Portosystemic anastomoses are areas of shared venous drainage between the portal venous system and systemic venous system. Recall that the portal venous system drains blood from most the gastrointestinal tract to the liver and subsequently to the superior vena cava, while the systemic venous system drains blood from the ends of the gastrointestinal tract, where nutrients are not generally absorbed, as well as the rest of the body's organs directly to the inferior or superior vena cava.
Areas of portosystemic anastomoses are particularly important to know in the setting of portal hypertension. This is because portal hypertension leads to backup of drainage in the portal venous system, causing blood to shunt into systemic circulation via portosystemic shunts. This may lead to venous engorgement at the sites of these anastomoses, subsequently leading to the clinical manifestations discussed here. Recall that portal hypertension most commonly occurs in the setting of liver cirrhosis, though may also be seen as a result of chronic portal vein thrombosis, hepatic drug toxicity, and long-standing right-sided heart failure.
In the esophagus, the left gastric vein, which is part of the portal system, anastomoses with the esophageal veins, which are part of the systemic circulation.
In the setting of portal hypertension, venous engorgement of the anastomosis between the left gastric and esophageal veins can lead to esophageal varices, which are particularly worrysome as they can rupture and cause significant gastrointestinal bleeding.
The peraumbilical veins, which are small veins within the falciform ligament that drain blood from the abdominal wall to the liver, anastomose with the epigastric veins which drain blood from the abdominal wall to the systemic circulation via the iliac veins.
Caput medusae is the appearance of engorged veins within the superficial abdominal wall that occurs as a result of portal hypertension causing backup and venous distention at the anastomosis of the paraumbilical and epigastric veins. It appears as multiple engorged veins radiating from the umbilicus. This gives it a similar appearance to the head of the Greek mythical character Medusa, who had snakes on her head in place of hair, hence the name which literally translates to "head of Medusa".
The superior rectal veins drain blood from the sigmoid colon and proximal rectum (above the pectinate line) to the liver. This represents the distal end of the gastrointestinal tract that drains to portal circulation. The middle and inferior rectal veins drain to the systemic circulation via the internal iliac veins. These veins anastomose in the rectum, where portal hypertension can lead to anorectal varices.
Anorectal varices are caused by venous dilation at the anastoosis between the superior rectal veins (portal) and middle and inferior rectal veins (systemic). This is important to recognize as it may lead to significant rectal bleeding in these patients. It is also important to recognize that this is a distinct clinical manifestation from hemorrhoids, as the two are often confused.
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