Numbness that stays in one leg is almost always a nerve problem, and the part of the leg that is numb tells you which nerve. A patch across the top of the foot points to the L5 nerve root in the lower back. A dead-feeling outer thigh points to a small nerve at the pelvis. A whole leg going strange with weakness and clumsiness points somewhere higher - the spinal cord or the brain. The MRI you need depends entirely on which of these it is.
This piece is the decision guide. It walks through the dermatome map that neurologists actually use, the red flags that turn a routine referral into an A&E trip, the three scans that answer nearly every case, and when it is safe to wait six weeks before scanning at all.
One-line answer
For most UK adults with new one-sided leg numbness, the correct first scan is a lumbar spine MRI, because a compressed lumbar nerve root is by far the most common cause. If the numbness sits in a peripheral nerve pattern below the knee, an MR neurogram of the knee or pelvis is better. If there is weakness of both legs, bladder change or brain symptoms, the answer changes - see the red flags section.
Where the numbness is tells you which MRI
Every part of the leg skin is supplied by a nerve root that exits the lower back at a specific level. The pattern of where you are numb maps back to which root is being pinched. This is the dermatome map that a neurology or spine consultant will run in their head in the first two minutes of the consultation:
- L2 - front of the thigh. Numbness across the upper anterior thigh points to an L2 root or, more commonly, the lateral femoral cutaneous nerve (meralgia paraesthetica). Lumbar MRI first, MR neurogram of the pelvis second if the spine is clean.
- L3 - medial thigh down to the inside of the knee. An L3 root problem is uncommon and usually from a high lumbar disc. Lumbar MRI answers it.
- L4 - medial lower leg down to the inner ankle. Often paired with a weaker quadriceps and a reduced knee reflex. L3-L4 disc on the MRI.
- L5 - dorsum of the foot, top of the big toe and outer shin. The commonest sciatica pattern. Paired with weak foot lift (see our foot drop guide). L4-L5 disc on the MRI.
- S1 - lateral foot, little toe and back of the calf. The other common sciatica pattern. Paired with a weak calf, reduced ankle reflex, difficulty going up on tiptoe. L5-S1 disc on the MRI.
If the numbness respects one of these bands, a lumbar spine MRI is the right first test. If it does not - if it is a stocking distribution below the knee, or a patch on the outside of the calf that stops in the middle of a dermatome - the nerve is being trapped somewhere in the leg itself, not at the spine.
Red flags that need urgent MRI
Some patterns take the decision out of your hands and turn a routine question into an A&E visit the same day. If any of the following are present, do not book a routine private MRI - go straight to A&E and ask for an emergency scan:
- Numbness that spreads from one leg to the other. Bilateral leg involvement suggests the spinal cord itself or the cauda equina.
- Numbness in the saddle area. Inner thighs, buttocks or groin going numb is a cauda equina emergency.
- Any change in bladder or bowel control. Difficulty starting to urinate, dribbling, incontinence, loss of the urge to go. Same day A&E.
- Rapidly progressive leg weakness. Losing power in the leg over hours or a few days, not gradually over weeks.
- Numbness after significant trauma or a fall from height. Rule out a spinal injury before anything else.
- Numbness with fever, night sweats, unexplained weight loss or a history of cancer. Infection or metastatic disease of the spine needs same-week imaging.
Cauda equina, in particular, has a narrow surgical window. The evidence is that decompression within 48 hours of urinary symptoms gives the best chance of a normal bladder afterwards. This is not a wait-and-see problem.
The three MRIs you might need
For one-sided leg numbness, one of three scans nearly always gives the answer. The trick is matching the scan to the pattern of numbness rather than defaulting to the same test for every patient.
| Scan | When it is the right first test | Private price, 2026 |
|---|---|---|
| Lumbar spine MRI | Numbness in a clear L2 to S1 dermatome, sciatica-type pain, back pain with leg symptoms | £450 to £750 |
| MR neurogram, knee or pelvis | Numbness in a peripheral nerve pattern - outer thigh, side of calf, front of ankle, sole - with a clean spine | £600 to £900 |
| Brain MRI | Numbness with brain symptoms - visual change, speech, balance, one-sided face involvement, sudden onset | £450 to £800 |
A common private-clinic mistake is to book a lumbar MRI on every leg-numbness patient and then send them home with a normal report. If the pattern is not in a spinal dermatome, the spine will be clean, and the patient goes away no wiser. The five minutes of history-taking that matches pattern to nerve is what makes the scan worth doing.
MR neurography for suspected nerve entrapment
Peripheral nerves can be pinched a long way from the spine. Two common examples:
- Piriformis syndrome. The sciatic nerve is compressed by the piriformis muscle deep in the buttock, producing sciatica-type numbness and pain with a completely normal lumbar MRI. Diagnosed with an MR neurogram of the pelvis.
- Common peroneal entrapment at the fibular head. Numbness on the outer calf and top of the foot after prolonged leg-crossing, kneeling, plaster casts, or rapid weight loss. Sometimes paired with weak ankle dorsiflexion. Diagnosed with an MR neurogram of the knee.
MR neurography uses different sequences to a standard MRI and needs a musculoskeletal or peripheral-nerve radiologist to report properly. Not every private clinic offers it, and a general radiologist reading a neurogram is worse than not doing the scan at all. Ask before you book.
The rare central causes: MS, cord lesion, small stroke
A small minority of one-sided leg numbness is not a nerve root or a peripheral nerve at all - it is the spinal cord or the brain. These are less common but important not to miss:
- A demyelinating plaque of multiple sclerosis in the thoracic or cervical cord can cause a patchy sensory level in one leg, often paired with subtle balance issues, Lhermitte sign (electric shock down the spine on neck flexion) or a previous episode of blurred vision. Needs a brain and whole-spine MRI with contrast.
- A spinal cord lesion - tumour, inflammation, or vascular - gives a sensory level, meaning numbness that stops at a horizontal line on the trunk. Rare but a same-week scan.
- A small cortical stroke in the sensory cortex can present as pure hemibody numbness including the leg. It is more common in people over 60 with vascular risk factors. Sudden onset over minutes is the giveaway. Brain MRI with diffusion.
