Over the last few years, immune checkpoint inhibitors have been increasingly utilized to treat solid malignant tumors and associated metastatic disease. However, there has also been a higher incidence of observed immune mediated adverse events, such as vasculitis of the central and peripheral nervous system.

A 27-year-old woman with triple negative breast cancer was started on neoadjuvant therapy with paclitaxel, carboplatin, and pembrolizumab followed by bilateral mastectomy in March 2022. She was then placed on adjuvant and maintenance therapy with pembrolizumab. A month later, the patient developed new onset left facial palsy and left arm weakness. She was found to have a right MCA subocclusive thrombus and multifocal right M2 and ICA narrowing with acute on subacute infarcts of the right MCA territory. No evidence of metastatic disease was noted. Prescribed Apixaban for presumed hypercoagulability of malignancy.

Three months later, she underwent 5-day course of IV immunoglobulin and was started on prednisone after she developed worsening left arm weakness and numbness with evolving acute on subacute right MCA infarcts. Vessel wall imaging showed enhancement of multiple large and medium intracranial vessels. Diagnostic cerebral angiogram confirmed CNS vasculitis. CSF analysis showed glucose 64, protein 17, 1 WBC, Immunoglobulin G index 0.5 and no oligoclonal bands. CSF infectious studies, rheumatologic evaluation, autoimmune encephalitis and paraneoplastic panels were all negative. Despite treatment, the patient required 5-day course of methylprednisolone 1g daily and was given induction therapy with Cyclophosphamide a month later.

Immune checkpoint Inhibitor-induced CNS Vasculitis is rare but likely underdiagnosed. As these therapies have been increasingly utilized, we have observed more immune mediated vascular and non-vascular adverse events. Having a high clinical suspicion can promote early aggressive treatment and hopefully improved outcomes.