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Head & Neck Radiation · Late Effects
Lhermitte's Sign — Electric Shock Down the Spine After Neck Radiation
A brief electric-shock sensation running down your spine when you bend your neck forward has a name: Lhermitte’s sign. After radiation to the neck it is a recognised, temporary effect for the great majority of people, and it settles on its own. This page explains why it happens, what it is not, and what to do while it lasts.
Medically reviewed by Dr. Kirti Ranjan Mohanty, Radiation Oncologist, MBBS · MD (Radiation Oncology), Senior Consultant · Last reviewed August 2026
- Why it happens — the insulation around nerve fibres inside the treated area thins temporarily, so the fibres misfire when the neck bends.
- Usually temporary — it typically starts one to four months after the last session and fades over weeks to months without treatment.
- Know what is not this — numbness or weakness present at rest, an unsteady walk or bladder changes are different signals that need same-day review.
- Rehab, not endurance — pillow height, screen height, controlled neck movement and physiotherapy make these months far more manageable.
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The Direct Answer
What causes the electric shock down my spine after radiation?
Radiation to the neck temporarily thins the myelin — the insulating sheath around nerve fibres in the length of spinal cord that sat inside the treated area. Bending your neck forward stretches those fibres very slightly. Without full insulation they misfire, and you feel a brief electric shock travelling down your spine.
The symptom has a name. Doctors call it Lhermitte’s sign, or Lhermitte’s phenomenon. It was described long before radiotherapy existed, because the same shock turns up in other conditions that affect myelin. In someone who has had radiation to the neck, the upper spine or the chest, it is one of the recognised early-delayed effects of treatment.
The word delayed is doing real work in that sentence. Lhermitte’s sign almost never appears during the treatment course itself. It typically starts one to four months after the last session, most often around the two-to-four-month mark. That is long enough afterwards that most people have stopped connecting anything new to the radiation at all — which is exactly why this symptom frightens people so badly. It arrives when you thought you were finished.
Three details separate it from almost everything else. It is triggered by movement, usually dropping the chin towards the chest. It is brief — a second or two, not a constant ache. And it stops when you straighten up. Anything that stays behind after you stop moving belongs in the next section, not this one.
Your radiotherapy is delivered at an NABH-accredited partner centre; CION Cancer Clinics coordinates your treatment plan, your oncology team and your care throughout. That coordination does not end at the last session. The follow-up review where you report a symptom like this is part of the same plan, not a favour you have to ask for separately.
Did you know?
The nerve fibres themselves are not cut or destroyed in Lhermitte’s sign. What changes is the insulation around them. The cells that maintain myelin are among the more radiation-sensitive cells in the treated area, and they take months to rebuild what they make — which is why radiation oncology guidance groups this symptom with the transient, early-delayed effects of radiotherapy rather than with permanent injury.
The Blunt Answer First
Is Lhermitte’s sign dangerous?
No, not in itself. Lhermitte’s sign after radiation is a transient effect that settles on its own in the great majority of people. It is not evidence that your spinal cord has been permanently injured. It is unpleasant, it is alarming, and it is worth reporting. It is not, by itself, damage.
That headline is worth reading twice, because the sensation is dramatic enough that most people assume the opposite. A shock running down the spine feels like something breaking. It is closer to a wire with worn insulation brushing against another wire — startling, self-correcting, and being repaired while you wait.
There is a second reassurance that patients rarely get told and should. Guidance from bodies such as ASTRO and NCCN on radiotherapy late effects treats this transient shock and the rare, much later problem of delayed radiation injury to the spinal cord as two separate entities. Having this symptom is not regarded as a warning that the rarer one is on its way.
What we will not do is promise you an outcome. Nobody can predict how any individual nerve recovers, and any page that tells you your symptom will definitely be gone by a fixed date is guessing. What we can give you is the pattern most people follow, and the specific signs that mean you are not in that pattern and need to be seen.
- Same-day review: numbness, tingling or weakness that stays — present while you are sitting still, not only when you bend your neck.
- Same-day review: new weakness in the arms or legs, dropping things, an unsteady walk, or new difficulty with stairs.
- Go to an emergency department now: any loss of control over urine or stool, or a band of tightness or numbness around the chest or abdomen.
- Call the team today: severe or worsening neck pain, especially alongside fever, or the sensation turning constant rather than brief.
- Call the team today: a shock sensation appearing for the first time more than a year after treatment, or one that had settled and has come back.
Not sure which of these you are looking at? That uncertainty is normal and it is a reason to call rather than a reason to wait it out. Describe what you feel, when it started and whether it stops when you stop moving — that is usually enough for our team to tell you whether this is a clinic call or a same-day assessment.
The Timeline
How long does Lhermitte’s sign last?
For most people it lasts a few weeks to about six months, then fades on its own. It usually starts one to four months after radiation ends, comes and goes rather than running continuously, and often becomes less intense — or needs a deeper bend of the neck to trigger — before it stops altogether.
| Stage | When | What people usually notice | What to do |
|---|---|---|---|
| During radiation | Weeks 1–7 of treatment | The shock is not typical here. Neck stiffness, skin soreness, dry mouth and swallowing trouble are the usual complaints. | Report anything neurological at the daily on-treatment review. |
| First month after | Weeks 1–4 post-treatment | Still uncommon. Fatigue, taste changes and swallowing recovery dominate this window. | Keep the first follow-up appointment even if you feel well. |
| Onset window | Month 1 to month 4 | The shock appears, usually on bending the chin to the chest. Brief, repeatable, and it stops on straightening. | Name it at the next review and start a two-line diary. |
| Settling window | Month 3 to month 9 | Episodes typically get less frequent, less intense, or need a deeper neck bend to set them off. | Continue neck range-of-motion work and the pillow and screen changes. |
| Beyond a year | Month 12 onwards | Most people no longer have it. A sensation that persists, returns or changes character is not assumed to be this. | Ask for a neurological assessment rather than waiting it out. |
Timings above describe the pattern set out in radiation oncology guidance on early-delayed radiotherapy effects (NCCN, ASTRO), indicative as of August 2026. They are a guide to what is usual across patients, not a prediction for any one person.
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A Frightening Symptom Deserves a Real Answer
Talk to a radiation oncologist about the shock sensation, how long it is likely to last, and what your follow-up plan should now include.
A Practical Sequence
What should I actually do about it?
Report it, then reduce what triggers it. Tell your oncology team at the next review so it is on record with a date. Then raise your screen and reading material towards eye level, use a pillow that keeps the neck neutral, move the neck through a controlled range instead of snapping the chin down, and make the shocks safe on stairs and in traffic.
1
Get it on record at the next review
Say the words plainly: a brief electric shock down the spine when I bend my neck. Ask for it to be documented with the date it started. That record matters if anything changes later.
2
Keep a two-line diary
Date, what triggered it, how long the shock lasted, and whether anything remained afterwards. Four weeks of this answers questions a single consultation cannot.
3
Stop bending the neck to look down
Raise the phone, the book and the laptop closer to eye level. Most everyday episodes come from reading, eating and looking at a screen with the chin dropped.
4
Fix the pillow before you fix anything else
A pillow that pushes the head forward keeps the neck flexed for hours at a stretch. Aim for the neck in line with the spine, whether you sleep on your side or your back.
5
Move the neck deliberately, not suddenly
Slow, controlled range-of-motion work is part of standard head and neck rehabilitation anyway. A physiotherapist can set a range that keeps the neck mobile without provoking a shock.
6
Make the shock safe when it happens
Do not drive or ride a two-wheeler while episodes are frequent. Hold the handrail on stairs. Sit down to wash your hair and to reach into a low cupboard.
7
Keep the rest of the follow-up protocol running
Dental reviews, fluoride tray use and speech and swallow therapy are part of the standard post-radiation protocol. A new nerve symptom is not a reason to pause any of them.
One thing this list is not. None of it asks you to change, add or stop anything you have been prescribed. Positioning, pacing, pillow height and supervised movement are the levers a survivor genuinely controls. Everything else is a conversation with the treating team, and it is a conversation worth having early rather than at the next scheduled slot.
The Wider Follow-Up
Where does this fit in the rest of my head and neck recovery?
Alongside it, not instead of it. Lhermitte’s sign is a nerve symptom inside a recovery that also involves your teeth, your saliva, your swallowing and your neck movement. Those parts of the protocol run to their own schedule. A shock sensation is not a reason to postpone a dental review or a swallowing therapy session.
Dental protocol
Your teeth are on a separate, non-negotiable clock
Daily fluoride tray use is lifelong protocol after head and neck radiation, not an optional extra. See Fluoride Trays After Head and Neck Radiation.
Jaw bone
Never let a tooth be pulled without telling the team
Extraction inside a treated field is planned differently because of a specific jaw-bone risk. See Tooth Extraction After Radiation and the Risk of Jaw Bone Damage.
Swallowing
Feeding recovery is staged and supervised
Coming off a feeding tube follows a protocol led by the speech and swallow therapy team. See Coming Off a Feeding Tube After Head and Neck Radiation.
Where the beam went
The treated field shapes what you feel
Upper-neck and skull-base treatment sits close to the spinal cord, which is part of why this symptom shows up. See Nasopharyngeal Cancer Radiation: What Is Different.
A word on what rehabilitation can and cannot promise. Physiotherapy, speech and swallow therapy and dental review are referrals your team makes as part of a defined post-radiation protocol, and they are aimed at protecting function and comfort. None of them comes with a promised result, and any service that offers you one is overselling. What they reliably do is catch problems early, while they are still small.
If you are unsure which parts of the protocol you are currently on, ask for the follow-up schedule in writing at your next appointment — or call us on 1800 202 8726 and we will walk you through what a standard head and neck follow-up plan contains.
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Common questions
Lhermitte’s Sign After Radiation: Your Questions Answered
What causes the electric shock sensation down my spine after radiation?
Radiation to the neck temporarily thins the myelin, the insulating sheath around nerve fibres in the length of spinal cord that sat inside the treated area. Bending the neck forward stretches those fibres very slightly. Without their full insulation they misfire, and the misfiring is felt as a brief electric shock running down the spine and sometimes into the arms or legs. The fibres themselves are not cut or destroyed. The cells that maintain myelin are simply among the more radiation-sensitive cells in the treated area, and they take months to rebuild what they make. This is why radiation oncology guidance groups the symptom with the early-delayed, transient effects of radiotherapy rather than with permanent injury.
Is Lhermitte's sign dangerous, and does it mean my spinal cord is damaged?
No, not in itself. Lhermitte's sign after radiation is a transient effect that settles on its own in the great majority of people, and it is not evidence that the spinal cord has been permanently injured. Guidance from bodies such as ASTRO and NCCN on radiotherapy late effects treats this transient shock and the rare, much later problem of delayed radiation injury to the cord as two separate things. Having this symptom is not regarded as a warning that the rarer one is coming. What no one can do is predict how any individual nerve recovers, so the honest position is a pattern most people follow rather than a promise about you.
How long does Lhermitte's sign last after neck radiation?
For most people it lasts from a few weeks to about six months and then fades on its own. It usually begins one to four months after the last radiation session, most often around the two-to-four-month mark. It comes and goes rather than running continuously, and it commonly becomes less intense, or needs a deeper bend of the neck to set it off, before it stops altogether. Longer courses do happen, and they are not automatically a bad sign. A shock sensation that is still present beyond about a year, or that had settled and has returned, should be assessed rather than waited out.
Why did it start months after my radiation actually finished?
Because it is a delayed effect, not an immediate one. The myelin that surrounds the nerve fibres is maintained by cells that turn over slowly, so the gap between the treatment and the moment the insulation is at its thinnest is measured in months rather than days. That is also why the symptom frightens people so much. It arrives at the point when most patients have stopped connecting anything new to the radiation, often just as they were starting to feel recovered. Mentioning the date it started at your next review is genuinely useful, because the timing is one of the things that helps your team recognise it.
What can I do at home to reduce the shocks?
Reduce what triggers them. Most everyday episodes come from dropping the chin towards the chest, so raise the phone, the book, the plate and the laptop closer to eye level. Choose a pillow that keeps the neck in line with the spine instead of pushing the head forward, because a bad pillow holds the neck flexed for hours. Move the neck slowly and deliberately rather than snapping it down. Keep a two-line diary of what triggers an episode and how long it lasts. Make the shocks safe as well: do not drive or ride a two-wheeler while episodes are frequent, and hold the handrail on stairs.
Which symptoms after neck radiation are not Lhermitte's sign and need urgent review?
Anything that stays when you are sitting still. Lhermitte's sign is triggered by movement, lasts a second or two, and stops when you straighten up. Numbness, tingling or weakness that is present at rest is a different signal. So is new weakness in the arms or legs, dropping things, an unsteady walk, or difficulty with stairs, and all of these need a same-day review. Loss of control over urine or stool, or a band of tightness or numbness around the chest or abdomen, means going to an emergency department now. Severe or worsening neck pain, particularly with fever, should be reported to the team the same day.