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Foot drop is the inability to dorsiflex the foot - so, to pull the foot up. Foot drop often presents as people saying that they are tripping over themselves. Essentially, it is hard to clear the foot, so people after they go for a long walk might find that they start to trip over themselves. As the foot drop gets worse, then that can become more pronounced and more profound and often people will develop a steppage gait - which is basically you have to heighten the leg up in order to clear the foot as you're walking.
Now, foot drop itself is not a disease, it's a sign of an underlying pathologic process. There are two general categories that we think of for foot drop. The first is people are plantarflex, so the calf is in spasm or it is spastic and it is not letting the dorsiflexors to overpower it, so the dorsiflexor muscles can't bring the foot up. The other reason is a lack of communication between the nerve and the muscles that are supposed to be bringing the foot up. Now, this lack of communication is what we're going to focus on at the moment and that lack of communication can happen anywhere along the path from the brain down into the foot. So, people could have central problems such as a stroke or a tumor or Parkinson's in the brain, then as the impulse comes down the spinal cord you can have spinal cord lesions. As the nerve exits the spine people can have what is called a "radiculopathy," or more colloquially a pinched nerve in the back that can certainly cause foot drop. Then as those nerves come out of the spine - the ones that go into the dorsiflexors - they branch out, they come together to form the sciatic nerve and people can have irritation to the sciatic nerve. The sciatic nerve then goes down the leg and branches again to the peroneal nerve, which the peroneal nerve can also have a lesion in it that could be causing a foot drop. And then you could have a problem at the neuromuscular junction as well and in the muscle itself. All of those things can lead to a foot drop.
When the problem is coming from the spine, typically will be an L5 radiculopathy - an irritation around the L5 nerve. And that irritation comes, in general, from the same kinds of things that can also cause pain going into the leg, which are things like herniated discs, like spondylolisthesis, facet joint arthropathy, spinal stenosis - essentially narrowing of the space where the nerves exit the spine, which can then put pressure or irritation around that nerve and then leads to the lack of communication between that nerve and the muscles that are supposed to do the job of lifting up the foot.
Often, but not always, there will be an accompaniment of pain, of numbness, tingling, but sometimes it can just present as foot drop in the absence of other symptoms.