Foot drop: making sense of its causes

A common problem for which patients consult neurologists like me is foot drop. As the name says, they have foot drop and hence are unable to dorsiflex their foot. As a result they are likely to catch their foot on the ground while walking and thus are prone to falls and this brings them to medical attention. Foot drop should be differentiated from frail foot. In foot drop, patients are unable to dorsiflex their foot while those who have a frail foot are unable to dorsiflex as well as unable to planter flex the foot (that is they are unable to pull their foot up or push down their foot as when you press down on a gas pedal).

Foot drop might occur suddenly (acutely) or may be more insidious and the causes for both vary. Before we discuss the causes of foot drop, it is helpful to know a little about the relevant anatomy. The muscle which helps to dorsiflex the foot is called tibialis anterior and it is supplied by a nerve called the peroneal nerve. The peroneal nerve is a branch of the sciatic nerve. The sciatic nerve is formed by the lower lumbar and sacral nerve roots and forms a part of the lumbosacral plexus. The peroneal nerve in the knee area is quite superficial as it cross the neck of the fibula (fibula is one of the bones in the lower leg along with the larger tibia). As the nerve is superficial it is prone to compression across the neck of the fibula.

So lets now discuss some of the common causes of foot drop. If suppose you suffer a fracture across the neck of the fibula, or have a gun shot wound to that area, or during knee surgery the peroneal nerve is accidently transected, you shall develop a foot drop. In all of the above the cause is injury to the peroneal nerve.

But peroneal compression may also occur due to other more subtle causes. One of the most common causes of foot drop is habitual leg crossing. This is most commonly seen in obese persons or in diabetics who lose weight.  After weight loss they can cross their legs more easily and may develop a foot drop. Why does this occur you may ask? Well the answer is simple, as I told you before the peroneal nerve is quite superficial and hence prone to compression. When you cross your leg, the nerve may get pinched against the other knee and if you do not relieve the pressure soon, you can develop a foot drop. A common scenario is that the person is sitting with his legs crossed on a long flight, or might have fallen asleep with his leg pressed against the side rails of the bed (this is common in hospitalized patients in the intensive care unit or also when patients are undergoing surgery in the OR), they wake up and find they have a foot drop. The good news is that the prognosis for this type of compression injury to the peroneal nerve is rather good. Once the pressure is released, these patients usually make a full recovery over a few days to weeks and their foot drop goes away.

They can though be other more proximal causes of foot drop. You can have compression or injury to the sciatic nerve or to the lumbosacral plexus (remember I told you, that the peroneal nerve is a branch of the sciatic nerve).  Compression of the sciatic nerve may at times be due to a tumor or mass in the pelvis or in the thigh or knee area.

Lower lumbar disc herniation may also result in a drop foot. This is commonly seen in L5 disc herniation. Patients usually present with radicular pain radiating down the leg, though sometimes this may be absent.

Diagnosis and management of foot drop: the diagnosis of foot drop is clinical and depending upon your examination findings, your doctor may or may not order other tests to confirm at what level is the problem. He may order a nerve conduction study and an EMG (needle study, electromyogram) to check for the the sciatic and peroneal nerves. If warranted an MRI of the lower back and of the plexus may be done.

The treatment depends upon the cause. If the foot drop is because of habitual leg crossing, then all what may be needed is to advise the patient not to cross his legs. The recovery is spontaneous. If a mass is the cause then well depending upon what it is, the treatment varies. Patients usually need a ankle foot orthosis or a foot drop splint. This splints the foot up and prevents fall.

I hope this is helpful to some of you.

Personal Regards,

Nitin Sethi, MD