Lower limb neuropathic presentations are difficult to localise for one reason: roots and peripheral nerves share too much of the same muscle and skin territory. Weak knee extension cannot separate an L3/4 radiculopathy from a femoral neuropathy, and weak dorsiflexion cannot separate L4/5 from a peroneal lesion. Weakness tells you that something neural is involved; on its own it almost never tells you where.

The lecture works from the same two questions used in the upper limb session — is this neurogenic, and if so what is the source? — and answers the second by reasoning across five domains: sensory pattern, proximal motor, distal motor, reflexes and symptom provocation. The motor domains carry most of the weight. Rather than grading more muscles, we look for the one muscle that sits in the root's supply but not the nerve's, or the nerve's but not the root's. That is what turns a finding into a discriminator, and it is why the standard sagittal screen needs a coronal movement added to it once weakness appears.

The framework is then applied to the three pairings that cause most of the difficulty in clinic — L3/4 against femoral, L4/5 against common peroneal, S1/2 against tibial — and tested against a real case: six years of anterior shin pain in a man with an incidental L3/4 stenosis on MRI. The examination did not fit the imaging, and following that mismatch rather than forcing it led to a deep peroneal neuropathy caused by a ganglion cyst. The session closes on what routinely blurs the picture: multi-root innervation, dermatomal overlap, variable plexus anatomy, pain inhibition, and genuine dual pathology.

KEY MESSAGES

• Precision in language is precision in reasoning — say whether you mean a nerve or a root, and be honest about how confident the findings let you be.
•    Weakness tells you a nerve is involved; a discriminator tells you where. Look for the muscle in one supply and not the other, and test the plane the standard screen leaves out.
•  Every test has a blind spot. Reflexes are specific but insensitive; neurodynamic tests confirm a sensitised nerve, not the site of the lesion.
•  Imaging is not diagnosis — a finding in roughly the right region is still incidental if the examination disagrees.
•  A picture that does not fit is information, not an inconvenience — follow the mismatch, and widen the differential when weakness is marked.