Burning Feet After a Pelvic Fracture: Is This Neuropathic Pain, Tolerance, or Something You Must Rule Out Tonight?
Mr. D is 42, six days after a motorcycle crash with an open-book pelvic fracture managed with external fixation and a sacral fracture seen on CT. He is on IV hydromorphone patient-controlled analgesia; in the last 24 hours he pressed the button 190 times and received 18 mg. Overnight he began describing 'burning and electric' pain in both feet and now says the bedsheet 'feels like sandpaper' on his legs. His pelvic pain is 3/10, but the leg and foot pain is 9/10. He is afebrile and hemodynamically stable. Sedation score is 1 of 4 (awake), and he has intermittent leg twitching. The surgical team wants to increase the PCA demand dose.
1. Before accepting 'new neuropathic pain' as the explanation, what must be ruled out in this specific patient? Explain the reasoning that links his injury to your assessment priorities.
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Anatomy drives the differential. A sacral fracture sits directly over the sacral plexus and nerve roots; bilateral burning feet plus skin hypersensitivity in a trauma patient is a nerve-injury description until proven otherwise.
Rule out, in this order:
- Cauda equina / sacral root injury. Check perineal and saddle sensation, rectal tone and voluntary anal contraction, bladder volume or post-void residual, bowel and bladder control, and lower-limb strength and reflexes. New urinary retention, incontinence, or saddle numbness converts this into an emergency MRI and spine-surgery call, not a PCA adjustment.
- Evolving hematoma or external-fixator complication compressing the plexus - examine pin sites, limb circumference, and pain with passive stretch.
- Bilateral compartment syndrome or vascular compromise - palpate dorsalis pedis and posterior tibial pulses, check capillary refill, and compare compartments; consider creatine kinase for rhabdomyolysis after major trauma.
- Drug-induced hyperalgesia/myoclonus - his twitching and rising opioid consumption make this plausible, but it is a diagnosis of pattern, not a substitute for the neuro exam.
Document your neuro exam by name (light touch, pinprick, vibration, proprioception, reflexes, strength, saddle sensation) because the change over the next 6 hours is what will drive the decision. Tell the surgical team your findings before agreeing to any dose increase - an increase in this picture can deepen sedation and mask a deteriorating exam.
2. Distinguish tolerance, withdrawal, opioid-induced hyperalgesia, and worsening nociception in this patient, and state which best fits the data.
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Tolerance - the same pain is present but the drug works less well; the patient needs more opioid for the original pain, and there is no spread of hypersensitivity beyond the injured area. Mr. D's pelvic pain has actually improved to 3/10, which argues against simple tolerance.
Withdrawal - the drug level is falling: yawning, rhinorrhea, lacrimation, piloerection, diarrhea, mydriasis, agitation, tachycardia. He is receiving escalating drug and is calm; withdrawal does not fit.
Opioid-induced hyperalgesia - pain becomes more diffuse and qualitatively different from the injury, worsened by dose escalation, with allodynia spreading beyond the injured dermatomes, myoclonus, and sometimes pupil changes. Mr. D's allodynia to a bedsheet, leg twitching, and 18 mg of hydromorphone in 24 hours are classic markers - but OIH is a secondary explanation that you can only accept once the sacral plexus is cleared.
Worsening nociception would be expected to localize to the injury (pelvis, sacrum), worsen with movement, and possibly come with fever or a rising white count.
Best fit: a genuine neuropathic component from the sacral injury, with a superimposed hyperalgesic/neurotoxic opioid effect driving the escalation. The two are not mutually exclusive, and the management differs from raising the demand dose: for a neuropathic driver you add a gabapentinoid and reduce, not increase, the opioid; for OIH you deliberately reduce the opioid and add an NMDA antagonist.
3. Outline a 24-hour pharmacologic and non-pharmacologic plan, including what you would tell the surgical team about the PCA.
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Pharmacologic
- Do not raise the demand dose. Convert the PCA to a lower total opioid exposure while maintaining comfort: reduce the demand dose, remove any basal infusion if present, and reassess every 2 hours with a sedation scale plus pain score.
- Add a gabapentinoid for the neuropathic component: gabapentin 100 mg PO three times daily, titrating upward over days as sedation and renal function allow. Expect a days-to-weeks onset - it is not a rescue drug, and the team must not judge it after one dose.
- Consider low-dose ketamine (e.g., 0.1-0.3 mg/kg/hour with monitoring, or sub-dissociative boluses) if the hyperalgesia is driving the escalation. It addresses the NMDA-mediated component and lets you come down on the opioid; it requires a monitoring plan, not a push-and-walk-away.
- Scheduled acetaminophen (max 3-4 g/day depending on hepatic status) and, if renal function and healing permit, an NSAID - both spare opioid.
Non-pharmacologic
- Loose bedding and a fan for allodynia; avoid vigorous rubbing, which worsens it.
- Repositioning, graded mobilization with PT, TENS, distraction, and sleep protection - sleep loss amplifies central sensitization.
- Teach him to describe the two pains separately, so the team stops treating one number.
Message to surgery: 'His pelvic pain is controlling well; the new pain is a different mechanism. Escalating the opioid will sedate him and can mask a sacral root deficit. I need the neuro exam documented and a spine consult if the saddle exam or bladder scan is abnormal.'