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Understanding Diabetic Foot Infections

depiction of a diabetic foot infection

We treat infections either surgically or with antibiotics, sometimes both. How we devise the treatment plan depends on what kind of infection we are dealing with, so let’s talk about the different kinds of infections.

Cellulitis (skin infection)

Cellulitis is the medical word used to describe an infection of the skin. Usually, there is a break in the skin, such as from a scratch, a wound, or an ulcer. Bacteria will then get into the skin, and quickly spread within the skin. The body will also quickly realize something is going on, so it will respond by sending more blood to the skin, so that white blood cells can reach the bacteria and fight off the infection. This is why the skin starts to turn red. Sometimes, our bodies can fight off this infection without the help of antibiotics. Unfortunately, diabetes causes white blood cells to work less efficiently, so usually skin infections tend to get worse without the help of antibiotics. Fortunately, within hours of receiving antibiotics, the cellulitis will go away.

Necrotizing fasciitis (flesh eating infection)

We sometimes hear this in the news, a healthy teenager playing in the forest suddenly becomes ill, gets airlifted immediately to the nearest hospital by helicopter, and receives an amputation because of the rapid spread of a flesh eating bacteria. This is rare, but can happen when an individual is unlucky enough to get a scratch and gets infected by a rare type of bacteria. There are millions of different types of bacteria, and they all vary with how aggressive they are. People with diabetes are less able to fight off bacteria than someone without bacteria, which means a bacteria that normally is not aggressive, but can become aggressive if it finds its way into the flesh of someone with diabetes.

The classic medical story of necrotizing fasciitis usually involves somebody who has an infection that worsens within hours, causing the individual to become so sick that they become unconscious. We can take a sample of the rotten flesh and look at it under the microscope. A pathologist will look at the layers of the skin including the fascia, the layer just underneath the skin. All of these layers will appear dead and flooded with white blood cells, hence the name necrotizing fasciitis. Young healthy people can get this kind of infection if they are extremely unlucky enough to have a rare and highly aggressive bacteria enter their skin. For those with diabetes, their immune system is weakened, so common and less aggressive bacteria can cause the same kind of destruction, although not as quickly. These individuals usually are sitting up and talking normally, feel fine, and say that their foot has been slowly worsening over the past few days, not hours. This is not classic necrotizing fasciitis, but the infection if examined under a microscope will also appear flooded with white blood cells and the layers of skin and fascia look dead. So, putting it all together, people with diabetes can get an infection that microscopically looks identical to classic necrotizing fasciitis, but are not as sick, and not as fast as classic necrotizing fasciitis. As you can imagine, necrotizing fasciitis is a scary thing, for both doctors and patients, so we don’t like to use this word unless it fits the classic picture. Instead, we call it a diabetic foot infection. I know this is confusing! It will take some time for our medical vocabulary to be updated and accurately reflect what we see nowadays.

So, looking at a foot with some kind of skin infection, it can look just like cellulitis, but underneath the bacteria is slowly eating at the flesh. Sometimes we can tell by looking through the wound. If the wound is small, but the redness is spreading up the leg and we suspect some flesh eating component, we can make a small cut on the skin and see if the flesh underneath is dead or not. If we determine that it is necrotizing fasciitis, the dead flesh must be removed surgically. The individual will also receive antibiotics for several weeks.

Osteomyelitis (bone infection)

Bones in diabetic feet can get infected through spread from an infected wound, or through the blood stream. Bone infection through the bloodstream is very very rare, so we will focus on spread from an infected wound. The diagnosis of infected bone is not straightforward, sometimes it is obviously infected, and sometimes it is not.

Obviously infected bone is when you see pus and bone fragments falling out of the wound. On the x-ray, the edges of the bone appear eaten away with fragments floating around. We would have to surgical remove all of the bone that appear dead. We then take a tiny sample of bone that appear healthy from the bones remaining. This sample will be sent to the lab, and they will tell us if there is any infection left, and if so, what bugs are growing. This process takes about a week. This will tell us if we antibiotics.

When we have a deep wound that is infected, if the infection is close to the bone, we have to order an x-ray to evaluate the bone. The x-ray may appear normal, which means either the bone is clean and healthy, or the bone is infected but it is too early for an infection to make changes big enough to be seen on x-ray. The next step depends on the patient’s situation, and the doctor’s judgement. If we are still suspicious for bone infection, we can order an MRI (magnetic resonance image). An MRI is an expensive test that can see the foot in 3-dimensions, and can see fat and fluid. Bones normally have fluid and fat in the bone marrow. Infected bones will have extra fluid, and loss of fat. If we find a bone infection on MRI that is not found on X-ray, we can conclude that this is an early bone infection, and the treatment varies quite a bit based on where the infection is, how big the wound is, patient’s risk factors (i.e. age, presence of other medical problems), surgeon experience, and patient and surgeon preference. We can do antibiotics alone, surgery alone, or both. Sometimes the findings on MRI are unclear (yea, the expensive MRI still has its limits!) in which case we can do nothing for now and watch the foot for signs of worsening.


An abscess is a collection of pus, usually caused by a bacterial infection. Think of it as a water balloon filled with pus, not water. To treat an abscess, we make a small cut to “pop” the balloon, and call it a day. However, only healthy immune systems can wall off infections like this. Diabetes impairs the body’s immune system, so instead of forming an abscess, the infection spreads throughout the layers of the skin and fascia, and start slowly eating away at the flesh. The impaired immune system can still wall off a little bit of the infection, which is why when we squeeze the foot in the operating room we can still see pus pouring out, but it is not like an abscess where a small incision will get rid of all the pus. In the diabetic foot, we make big long incisions and have to really work and explore the foot to find where the pus is spreading.

Putting it all together

I know this has been a lot of information, and I will keep updating this blog post to make it easier to understand. But right now I will attempt to put all of this information together in a big picture way.

Diabetic foot infections always have a starting point, such as from a small wound or ulcer. From there, it can either spread out, or go deep, or both. The body’s layers are as follows: skin, fascia, fat, muscles and tendons, then bone. In order to get to bone, the infection has to spread deep. That sounds like a lot of stuff, and in some parts of the body, it is. However, feet have very little stuff separating the skin from the bone, which is why it is so easy for diabetic feet to get bone infection.

Representation of the structures in and around our big toe.

1) Skin

Ok, so we start off with a wound, if left untreated, the first thing to get infected is the skin. If the infection spreads outwards and not any deeper, we have cellulitis. There is no pus, we only see redness.

2) Fascia

Next, infection can spread deep, to fascia, and we can start to see a bit of pus. In the diabetic foot, there is no medical term specifically for infection of the fascia. Remember, necrotizing fasciitis is a term used only when there is: rapidly spreading outwards within fascia, caused by a rare aggressive flesh eating bacteria, and the individual becomes very sick. In the diabetic foot, usually the infection has a small amount of flesh eating, progresses slowly, and the individual is not feeling very sick. This is NOT the classic necrotizing fasciitis, instead we simply call it diabetic foot infection.

The ulcer allows bacteria to infect the body, starting with the skin, then the fascia.

3) Muscle/Tendon

We don’t have a lot of muscle in our feet, but there are a lot of tendons. Our tendons usually have a sheath around it, kind of like a deflated long balloon. When infection spreads deep to tendon, it can decide to spread outwards, along the tendon and its sheath. We then call this infectious tenosynovitis.

Infection likes to spread outwards as indicated by the arrows. However, because of continued walking, the ulcer is deep enough to damage the tendon. The bacteria will not pass up this open invitation to go deeper and eat at the tendon. Infected tendon is called infectioustenosynovitis.

4) Bone

Bones can move because they are attached to muscles and tendons, so naturally bones are the deepest structure in the body. If the infection spreads deep enough to get to bone, and we see evidence of bone infection either by x-ray or MRI, we call it osteomyelitis.

Even more walking allows the ulcer to get deeper, and with enough time, bacteria will slowly eat its way into the bone. Infected bone is called osteomyelitis. The infection will still continue to spread outwards at the same time.

5) Joints

Bones are attached to other bones through ligaments. Ligaments vary in shapes and sizes, but generally underneath ligaments is a structure called a joint capsule. Think of it as a balloon that seals off the ends of two bones, reinforced by distinct rubber bands. Sometimes infections can spread deep past the ligaments and get within the joint capsule, which we call septic arthritis. We can drain the joint, but now we are concerned about possible osteomyelitis of not one but TWO bones.

The big toe joint has a special bone called the sesamoid, which was already destroyed in this scenario. Unfortunately, now the infection can spread into the joint, which means it now has access to two bones. Infected joints are called septic arthritis.

As seen in the images above, there is very little distance from ulcer to bone. From there, the joint of the big toe is directly underneath. We suddenly have everything between the skin and the joint involved. Instead of naming everything that is involved, we can focus on the most concerning diagnoses, which would be septic arthritis. Treating the septic arthritis will include treating everything else involved.

Diabetic foot infections in reality

In reality, it is very difficult for infections to spread so deeply on its own. Think of bacterial infection as a fluid, they travel along the path of least resistance. It is easier to spread along the layer that it is already in, rather than digging deep into the foot. How does it get deep into bone? That is because of neuropathy. Half of people with diabetes will eventually develop neuropathy. Neuropathic feet do not have the gift of pain, so calluses will keep digging into the foot, and if the individual does not catch it in time, it can dig deep to bone.

If septic arthritis go untreated, bacteria will eat its way into the bones that form the joint. Now, we have to septic arthritis and osteomyelitis of multiple bones.

Believe it or not, even when the infection is as bad as the image below, we can still to save the foot, possibly even the toe! Ideally, one would never reach this point by performing daily foot inspection and undergoing routine foot care with their podiatrist.

Once in the bone, bacteria will continue to eat away at the bone causing more and more destruction. You can see bits of bone fragments start dropping out of the ulcer underneath.

These pictures do not include every detail. The different types of infections also don’t always happen through a foot ulcer. But in diabetic feet, this is a very common way in which we get these types of infections, and the progression as described above is also very common. I hope this gives you a sense of how infections work and I hope I’ve demystified some of the medical jargon that we use.

Dr Haywan Chiu, DPM Haywan Chiu

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