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 sympathetically maintained pain, and how settled that idea actually is
• Why a temperature rise confirms the block was technically successful — and why that is not the same as proving your pain is sympathetically driven
• 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 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 ganglia sitting against the front of the lumbar spine that nobody has examined, because it does not appear on a bone film. This article explains what that chain does, why it produces the symptoms you feel, what a lumbar sympathetic block can honestly tell you — and where the evidence for it is weaker than most clinics admit.

The nerve chain that no film of your back will show

The lumbar sympathetic chain is not inside the spinal canal. It runs along the anterolateral surface of the lumbar vertebral bodies — the front-and-side face of the bones — from roughly L1 through L5, separated from the psoas muscle by a thin fascial plane. 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 separation of function is the entire reason the procedure is diagnostically useful, and it is worth holding on to as you read the rest of this page. Everything the sympathetic chain does is vascular and sudomotor — blood flow and sweat. Everything a compressed nerve root does is sensory and motor — numbness, weakness, a dermatomal stripe of pain. They are different systems with different fingerprints.

The mechanism people argue about

The leading explanation for sympathetically maintained pain is that after an injury, pain-carrying fibers become abnormally sensitive to the body’s own norepinephrine. Sympathetic outflow that would ordinarily just adjust blood flow instead drives the pain signal directly.

That model should be labeled honestly: it is contested. It comes largely from animal work, it does not apply to every patient, and it remains genuinely debated in humans. Any physician who presents adrenergic coupling as settled biology is overselling it. It is a working hypothesis that explains a great deal of what is seen at the bedside — not a proven chain of causation in your particular limb.

Why that anatomy produces the symptoms you feel

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 intolerable bedsheet. When a limb becomes so sensitized that light touch registers as pain, a sheet across the toes is a genuine problem, not an exaggeration. It is also the finding that makes rehabilitation difficult, 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 lumbar MRI. The MRI shows a disc bulge — a finding that a large share of pain-free adults also have, with no symptoms whatsoever — and the pain is labeled radiculopathy. A nerve medication, a home exercise sheet, next patient. Nobody puts a hand on the limb, measures its temperature, or notices that the pain ignores dermatomes, wraps the foot, and worsens in cold. None of that behaves like a compressed 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. Somebody correctly recognizes complex regional pain syndrome and immediately sells a series. Six blocks. Twelve blocks. Booked before the first one has told anybody anything. That is a subscription, not a diagnostic plan.

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.

Closing the differential before anyone reaches for a needle

Several conditions produce a painful, discolored, swollen lower limb, and some of them are dangerous. They have to be excluded first.

• A hot, swollen, discolored calf is a deep vein thrombosis until proven otherwise. That is an ultrasound.
• A cold, pulseless foot with claudication is peripheral arterial disease. That is a vascular study.
• Symmetric, stocking-distribution burning in both feet is more likely small fiber or metabolic neuropathy. Glucose and A1c belong in that workup, because a metabolic driver will not be blocked away.
• Cellulitis, an occult nonunion of the original fracture, and chronic venous insufficiency also sit on this list, alongside true lumbar radiculopathy.

When those are excluded, the diagnosis rests on the Budapest criteria: continuing pain disproportionate to the injury that caused it; at least one reported symptom in three of the four categories (sensory, vasomotor, sudomotor/edema, motor/trophic); at least one sign the physician can observe on the day of examination in two of those categories; and no other diagnosis that better explains the picture.

This is an established, validated standard. Harden and colleagues, publishing in Pain in 2010, tested the criteria in 113 patients with CRPS-I and 47 with non-CRPS neuropathic 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 designed to reach the highest specificity, 0.79, and it buys that number at the cost of sensitivity — meaning more true cases missed.[^3] The clinical version belongs at the bedside; the research version belongs in trial enrollment.

One distinction matters for your workup. What is described above is type one — following a fracture, a crush injury, or immobilization, with no identified nerve injury. That is the form the validation cohort was built from. Where a named nerve was cut or damaged, it is type two, and electrodiagnostic testing belongs in that workup first. Both types are described in more detail on the complex regional pain syndrome treatment page.

What a technically successful block proves — and what it does not

The procedure itself is straightforward to describe. You lie prone. Temperature probes go on both feet and record a baseline before anything is injected. The C-arm angles obliquely until the anterolateral corner of the vertebral body is in profile. The needle advances to that corner at L2, L3, or L4, with depth confirmed on a lateral view — just anterior to the vertebral body, in front of psoas. Then contrast, under live imaging.

Correct spread is a smooth longitudinal column hugging the anterior vertebral margin. Feathering into psoas means the needle is too shallow. Instant washout means a vessel. Tracking toward the neuroforamen means too posterior. Only a correct contrast pattern earns the local anesthetic. The full technical sequence, positioning and recovery are laid out on the lumbar sympathetic nerve block procedure page.

Now the anatomy pays off. A technically successful block raises the skin temperature of that foot, because vasomotor tone has been released — while strength and light touch remain intact. No numbness, no weakness.

That last detail is also how contamination is caught. If light touch or strength changes afterward, anesthetic has reached a nerve root or the somatic plexus, the test is no longer clean, and whatever relief follows cannot be credited to the sympathetic chain. Sensory and motor testing after the block is what makes the result readable at all.

Here is the part that gets blurred in most consent conversations: a temperature rise proves the block worked technically. It does not prove your pain is sympathetically maintained. Relief afterward is suggestive, not conclusive. A single unblinded block carries a high false-positive rate from placebo response, systemic absorption of the anesthetic, and somatic spread. The result is read alongside the rest of the clinical picture — never as a verdict on its own.

The outcome evidence deserves your skepticism

This is the section most clinic pages leave out.

In 2016, O’Connell and colleagues assembled for Cochrane every randomized trial of local anesthetic sympathetic blockade in this syndrome. The yield was 12 studies and 461 participants — every one judged at high or unclear risk of bias, with the evidence graded low to very low quality. Two small trials compared the block against placebo or sham (32 participants total) and did not demonstrate significant short-term benefit. The authors’ conclusion: the limited data available do not suggest the block is effective for reducing 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 span the whole field of treatments for this syndrome — bisphosphonates, steroids, ketamine, immunoglobulin, mirror therapy, stimulation and more. Sympathetic blockade receives a single unpooled sentence noting an expanding role, and the review itself calls trial quality variable.[^2] That does not overturn Cochrane.

Reported adverse events in the Cochrane trials were minor,[^1] but the risks are real and should be named: bleeding, infection, intravascular or intrathecal injection, puncture of the kidney or ureter, somatic nerve injury, genitofemoral neuralgia, post-sympathectomy neuralgia, and transient drops in blood pressure. Relief from a single block is often short — hours to days.

The honest position is this: it is a diagnostic trial that can buy a therapeutic window, not an open subscription. The value lies in what gets done inside that window — desensitization, graded motor imagery, loading the limb, moving it.

What this means for your evaluation

If a limb burns, swells, sweats asymmetrically, or changes color after an injury, the sequence that matters is: examine the limb, close the differential, apply the criteria — then decide about a needle. Not the reverse.

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 is consistent with a sympathetically maintained component — suggestive, not proof. Second, how long a single block buys, which determines how the rehabilitation window gets scheduled.

The rehabilitation itself is the hard part, and it is worth saying plainly. Desensitization asks you to do the exact thing the limb has spent months refusing — be touched, every day. In practice there are plateau weeks and real discouragement, and a repeat block sometimes returns less than the first. Tolerating a shoe tends to come back long before tolerating cold does, and cold sensitivity may not fully resolve. These are practice observations, not trial results. Individual results vary.

A note on adjuncts. Diet, specialized pro-resolving mediators, and photobiomodulation are investigational and adjunctive for this condition — nothing more. They are used at the Padda Institute to lower general inflammatory load, not because a trial in this syndrome supports them. There is none that does, and they are not priced as treatment for this condition.

Frequently asked questions

Key takeaways

• The lumbar sympathetic chain runs along the front-and-side surface of the lumbar vertebral bodies and controls blood vessel caliber and sweating in the leg — which is exactly why the burning, color change, swelling and one-sided sweating cluster together after an injury.
• The idea that injured pain fibers become abnormally sensitive to the body’s own norepinephrine is the leading explanation, but it is drawn largely from animal work and remains debated in humans.
• Diagnosis rests on the validated Budapest criteria after clot, arterial disease and metabolic neuropathy are excluded — not 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 has a high false-positive rate.
• The strongest review to date found low to very low quality evidence and no demonstrated pain benefit, so the block belongs as a short diagnostic trial that buys a rehabilitation window — never as an open-ended series booked in advance.

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

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.

[^1]: O’Connell NE, et al. Cochrane Database Syst Rev. 2016;7(7):CD004598.

[^2]: Duong S, et al. Can J Anaesth. 2018;65(6):658-684.

[^3]: Harden NR, et al. Pain. 2010;150(2):268-274.

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 274671161010021465185800459847202132. 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 294928261010071263001810915. 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 204936331010162010040302914601. PubMed

Dr. Gurpreet Singh Padda, MD, MBA, MHP