Background. Multiple sclerosis (MS) and Guillain-Barré syndrome (GBS) are autoimmune demyelinating disorders affecting different parts of the nervous system: the central nervous system in MS and the peripheral nervous system in GBS. Despite partially overlapping immunological and genetic mechanisms, the coexistence of these two conditions in a single patient is extremely rare.

Objective. To analyse a case of Guillain-Barré syndrome developing in a patient with multiple sclerosis during long-term anti-CD20 therapy

Materials and Methods. A retrospective analysis of clinical data, laboratory findings, neuroimaging results and neurophysiological studies was performed in a patient with multiple sclerosis who developed Guillain-Barré syndrome during prolonged anti-CD20 treatment.

Results. The patient had a five-year history of multiple sclerosis and had been receiving anti-CD20 therapy due to high disease activity. Acute deterioration was characterised by fever, bulbar dysfunction and progressive limb weakness resulting in peripheral tetraparesis. Initial cerebrospinal fluid examination revealed neutrophilic pleocytosis suggesting infection; subsequent analysis demonstrated lymphocytic predominance and infectious aetiology was not confirmed. Magnetic resonance imaging showed no evidence of active demyelinating lesions. Electroneuromyography demonstrated axonal involvement of peripheral nerves supporting the diagnosis of Guillain-Barré syndrome.

Conclusions. The coexistence of multiple sclerosis and Guillain-Barré syndrome is rare but should be considered in clinical practice. Patients receiving long-term anti-CD20 therapy require careful monitoring for potential infectious and autoimmune complications.