Lower GI Bleeding: Diverticula, Angiodysplasia, and How Doctors Find the Cause

When you see bright red blood in your stool or on the toilet paper, it’s natural to panic. But not all lower GI bleeding is the same. Two of the most common causes - diverticula and angiodysplasia - look very different, act differently, and need totally different approaches. Knowing how they’re diagnosed and treated can make all the difference in your recovery.

What Exactly Is Lower GI Bleeding?

Lower gastrointestinal (GI) bleeding means blood is coming from somewhere in your colon, rectum, or anus - anywhere past the ligament of Treitz, which is deep in your small intestine. The most obvious sign is hematochezia: bright red or maroon blood in your stool. Sometimes it’s just a few streaks; other times, it’s a full bowl of blood. This isn’t the same as melena (black, tarry stools), which usually points to bleeding higher up, like in your stomach or duodenum.

It’s more common than you think. About 20 to 33% of all GI bleeds happen below the small intestine. In people over 60, it’s one of the top reasons they end up in the hospital. The good news? Most of the time, the bleeding stops on its own. The challenge is figuring out why it started - because the cause determines everything that comes next.

Diverticula: The Silent Bleeder

Diverticula are small pouches that form in the wall of your colon, especially in the sigmoid area. They’re super common - over half of people over 60 have them. Most never cause problems. But when one of these pouches develops a weak spot near a blood vessel, that vessel can rupture. And when it does? It bleeds. Hard.

This isn’t diverticulitis. That’s when the pouches get inflamed and painful. Diverticular bleeding is different. It’s painless. Sudden. And sometimes massive. People describe it as “like a faucet turned on.” You might feel fine one minute, then suddenly dizzy, weak, and covered in blood.

Why does this happen? The blood vessels that feed the colon run right next to the inside of the bowel wall. When a diverticulum forms, the vessel gets stretched over the top of the pouch. It’s like a rubber band being pulled too tight - eventually, it snaps. This is why the bleeding is often so dramatic. About 30 to 50% of hospitalizations for lower GI bleeding are due to diverticula.

Angiodysplasia: The Slow Leak

Now, think of a completely different scenario. You’ve been feeling tired for months. Your skin feels pale. You get winded climbing stairs. You don’t see blood in your stool - not even once. But your blood tests show you’re anemic. That’s often angiodysplasia.

Also called vascular ectasia or AVM (arteriovenous malformation), angiodysplasia is when small blood vessels in the colon grow abnormally. They form tangled, fragile networks - kind of like spider veins inside your intestine. These aren’t caused by inflammation or injury. They just happen over time, especially as you age. Over 80% of cases occur in people 65 and older, with the average patient being 72.

The bleeding is slow. It drips. It doesn’t flood. But over weeks or months, it adds up. That’s why people with this condition often have iron deficiency anemia before they ever notice blood in their stool. It’s easy to miss. Many patients go years with unexplained fatigue before someone connects the dots.

There’s also a surprising link to heart disease. If you have aortic stenosis - a narrowing of the valve that pumps blood out of your heart - the turbulent flow can damage a key clotting protein called von Willebrand factor. That makes you more likely to bleed from these fragile vessels. So if you’re an older patient with heart valve issues and unexplained GI bleeding, doctors should definitely consider this.

How Do Doctors Figure Out What’s Causing the Bleed?

The first thing they do? Stabilize you. If you’re dizzy, your heart is racing, or your blood pressure is low, they’ll give you fluids and maybe a blood transfusion. Then they’ll check your hemoglobin. If it’s below 10 g/dL, you’ve lost a significant amount of blood.

After that, the real detective work begins. Here’s how it usually goes:

1. Colonoscopy - This is the gold standard. It’s done within 24 hours of admission. A flexible scope is inserted through the anus, letting the doctor see the entire colon. If they spot a bleeding diverticulum - a red, oozing pouch - they can treat it right away. Same with angiodysplasia: they can zap the abnormal vessels with heat or electricity.
2. CT Angiography - If the colonoscopy doesn’t find anything but you’re still bleeding, this scan looks for active bleeding in real time. It can detect leaks as small as 0.5 mL per minute. It’s especially helpful when you’re too unstable for a scope.
3. Capsule Endoscopy - Sometimes the bleed is coming from your small intestine. You swallow a tiny camera in a pill. It takes pictures as it travels. It finds the cause in about 62% of cases where colonoscopy was negative.
4. Device-Assisted Enteroscopy - This is more invasive. A special scope with balloons is used to reach deep into the small bowel. It’s not for everyone - it needs an expert and takes longer - but it finds the problem in 71% of tricky cases.

One thing to know: colonoscopy doesn’t always catch angiodysplasia. These lesions are small and can be hard to see. A 2022 study found that using AI-assisted colonoscopy improved detection by 35%. That’s huge. If your first colonoscopy was negative but you’re still anemic, ask if AI tools were used. If not, a repeat scope with better tech might be needed.

How Is Each Condition Treated?

For diverticula, about 80% of bleeds stop without any treatment. Doctors just monitor you. If it doesn’t stop, they go in with a scope. They inject epinephrine to shrink the blood vessel, then use heat (thermal coagulation) to seal it. It works in 85-90% of cases. But rebleeding is common - about 1 in 4 people will bleed again. That’s why some patients need surgery to remove the affected segment of colon.

Angiodysplasia is trickier. Even after successful endoscopic treatment, up to 40% of people bleed again within two years. That’s why doctors are turning to medical options. Thalidomide - yes, the same drug used for leprosy and multiple myeloma - has shown surprising results. A 2019 trial found that 100 mg daily cut transfusion needs by 70%. It’s not a cure, but it helps. Octreotide, a hormone-like drug, is also used as a short-term fix. Both are used off-label but are backed by solid data.

What Happens After the Bleeding Stops?

You might think the story ends once the blood stops. But it doesn’t. For diverticula, you’re not at higher risk of future bleeds just because you had one. But you still need to manage your colon health - eat fiber, stay hydrated, avoid NSAIDs. No need for routine repeat scopes unless symptoms return.

For angiodysplasia, you’re in it for the long haul. Recurrent bleeding is common. You’ll likely need ongoing iron supplements. Some people need repeat endoscopies every year or two. Others benefit from long-term thalidomide. If you’re older and have heart disease, your doctor might check your von Willebrand factor levels. That’s not routine, but it’s important if you keep bleeding.

One thing you should know: many patients with angiodysplasia go years without a diagnosis. A survey of 243 people on a GI support forum found they had, on average, 3 negative colonoscopies before their angiodysplasia was finally found. That’s not because doctors missed it - it’s because the lesions are tiny, intermittent, and easy to overlook. If you’re still anemic after a clean colonoscopy, don’t give up. Push for capsule endoscopy or repeat scope with AI assistance.

What Should You Do If You Notice Blood?

Don’t wait. Don’t assume it’s hemorrhoids. Even if it’s a one-time thing, get checked. Lower GI bleeding can be a sign of something serious - or something manageable. Either way, early diagnosis saves lives.

Here’s what to do:

• Call your doctor or go to urgent care if you see bright red blood in your stool.
• Don’t take aspirin, ibuprofen, or other NSAIDs - they can make bleeding worse.
• Stay hydrated. Dehydration makes your blood pressure drop faster.
• Write down details: How much blood? Was it painful? Did you feel faint? Any recent weight loss or fatigue?
• If you’re over 60 and have heart disease, mention it. That’s a big clue.

Most people recover fully. But knowing the difference between a one-time diverticular bleed and a slow leak from angiodysplasia changes everything. One might be a quick fix. The other needs a long-term plan.