Occasionally a person who has been snakebitten on the foot will recover in every way but one: months, or even years, after the injury, the leg will tend to swell up again – sometimes at very inconvenient moments. It doesn’t get red, there’s no sign of infection, but it’s all puffy and maybe it even throbs with pain. What’s with all that?

What I’m going to say next is based on what doctors know from science, but I have to tell you right up front that we have not yet done the precise research to fill in all the details. Also, in any one person’s particular case there might be other factors involved, an infection for example, that would need treatment outside of what I’m writing. So definitely talk this over with your doctor before coming to any conclusions about your own swollen foot.

The people who complain to me about this situation are mostly over the age of 60 years. A few are younger, in which case I want to know right away if they have any special risk factors, like diabetes, a heart condition, other leg injuries, or serious obesity. (Those are conditions that can make puffy feet worse, so we have to take care of them at the same time.) The puffy-foot-years-later thing hardly ever happens to pediatric snakebite patients. Ah, youth.

What folks complain about follows a couple of common themes: temperature and gravity. The leg may tend to swell during a hot shower, or a summer day. It gets especially painful if the person is seated in one position for a long time – like on an airplane, or in a tight seat at the theater. Sitting in a hot tub seems to bring out the worst of all. The problem is mostly on the side of the old snakebite, but sometimes there is a small amount of swelling on the other side, also – this is a hint that there is more than just the old injury involved, and that aging may be a factor.

Sometimes the person has been told, “you have vascular disease,” which translates to “you have leaky vessels that aren’t doing such a great job carrying the fluids uphill from your leg to your heart.” Leaky vessels are one of those lovely things that older folks have to put up with more than younger ones, and they might very well explain the slight puffiness on the other side. But then, why one side more than the other?

To understand why, we have to think about what goes on at a microscopic level, inside the tissues of the leg. There are three sets of vessels in there. The first is the arterial vessels, which carry blood at high pressure, downhill from the heart into the leg. You can feel a pulse if you press just right, over an artery. Arteries branch into smaller and smaller vessels as they reach the soft tissues, and we re-name them as they get smaller: arterioles. Finally, arterioles divide to become the tiniest blood vessels of all: blood capillaries. Normally, fluid can leak out of the capillaries to carry nutrients to the tissues, but the red blood cells are too big to squeeze out, so they move on by. When a snakebite is fresh, though, venom in the soft tissues may damage the blood capillaries. Some of the red blood cells then spill out of the vessels. This can result in a big ugly hemorrhage under the skin, together with awful swelling on the first day after a bite. Once the arterioles and capillaries have healed, later, the hemorrhage can clear up and the skin goes back to a normal appearance. So, the puffy-leg-later problem is not because of red blood cells leaking out – we can tell that from the color alone.

The second set of vessels in there is the venous set of blood vessels. On the way back uphill to the heart, blood in the capillaries merges into little venules, which merge in turn into big veins, big enough that you can see the shallow ones through your skin. Mash on a vein and you’ll know right away that they are low pressure – no pulsing like an artery, just squishiness. Still, there is enough flow through the veins to carry all of the red blood cells back upstream. After a bite, some venom is carried uphill through the veins, particularly the smaller toxins that can squeeze in through the capillaries. The veins eventually find their way back to the heart, where blood exits through the arteries for another trip around.

The third set of vessels in your legs is not referred to as “blood vessels,” because they don’t normally carry any of the red blood at all. These are the lymphatic vessels. Lymphatic vessels, like veins, are a low pressure system that starts in the tissues and then flows uphill, and tiny lymphatic capillaries merge into bigger vessels as they go. In this case, what is inside the vessels is called lymph, and it consists of the juices that have leaked out of the capillaries, together with other materials that may be floating around in the tissues. Venom, for instance, or broken-down old red blood cells from a snakebite hemorrhage. (When the lymph finally reaches the top of its climb, it pours into a vein near the base of your neck. That is one of the ways that venom gets into the bloodstream after a snakebite.)

Lymphatic vessels are super-thin and easily damaged. You can’t see or feel them from outside of the body, except under special circumstances. One of those circumstances is right after a snakebite: the venom gets into the lymphatic vessels, then injures the lymphatic vessel walls on its way up. The vessel becomes red and tender to the touch, and it starts leaking, which causes swelling nearby. When venom reaches the lymph nodes in the groin, or in the armpit, these also become tender, and they swell up painfully. For a while after that, the bitten leg or arm remains swollen, partly because the lymphatic vessels (and maybe some veins) are damaged and cannot carry enough fluid. During this time, nurses and physical therapists may encourage snakebitten people to keep the leg elevated, so that gravity will help the extra fluid to smoosh its way back, slowly, through the vessels and the damaged tissue itself.

Fortunately, the body knows how to grow replacement vessels. Especially in young people, it is common for a snakebitten leg to be back down to its normal size within a couple of weeks. But for us older folks the process is not always perfect. It may take months before the bitten area appears right, again, and even then our re-grown vessels are not as well put-together as they once were. They may be tangly, or they may be missing some of their old one-way flow valves.

Now think about the situation when you sit too long in one position in a small seat, with your legs bent for hours – your own weight is mashing your veins and your lymphatics partly closed. When pressure builds up in the vessels around your ankles, and blood and lymph can’t flow up the leg, the lymph accumulates in the soft tissues. It happens on both sides at once, but the snake-bitten side has a harder time of it, so the swelling on that side is worse. And if that seat is in a hot tub, the temperature is warm enough to make your heart pump harder. Your arteries carry more blood down to your bent legs, more fluid gets trapped, swelling happens faster, and – voilà – that old snakebitten foot starts to throb with each heartbeat.

What can you do about this? Simple stuff, mainly. As of today, there is no permanent cure, but there are several things that help:

• Elevation: put your foot up, in a comfortable position with the knee straight.

• Pressure: wear a pressure stocking (you can buy these at many drug stores). Most people use them during the day only, but you can keep it on at night, too, if it feels good. I’ve had patients tell me that cowboy boots help, for walking, although they can become hard to remove. Or a snug set of pantyhose sometimes is all you need.

• Exercise: wiggle those toes, rotate that ankle, and get up and stretch whenever you can. Work the leg muscles. Avoid staying in one position too long.

• Massage: this should start at the toes and work upward, the direction you want the fluid to move.

Finally, do talk with your doctor about other risk factors. Maybe you didn’t have diabetes in the past, but that can change. Or maybe persistent pain is a clue to a new infection in the previously-damaged area.

Cheers. And after you pull on your cowboy boots, step outside and enjoy a walk – you only live once.