A leg aneurysm often looks like a lump or bulge near a joint, and in about 60% of cases behind the knee, you can feel a distinct pulsating mass. Depending on its location and size, the bulge may be visible under the skin or only detectable by touch. Many leg aneurysms cause no visible changes at all until they start producing complications, which is when the most dramatic visual signs appear.
Where Leg Aneurysms Form
The two most common locations for a leg aneurysm are the popliteal artery (behind the knee) and the femoral artery (in the groin or upper thigh). A popliteal aneurysm is the most frequent peripheral aneurysm, and it’s defined as a widening of that artery beyond about 10 millimeters in diameter. A femoral artery aneurysm involves the artery stretching to roughly twice its normal size, and surgery is typically considered once it exceeds about 2.5 centimeters (around 1 inch) across.
What You Can See and Feel
Behind the knee, a popliteal aneurysm may appear as a soft, rounded bulge in the crease or hollow at the back of the joint. The hallmark feature is pulsation: if you press your fingers gently into the area, you may feel a rhythmic throbbing that matches your heartbeat. Not everyone can detect this on their own, especially if the aneurysm is small or if there’s extra tissue in the area.
In the groin, a femoral aneurysm presents as a lump that may also pulsate. This bulge can look and feel remarkably similar to a hernia, which is one reason it’s frequently misidentified. Your doctor may notice it during a routine physical exam before you ever see it yourself.
If the aneurysm compresses the popliteal vein (the large vein that runs alongside the artery behind the knee), your calf on that side can become noticeably swollen. This swelling can mimic the appearance of a blood clot in the leg, adding another layer of confusion.
How It Differs From a Baker’s Cyst
A Baker’s cyst is the most common mass found in the back of the knee, especially in people with arthritis or knee injuries. Because both a Baker’s cyst and a popliteal aneurysm sit in the same area, they’re easy to confuse. The key difference is pulsation. A Baker’s cyst feels like a firm, fluid-filled sac that doesn’t throb. A popliteal aneurysm pulses in sync with your heartbeat. That said, case reports describe aneurysms being misdiagnosed as cysts, particularly in patients with inflammatory joint disease. An ultrasound is the reliable way to tell them apart.
Skin Changes That Signal Complications
The most alarming visual signs of a leg aneurysm don’t come from the aneurysm itself. They come from what happens when blood clots form inside it and break loose, traveling downstream to block smaller arteries in the foot and toes.
Blue toe syndrome is one of the clearest warning signs. One or more toes turn a deep purple or blue color, become painful, and may feel cold to the touch. The rest of the foot can still have detectable pulses because the blockages affect only the tiniest vessels. This combination of blue or purple toes with otherwise normal foot pulses is a strong indicator that something upstream, like an aneurysm, is showering small clots into the circulation. In more widespread cases, you may also notice a mottled, net-like pattern of reddish-purple discoloration on the skin of the lower leg or foot (a pattern called livedo reticularis), along with tiny red or purple dots (petechiae) or small areas of skin breakdown.
What a Blocked Aneurysm Looks Like
When a leg aneurysm suddenly clots off entirely, cutting blood flow to the lower leg, the changes are rapid and unmistakable. The leg or foot turns pale or waxy white. The skin feels cold compared to the other side. You lose the ability to feel a pulse at the ankle or on top of the foot. Pain comes on suddenly and intensely.
As the blockage worsens, numbness and tingling set in, followed by difficulty moving the foot or toes. In the most severe stage, the skin develops a fixed bluish-purple discoloration that doesn’t blanch when you press on it. Muscle stiffness may follow. This progression, from pale and cold to numb and blue, represents a surgical emergency. At the most advanced stage, the tissue damage can become irreversible.
What an Ultrasound Reveals
Because many leg aneurysms aren’t visible from the outside, ultrasound is the standard tool for confirming one. On the screen, the aneurysm appears as a widened, balloon-like segment of the artery. Inside, there’s often a layer of clotted blood lining the walls, called mural thrombus. In aneurysms that haven’t yet needed surgery, this clot layer fills an average of about 60% of the aneurysm’s interior. In those that do require repair, the clot burden tends to be higher, around 75%. The ultrasound also shows how blood flows through the remaining open channel and whether smaller arteries below the knee are still receiving adequate supply.
If you’ve noticed a pulsating lump behind your knee or in your groin, persistent swelling in one calf without an obvious cause, or unexplained blue discoloration in your toes, these are all patterns worth getting evaluated with imaging rather than watching and waiting.