Bilateral Foot Drop as the Presenting Manifestation of Systemic Lupus Erythematosus-Associated Mononeuritis...

Peripheral nervous system involvement in systemic lupus erythematosus (SLE) is uncommon, and mononeuritis multiplex as the presenting neurological manifestation is particularly rare. Early recognition may be difficult when the initial clinical picture resembles focal compressive neuropathy.

A 31-year-old previously healthy woman developed acute right foot numbness and foot drop after right knee pain and earlier nonspecific arthralgia. Initial assessments favored peroneal neurapraxia. Over the following 3 weeks, her deficits progressed to persistent right foot drop followed by new left lower limb numbness and weakness, resulting in asymmetric bilateral foot drop. Electrophysiological studies demonstrated an asymmetric, non-length-dependent motor and sensory axonal neuropathy consistent with mononeuritis multiplex. Magnetic resonance imaging of the lumbosacral spine excluded a compressive lumbosacral explanation and focused ultrasound showed no compressive lesion involving the right common peroneal nerve. Serological evaluation revealed high-titer antinuclear antibody, positive anti-double-stranded DNA, anti-Ro, anti-ribosomal P antibodies, elevated inflammatory markers, and a positive lupus anticoagulant screen, with negative infectious screening and no evidence of active lupus nephritis. She was treated with intravenous methylprednisolone, intravenous immunoglobulin, hydroxychloroquine, and rituximab, followed by oral corticosteroid tapering and rehabilitation. Her pain improved early, distal motor function partially recovered, and she later demonstrated meaningful functional improvement with orthotic and rehabilitation support.

This case suggests that SLE may initially present predominantly as mononeuritis multiplex, with evolving bilateral foot drop as the dominant neurological and function-threatening manifestation, through a suspected vasculitic mechanism, even when more typical renal, serosal, or major mucocutaneous features are absent at neurological presentation. In patients with progressive or sequential foot drop, especially when the presentation becomes bilateral or multifocal, prompt electrodiagnostic testing and evaluation for systemic autoimmune disease are essential to avoid diagnostic delay and allow timely immunotherapy.