Foot Drop

Foot drop is due to weakness of tibialis anterior, a muscle supplied by the common peroneal nerve ( L4/5 roots ).

Anatomy of sciatic nerve:

It is derived from spinal nerves L4 through S3. It supply adductor magnus in thigh and finally gives off common peroneal branch and tibial nerve.

Common Peroneal Nerve supplies

• Tibialis anterior causing dorsiflexion
• Peroneal longus causing eversion
• Extensor digitorum longus and extensor hallucis longus causing extension of toes
• Sensory supply to anterolateral aspect of leg and dorsum of foot

Tibial Nerve supplies

• Tibialis posterior causing inversion of foot
• Gastronemius, soleus and plantaris causing planter flexion
• Flexor digitorum longus and flexor hallucis longus causing flexion of toes
• Sensory supply to sole of the foot

Causes of foot drop:

Without weakness of inversion

• Common Peroneal nerve palsy

With weakness of inversion

• Sciatic nerve palsy ( give rise to common peroneal nerve and tibial nerve )
• Peripheral neurone disease
• L4/5 root lesion including Cauda equina lesion
• Spinal cord lesion
• Cortical lesion

How to differentiate?

Weak dorsiflexion and eversion without weak inversion. ( patient will also have weak externsor hallucis longus ) - Common Peroneal Nerve lesion.

If there is involvement of inversion, then the patient has sciatic nerve lesion, L4/5 root/ plexus or cord lesion ( depending on whether it is LMNL or UMNL ).

L4/5 lesion alone causes weak dorsiflexion, eversion and inversion. It also causes weakness of hip abduction and sensory loss in L4/5 distribution.