Severe Progressive Multifocal Axonal Neuropathy After a Prolonged Fibrotic Interstitial Lung Disease Phase in MPO-ANCA-Associated Vasculitis: A Case Report

Microscopic polyangiitis (MPA) is an antineutrophil cytoplasmic antibody (ANCA)-associated small-vessel vasculitis that may affect multiple organs. Interstitial lung disease (ILD), particularly with a usual interstitial pneumonia (UIP) pattern, may precede systemic vasculitis and delay diagnosis. We report a patient with initially undifferentiated fibrotic ILD identified in 2023, with radiological suspicion of UIP/IPF. The initial immunological evaluation in December 2023 was inconclusive: ANCA testing by indirect immunofluorescence was non-diagnostic, while antigen-specific MPO-ANCA and PR3-ANCA assays were negative. Over the following two years, the patient developed recurrent fever, progressive respiratory symptoms, systemic inflammation, anaemia, renal impairment, and severe bilateral lower limb weakness leading to loss of ambulation. Electroneurography demonstrated a severe, predominantly axonal sensorimotor neuropathy with multifocal involvement, while sural nerve biopsy revealed vascular inflammatory changes, raising the possibility of vasculitic neuropathy, although chronic thrombotic microangiopathy remained a possible coexisting process. In October 2025, markedly elevated MPO-ANCA levels were detected during evaluation of progressive multisystem disease. Whether this change reflected seroconversion during disease evolution or differences related to earlier testing cannot be determined retrospectively. The diagnosis of MPA was supported by the combination of markedly elevated myeloperoxidase (MPO)-ANCA levels, fibrotic ILD, renal dysfunction, systemic inflammation, severe multifocal neuropathy, and the nerve-biopsy findings. Treatment with high-dose corticosteroids, rituximab, intravenous immunoglobulin, and rehabilitation was followed by partial neurological recovery, although significant lower limb deficits persisted. This case illustrates the diagnostic challenges posed by severe MPO-ANCA-associated vasculitis following a prolonged fibrotic ILD phase. However, the available evidence does not establish whether the initial pulmonary fibrosis was an early manifestation of vasculitis or a coexisting fibrotic lung disease. The case also demonstrates that meaningful functional improvement may occur despite severe electrophysiological abnormalities.