Evaluation of Foot Drop

Definitions:
When there is paralysis of the dorsal extensor muscles of the foot and the toes (tibialis anterior, extensor digitorum longus, and extensor hallucis longus), which are innervated by the deep peroneal nerve, foot drop occurs. Because the tibialis anterior muscle is innervated from the L4-S1 roots, especially the L5 and to a lesser extent L4 root, through the sciatic and ultimately the deep peroneal nerves, a lesion in any of these can cause foot drop. The toe extensors are primarily innervated from L5, with some contribution from S1.

Causes of Foot Drop:

Peripheral Causes:

  1. Peroneal nerve injury
    superficial peroneal nerve: supplies the peroneus longus and brevis muscles (L5, S1), weakness in which causes loss of foot eversion and plantar flexion, but NOT foot drop; there is often sensory loss in the lateral aspect of the lower half of the leg and foot; deep peroneal nerve: supplies the tibialis anterior, extensor digitorum longus, extensor hallucis longus, peroneus tertius, extensor digitorum brevis, and the first dorsal interosseous muscles, weakness in which causes isolated foot drop; sensory loss is minimal, affecting the great toe web space common peroneal nerve: supplies all of the above muscles, except for the tibialis posterior (foot inversion); damage to this nerve causes foot drop, because it supplies all of the foot and toe extensors; the patient cannot dorsiflex the foot and the toes will drag when the patient walks; there is sensory loss in the lateral aspect of the lower half of the leg and foot; painless foot drop is more likely due to peroneal neuropathy than to radiculopathy.
  2. L5 Radiculopathy (less commonly L4)- this is most often caused by a disc herniation at the L4-5 interspace; weakness affecting the peroneus, toe extensors, possibly the tibialis anterior; the patient has trouble supporting weight on the heel or there may be a foot drop, with the patient describing the toes becoming caught on the carpet; pain and sensory loss over the anterolateral aspect of the affected leg below the knee and extending to the dorsum of the foot and toes, including the big toe
  3. Lumbosacral plexus neuropathy:
    idiopathic plexitis
    diabetic plexus neurpathy
    vasculitis
    trauma
    post radiation therapy
  4. Peripheral neuropathy: the most common inherited disorder is Charcot-Marie-Tooth syndrome, or peroneal muscular atrophy; other causes: diabetes, alcohol, and Guillain-Barre syndrome account for 90% of cases

Central Causes:

cortical lesion in the paracentral lobule of the motor strip: parasagittal meningioma, metastasis; sensation spared
spinal cord injury