Antineutrophil Cytoplasmic Antibody (ANCA)-Associated Vasculitis (AAV) Masquerading As Mononeuritis Multiplex: A Case Report

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a group of disorders that causes severe small-vessel inflammation with systemic manifestations. There are three subtypes of AAV, namely granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic GPA (EGPA). The most commonly affected organs are the upper and lower respiratory tract and the kidneys with occasional and varied neurological manifestations. Here we report a case of a 61-year-old female who presented with a one-month history of numbness, paresthesia, and asymmetric distal weakness of both lower limbs without any bladder or bowel involvement. Similar complaints appeared in her upper limbs three days prior to admission. She also suffered from myalgia, arthralgia, reduced appetite, and lost 8-10 kg weight over the past six months. Her nerve conduction study (NCV) revealed asymmetrical, predominantly motor, mixed, axonal and demyelinating polyneuropathy affecting both lower limbs, which was suggestive of mononeuritis multiplex. After a detailed workup, she tested strongly positive for cytoplasmic ANCA (c-ANCA). Although there was no clinical involvement of the respiratory tract, a contrast-enhanced computed tomography scan of the thorax and abdomen showed multifocal subpleural and lung parenchymal soft tissue lesions and mediastinal and bilateral hilar lymphadenopathy suggestive of a granulomatous lesion. She was diagnosed with the GPA variant of ANCA-associated vasculitis. Remission induction was achieved with high-dose methylprednisolone and cyclophosphamide along with alternate-day cotrimoxazole. Remission was maintained with tapering doses of steroid and mycophenolate mofetil with a slow but sustained recovery. On follow-up after one year, she walked without support with mild residual burning paresthesia in both feet. This case highlights the fact that neurological symptoms can be the presenting manifestation of AAV, and clinicians should have a high level of suspicion for AAV in patients presenting with mononeuritis multiplex, especially after ruling out common causes. By considering such etiologies, it may be possible to diagnose the condition at an earlier stage and initiate treatment to prevent potential pulmonary or renal damage.