Immune checkpoint inhibitors (ICI) are a form of immunotherapy that regulates the body's immune system. By doing so, they enhance immune response, particularly T-cells, to target and destroy cancer cells. However, their use has been rarely linked with neurological adverse effects including Neuromyelitis Optica spectrum disorder (NMODS). However, there has yet to emerge a unifying pathophysiology explaining the development of NMOSD following ICI therapy. The benefits of immunotherapy in cancer treatment must be weighed against the risk of autoimmune reactions.

A 49-year-old Caucasian female with triple negative invasive ductal carcinoma presented with acute onset lower extremity weakness and paresthesia, visual disturbances, urinary retention, bowel dysfunction, and perianal anesthesia 6 months after receiving PD-1 inhibitor Pembrolizumab. MRI spine revealed numerous areas of increased T2 signal intensity at the lower cervical, thoracic, and lumbar spine, suggestive of demyelinating disease and serum analysis showed anti-aquporin-4 (AQP4) antibodies. She was diagnosed with NMOSD based on diagnostic criteria.

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This is the first case of AQP4-Ab positive NMODS in a patient with breast cancer after starting pembrolizumab therapy. Immune checkpoint inhibitors might trigger neuroinflammatory diseases such as NMOSD. Clinicians must be vigilant for such adverse effects.