LESIONS

A Guide to Localisation in Neurology

Case 12. A floppy ankle

Outcome

The patient was diagnosed with a compressive CPN palsy on clinical grounds. Imaging was not felt needed to look for spinal or root problems that could mimic this.

He was given physiotherapy exercises and an orthotic splint device and encouraged to not put any pressure on the lateral shin (e.g. avoiding kneeling or sitting cross-legged).

His inflammatory bowel disease was treated effectively with medications and he was able to get home.

Nerve conduction studies were not felt likely to change management in the initial stage, but the patient's healing was somewhat prolonged, with ongoing issues 3 months after discharge - so these were performed several months later. They showed incomplete recovery and evidence of axonal damage, indicating a more severe compressive palsy and suggesting an uncertain prognosis for complete recovery. However, following these tests, things continued to improve, and he was able to resume running and normal activities.

Compressive foot drop due to CPN palsy, triggered by profound weight loss and prolonged bed rest due to colitis.

1. Foot drop can be seen in lesions anywhere between the cortex and the periphery, but knowing the relevant anatomy in terms of myotomes, dermatomes and peripheral nerve territories is necessary to localise the cause
2. Common peroneal nerve palsy affects dorsiflexion and eversion but not inversion or plantarflexion. If those are also weak, consider a more proximal lesion including L5 radiculopathy
3. Weight loss predisposes to compressive palsies as does prolonged immobility and sitting or lying cross legged
4. The prognosis is often good, with spontaneous improvement over several months - unless full-thickness axonal damage has been sustained, as in this case, which leads to a prolonged and sometimes incomplete recovery