Mononeuritis Multiplex Presenting with Retroperitoneal Inflammation: A Case-Based Review in the Light of Diagnostic and Therapeutic Challenges.

Background: Mononeuritis multiplex (MNM), is an uncommon, peripheral neuropathy, presenting with asymmetric involvement of multiple individual nerves. It is frequently associated with vasculitic disorders and other immune-related conditions. Early manifestations can often be non-specific, which might complicate diagnosis. Case presentation: A 56-year-old woman with no significant medical history developed sudden abdominal and right gluteal pain followed by progressive and asymmetric sensorimotor deficits. Initial imaging demonstrated fat stranding and infiltration in the retroperitoneum, lateral pelvic walls, and lumbosacral plexus, suggestive of an inflammatory process. Laboratory testing demonstrated marked leucocytosis, eosinophilia, elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), sterile pyuria, hypoalbuminemia and elevated alpha- and beta-globulins. Markers of infectious and autoimmune diseases, as well as neoplastic markers were unrevealing. Nerve biopsy was inconclusive. Therefore, a diagnosis of immune-mediated vasculitic neuropathy was favoured but not confirmed. She was treated with high-dose oral corticosteroids tapered over 12 months, with additional antibiotics to reduce risk of infection. At follow-up, she remained ambulatory and independent, with mild residual deficits and intermittent inflammatory marker elevation. Conclusions: This case illustrates how MNM presenting with gradual evolution, mixed infectious and autoimmune features, and in the context of retroperitoneal inflammation, can complicate the diagnostic process. In absence of biopsy confirmation, management required balancing empirical immunosuppression with infection risk. Comparison with prior reports underlines the importance of complete autoimmune workup, biopsy and imaging techniques such as positron emission tomography scan in minimising diagnostic uncertainty.