Define Acute Appendicitis. Describe its pathogenesis, clinical features, investigations and diagnosis.

The Concept

Acute appendicitis is inflammation of the vermiform appendix and is the commonest abdominal surgical emergency. Almost the entire disease can be understood from a single initiating event — obstruction of the appendiceal lumen — which sets off a predictable cascade ending in gangrene and perforation if untreated. Knowing this sequence explains the symptoms, the signs, the urgency, and the complications.

Pathogenesis — a Logical Sequence

The lumen becomes obstructed — most often by a faecolith, or by lymphoid hyperplasia (common in the young), and rarely by a tumour or worms. Behind the obstruction, mucus and bacteria accumulate, raising intraluminal pressure. This distension compresses first the veins and lymphatics, causing ischaemia; the ischaemic mucosa ulcerates, allowing bacterial invasion of the wall (suppurative appendicitis). Progressive ischaemia leads to gangrene and then perforation, which either forms a walled-off appendicular mass or abscess or spills into the peritoneum causing generalised peritonitis.

Clinical Features — WHY the Pain Shifts

The classic history of migrating pain is explained by the two nerve supplies of the peritoneum. Initially the inflamed appendix produces poorly localised, colicky central (periumbilical) pain — because the appendix is a midgut structure whose visceral afferents refer to the T10 dermatome. As inflammation reaches the parietal peritoneum of the right iliac fossa, the pain becomes sharp, constant and localised to the right iliac fossa (McBurney's point). Accompanying features are anorexia (almost invariable), nausea, vomiting and a low-grade fever.

Examination Signs

• Tenderness and guarding at McBurney's point (two-thirds along from umbilicus to the anterior superior iliac spine), with rebound tenderness.
• Rovsing's sign — pressure in the left iliac fossa produces pain in the right iliac fossa.
• Psoas sign (pain on hip extension — retrocaecal appendix) and the obturator sign (pain on internal rotation of the flexed hip — pelvic appendix).
• Signs of peritonitis (rigidity, absent bowel sounds) if perforated.

Investigation

The diagnosis is chiefly clinical, supported by a neutrophil leucocytosis and raised CRP. A urine dipstick helps exclude a urinary tract infection, and a pregnancy test is mandatory in any woman of childbearing age to exclude an ectopic pregnancy. Ultrasound or CT is valuable in atypical presentations, in women, and in the elderly; scoring systems such as the Alvarado score aid decision-making.

Differential Diagnosis & Management

The differential is wide — mesenteric adenitis, ectopic pregnancy, ovarian cyst torsion, pelvic inflammatory disease, ureteric colic, Meckel's diverticulitis and Crohn's disease. Treatment is appendicectomy (laparoscopic or open through a Lanz/grid-iron incision) with intravenous fluids and antibiotics; selected uncomplicated cases may be treated with antibiotics alone.

Variations in Position & Presentation

The appendix has a variable position, and this explains atypical presentations. A retrocaecal appendix (the commonest position) may give flank or right-loin pain with a positive psoas sign and relatively little anterior tenderness; a pelvic appendix lies near the bladder and rectum, causing suprapubic pain, diarrhoea or urinary symptoms and a positive obturator sign; and a post-ileal appendix may produce vomiting and diarrhoea that mislead. Awareness of these positions prevents missed diagnoses, especially where the classic McBurney's-point tenderness is absent.

Special Groups — WHY Diagnosis Is Harder

Appendicitis is more dangerous and harder to diagnose at the extremes of life and in pregnancy. In the very young and the elderly, presentation is often atypical and perforation more frequent because diagnosis is delayed. In pregnancy, the enlarging uterus displaces the appendix upward, shifting the pain, and the gravid patient's symptoms overlap with normal pregnancy — so a high index of suspicion and ultrasound are used to avoid the serious fetal risk of a perforated appendix.

Complications & the Bottom Line

Untreated appendicitis progresses to perforation, then to a walled-off appendicular mass or abscess or to generalised peritonitis, with rarer sequelae such as portal pylephlebitis and sepsis. The bottom line is that appendicitis remains a clinical diagnosis demanding prompt appendicectomy: the cost of a modest negative-appendicectomy rate is accepted because the cost of a missed, perforating appendix is far greater.

• Diagnosis is mainly clinical + leucocytosis/CRP; pregnancy test mandatory in women; imaging (US/CT) if atypical.
• Treatment = appendicectomy (laparoscopic/open) + IV fluids + antibiotics.
• Complications: perforation, appendicular mass/abscess, peritonitis.

Pain shifts from umbilicus to right iliac fossa as peritoneum is involved.

• Commonest abdominal surgical emergency; caused by luminal obstruction (faecolith, lymphoid hyperplasia) → distension → ischaemia → gangrene → perforation.
• Pain shifts from periumbilical (visceral, T10) to right iliac fossa/McBurney's point (parietal); with anorexia, nausea, low fever.
• Signs: RIF tenderness/guarding/rebound, Rovsing's, psoas and obturator signs.
• Mainly clinical + leucocytosis/CRP; urine dip + pregnancy test (exclude ectopic); US/CT if atypical.
• Treat by appendicectomy + fluids + antibiotics; wide differential especially gynaecological in women.