Why One Foot Burns, Swells, and Changes Color After the Cast Comes Off

What this video covers

• Where the lumbar sympathetic chain sits and what it actually controls in the leg
• The leading theory of pain kept alive by the sympathetic nerves, and how settled that idea really is
• Why a rise in foot temperature shows the block worked, and why that does not prove your pain is driven by the sympathetic nerves
• What the Cochrane review and newer randomized trials really say about how well it works
• The real risks, and why a short diagnostic trial beats endless repeat injections
• MEDICAL DISCLAIMER: This content is for educational purposes only and is not medical advice. It does not substitute for professional diagnosis or treatment. Always consult a licensed healthcare provider regarding your condition. Viewing this video does not establish a doctor-patient relationship.

A foot that burns, swells and changes color after a healed fracture points to the lumbar sympathetic chain. That chain is a string of nerve clusters (ganglia) along the front of the low back spine. It controls the leg’s blood flow and sweat. A lumbar sympathetic block tests it. When the foot warms, it shows the block reached the chain. It does not prove the pain is driven by the sympathetic nerves.

A broken ankle heals. Six weeks in a cast, the bone knits, the films look perfect. Then the cast comes off and the foot is wrong. It burns. It changes color through the day — dusky, then flushed. One sock comes off damp and the other does not. A bedsheet across the toes is intolerable.

Everything you describe goes into the chart as unexplained. It is not unexplained. It comes from a chain of nerve clusters sitting against the front of the low back spine that nobody has examined, because it does not show on a bone X-ray. This article explains what that chain does and why it causes the symptoms you feel. It also covers what a lumbar sympathetic block can honestly tell you, and where the evidence for it is weaker than most clinics admit.

What is the lumbar sympathetic chain, and why doesn’t it show on an X-ray?

The lumbar sympathetic chain is not inside the spinal canal. It runs along the front-and-side face of the low back spine bones, from about L1 through L5. A thin sheet of tissue separates it from the psoas muscle. Blocks target the L2 through L4 levels.

These fibers do two jobs in the leg. They set the caliber of its blood vessels, and they drive the sweat glands of the foot. They carry no motor supply and no touch sensation. Nothing about walking, gripping the floor, or feeling a sock belongs to them.

That split in jobs is the whole reason the procedure is useful as a test, so hold on to it as you read. Everything the sympathetic chain does involves blood flow and sweat. Everything a pinched nerve root does involves feeling and movement: numbness, weakness, a stripe of pain down one strip of skin (a dermatome). They are different systems with different fingerprints.

The mechanism people argue about

The leading explanation for sympathetically maintained pain goes like this. After an injury, pain-carrying fibers become far too sensitive to the body’s own norepinephrine, a stress chemical. Sympathetic signals that would normally just adjust blood flow now drive the pain signal directly.

That model should be labeled honestly: it is contested. It comes mostly from animal work. It does not apply to every patient, and it is still truly debated in humans. Any physician who presents it as settled biology is overselling it. It is a working idea that explains much of what we see at the bedside. It is not a proven chain of cause and effect in your own leg.

Why does the foot burn, sweat and change color after an injury?

Once you know the chain controls vessel caliber and sweat, the symptom list stops looking random.

The color changes. Dusky one hour, flushed the next, is what a limb looks like when vascular tone is being driven erratically rather than held steady. That is a vasomotor sign, not a tissue-damage sign — which is why the foot can look alarming on a day the X-ray is perfect.

The one damp sock. Sudomotor fibers to the foot run in this same chain. Asymmetric sweating — one side wet, the other dry — is one of the cleanest bedside clues that the problem sits in the autonomic supply to that limb rather than in the bone or the joint.

The temperature difference. A cold foot on the injured side, or a warm one, is the same vasomotor story read through a thermometer instead of through your eyes. It is also why the symptoms often worsen in the cold.

The pain that ignores the map. This is the tell that most often gets missed. Pain from a compressed nerve root follows a dermatome — a stripe down the leg with a predictable pattern. Sympathetically maintained pain does not. It wraps the foot the way a sock does. It is out of proportion to the original injury. It does not respect the anatomy of a nerve root because it is not coming from one.

The bedsheet you can’t stand. When a limb becomes so sensitive that light touch feels like pain, a sheet across the toes is a real problem, not an exaggeration. It is also what makes rehab hard, for reasons covered below.

What the standard approach misses — in both directions

Patients with this picture tend to get failed twice, in opposite directions.

The first failure is the imaging reflex. Burning pain down one leg triggers a low back MRI. The MRI shows a disc bulge, a finding that many adults without pain also have, with no symptoms at all. So the pain gets labeled radiculopathy. A nerve pill, a home exercise sheet, next patient. Nobody touches the limb, takes its temperature, or notices that the pain ignores the nerve’s strip of skin, wraps the foot and gets worse in the cold. None of that acts like a pinched nerve root.

This is not a criticism of imaging or of the physicians ordering it. An MRI answers the question it was asked. The problem is that the wrong question was asked, because nobody examined the limb first.

The second failure is the opposite sin. Someone correctly spots complex regional pain syndrome and right away sells a series. Six blocks. Twelve blocks. All booked before the first one has told anybody anything. That is a subscription, not a plan to find answers.

A lumbar sympathetic block is a question, not a course of therapy. What it earns you is information and, sometimes, a window. What that window is used for is the part that actually changes the outcome.

How is CRPS in the foot diagnosed? Closing the differential first

Several conditions cause a painful, discolored, swollen leg, and some are dangerous. They have to be ruled out first.

• A hot, swollen, discolored calf is a deep vein thrombosis until proven otherwise. That is an ultrasound.
• A cold foot with no pulse and leg pain when you walk is peripheral arterial disease. That needs a blood vessel study.
• Burning in both feet, evenly, in a sock pattern, is more likely small fiber or metabolic neuropathy. Glucose and A1c belong in that workup, because no block will fix a metabolic cause.
• Cellulitis (skin infection), an occult nonunion (a hidden break that never healed), and chronic venous insufficiency (weak leg veins) also sit on this list. So does true nerve root pain from the low back.

Once those are ruled out, the diagnosis rests on the Budapest criteria. First, pain that keeps going and is out of proportion to the injury that caused it. Second, at least one reported symptom in three of four groups: sensory, vasomotor, sudomotor/edema and motor/trophic. Third, at least one sign the physician can see on exam day in two of those groups. And fourth, no other diagnosis that better explains the picture.

This is an established, validated standard. Harden and colleagues, writing in Pain in 2010, tested the criteria in 113 patients with CRPS-I and 47 with other nerve pain. The older IASP criteria had near-perfect sensitivity (1.00) but poor specificity (0.41). They caught everyone, including many people who did not have the condition. The Budapest clinical version raised specificity to 0.68 while holding sensitivity at 0.99. The research version was built to reach the highest specificity, 0.79. It pays for that number with lower sensitivity, which means more true cases missed.³ The clinical version belongs at the bedside. The research version belongs in choosing people for trials.

One distinction matters for your workup. What is described above is type one. It follows a fracture, a crush injury or being immobilized, with no known nerve injury. That is the form the validation cohort was built from. When a specific nerve was cut or damaged, it is type two, and nerve testing belongs in that workup first. Both types are described in more detail on the complex regional pain syndrome treatment page.

What does a lumbar sympathetic block prove, and what does it not?

The procedure itself is simple to describe. You lie face down. Temperature probes go on both feet and record a baseline before anything is injected. The X-ray arm (C-arm) tilts until the front-side corner of the spine bone shows in profile. The needle goes to that corner at L2, L3 or L4. A side view confirms the depth: just in front of the spine bone, in front of the psoas muscle. Then dye goes in, under live imaging.

The right spread is a smooth column hugging the front edge of the spine bone. Dye feathering into the psoas means the needle is too shallow. Dye that washes out instantly means a blood vessel. Dye tracking toward the nerve opening means too far back. Only the right dye pattern earns the numbing medicine. The full steps, positioning and recovery are on the lumbar sympathetic nerve block procedure page.

Now the anatomy pays off. A block that works raises the skin temperature of that foot, because the grip on the blood vessels has been released. Strength and light touch stay intact. No numbness, no weakness.

That last detail is also how a messy result gets caught. If light touch or strength changes afterward, numbing medicine reached a nerve root or other nerves. Then the test is no longer clean, and any relief can’t be credited to the sympathetic chain. Testing feeling and strength after the block is what makes the result readable at all.

Here is the part most consent talks blur: a temperature rise proves the block worked technically. It does not prove your pain is sympathetically maintained. Relief afterward is a hint, not proof. A single unblinded block (you know what you got) has a high false-positive rate. Placebo, numbing medicine soaking into the blood, and spread to other nerves can all fake it. The result is read along with the rest of the clinical picture, never as a verdict on its own.

Does a lumbar sympathetic block work? The outcome evidence

This is the section most clinic pages leave out.

In 2016, O’Connell and colleagues gathered for Cochrane every randomized trial of sympathetic blocks with numbing medicine in this syndrome. They found 12 studies and 461 people. Every one was judged at high or unclear risk of bias, and the evidence was graded low to very low quality. Two small trials compared the block with placebo or sham (32 people total) and did not show significant short-term benefit. The authors concluded that the limited data do not suggest the block reduces pain in this syndrome.¹

A 2018 review in the Canadian Journal of Anesthesia by Duong and colleagues is sometimes quoted as a rebuttal. It is not one. Its 35 randomized trials cover every kind of treatment for this syndrome: bisphosphonates, steroids, ketamine, immunoglobulin, mirror therapy, stimulation and more. Sympathetic blocks get a single sentence, with no pooled data, noting a growing role. The review itself calls the trial quality uneven.² That does not overturn Cochrane.

Side effects reported in the Cochrane trials were minor,¹ but the risks are real and should be named. They include bleeding, infection, injection into a blood vessel or the spinal fluid, puncture of the kidney or ureter, and injury to a nearby nerve. They also include nerve pain in the groin (genitofemoral neuralgia), nerve pain after the block (post-sympathectomy neuralgia), and short drops in blood pressure. Relief from a single block is often short: hours to days.

The honest position is this. It is a test that can buy a treatment window, not an open subscription. The value lies in what gets done inside that window: desensitization (retraining the skin to tolerate touch), graded motor imagery, loading the limb and moving it.

What this means for your evaluation

If a limb burns, swells, sweats on one side or changes color after an injury, the order that matters is this. Examine the limb. Rule out the look-alikes. Apply the criteria. Then decide about a needle. Not the other way around.

A first block should be ordered to answer a question, not to treat. Two pieces of information come back from it. First, whether the response fits with a sympathetic part to the pain: a hint, not proof. Second, how long one block lasts, which sets how the rehab window gets scheduled.

The rehab itself is the hard part, and it is worth saying plainly. Desensitization asks you to do the very thing the limb has refused for months: be touched, every day. In practice there are stuck weeks and real discouragement, and a repeat block sometimes gives back less than the first. Wearing a shoe tends to come back long before handling cold does, and cold sensitivity may never fully go away. These are practice observations, not trial results. Individual results vary.

A note on add-ons. Diet, specialized pro-resolving mediators (fats that help end inflammation) and red-light therapy (photobiomodulation) are adjunctive for this condition, nothing more. The Padda Institute uses them to lower overall inflammation, not because a trial in this syndrome backs them. No such trial exists, and they are not priced as treatment for this condition.

Frequently asked questions

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Key takeaways

• The lumbar sympathetic chain runs along the front-and-side face of the low back spine bones. It controls blood vessel size and sweating in the leg. That is exactly why burning, color change, swelling and one-sided sweating show up together after an injury.
• The leading explanation is that injured pain fibers become far too sensitive to the body’s own norepinephrine. But it comes mostly from animal work and is still debated in humans.
• The diagnosis rests on the validated Budapest criteria, after clot, artery disease and metabolic neuropathy are ruled out. It does not rest on an MRI showing a disc bulge.
• A temperature rise proves the block worked technically. It does not prove your pain is sympathetically maintained. And a single unblinded block (you know what you got) has a high false-positive rate.
• The strongest review to date found low to very low quality evidence and no proven pain benefit. So the block belongs as a short test that buys a rehab window. It should never be an open-ended series booked in advance.

This article is educational and is not a substitute for evaluation, diagnosis, or treatment by a physician. Individual results vary. Do not start, stop, or change any medication without consulting your physician. To be evaluated, request an appointment at painmd.tv, call (314) 481-5000, or text (314) 886-5902.

References

1. O’Connell NE, Wand BM, Gibson W, Carr DB, Birklein F, Stanton TR. Local anaesthetic sympathetic blockade for complex regional pain syndrome. Cochrane Database Syst Rev. 2016;7(7):CD004598. PMID 27467116. PubMed
2. Duong S, Bravo D, Todd KJ, Finlayson RJ, Tran Q. Treatment of complex regional pain syndrome: an updated systematic review and narrative synthesis. Can J Anaesth. 2018;65(6):658-684. PMID 29492826. PubMed
3. Harden NR, Bruehl S, Perez RSGM, Birklein F, Marinus J, Maihofner C, Lubenow T, Buvanendran A, Mackey S, Graciosa J, Mogilevski M, Ramsden C, Chont M, Vatine JJ. Validation of proposed diagnostic criteria (the “Budapest Criteria”) for Complex Regional Pain Syndrome. Pain. 2010;150(2):268-274. PMID 20493633. PubMed

Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP — Board Certified in Anesthesiology, Pain Medicine, Interventional Pain Management, Addiction Medicine, and Obesity Medicine. Last reviewed July 2026.

Dr. Gurpreet Singh Padda, MD, MBA, MHP