Peroneal Palsy (foot drop)

lifting the foot up is difficult or impossible

Condition

Compression of the common peroneal nerve as it passes around the head of the fibula bone (the bone you can feel just below and outside of the knee. This nerve supplies some of the muscles which can lift up the foot. It also supplies sensation of the back of the foot and part of the outside of the lower leg. The nerve is usually damaged when injuring the head of the fibula or following prolonged pressure on it (when crossing the legs the head of the fibula might rest in the knee cap of the other leg trapping the nerve). This can happen even in sleep. In rare cases the pressure can be caused by a tumour pressing on or arising from the nerve (Schwannoma or neurofibroma)

Symptoms

Patients notice that the foot is flapping when walking or that they trip over their own foot. This is because the muscles which are lifting the foot upwards are paralysed. This might only be partial and very mild forms may go unnoticed. Sensation is often normal. It is not associated with any pain and other causes should be considered such as a slipped disc in the lower back at L4/5 if there is any pain radiating down the leg.

Clinical signs

Weakness of lifting up the big toe (extensor hallucis longus muscle or EHL) and foot (tibialis anterior muscle), lifting the foot up and outwards (peroneal muscles) and loss of sensation of the back of the foot.

Complications

Permanent numbness and weakness of the foot.

Investigations

Taking an accurate history and a thorough clinical examination are important. A trapped nerve in the upper lumbar spine (L4/L5) can cause similar symptoms but in contrast to the peroneal palsy often causes pain (sciatica).

Electrophysiological tests (EMG and nerve conduction test) will be able to show the cause. It is, however, important to know that the changes of these tests usually take 3-4 weeks to develop and therefore the test is unreliable if performed too early.

An MRI scan of the lumbar spine is indicated if there is a clinical doubt or if the electrophysiological tests do not confirm the peroneal palsy. In cases of progressive symptoms an MRI scan of the region just below the knee is indicated to see whether there is an underlying lesion.

Treatment

Non-surgical treatment often means support of the foot with a calliper or foot support to avoid tripping over the foot. Physiotherapy will also help to strengthen the muscles. Surgery exploring and decompressing the nerve is only indicated in a very few cases where conservative treatment has failed and the patient is significantly disabled from this. If an underlying tumour is found, it should be surgically explored.

Outlook

The majority of cases improve with time, in particular when the paralysis is incomplete (mild to moderate weakness).