Acrodynia and CASPR2 Autoimmunity in Elemental Mercury Toxicity.

Elemental mercury exposure is associated with neurologic manifestations, which can improve after source removal with time. Autoimmune complications from mercury intoxication are rarely reported and may lead to worsening or persistent neurologic symptoms. A 2-year-old girl, in addition to her parents and sibling, developed cough and congestion initially attributed to a respiratory infection. For the next month, the child experienced intermittent episodes of sweating and increased fussiness, then later developed a pruritic erythematous rash to her distal extremities. She was hospitalized and, after toxicology testing revealed elevated whole blood (11 mcg/L) and urine mercury concentrations (79 mcg/g Cr), was treated with oral dimercaptosuccinic acid (DMSA). The source of mercury exposure was later identified as an accidental spill of elemental mercury that occurred in her residence weeks before the onset of her symptoms. Her family temporarily moved away from their residence while mercury remediation was initiated, but continued episodes of altered mental status necessitated hospital readmission. A lumbar puncture was performed, additional chelation with N-acetylcysteine was administered, and an autoimmune encephalitis panel was sent to a reference laboratory. The patient's repeat urine mercury concentration was undetectable, but she had still not returned to her baseline mental status. The autoimmune encephalitis panel was positive for contactin-associated protein 2 (CASPR2) IgG antibodies. After treatment with intravenous immunoglobulin, high-dose methylprednisolone, and oral prednisolone, her mental status gradually improved and returned to baseline over the next several weeks. CASPR2-IgG autoimmune encephalitis is a rare complication of elemental mercury intoxication, characterized by neuropsychiatric symptoms that persist despite traditional chelation treatment and systemic clearance of mercury. Treatment of affected patients may involve the administration of intravenous immunoglobulin and high-dose corticosteroids.