Foot Drop - Pathophysiology

Pathophysiology

The causes of foot drop, as for all causes of neurological lesions, should be approached using a localization-focused approach before etiologies are considered. Most of the time, foot drop is the result of neurological disorder; only rarely is the muscle diseased or nonfunctional. The source for the neurological impairment can be central (spinal cord or brain) or peripheral (nerves located connecting from the spinal cord to an end-site muscle or sensory receptor). Foot drop is rarely the result of a pathology involving the muscles or bones that make up the lower leg. The muscle that is designed to pick up the foot is the anterior tibialis. It is innervated by the common peroneal nerve, which branches from the sciatic nerve. The sciatic nerve exits the lumbar plexus with its root arising from the fifth lumbar nerve space. Occasionally, spasticity in the muscles opposite the anterior tibialis exists in the presence of foot drop, making the pathology much more complex than foot drop. Isolated foot drop is usually a flaccid condition. There are gradations of weakness that can be seen with foot drop, as follows: 0=complete paralysis, 1=flicker of contraction, 2=contraction with gravity eliminated alone, 3=contraction against gravity alone, 4=contraction against gravity and some resistance, and 5=contraction against powerful resistance (normal power). Foot drop is different from foot slap, which is the audible slapping of the foot to the floor with each step that occurs when the foot first hits the floor on each step, although they often are concurrent.

Treated systematically, possible lesion sites causing foot drop include (going from peripheral to central):

1. Neuromuscular disease;
2. Peroneal nerve (common, i.e., frequent)—chemical, mechanical, disease;
3. Sciatic nerve—direct trauma, iatrogenic;
4. Lumbosacral plexus;
5. L5 nerve root (common, especially in association with pain in back radiating down leg);
6. Spinal cord (rarely causes isolated foot drop)—poliomyelitis, tumor;
7. Brain (uncommon, but often overlooked)—stroke, TIA, tumor;
8. Genetic (as in Charcot-Marie-Tooth Disease and hereditary neuropathy with liability to pressure palsies);
9. Nonorganic;
10. Cauda Equina Syndrome. Spinal cord damage caused by nerve impingement due to badly bulging disc.

If the L5 nerve root is involved, the most common cause is a herniated disc. Other causes of foot drop are diabetes, trauma, motor neuron disease (MND), adverse reaction to a drug or alcohol, and multiple sclerosis.