A “foot-drop” is a medical term which–thankfully–does not mean that the foot suddenly disconnects from the leg. Rather, it means that when the leg is lifted from the ground, the foot droops downward at the ankle.

The muscles that are supposed to prop up the foot have become so weakened that they cannot overcome gravity’s downward pull. When people with this problem try to walk, they have to either hike the leg higher to clear their drooping toes or else risk tripping over them.

What is to blame for this inconvenient symptom?

In truth, there are multiple possible causes, but one of the most common culprits is injury to a nerve-bundle in the leg known as the peroneal nerve. To understand how this nerve-bundle can get in trouble, a quick review of the bones of the leg is helpful. There is just one bone, a big one, that connects the hip to the knee, and that is the femur. There are two bones that connect the knee to the ankle. The tibia is the larger one and lies more to the inside, while the fibula is the thinner one and lies more to the outside. That’s the extent of the bony anatomy we need to know.

The nerve-fibers constituting the peroneal nerve travel with the huge sciatic nerve that runs behind the femur from the buttock to the lower thigh. That’s where the “common peroneal nerve” splits out from the pack and runs along the outside of the knee, tucking behind the head of the fibular bone (a knobby protrusion just beyond the knee) and then snaking around the neck of the fibula just below its head. The neck of the fibula forms the floor of the fibular tunnel that the common peroneal nerve must pass through. Within this tunnel the common peroneal nerve is particularly vulnerable to injury.

Also within this tunnel the common peroneal nerve splits into two branches, the “deep peroneal nerve” (farther from the leg’s surface) and the “superficial peroneal nerve” (closer to the leg’s surface). Because the two branches have different connections to muscles and skin, injury to one produces different impairments than are produced by injury to the other.

The deep peroneal nerve is responsible for cocking up the ankle and toes, so injury to this branch produces weakness or paralysis of the muscles responsible for these actions.

There is just a tiny patch of skin, located between the big toe and the toe next to it, connected to the deep peroneal nerve, so damage to this branch produces numbness limited to this small area.

The superficial peroneal nerve, by contrast, is responsible for skin sensation on most of the outside of the calf and top of the foot, so these areas can become numb when the superficial peroneal nerve is injured. This branch is also responsible for lifting the outside edge of the foot, so this action is gone when the superficial peroneal nerve is not functioning properly.

Impairments due to injury of the common peroneal nerve (the parent of the two branches) are the sum of the impairments associated with each of the branches.

So this means that the ankle and toes cannot cock upwards, the outside edge of the foot cannot lift, and there is numbness on the outside of the calf and top of the foot.

“Peroneal neuropathy” means impairment of the peroneal nerve. Peroneal neuropathies are the most common neuropathies (of the kind that affects just one nerve at a time) in the lower extremities. Investigators at the Louisiana State University Health Sciences Center recently collected a series of 318 patients with peroneal neuropathy who required surgery, while Italian researchers collected another 69 cases that included those who didn’t need surgery. From these two tabulations of cases a good picture emerges of the more common causes of peroneal neuropathy.

Many were due to physical traumas. Some of the traumas were severe enough to break or dislocate bones, while others involved deep cuts in the soft tissues, and still others involved just a stretch or bruise. Another common cause was surgical operations. Some of the surgeries were to the nearby knee, but others were performed on more distant structures, like the hip, the abdomen or even the chest.

Many cases were due to excessive external pressure being applied to the nerve. This occurred in different ways. For example, in prolonged leg-crossing the knee of the bottom leg pushes steadily against the peroneal nerve of the crossing leg. Peroneal neuropathies seen in bedridden patients were presumably due to lying on the fibular tunnel for too long without a shift in position. Other patients had entrapment or pinching of the nerve within the fibular tunnel unrelated to external pressure.

A surprisingly large group of patients had peroneal neuropathy due to weight loss, also known as “slimmer’s paralysis.”

More than one factor might have been at play in these cases, including lack of nutrients, pressure on the nerve, or both.

Researchers and clinicians find that in some people an apparently isolated peroneal neuropathy is actually the leading edge of a more widespread polyneuropathy. “Polyneuropathy” means that peripheral nerves are impaired in a more diffuse pattern–not just single nerves in single places. So in some cases of apparent peroneal neuropathy further investigations turn up polyneuropathy due to other causes, for example, diabetes, excessive alcohol consumption or genetic factors.

How are cases evaluated? The physician’s evaluation starts with the time-honored methods of history-taking and physical examination. As part of the physical examination the doctor inventories which muscles are weak (and which are not) and maps out areas of numbness affecting the skin. Additional testing with electromyography and nerve conduction studies, which check on electrical functions of the muscles and nerves, often provides valuable information, including whether additional nerves are affected and how bad the impairments are.

How about treatment? Treatment varies according to what caused the peroneal neuropathy in the first place, but let’s consider a typical case unrelated to severe trauma. Nonsurgical approaches are usually tried first, including avoidance of further pressure on the peroneal nerve, improved nutrition and supplementation of the diet with vitamins. A simple brace applied to the ankle improves walking. In many cases the nerve recovers without anything more drastic being done. But if these conservative treatments fail (and the peroneal neuropathy is not part of a more widespread polyneuropathy) then surgical exploration of the fibular tunnel is often indicated. If the nerve is pinched, then the surgeon frees up the nerve from whatever was pinching it.