Acute anti-Ma2 paraneoplastic encephalitis associated to pembrolizumab: a case report and review of literature.

Anti-Ma2 encephalitis is a rare neurological disorder with a predominant involvement of brainstem, limbic and diencephalic structures. Although an unspecific encephalopathy is the usual form of presentation, acute-onset neurologic symptoms and other atypical manifestations have been described and account for the challenging diagnosis of this entity. Despite being usually detected as a paraneoplastic syndrome in patients with early-stage tumors or without a previous history of malignancy, a growing concern has arisen from several cases reported in metastatic patients under treatment with immune checkpoint inhibitors. We report what to our knowledge is the first known case of anti-Ma2 encephalitis associated to pembrolizumab and presenting as an acute-onset focal neurological syndrome, consisting on acute global aphasia, right upper limb paresia, hypoacusia, sleep disorder, decreased conscious level and a motor focal status that was refractory to anticonvulsant therapy. A brain MRI scan showed a focal alteration of the cortical-subcortical signal on the left parietal lobe. CSF study found a significant hyperproteinorrhachia and electroencephalography showed lateralized periodic discharges (LPDs), suggestive of a diffuse encephalopathy. A positive result for anti-Ma2 antibodies was obtained both in blood and CSF samples through indirect immune-fluorescence (IFI) and later confirmed by western-blot technique. Our patient obtained a mild response to steroid therapy and a significant improvement after the administration of intravenous immunoglobulins. The hypothesis that checkpoint inhibitors may trigger the expression of previously subclinical paraneoplastic events, through the strengthening of cytotoxic T cells-mediated immune response, is supported by our finding of preexisting anti-Ma2 antibodies in preserved blood samples obtained before the initiation of pembrolizumab in our patient. Further research is needed to reveal if the detection of onconeural antibodies prior to a treatment with checkpoint inhibitors may be used as a predictive biomarker of neurologic immune-related high-grade toxicity.