Overview
The appendix is a narrow, hollow and blind-ended muscular tube which arises from the posteromedial surface of the caecum. It contains a large amount of lymphoid tissue, although its complete role in the immune system is not fully understood. It is supported by the mesoappendix, a fold of mesentery which suspends the appendix from the terminal ileum
Origin
At the origin of the taenia coli, 2 cm inferior to the ileocaecal valve. Its position within right iliac fossa region of the abdomen corresponds to a surface landmark known as McBurney’s point.
Orientation
The orientation of the appendix is highly variable and is described in relation to the ileum, caecum or pelvis.
- Pre-ileal – anterior to the terminal ileum – 1 or 2 o’clock.
- Post-ileal – posterior to the terminal ileum – 1 or 2 o’clock.
- Sub-ileal – parallel with the terminal ileum – 3 o’clock.
- Pelvic – descending over the pelvic brim – 5 o’clock.
- Subcecal – below the cecum – 6 o’clock.
- Paracecal – alongside the lateral border of the cecum – 10 o’clock.
- Retrocecal – behind the cecum – 11 o’clock (most common position)
Arterial Supply
- Derived from the embrylogic midgut, therefore supplied via branches of the superior mesenteric vessels
- Appendicular artery – branch of ileocaecal artery (branch of superior mesenteric artery), descends posterior to terminal ileum to enter mesoappendix and runs to the tip of the appendix
Venous Drainage
- Appendicular vein – contained within the mesoappendix, drains to the ileocolic vein
Innervation
- Ileocolic branch of the superior mesenteric plexus – supplies sympathethic and parasympathetic fibres which accompany the ileocolic artery
Lymphatics
- Contains lymphatic tissue within its walls
- Lymphatic fluid from the appendix drains into lymph nodes within the mesoappendix and into the ileocolic lymph nodes (which surround the ileocolic artery).
Variants
- Valve of the vermiform appendix (Gerlach’s valve) – semi-circular fold of mucous membrane at the opening of the appendix – occassionally seen
- Absent appendix (1 in 100,000)
- Subhepatic cecum
Radiological Appearance
General Features
- Diameter < 6 mm
- Wall thickness < 2 mm
MRI
- T1: Low luminal signal intensity
- T2: Low luminal signal intensity