Foot drop is one of the small number of neurological signs where the calendar itself is part of the diagnosis. Caught inside a few weeks, most patients recover fully. Left for three months, the nerve fibres begin to die and the weakness becomes permanent. The one investigation that resolves the question of what to do next is almost always a lumbar MRI scan – and in the UK in 2026, waiting the NHS routine queue for it is the single most common mistake we see.
This guide is about knowing when your foot drop is the kind that needs a scan this week, what that scan will show, and how to get it done fast enough to matter.
One-line answer
If you have new or progressive foot drop, you need an urgent lumbar MRI within days, not weeks – because the most common cause is a lumbar disc compressing the L5 nerve root, and the window for full recovery closes quickly.
What foot drop actually is
Foot drop is weakness of dorsiflexion – the movement that lifts the front of your foot up towards your shin. When the muscles that pull the foot up (mainly tibialis anterior) can no longer overcome gravity, the toes drag on the floor as you walk. The compensation is a high-stepping “steppage” gait, where you lift the whole leg exaggeratedly to clear the toes, and the foot slaps down instead of rolling heel-to-toe. Patients trip on carpets, catch their toe on kerbs, and quietly give up wearing loose shoes.
There are two anatomical patterns worth knowing:
- L5 pattern. Numbness across the top of the foot and outer shin, weakness of great-toe extension as well as foot lift, often preceded by back and buttock pain. The lesion is at the lumbar spine.
- Common peroneal pattern. Numbness confined to the top of the foot, no back pain, weakness of foot lift and eversion but not toe extension in a pure form, and often a story of prolonged crossed-leg sitting, a leg cast, or bariatric weight loss. The lesion is at the fibular head of the knee.
Three main causes, and the right imaging for each
Foot drop is a sign, not a diagnosis. The reason imaging matters is that the same weakness can come from three quite different problems, each with a different scan and a different treatment.
| Cause | Typical clues | First imaging |
|---|---|---|
| L4/5 disc herniation compressing L5 root | Back pain, sciatica, numb top-of-foot and outer shin, weak great-toe lift | Lumbar MRI, urgent |
| Common peroneal nerve compression at knee | No back pain, history of crossed legs, plaster cast, kneeling job, or rapid weight loss | Knee MRI or high-resolution nerve ultrasound |
| Central cause (stroke, MS lesion, tumour) | Sudden onset with facial or arm involvement, brisk reflexes, upgoing plantar | Brain MRI, urgent |
| Motor neurone disease, peripheral neuropathy | Bilateral, progressive over months, fasciculations, sensory changes elsewhere | Neurology referral, EMG/nerve conduction, then MRI as directed |
In UK primary care in 2026, roughly seven in ten new foot drops presenting to a GP turn out to be L5 nerve root compression from a lumbar disc. That is why the lumbar MRI is the default first scan when the clinical picture fits – and it is the scan the NHS routine queue delays worst.
Timing matters: the recovery window
This is the part every patient should understand before choosing to wait. Nerve fibres compressed by a disc do not fail all at once. They lose function in stages:
- Hours to days: the nerve is irritated but structurally intact. Decompression at this stage typically returns full strength within weeks.
- Two to six weeks: a mixed picture. Most patients still recover well after urgent decompression, but a proportion are left with a residual weakness.
- Three months and beyond: Wallerian degeneration is well established. Axons have died. Even after successful surgery, the foot drop may only partially recover, and some patients need a permanent ankle-foot orthosis.
The medical literature is consistent on this. Time from onset of foot drop to decompression is the single strongest predictor of recovery, ahead of age, disc size, or surgical technique. Waiting nine weeks for an NHS MRI, then another six for a spinal opinion, then another four for a theatre slot, is a pathway that quietly writes off the recovery window.
What the MRI actually shows
A lumbar MRI in a patient with a suspected L5 foot drop is looking for a small number of specific findings. A competent report will address each of them:
- Disc extrusion or sequestration at L4/5. A fragment of disc pushed backwards into the spinal canal, or migrated up or down, compressing the exiting nerve root.
- Nerve root compression grade. Radiologists grade the degree of root effacement – mild, moderate or severe. Severe compression with corresponding clinical weakness is a surgical indication.
- Ligamentum flavum thickening and facet joint hypertrophy. Chronic canal narrowing that can compress the L5 root even without a fresh disc event, more common in patients over 60.
- Central canal versus foraminal. A far-lateral or foraminal disc can compress L5 as it exits at L5/S1 rather than at L4/5. Missing this is a classic reporting error.
- Signs of cauda equina. Any hint of compression of the whole cauda equina below L1 is a same-day surgical emergency.
The post-scan pathway
The MRI report is only useful if it slots into a plan. The three routine post-scan pathways are:
- Physiotherapy and watchful waiting. Reserved for mild L5 compression with mild weakness, where the patient has near-full strength and no progression. Tibialis anterior strengthening, gait retraining, and reassessment at four weeks.
- Ankle-foot orthosis (AFO). A moulded plastic brace worn inside the shoe that holds the foot in dorsiflexion. Useful either as a bridge while the nerve recovers or as a long-term solution if surgery is declined or unhelpful.
- Microdiscectomy or lumbar decompression. Day-case or overnight spinal surgery to remove the disc fragment pressing on the L5 root. For a foot drop caused by a clear disc extrusion, this is the treatment with the strongest evidence, and the sooner it is done after onset the better the outcome.
