Possible sites of lesions causing foot drop, working from superior to inferior:
- L5 nerve root
- Sciatic nerve
- Common peroneal nerve
- Deep peroneal nerve
- Superficial peroneal nerve
Working through the other symptoms associated with different sites, in reverse order of the above:
Superficial peroneal nerve:
- Weakness of ankle eversion (peroneus longus/brevis muscles)
- Sensory loss over anterolateral aspect of lower leg and dorsum of foot, with sparing of first web space
Deep peroneal nerve:
- Weakness of ankle eversion (peroneus longus and brevis)
- Weakness of ankle dorsiflexion (tibialis anterior)
- Weakness of toe extension (extensor digitorum longus, extensor hallucis longus)
- Sensory loss in first web space
Common peroneal nerve:
- Any of the features of superficial and/or deep peroneal nerve lesions (logically enough, since the common peroneal nerve is the nerve that divides to form those two nerves)
- Any of the features of superficial and/or deep peroneal nerve lesions
- Pain in the back of the thigh and calf (eponymously)
- Sometimes weakness of plantarflexors and knee flexors – depends on site & severity of lesion
L5 nerve root:
- Weakness of hip abduction (gluteals)
- Pain and sensory loss in side of thigh, lower leg, dorsum of foot, toes 1 – 3.
- Features from superficial/deep peroneal nerve lesions.
Painless isolated foot drop is almost always peroneal mononeuropathy. Painful foot drop can have a variety of causes (L5 radiculopathy, trauma, lumbar plexopathy, mononeuritis multiplex). Hyperreflexia/Babinski’s sign indicate upper motor neurone lesion.
Prolonged leg stretching or ankle stretching can cause peroneal neuropathy, as can habitual leg crossing, prolonged squatting or kneeling, or braces/POP/leg positioning or supports due to surgery (all these cause external pressure, the most common cause of peroneal neuropathy). So can direct trauma (remember fibular neck fracture).
Acute bilateral foot drop – immediate neurological referral
Unilateral foot drop with widespread neuropathy, or fasciculations – neurological referral within one week.
Suspected compartment syndrome – immediate surgical referral.
Otherwise – Advise avoid leg crossing, squatting or kneeling, and wearing flat shoes that support the ankles. In severe cases, consider ankle foot orthosis to support foot while walking to reduce falls risk, and physio. Review in two months – resolution normally expected in 2 – 3 months so refer to neurologist if not improving in 2 months.
(BMJ 2015;350:h1736 doi:10.1136/bmj.h1736)