LESIONS

A Guide to Localisation in Neurology

Case 12. A floppy ankle

What is the lesion?

Two relevant points should be noted in this patient’s history.

1. He recently lost dramatic amounts of weight, which removes the padding from soft tissue around the CPN as it winds around the fibular neck
2. He also spent lots of time in bed, which could predispose to pressure over the nerve at the fibular neck, particularly if lying with legs crossed in the bed

It is likely therefore that the problem is a compressive CPN palsy.

We should keep an open mind however; certain disorders can affect the CPN intrinsically, i.e. not from compression, and this patient has been quite unwell, with an inflammatory state, and subsequently given immunosuppressive therapies.

An important example is vasculitis, when the CPN is damaged by microvascular inflammation leading to infarction – this is usually very painful, and while there may initially only be one involved nerve, soon there is often involvement of other nerves, i.e. mononeuritis multiplex.

Another consideration would be deep vein thrombosis, which the patient is at risk of (reduced mobility, dehydration, and systemic inflammation), but there are no reported clinical features of this. It would be an unusual cause of foot drop, but there are reports of this.

While he is immunosuppressed, an infectious cause of foot drop would be unusual without other clinical features supporting this.