Foot drop occurs when the peroneal nerve is injured, which is a branch off the sciatic nerve. The peroneal nerve gives power the the tibialis anterior muscle which is a very important muscle because it lifts up our foot and toes as we walk so that we dont trip. If you've ever seen someone dragging their foot, or sort of lift up their leg and slapping their foot onto the ground, or someone that wears an ankle-foot brace ("AFO"), there is a good chance its because they have a peroneal nerve palsy.
A peroneal nerve palsy is the most common nerve injury in the lower extremity (legs). As the peripheral nerves travel from your spinal cord to the tips of your toes, they are covered by a "myelin sheath" which provides insulation and helps conduct information. Nerves are best thought of as electrical wires, because they are actually conducting a current, and the pattern of the current sends information as a signal and allows our muscles to contract, or allows our brain to understand what is happening all the way down by our toes. The nerves are not only encased in a myelin sheath to improve electrical conduction, but they are also covered in a protective layer called the "endoneurium" and then another layer called the "Perineurium" and finally another outer outer layer called the "Epineurium". All of these layers of packaging help to protect the nerve, and yet even with all this protection, the nerve can still get injured.
Nerve injury is classified based on the Seddon Classification of Nerve Injury. The most mild injury occurs when the myelin sheath is damaged so that electrical signals are not conducted as rapidly as normal, however, the nerve itself is ok. This can occur when the nerve is stretched or squished for a short period of time. This is called Neuropraxia. The next worst injury is called Axonotemesis, which occurs when the nerve itself is damaged, but the protective covering is preserved. In this scenario, the nerve beyond the point of injury dies (almost link the stem of a plant that gets squished), however, a new branch of the nerve will grow back, and the growth is guided by the protective covering which is still intact. The new nerve bud has a path to follow. Recovery is often quite good. The death and subsequent regrowth of a new nerve bud is called Wallerian Degeneration. Neurotemesis is the most severe injury because the nerve and its protective sheath are both injuried, which can occur if the nerve is cut. Recovery from this injury is not reliable and may or may not occur at all. This is because the new bud doesnt know where to go because the protective sheath was also disrupted and a scar will form within the sheath and block the new nerve from growing.
Nerve injury can occur with compression (its squished) or a nerve can be lacerated (cut). A lacerated nerve immediately jumps to neurotemesis type injury. But a compressed nerve starts with neuropraxia (if the nerve compression only lasts a few days) but can progress to axonotemesis with endoneurium inflammation if compression lasts for weeks, or axon degeneration and scar formation if it lasts even longer (or if there is severe compression after just a short amount of time).
Neurodiagnostic tests are best used to evaluate the function of the nerve. These tests calculate the conduction speed of the nerve (how fast it can send information via electrical signals) and thus determines how injured the nerve is. These tests should be ordered immediately in cases of chronic nerve compression, but should not be ordered until 2-6 weeks after an acute nerve injury (like after a car accident or post-surgical finding). The test can then be repeated in 3 months to determine if healing is occurring.
Based on the type of nerve injury and the results of neurodiagnostics, your doctor can determine the best treatment. Many of the compressive neuropathies, especially cases of neuropraxia, get better with nonoperative treatment (which is often times watching and waiting for the body to heal itself). However in cases where the nerve is lacerated, the best outcomes occur when surgery is performed within the first 72 hours. Surgery reconnects the two cut ends and repairs the surrounding protective layers to prevent scar tissue from invading the channel where the nerve can regrow. If a chunk of nerve is destroyed, a "nerve tube" can be sown into the two nerve ends and reconnect the nerve if the gap between the two ends is less than 3 cm.
1) Sedden Classification Paper on Nerve Injuries. See full article.
2) Sunderland Classification paper on Nerve Injuries. See full article.
3) Recovery potential after neuropraxia, axonotemesis, and neurotemesis. See full article.
6) review paper.