Arteriovenous Malformations (AVMs) are abnormal connections within the body’s vascular network. They occur when arteries, which carry oxygenated blood, connect directly to veins, bypassing the necessary capillary system. AVMs can develop in various locations, but they are particularly relevant in the gastrointestinal tract. The colon is one of the most frequent sites for these vascular anomalies, often leading to chronic health concerns.
Defining Colonic AVMs
A Colonic AVM is an acquired vascular lesion characterized by a cluster of fragile, dilated blood vessels in the wall of the large intestine. In the colon, the term AVM is frequently used interchangeably with angiodysplasia. These lesions are typically small, measuring less than five millimeters, and are formed by an abnormal tuft of vessels located in the submucosa or mucosa layer of the colon wall.
Normal colonic blood flow moves from the artery through a capillary bed and then into a vein, which regulates pressure. In an AVM, this capillary network is absent, causing high-pressure arterial blood to flow directly into the low-pressure venous system. This chronic exposure causes the veins to become tortuous, dilated, and extremely fragile, making them susceptible to rupture and bleeding. Colonic AVMs are most commonly found in the right side of the colon, specifically the cecum and ascending colon.
Primary Causes and Contributing Factors
Colonic AVMs are degenerative lesions strongly associated with advanced age, usually appearing in individuals over 60. The prevailing theory for their development centers on chronic, low-grade venous obstruction and mechanical stress on the colon wall. Repeated increases in tension or pressure within the colon are believed to intermittently obstruct the small submucosal veins.
This obstruction is particularly impactful in the right colon, which has the largest diameter and experiences the highest wall tension (Laplace’s Law). Over many years, this recurrent stress and compromised venous drainage cause the veins to gradually dilate and become twisted. This eventually leads to the formation of a direct, fragile connection between the artery and vein, explaining the age-related prevalence of these lesions.
Certain underlying health conditions can contribute to the formation of AVMs or increase the risk of bleeding. Chronic kidney disease, especially in patients requiring dialysis, is a known risk factor for developing multiple vascular lesions. The link is thought to involve circulating toxins or hormonal imbalances that affect vascular integrity.
Another notable association is Heyde’s syndrome, which involves aortic stenosis and gastrointestinal bleeding from angiodysplasia. Turbulent blood flow across the narrowed aortic valve causes mechanical destruction of Von Willebrand factor, a protein essential for blood clotting. This deficiency increases the risk of bleeding from the already fragile AVM lesions.
Recognizable Symptoms and Complications
The primary clinical manifestation of a Colonic AVM is gastrointestinal bleeding, which is often chronic and intermittent. Since the bleeding may be slow and minimal, it can be occult, meaning it is not visible in the stool. This chronic blood loss frequently leads to iron deficiency anemia, causing symptoms like persistent fatigue, weakness, and pallor.
When bleeding is more pronounced, patients may notice changes in their stool appearance. Bleeding from the right colon often results in melena (black, tarry, foul-smelling stool) caused by the digestion of blood. If the AVM is closer to the rectum, the blood may appear bright red, a condition called hematochezia. Bleeding from a Colonic AVM is typically painless.
Diagnosis and Treatment Overview
Diagnosing a Colonic AVM usually begins with an evaluation for the source of gastrointestinal bleeding or unexplained anemia. The definitive diagnostic tool is a colonoscopy, which allows a physician to visually inspect the entire large intestine. During the procedure, AVMs typically appear as flat, bright red spots or small, spider-web-like clusters of vessels on the inner lining of the colon.
If the bleeding is obscure or cannot be located by colonoscopy, other imaging methods may be used. These include capsule endoscopy, which involves swallowing a pill-sized camera, or angiography, which involves injecting dye into the blood vessels. Angiography is often reserved for cases of severe, active bleeding to pinpoint the exact location.
The main goal of treatment is to stop the bleeding from the fragile vessels. The most common and effective primary treatment is endoscopic therapy performed during a colonoscopy. Techniques such as thermal coagulation or argon plasma coagulation (APC) apply heat or an electrical current to the lesion, effectively sealing the abnormal blood vessels. These methods are highly targeted.
For patients with chronic bleeding, treatment also involves managing the resulting anemia. This requires oral iron supplementation to restore iron stores, and in severe cases, blood transfusions may be necessary. Surgical resection of the affected segment of the colon is generally considered a last resort, reserved for extensive lesions or when repeated endoscopic treatments fail to control severe, persistent bleeding.