If you have been living with unexplained left leg swelling, heaviness, or discomfort, and every scan you have had so far has come back "normal," you are not alone. One of the most common questions we hear from patients is how a condition like May-Thurner syndrome can exist for years without showing up on standard imaging. The answer comes down to what these routine tests are actually designed to look for, and why iliac vein compression tends to fall outside of that scope.
What Is May-Thurner Syndrome?
May-Thurner syndrome occurs when the right iliac artery crosses over and presses against the left iliac vein, pinning it against the spine. Over time, this compression can narrow the vein, slow the return of blood from the leg, and set the stage for chronic swelling, heaviness, skin changes, or even deep vein thrombosis (DVT). Because the compression itself does not always cause obvious symptoms until it progresses, many people live with the underlying anatomy for years before anything prompts a closer look.
Why Standard Imaging Often Comes Back Clean
A Standard CT Scan Is Built to Look at Structure, Not Flow
A routine CT scan of the abdomen or pelvis is excellent at identifying masses, organ abnormalities, or obvious blockages, but it is a static, single-moment image. It does not measure how blood is actually moving through the vein in real time. A vein can appear structurally intact on a CT scan while still being functionally compressed enough to cause a meaningful reduction in blood flow. Unless a radiologist is specifically looking for iliac vein compression, and specifically measuring the degree of narrowing, this finding can easily be read as incidental or simply go unmentioned in the report.
A Standard Ultrasound Has Trouble Reaching the Iliac Veins
A duplex ultrasound is a reliable first step for evaluating the veins in the leg itself, but the iliac veins sit deep in the pelvis, often obscured by bowel gas, body habitus, or overlying tissue. Getting a clear, reliable view of this specific segment with an external ultrasound probe is technically difficult, even for experienced sonographers. As a result, compression at the iliac level is one of the more common blind spots in an otherwise thorough venous ultrasound study.
Patient Positioning Can Hide the Compression
Many imaging studies are performed with the patient lying flat, and in that position, the degree of compression on the vein can look less severe than it actually is when a person is upright or moving throughout the day. This is part of why symptoms that seem disproportionate to what a static scan shows are not necessarily being imagined. The vein may be behaving very differently once gravity, posture, and daily activity are factored back in.
It Frequently Mimics Other Conditions
Leg swelling, heaviness, and pelvic discomfort are also associated with a long list of other issues, from general venous insufficiency to musculoskeletal causes to conditions unrelated to circulation entirely. When a standard scan does not immediately point to an obvious answer, iliac vein compression is often further down the list of possibilities that gets investigated, especially in a routine visit without a vascular-focused evaluation.
The Diagnostic Tools That Actually Reveal It
Because May-Thurner syndrome is fundamentally a flow and compression problem rather than a structural mass or lesion, identifying it usually requires imaging that looks at the vein from the inside and in motion, rather than from the outside in a single frame.
Venography gives us a real-time, dynamic view of how blood is actually moving through the vein. By introducing contrast directly into the venous system and watching it flow under live imaging, we can see hesitations, backups, or areas where blood is clearly struggling to get past a point of compression, something a static image cannot capture. Learn more about how a venogram works.
Intravascular ultrasound (IVUS) goes a step further by placing a small imaging catheter directly inside the vein itself. This gives us a detailed, cross-sectional view of the vessel wall from the inside, allowing us to measure the exact percentage of compression rather than estimate it from an external image. IVUS is widely considered one of the more precise tools for confirming a May-Thurner diagnosis when other imaging has been inconclusive. You can read more about intravascular ultrasound (IVUS) and how we use it during a diagnostic workup.
Together, these two studies allow us to see both the functional impact (how the blood is actually flowing) and the structural cause (how compressed the vein actually is), which is a combination that a standard outpatient CT or ultrasound is simply not designed to provide.
When It Might Be Worth a Closer Look
Not everyone with a compressed iliac vein has symptomatic May-Thurner syndrome, and not everyone with leg swelling has this condition. But a closer vascular evaluation is often worth considering if you have experienced:
- Swelling, heaviness, or aching that affects primarily the left leg
- Symptoms that have persisted despite a "normal" CT scan or ultrasound
- A prior unexplained DVT, particularly in the left leg
- Chronic left leg symptoms that developed or worsened after pregnancy
- Visible varicose veins or skin changes in the left leg without an obvious explanation
If any of this sounds familiar, it may be connected to broader patterns we see in patients dealing with chronic leg pain and swelling that has gone unexplained through standard testing.
Our Approach to Finding What Standard Imaging Misses
At Illiana Vascular Center, we regularly see patients who have already had imaging that came back normal, only to find a clear, treatable cause once we look more closely with venography and IVUS. Dr. Dominic J. Tolitano brings decades of surgical and vascular experience to these evaluations, and treatment options such as iliac vein balloon angioplasty or iliac vein stenting are only considered once the compression has been clearly confirmed and measured, not assumed from symptoms alone.
A normal CT scan or ultrasound does not always mean there is nothing there. Sometimes it simply means the test was not designed to see it in the first place.