Efgartigimod-responsive autoimmune peripheral neuropathy with concurrent anti-GM2 IgG and anti-contactin-1 IgG antibodies: a rare case report

Immune-mediated peripheral neuropathies arise from dysregulated immune responses targeting peripheral nerves. Contactin-1 (CNTN1) antibodies, typically of the IgG4 subclass, are associated with autoimmune nodo/paranodopathies and often respond poorly to intravenous immunoglobulin (IVIG). Anti-GM2 IgG antibodies are rare and are usually linked to complement-mediated axonal injury. The coexistence of these antibodies has not, to our knowledge, been previously reported. A 73-year-old man developed rapidly progressive quadriparesis, bulbar dysfunction, and respiratory failure four days following lumbar decompression surgery. Examination revealed generalized weakness, hypotonia, and areflexia. Electrophysiological studies showed widespread but heterogeneous involvement of peripheral nerves, with prolonged distal motor latencies in selected nerves, reduced F-wave persistence, and decreased compound muscle action potential amplitudes, suggesting a mixed demyelinating and axonal pattern. Serological testing identified dual positivity for anti-GM2 IgG and anti-CNTN1 antibodies, with a serum IgG4 level of 1.81 g/L, within the reference range. Despite IVIG treatment, the patient’s condition did not improve, and he became dependent on mechanical ventilation. Treatment with the neonatal Fc receptor antagonist efgartigimod, combined with methylprednisolone 40 mg/day, led to rapid clinical improvement, successful weaning from the ventilator, and recovery of independent ambulation. To our knowledge, this is the first reported case of concurrent anti-GM2 IgG and anti-CNTN1 IgG positivity in a single patient. This case describes a previously unrecognized antibody combination in immune-mediated peripheral neuropathy, suggesting a possible overlap of distinct immune-mediated mechanisms. Neonatal Fc receptor blockade may represent a mechanism-based therapeutic option in selected patients with severe or treatment-refractory disease.