What is the anatomy of the appendix?

Anatomy of the Appendix

The vermiform appendix is a blind-ended tube located at the junction of the small and large intestine near the ileocecal valve, with important anatomical and physiological characteristics that have clinical significance in diagnosing and treating appendicitis. 1

Anatomical Location and Structure

• The appendix is situated at the posteromedial wall of the cecum, approximately 2-3 cm below the ileocecal valve, serving as an important anatomical landmark during surgical procedures 2, 3
• The base of the appendix is typically located at the convergence of the three taeniae coli of the cecum, which can guide surgeons during appendectomy 3
• The appendix is a tubular structure with an average length of 9 cm, though it can range from 2-20 cm in adults 4
• The lumen of the appendix communicates with the cecum and contains lymphoid tissue that contributes to immune function 4

Anatomical Variations

• The position of the appendix can vary significantly, with several recognized positions:
  • Retrocecal/retrocolic (behind the cecum) - most common position (65%)
  • Pelvic/descending (pointing toward the pelvis)
  • Subcecal (below the cecum)
  • Pre-ileal and post-ileal (anterior or posterior to the ileum)
  • Lateral pouch type (situated between the cecum and lateral abdominal wall) 5
• These variations can significantly impact clinical presentation and diagnostic challenges in appendicitis 5
• The mesoappendix, a peritoneal fold containing the appendicular artery, can also have variable attachments and positions 5

Blood Supply and Lymphatic Drainage

• The appendix is supplied by the appendicular artery, a branch of the ileocolic artery, which runs in the mesoappendix 4
• The appendicular artery is an end artery with limited collateral circulation, making it susceptible to ischemia when inflamed 4
• Lymphatic drainage occurs through lymph nodes in the mesoappendix and ileocolic lymph nodes 4

Imaging Identification

• On ultrasound, the appendix appears as a blind-ending tubular structure arising from the base of the cecum 3
• Normal appendix diameter should be less than 6 mm when compressed 6
• CT imaging can visualize the appendix as a tubular structure arising from the cecum, with wall thickness normally less than 3 mm 6
• MRI can identify the appendix with high sensitivity, especially in pregnant patients where the appendix visualization rate reaches 80% compared to only 7% with ultrasound 6
• The T1 bright appendix sign (high-intensity signal filling more than half the length of the appendix on T1-weighted imaging) is a specific sign for a normal appendix on MRI 6

Physiological Function

• The appendix contains abundant lymphoid tissue (gut-associated lymphoid tissue or GALT) that plays a role in immune function 4
• Recent evidence suggests the appendix serves as a reservoir for beneficial gut bacteria, helping to repopulate the intestine after gastrointestinal illness 4
• The appendix may function as a "safe house" for commensal bacteria, contributing to immune defense against pathogens 4

Clinical Significance

• Intra-operative identification of the appendix can be challenging due to anatomical variations, especially in cases of atypical location 2
• The surgeon's macroscopic judgment of early grades of acute appendicitis is often inaccurate and highly variable 7
• Studies show that 19-40% of visually normal appendices are actually pathologically abnormal, highlighting the limitations of visual assessment during surgery 7
• The World Society of Emergency Surgery provides a weak recommendation supporting appendix removal when it appears normal during surgery for suspected appendicitis in symptomatic patients 7

Surgical Considerations

• Routine histopathology after appendectomy is strongly recommended to identify unexpected findings 6
• Operative findings and intra-operative grading correlate better than histopathology with morbidity and overall outcomes 6
• The decision to remove a normal-appearing appendix should weigh the risk of leaving a potentially abnormal appendix against the risk of appendectomy 7
• Risks of leaving an apparently normal appendix in situ include subclinical inflammation with persisting symptoms and missed appendiceal malignancy 7

Understanding the anatomy of the appendix is crucial for accurate diagnosis and appropriate surgical management of appendicitis, particularly given the significant anatomical variations that can complicate both diagnosis and treatment.