Anatomical Classification of Caterpillar Hump of RHA and its Surgical Importance (Nagpur Classification)

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Executive Summary

The "caterpillar hump," also known as the "Moynihan hump," is a critical anatomical variation of the Right Hepatic Artery (RHA) characterized by a tortuous, U-shaped loop. During hepatobiliary surgeries such as laparoscopic cholecystectomy, this variation presents a significant risk: the RHA may be mistakenly identified as the cystic artery (CA) and ligated, leading to necrosis of the liver's right functional lobe.

A retrospective study of 600 laparoscopic videos conducted in Nagpur, India, identified the caterpillar hump in 3.5% of cases. The study proposes the Nagpur Classification, which categorizes the hump into three primary types—Supracystic, Paracystic, and Infracystic—based on its relationship to the cystic duct (CD). The Paracystic type represents the highest surgical risk due to the presence of short cystic arteries and the hump's close proximity to the dissection site. Surgeons are advised to visualize the RHA above and below the origin of the cystic branch to prevent vascular catastrophes and biliary injuries.

Anatomical Overview

Normal RHA Anatomy

In standard anatomy, the Right Hepatic Artery originates from the hepatic artery proper, crosses anterior to the portal vein, and passes behind the common hepatic duct (CHD) to enter Calot’s triangle. It typically gives off the cystic artery within this triangle before turning upward toward the right lobe of the liver.

The Caterpillar (Moynihan) Hump

The caterpillar hump is a rare anomaly where the RHA follows a tortuous course, producing a loop that may point downward, upward, to the right, or to the left. This loop can occur both inside and outside Calot's triangle and brings the RHA into dangerously close proximity to the gallbladder and cystic duct.

The Nagpur Classification of Caterpillar Humps

The Nagpur Classification defines three distinct types of humps based on their anatomical relationship to the cystic duct and the resulting length and number of cystic arteries.

1. Supracystic Hump

  • Position: The hump is situated superior to the cystic duct, nearer to the hilum of the liver.

  • Vascular Characteristics: The cystic artery is typically long, originating from the inferior part of the loop.

  • Surgical Risk: Because the hump remains away from the primary surgical dissection area and the cystic artery is long, the risk of accidental ligation or injury is relatively low.

2. Paracystic Hump

  • Position: The hump is located at the same level as the confluence of the cystic duct and the CHD.

  • Vascular Characteristics: This type usually yields two very short branches (anterior and posterior cystic arteries).

  • Surgical Risk: This is the most dangerous variant. The proximity of the hump to the cystic duct often leads to the RHA being mistaken for the CA. Furthermore, the short cystic arteries can be easily avulsed from the RHA if excessive traction is applied to the gallbladder, resulting in brisk bleeding.

3. Infracystic Hump

  • Position: The hump is located below the confluence, toward the duodenum.

  • Vascular Characteristics: The cystic artery is typically single and long, originating from the ascending part of the hump.

  • Surgical Risk: Similar to the supracystic type, the risk of mishap is lower because the hump is further from the immediate site of dissection.

Additional Sub-Classifications

The Nagpur Classification further categorizes humps by:

  • Loop Position: Anterior or posterior in relation to the CHD/CBD.

  • Quantity: Single hump or double hump.

  • Arterial Supply: Single cystic artery or double cystic artery.

Clinical Study Results and Incidence

The Nagpur study reviewed 600 videos of laparoscopic cholecystectomy and CBD exploration from 2012 to 2021.

Study Findings (Nagpur Study)

Surgical Significance and Risk Mitigation

The presence of a caterpillar hump fundamentally alters the safety profile of cholecystectomy.

Primary Surgical Risks

  • Inadvertent Ligation: An unusually large "cystic artery" seen through a laparoscope should be suspected of being a caterpillar hump. Ligation leads to hepatic necrosis.

  • Avulsion and Hemorrhage: Short cystic arteries originating from a Paracystic hump may be torn from the RHA during gallbladder traction, causing uncontrolled bleeding.

  • Multiple Branches: Occasionally, the hump supplies the gallbladder through multiple small twigs rather than a single artery; the RHA can be injured while attempting to secure these individual branches.

  • Biliary Injury: Vascular injury is often a precursor to iatrogenic extrahepatic biliary injury and remains a primary reason for conversion to open surgery (incidence 0–1.9%, mortality 0.02%).

Clinical Recommendations for Surgeons

  • Verification of Anatomy: Surgeons must locate the origin of the cystic artery and visualize the RHA both above and below this origin before any ligation.

  • Caution with Traction: Excessive cephalad traction on the infundibulum can displace an Infracystic hump into a Paracystic position, potentially confusing the anatomy.

  • Caliber is Unreliable: The size of the vessel is not a reliable indicator; a vessel resembling a cystic artery in its course and caliber may still be the RHA.

  • Technological Aids: The use of high-quality optics, safe energy sources, and infrared imaging (such as ICG) can significantly improve the visualization of Calot’s anatomy and identify these variants.

Conclusion

The Nagpur Classification provides a structured framework for identifying the vascular risks associated with the caterpillar hump of the RHA. By recognizing the Paracystic variant as a high-risk anatomical configuration, surgeons can exercise necessary caution to prevent life-threatening vascular and biliary complications. Awareness of these variations is essential for maintaining "surgical conscience" during laparoscopic procedures.