Ulcerative Colitis (UC)

• Starts at the rectum and extends proximally; left-sided colitis causes LIF pain
• Features: Bloody diarrhoea, mucus, urgency, tenesmus, cramping pain, weight loss, fever in severe disease
• Examination: LIF tenderness; extraintestinal features (uveitis, arthropathy, erythema nodosum, pyoderma)
• Investigations: Colonoscopy + biopsy; CRP/ESR/FBC; stool calprotectin

Crohn's Disease

• Can affect any part of the GI tract; left colon involvement causes LIF pain
• Features: Colicky pain, diarrhoea (may be non-bloody), weight loss, perianal disease, fistulae
• Key differentiator from UC: Skip lesions, transmural inflammation, cobblestone mucosa, perianal involvement

5. Constipation

• Often left-sided, especially in older patients or those on a low-fibre diet
• Features: Chronic dull aching LIF pain, bloating, infrequent hard stools
• Examination: Palpable faecal loading in descending/sigmoid colon
• Management: Dietary fibre, laxatives; always exclude organic cause first

6. Bowel Obstruction / Volvulus

• Sigmoid volvulus is the most common large bowel volvulus; the sigmoid twists on its mesentery
• Features: Acute onset severe colicky pain, abdominal distension, absolute constipation, vomiting
• AXR: "Coffee bean" sign — massively dilated sigmoid loop
• Management: Endoscopic decompression ± sigmoid colectomy if recurrent

7. Ischaemic Colitis

• Left colon most vulnerable (watershed area at splenic flexure — Griffiths' point)
• Clinical features: Sudden onset crampy LIF/left-sided pain, rectal bleeding, diarrhoea; often in older patients with cardiovascular risk factors or post-aortic surgery
• Examination: LIF tenderness without peritonism (usually)
• Investigations: CT (bowel wall thickening, "thumbprinting"), colonoscopy
• Key associations: Atherosclerosis, atrial fibrillation, post-AAA repair, low-flow states

8. Epiploic Appendagitis

• Inflammation/torsion of a colonic epiploic appendage (fat-filled peritoneal outpouching)
• Features: Acute, localised, sharp pain — often LIF; no fever, no leucocytosis, no change in bowel habit
• CT finding: Oval fatty mass with perilesional fat stranding and central dot sign
• Management: Self-limiting; NSAIDs for analgesia; no surgery needed

II. Gynaecological Causes (Females)

9. Ectopic Pregnancy ⭐ (Surgical emergency)

• Implantation outside the uterus (95% in fallopian tube)
• Features: Acute lower abdominal pain (unilateral), amenorrhoea, vaginal bleeding, shoulder-tip pain (haemoperitoneum → diaphragmatic irritation); haemodynamic instability if ruptured
• Examination: Cervical excitation, adnexal tenderness; signs of shock if ruptured
• Investigations: β-hCG positive; transvaginal ultrasound (empty uterus + adnexal mass); free fluid in pouch of Douglas
• Risk factors: Previous ectopic, PID, tubal surgery, IUD, infertility treatment, smoking
• Management: Medical (methotrexate) if stable and criteria met; surgical (salpingotomy/salpingectomy) if ruptured or unstable

10. Ovarian Cyst / Rupture / Haemorrhage

• Simple physiological cysts are common; complications produce acute LIF pain
• Rupture: Sudden sharp pain, free fluid on USS
• Haemorrhagic cyst: Pain can be severe; USS shows mixed echogenicity "fishnet/cobweb" pattern
• Management: Most self-resolving; surgical if persistent, large (>5 cm), or haemodynamically significant

11. Ovarian Torsion ⭐ (Surgical emergency)

• Adnexa twists on its pedicle, compromising blood supply
• Features: Acute onset severe unilateral lower abdominal pain, often colicky/intermittent, nausea and vomiting; may be associated with an underlying cyst/dermoid
• Examination: Exquisite unilateral adnexal tenderness; low-grade fever if ischaemia is advanced
• Investigations: Transvaginal USS with Doppler — absent or reduced ovarian blood flow (though normal flow does NOT exclude torsion)
• Management: Emergency laparoscopy; detorsion ± oophoropexy; oophorectomy if ovary non-viable

12. Pelvic Inflammatory Disease (PID) / Tubo-Ovarian Abscess (TOA)

• Ascending infection of upper genital tract (usually Chlamydia trachomatis, Neisseria gonorrhoeae, or polymicrobial)
• Features: Bilateral (sometimes unilateral) lower abdominal pain, vaginal discharge, fever, abnormal uterine bleeding, dyspareunia; cervical motion tenderness (CMT) on bimanual examination
• TOA: More severe presentation — pelvic mass, high fever, peritonism
• Fitz-Hugh–Curtis syndrome: RUQ pain from perihepatitis (violin-string adhesions on liver capsule)
• Investigations: High vaginal swab, NAAT for chlamydia/gonorrhoea; pelvic USS; FBC/CRP
• Management: IV/IM antibiotics (ceftriaxone + doxycycline + metronidazole); surgical drainage if TOA fails medical treatment

13. Endometriosis

• Ectopic endometrial tissue in the pelvis (ovaries, fallopian tubes, pouch of Douglas, sigmoid colon)
• Features: Chronic cyclical pelvic/LIF pain, dysmenorrhoea, dyspareunia, dyschezia (painful defaecation), infertility; pain peaks perimenstrually
• Examination: Tender nodularity in posterior fornix; fixed, retroverted uterus
• Investigations: Pelvic USS (endometrioma — "chocolate cyst" with ground-glass appearance); MRI; laparoscopy is gold standard
• Management: Hormonal suppression (COCP, GnRH analogues, progestins); surgical excision

14. Mittelschmerz

• Mid-cycle ovulatory pain from follicular rupture and peritoneal irritation by follicular fluid
• Features: Unilateral, brief (hours to days), mid-cycle, sharp/dull lower abdominal pain; no systemic features
• Management: Reassurance; NSAIDs

15. Uterine Fibroids (Leiomyomas)

• Can cause chronic pelvic/LIF pain; acute pain from red degeneration (especially in pregnancy) or torsion of a pedunculated fibroid
• Features: Heavy menstrual bleeding, pelvic pressure, urinary frequency, constipation
• Investigations: USS — heterogeneous uterine mass; MRI for surgical planning

III. Urological Causes

16. Ureteric Colic (Left)

• Left ureter runs through the LIF; a calculus can impact at the pelviureteric junction, iliac vessels crossing, or vesicoureteric junction
• Features: Severe, colicky, "loin-to-groin" pain radiating to the testis/labium majus; patient is restless (cannot find comfortable position); nausea, vomiting, microscopic haematuria
• Examination: Loin/flank tenderness; no peritonism
• Investigations: CT KUB (gold standard); urine dip (haematuria, leucocytes if infection); urine culture
• Management: Analgesia (NSAIDs ± opiates); alpha-blocker (tamsulosin) for medical expulsive therapy; urological intervention (ureteroscopy/ESWL) if stone ≥10 mm or obstruction with infection

17. Urinary Tract Infection (UTI) / Pyelonephritis

• UTI: Dysuria, frequency, urgency, suprapubic/LIF pain, haematuria; low-grade fever; positive dipstick
• Pyelonephritis: Higher fever, rigors, loin pain/tenderness, systemic illness; MSU for culture; IV antibiotics if severe

IV. Vascular Causes

18. Abdominal Aortic Aneurysm (AAA) — Leaking/Ruptured

• Do not miss — frequently misdiagnosed as renal colic or diverticular disease
• Features: Sudden severe abdominal/back/flank pain; may radiate to LIF; haemodynamic instability (hypotension, tachycardia, collapse); pulsatile abdominal mass on palpation
• AAA >5.5 cm at risk of rupture
• Management: Immediate surgical/endovascular repair; do NOT delay for imaging if shocked

19. Iliac Artery Aneurysm / Occlusion

• Isolated left iliac aneurysm can present with LIF pain ± buttock/thigh claudication
• Left iliac artery occlusion causes acute limb ischaemia + LIF discomfort

V. Musculoskeletal / Abdominal Wall

20. Inguinal Hernia

• Bowel (often sigmoid colon) or omentum herniation through the inguinal canal
• Features: Groin bulge ± LIF pain; worsens with straining/coughing; reducible (uncomplicated) or tender/irreducible (incarcerated/strangulated)
• Strangulated hernia: Ischaemic bowel — severe pain, vomiting, systemic illness → surgical emergency
• Direct vs indirect: Direct through Hesselbach's triangle; indirect through deep inguinal ring (congenital)

21. Psoas Abscess

• Collection within the psoas muscle; primary (haematogenous — Staphylococcus aureus) or secondary (adjacent pathology — Crohn's disease, diverticulitis, vertebral osteomyelitis/TB)
• Features: Insidious onset LIF/flank pain, fever, malaise; patient lies with hip flexed; positive psoas sign
• Investigations: CT demonstrates psoas enlargement with fluid collection; USS-guided aspiration
• Management: CT/USS-guided percutaneous drainage + IV antibiotics; treat underlying cause

22. Rectus Sheath Haematoma

• Bleeding into the rectus abdominis muscle sheath (usually inferior epigastric artery rupture)
• Causes: Anticoagulants (warfarin/DOAC), trauma, vigorous coughing, post-surgical
• Features: Acute lower abdominal pain, tender palpable mass that does NOT cross the midline; Fothergill's sign (mass remains palpable on tensing rectus)
• Investigations: USS/CT confirms haematoma
• Management: Usually conservative; may need embolisation or surgical evacuation if expanding

VI. Other / Rarer Causes

Summary: Structured Approach by Acuity

Investigations Framework

• Ultrasound (pelvic/transabdominal): First-line in females (gynaecological pathology, free fluid); also useful for urological causes
• CT abdomen/pelvis (with IV contrast): Gold standard for acute LIF pain — best for diverticulitis, bowel obstruction, vascular pathology, abscess, renal calculi
• AXR: Bowel obstruction, volvulus (coffee-bean sign), perforation (under diaphragm on erect CXR); calcified stones (70% visible)
• MRI pelvis: Chronic pain, endometriosis staging, pelvic masses