Unilateral foot drop is a common complaint. Careful examination of the anatomical pattern of the weakness helps identify the most likely location of the causative lesion. In some cases, clinical weakness is subtle and the pattern of weakness is more easily identified on needle electromyography (EMG) examination of these muscles.
A corticospinal tract lesion can present with preferential weakness of unilateral hip flexion, knee flexion, and ankle dorsiflexion because the extensor muscles of the legs also receive innervation from the lateral vestibulospinal tract. While this weakness is not consistent with an isolated foot drop, the ankle dorsiflexion weakness may be the primary symptom noticed by the patient. In addition to assessing for subtle hip flexion and knee flexion weakness, a careful examination for other upper motor neuron signs such as spasticity and hyperreflexia should be performed.
In a patient with foot drop, the first movements to assess for weakness are ankle eversion and inversion. These movements need to be performed with the foot in a neutral or dorsiflexed position, which usually requires the examiner to dorsiflex the foot. If the foot remains plantarflexed, the gastrocnemius muscle can participate in eversion or inversion, potentially causing the examiner to mislocalize the lesion.
The common peroneal (fibular) nerve bifurcates into the deep peroneal and superficial peroneal nerves. The superficial peroneal nerve innervates the peroneus longus, which everts the foot and carries sensory information from the lower lateral leg and the dorsum of the foot. In this context, a patient with a deep peroneal neuropathy would have a foot drop without ankle eversion weakness or obvious sensory loss. The only sensory fibers carried by the deep peroneal nerve convey sensation between the great and second toe, and thus this sensory distribution would need to be carefully assessed in a patient with foot drop but preserved eversion.
A common peroneal (fibular) neuropathy is a frequent cause of foot drop. The typical location of the lesion is where the common peroneal nerve crosses the head of the fibula. This frequently occurs in the context of excessive crossing of the legs, particularly in patients who have recently lost a significant amount of weight. A patient with a common peroneal neuropathy will have numbness over the lower lateral leg and the dorsum of the foot and weakness of ankle dorsiflexion and eversion, but inversion will be normal because the tibialis posterior muscle, which inverts the ankle, is innervated by the tibial nerve.
An L5 radiculopathy is the other frequent cause of foot drop. The presence of radicular back pain suggests an L5 radiculopathy. On examination, the presence of ankle inversion weakness distinguishes an L5 radiculopathy from a common peroneal neuropathy.
Sciatic neuropathy is a rare cause of foot drop (note that the term “sciatica” usually refers to an L5 radiculopathy and not a sciatic neuropathy). The sciatic nerve bifurcates into the tibial and common peroneal nerves. A sciatic neuropathy can involve the fibers that will travel in both of these nerves or preferentially involve the common peroneal nerve fibers. When both the tibial and common peroneal fibers in the sciatic nerve are affected, there is ankle plantarflexion (gastrocnemius muscle) weakness due to tibial nerve involvement, which would not be expected in a common peroneal neuropathy. Involvement of the gastrocnemius muscle is also not seen with an L5 radiculopathy, since the gastrocnemius muscle is innervated by the S1 nerve root. More difficult is distinguishing a sciatic neuropathy with preferential involvement of the common peroneal fibers from a common peroneal neuropathy. In this case, the key factor is that the common peroneal nerve fibers contained within the sciatic nerve innervate the short head of the biceps femoris, part of the hamstring muscle. Involvement of this muscle, therefore, implicates the sciatic nerve and is inconsistent with a common peroneal neuropathy. Because the hamstring is a very strong muscle, the examiner may not be able to appreciate weakness of this muscle on examination. The presence of denervation of the short head of the biceps femoris on needle EMG will make the distinction.
A sciatic neuropathy with involvement of tibial and common peroneal fibers needs to be distinguished from a lumbosacral plexopathy. The superior gluteal nerve, which innervates the gluteus medius (responsible for leg abduction) and tensor fascia lata muscles, arises from the lumbosacral plexus but not the sciatic nerve. Similarly, the inferior gluteal nerve, which innervates the gluteus maximums (responsible for leg extension), arises from the lumbosacral plexus but not the sciatic nerve. The presence of either hip abduction weakness or hip extension weakness in a patient with ankle dorsiflexion and ankle plantarflexion weakness indicates a lumbosacral plexopathy instead of a sciatic neuropathy.