Thank you for your question to HCM.
Causes of foot drop can be either in lumbar spine where nerves come out, along the course of nerve in the leg or the muscles and joints.
Foot drop from a lumbar disc herniation or stenosis generally is associated with radicular pain
or numbness but may occur in isolation. Peripheral nerve
lesions can involve common or deep peroneal nerve that may lead to foot drop. Joint and muscle problems lead to deformity rather than a true foot drop.
A detailed examination by a neurologist
can differentiate between these cause. A lumbar disc is obvious on MRI of the lumbar spine whereas nerve problems can be localized by Electromyography
and Nerve conduction testing (EMG/NCV). Muscle weakness
in the absence of nerve involvement can be detected on examination. So plan would be EMG/NCV and MRI of lumbar spine if these are not already done.
Hope this is helpful. I would be happy to answer any questions that you may have.