What are slow and fast flow lesions?
Slow-flow anomalies are either capillary, venous, or lymphatic, or a combined form. Fast-flow anomalies are various arterial anomalies; either simple channel types, single channel types or combinations. People sometimes say “mixed” but I never liked the word; they’re not really mixed, they’re combined. The most fearsome vascular anomaly is a combined arteriovenous malformation. There are combined capillary-lymphatic; or capillary-venous malformations. There are combined lymphatico-venous lesions and you can have all three types in Klippel-Trenaunay syndrome, i.e. capillary-lymphatico-venous malformation.

John Mulliken, MD

 

When we look at any random vascular anomaly, there’s a diagnosis decision tree that we go through to try and diagnose these. At the top level you have to decide whether it’s a high-flow lesion versus a low-flow lesion. Usually it’s a high-flow lesion if it involves an artery of some sort, whereas if it’s a low-flow lesion, it usually involves some sort of vein or lymphatic channel. Arteries are the vessels that take blood away from the heart; they’re high-pressure and hence high-flow. The high-flow lesion is typically what we call the arteriovenous malformation. With the low-flow lesions, the venous and the lymphatics, it just involves the veins or the lymphatic vessels that we have in our body. Both of those vessels are low-pressure, hence they’re also low-flow. Venous malformations tend to involve only veins. Lymphatic malformations tend to involve only the lymphatics, and again, the best way to think about it is malformation; it’s like a congenital abnormality. The vessels just didn’t develop right while the embryo and the fetus were developing, and then they developed these malformed vessels that formed what we now call malformations.

Horacio Padua, MD

 

What do low-flow and fast-flow anomalies look like on imaging?
I think the easiest way to divide these is into low-flow or high-flow. A low-flow malformation would be something like a venous malformation which would effectively look like little sacs on the MRI or ultrasound, you see these little channels with very low flow within it. [Low flow has to do with] the velocity (or the speed at which fluids flow). For example, the venous malformation, the blood will be within the sac, it will swirl very slowly and empty slowly into the vein. In the lymphatic, you basically have lymph being produced by the lymphatic endothelium and it effectively just sits in a big sac, either a big sac or some are microcystic. That is low-flow based on velocity.

On the high-flow side we try and divide up into tumors and AVMs. The tumors are the ones that often have a mass effect. The majority of these will be diagnosed in early life, in infancy, and they tend to be infantile hemangiomas. If you see a mass with multiple high-flow vessels in an older child, it’s very unlikely to be a hemangioma. On the other hand, if you see a high-flow lesion but there isn’t a mass associated with it, then that is more consistent with an arteriovenous malformation. The high flow is caused by blood going straight from an artery into a vein, so you’re getting large veins because of the diversion of flow instead of going to the capillaries first… [We look at] High-flow, low-flow, mass, and then—unfortunately a lot of the rarer types become pattern recognition.

Gulraiz Chaudry, MB, ChB, MRCP, FRCR

 

What are different types of low-flow lesions?
There’s a spectrum. You can have combinations of lymphatic and venous lesions, and then you can have the pure venous or pure lymphatic lesions, and then within those subgroups of venous and lymphatic malformations there are also different subtypes. Venous malformations tend to be on their own, but there’s a variant that we call glomuvenous malformation, which are somewhat more genetically related and genetically transmitted and have a different clinical presentation. The lymphatics come in a couple of different types. In macrocystic lymphatic malformations, they develop these large cystic fluid-filled structures while in the microcystic lymphatic malformations, the malformation is comprised of thousands of microscopic cysts where all put together they almost look like a firm mass. It’s different because macrocystic lymphatic malformations can be treated very readily with the techniques that I use. Microcystic can be but they don’t respond as well as the macrocystic lymphatics do.

Horacio Padua, MD

 

Are there any common features of low-flow lesions?
The common things that bring venous malformations in are pain and swelling. The venous malformations, especially if they’re close to the skin, also tend to be very blue. They’ll have a very bluish hue to the skin, which can sometimes be quite noticeable. Patients do come in for the cosmetic part, but for the most part it’s really pain and swelling that brings venous lesions in.

Lymphatics are usually not painful, but they can be much more disfiguring. They can present as quite large masses, and that can be visible from the outside or if they’re internal can sometimes actually cause disruption of certain organs; usually it’s the bowel in the belly. The cysts can get so large that they can actually obstruct the bowel. They can be in the neck in the airway, and it can get so large that you essentially get large masses in the neck. That’s probably one of the more common things I see, a large lymphatic malformation in the neck in babies. Again, those tended to be operated on in the old days, but now most people are buying into using sclerotherapy to treat these now.

Horacio Padua, MD

 

What is an AVM?
An AVM is where there’s a direct connection from arteries to veins without intervening normal tissue and normal capillaries, which are vessels where oxygen exchange would normally take place. The result of that is essentially a short circuit where you have blood under high arterial pressure going into the AVM and then immediately back to the heart through the venous part of the AVM. It’s a parallel short circuit of blood within the broader circulation, and depending on the size of it that can take up a sizeable fraction of the total blood in the body. For a newborn, it’s a very fast-flow high-volume lesion that can literally put the newborn into heart failure within a few days of birth. Those are the very rare cases where we would have to intervene within the first few days of life to prevent heart failure, and the idea is to shut as much of that direct arteriovenous connection as possible, just to slow the flow enough so that enough cardiac output is going to the rest of the body that the heart doesn’t go into failure. That’s one extreme.

Darren Orbach, MD, PhD

 

Are there any common features to high-flow lesions?
The arteriovenous malformations usually present with pain, that’s their most common presentation. But there’s a whole host of other things if the AVM is not treated and allowed to progress: swelling can ensue, problems with deformity can also happen. Basically, the AVM can grow; it’s not a cancer, but it can behave like one where it grows and starts affecting other parts of the body, (i.e. bones, muscles, joints), and it can be rather aggressive. So out of all the lesions that we treat, the AVM is probably the most difficult one to treat because even after surgery, if you think you’ve taken it all out, there is a propensity for it to grow back because you can never really take all of it out. Looking at these from our research, we know that interspersed between stuff that you can see is compressed stuff that you can’t see, and what happens is when you take out the stuff you can see, all that blood diverts into the stuff that you couldn’t see at the time, and now you basically have a new lesion to deal with.

Horacio Padua, MD