Curriculum · Neurology
Localisation of foot drop
What it is
Foot drop is dorsiflexion weakness. Localizing it rests on which nerve or root supplies which action: L5 supplies inversion, eversion and dorsiflexion; the peroneal nerve supplies eversion and dorsiflexion; the tibial nerve supplies inversion and plantarflexion; and the sciatic nerve supplies all of these plus the hamstrings. The tibial nerve has two I's in it, and I is for inversion: it supplies the tibialis posterior, which inverts the ankle, but not the tibialis anterior, which does dorsiflexion, so a tibial neuropathy is not going to cause foot drop.
Causes and risk
Most of the time foot drop is caused by either a common peroneal neuropathy at the fibular head or an L5 radiculopathy. Past those two the causes group by site.
- Muscular: no sensory involvement, muscular tenderness, and a distribution that depends on the compartment involved. Inflammatory and drug-induced.
- Peripheral nerve lesions: lower motor signs with patchy sensory deficits based on the nerves involved. Trauma, post fracture or iatrogenic; compression from a cast or abnormal posturing; metabolic causes such as diabetes, B12 deficiency, alcoholic and heavy metals; infectious causes such as polio, leprosy and syphilis; and hereditary causes such as Charcot-Marie-Tooth.
- Nerve roots and plexuses: radicular pain, spine tenderness, and deficits following the dermatome and myotome of the roots involved. Mechanical lesions such as disc herniation, spinal stenosis, a compressive tumor, vertebral fracture or abscess, and inflammatory causes such as GBS.
For the common peroneal neuropathy the compressive causes named are the fibular head, crossed legs, prolonged kneeling and a Baker cyst, with trauma at the knee. For L5 they are lumbar disc herniation and spondylosis. For sciatic neuropathy they are compression at the buttock, a toilet seat, being bedbound, trauma from an intragluteal injection, or trauma at the pelvis or hip.