Case Presentation: Mononeuritis multiplex (MM) was first described in 1960 as a hereditable neuropathy favoring the brachial nerve. MM is now known as the painful, asymmetrical destruction of at least two separate peripheral nerves caused by an array of disorders including diabetes mellitus, systemic lupus erythematous, rheumatoid arthritis, and various vasculitides. This clinical case describes a complex presentation of MM, the vasculitic neuropathy variant. Briefly, a 61-year-old African-American female with diabetes mellitus type II and rheumatoid arthritis was recently admitted to the hospital with complaints of severe joint pain and bilateral fingertip necrosis. Skin biopsy revealed gross morphologic changes consistent with cryoglobulin vasculopathy. Immunofluorescent studies were negative for C3 complement but did reveal a non-specific perivascular staining pattern of IgA, IgM, and IgG. She was diagnosed with a severe rheumatoid arthritis flare complicated by cryoglobulin vasculitis, treated with rituximab, and discharged to a skilled nursing facility on high-dose prednisone. One month later, she was re-admitted to the hospital for aspiration pneumonia with dysphasia and rapidly declining muscle strength. Physical exam revealed severe myopathy with areflexia, poor phonation, and worsening end-extremity necrosis. She received a 5-day course of IVIg with continuation of high-dose prednisone.

Discussion: Despite this treatment, her clinical condition worsened with rapid decline in peripheral nerve and muscle function. After an extensive work up for myopathy with neuropathy, she tested strongly positive for acetylcholine receptor antibody and was given the diagnosis of myasthenia gravis. A repetitive nerve conduction study (RNCS) was performed for diagnosis confirmation. RNCS was negative for decreased nerve conduction amplitude after repetitive stimulation. Based on these results, her diagnosis was revised to mononeuritis multiplex in the setting of severe rheumatoid arthritis with confirmed cryoglobulin vasculitis. Unfortunately during her hospital course, the patient developed a life-threatening brain hemorrhage and was transferred to the neurology intensive care unit, where she remains in critical condition.

Conclusions: Although rare, this case illustrates the possibility of false positive acetylcholine receptor antibody tests in the setting of severe auto-inflammatory disease. Even though the prognosis for this patient is poor, it is important to understand the value of confirmatory tests when the diagnosis is uncertain, particularly in cases such as these. Early diagnosis and treatment of mononeuritis multiplex secondary to vasculitis is critical for halting disease progression and improving patient outcomes.