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Neurogenic Osteoarthropathies

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❓What❓

  • “Neurogenic Osteoarthropathies”: 2 “sister” syndromes of “pain out of proportion to exam” that are frequently mistaken for infection given the severity of pain as well as local inflammatory features of both the skin & bone

  • Syndrome #1 = Charcot foot: in patients with polyneuropathy or focal neuropathy (e.g. prior severe trauma to foot), neuropathy results in a dysregulated local inflammatory cascade that causes significant inflammation in the soft tissues (i.e. cellulitis mimicker), bones (i.e. osteomyelitis mimicker), & joints (i.e. septic arthritis mimicker). After unchecked inflammation, the bones become eroded & joints unstable

  • Syndrome #2 = Complex Regional Pain Syndrome (CRPS): another syndrome of neurogenic inflammation, but instead of being related to underlying neuropathy, CRPS is triggered by some form of trauma in ~90% & nothing in ~10%

❓Epidemiology❓

  • Charcot foot triggers: patients often don’t recall inciting foot trauma due to underlying neuropathy & inability to feel pain (”I bumped my foot” “I heard a snap”), or may have been more active than usual (i.e. repetitive occult trauma)

  • CRPS triggers: most commonly due to fracture, crush injury, surgery, or sprain

❓Presentation❓

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Left: chronic Charcot deformity. Right: acute Charcot foot with focal ankle cellulitis

Clinical, Shared

  • Pain = Center-of-Gravity: allodynia (pain to non-painful stimulus) & hyperalgesia (exaggerated pain to painful stimulus) are prominent features. In the CRPS patient without background neuropathy, the pain is worse with weight-bearing & movement, but characteristically always present, whereas Charcot pain is often mostly resolved with offloading & immobilization. (Exception to this is if neuropathy is so severe that patient cannot feel the foot.)

  • Edema ⊕ Neurogenic Arteriolar Flushing: the edematous “red leg” quickly disappears on leg elevation owing to loss of gravity-mediated blood pooling in dilated arterioles, thus differentiating the syndromes from cellulitis

👈 Acute on Chronic Charcot foot: a young man w/ history of pediatric trauma to foot (run over by vehicle) s/p remote ankle reconstruction who presented w/ acute-on-chronic right ankle pain, found to have pain not just over local cellulitic site, but also over the foot & circumferential leg tender to minimal palpation. Video shows subtle arteriolar flushing of the dorsal foot that improves on elevation & returns upon bed contact. WBC was normal & CRP minimally elevated

Clinical, Charcot-specific

  • Gross foot deformity → dislocation

Clinical, CRPS-specific

  • Acute: skin color change (mostly livido vs. hyperemia), sensory changes (hyperesthesia), focal dysautonomia (asymmetric sweating)

  • Chronic: trophic skin & soft tissue changes (↑ hair growth, ↑/↓ nail growth, contraction/fibrosis of joints/fascia, skin atrophy, brawny skin), motor changes (↓ ROM, weakness, tremor)

Laboratory

  • Charcot: minimally ⊕ inflammatory markers (median CRP 5.8 mg/L, ESR 11 mm/hr, WBC 7.0)

  • CRPS: ⊖ inflammatory markers

Radiographic

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  • Both: in the very early stages of both syndromes, X-ray will be ⊖ but MRI will be ⊕ for bone marrow & soft tissue edema (CRPS = diffuse/patchy marrow edema that fluctuates/migrates; Charcot = more focal, subchondral edema that slowly regresses with joint offloading)

  • Charcot: if joint is not immobilized, X-ray will then show fractures, subluxation/dislocations, & bony debris (stage 1) → bone fragmentation, osteolysis & erosions (stage 2) → mature fracture callus (stage 3)

  • CRPS: ⊖ for dislocations & erosions

❓Diagnosis❓

Path to the Problem 🗻

  • “Pain out of proportion to exam”

  • Disproportionately ⊖ inflammatory markers

⚪ Inflammatory markers have extremely high NPV for infectious etiologies when there is high suspicion for clinical Dx of neurogenic osteoarthropathy syndromes. Conversely, if inflammatory markers are significantly ⊕, infection must be seriously considered

Clinical Diagnosis 🦶

  • Charcot: neuropathy ⊕ unilateral “red leg” without alternative explanation (i.e. rule-out infection)

  • CRPS = Budapest criteria: continuing pain disproportionate to any inciting event, as well as 3/4 criteria (sensory = hyperesthesia, allodynia; vasomotor = skin color changes; sudomotor/edema = edema, sweating changes; motor/trophic = ↓ joint ROM, motor function, trophic changes)

❓Treatment❓

  • Charcot: joint immobilization → amputation if superimposed infection vs. joint instability

  • CRPS: intensive physiatry, CBT, NSAIDs, neuropathy-directed pain (e.g. gabapentin, amitriptyline), topical lidocaine vs. capsaicin

📚 References 📚