Foot drop is weakness at the ankle that causes the foot to drop down when walking, so that the patient has a noticeable limp. In advanced cases the patient can trip over their own toes.

Although the condition can arise for many reasons, two common causes are:

  • Spinal nerve root injury, often L5 but not infrequently L4
  • Peroneal nerve damage

Symptoms

If the foot drop is associated with shooting pains or tingling sensations from the buttock down through the leg, then this is the classic indicator of spinal nerve involvement.

However, absence of pain is not diagnostic. The foot drop could still be due to either spinal or peroneal nerve damage.

Any history of injury or pressure to the lateral aspect of the knee strongly suggests peroneal nerve involvement.

Foot drop may come on suddenly or develop insidiously, which without pain, might not be detected until quite advanced. The onset doesn’t distinguish between spinal or personal nerve causes.

Signs on examination

Patients with symptomatic foot drop will exhibit weakness of ankle dorsiflexion of grade 3/5 or less on manual motor testing (just able to overcome gravity but not resistance).

There are a couple things we can do to distinguish between spinal and peroneal nerve lesions. Unfortunately, sensory changes do not distinguish between L5 nerve root and peroneal nerve injury, but motor function can.

  • Trendelenburg test

The patient stands on one leg. If the pelvis sags down on the side of the raised leg, then this indicates weakness of the hip abductors of the limb on which they are standing. This likely indicates injury to the L5 nerve root which supplies the hip abductors via the superior gluteal nerve.

Note that for patients with poor balance this test can be modified by holding their hands as they stand on one leg. If they exert a lot of pressure on the opposite hand, the test is also positive.

  • Foot inversion and dorsiflexion

If there is weakness of both, it would suggest damage of the L4 spinal nerve root, which is implicated in many cases of foot drop.

  • Rub the peroneal nerve at the knee where it passes round the neck of the fibula

If there is tenderness, or pain/tingling in the leg and dorsum of the foot (Tinel’s sign) then the nerve is likely the cause of the foot drop.

Diagnostic tests

If there is sciatic pain then there is likely injury to the L4 or L5 spinal nerve root caused by a protruding disc or degenerative stenosis.

In the absence of sciatic pain, the cause of foot drop may not be obvious but an MRI is still indicated to look for spinal nerve root involvement.

In both cases the patient needs a lumbar spine MRI scan fairly urgently.

Call the practice and I can arrange this and see them in a few days.

If the MRI is clear, or there is uncertainty, electrophysiological studies are useful to pinpoint the lesion to a nerve root or the peroneal nerve.

Prognosis in the event of spinal nerve root injury

The key factor here is severity of the weakness and how long it has persisted.

Severe weakness (grades 0-1/5) and/or duration of more than 4 – 6 weeks generally indicate a poor prognosis, though recovery has been seen in a few such cases after surgery.

The threshold of a few weeks for poor outcome is not long, so, it is best to get an accurate diagnosis and treatment plan quickly.

As always if you have any questions about a patient, please feel free to call me on 02 9650 4893.

I will return your call and discuss how we can help.

You can also reach me via ralphstanford@powspine.com.au.


Resources for Referrers

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Dr Ralph Stanford is a spinal surgeon with over 20 years’ experience. He is adept in all surgical approaches to successfully treat his patients.

Dedicated to lifelong learning and sharing his expertise with others, Ralph attends spinal conferences worldwide and regularly consults with colleagues about successful treatments.

As a teacher Dr Ralph Stanford is Conjoint Senior Lecturer at the University of New South Wales; Supervisor of Training for Orthopaedic Trainees, Prince of Wales Hospital; and Education Secretary for the Spine Society of Australia.

As a researcher and scientist, he is a well-published author and Honorary Senior Scientist at Neuroscience Research Australia (NeuRA). He is also a current member of Spinal PFET.


Posted 21 June 2021


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