After a 34-year-old female developed a headache and high fever, she was diagnosed with aseptic meningitis. On admission, neurological examinations revealed cerebellar limb ataxia, horizontal gaze paretic nystagmus, and pyramidal tract signs. Laboratory tests showed hyponatremia (129 mEq/l). Five days after admission, convulsions in the upper limbs due to the severe hyponatremia (108 mEq/l) were noted. In addition, serum antidiuretic hormone levels were markedly increased to 18.5 pg/ml. Brain MRI showed multiple small inflammatory lesions in the subcortical cerebral white matter, thalamus, and around the third ventricular diencephalic regions. Pulse corticosteroid treatment promptly improved her symptoms. Although tests for serum anti-aquaporin 4, anti-myelin oligodendrocyte glycoprotein, and anti-voltage-gated potassium channel antibodies were negative, cerebrospinal fluid samples tested positive for anti-N-methyl-D-aspartate (NMDA) receptor antibodies. Oral prednisolone administration was continued, but she developed paresthesia in her upper and lower extremities and gaze-evoked nystagmus three months after the first attack. MRI showed that the previously observed high-intensity regions were decreased, but a new area of high intensity was observed in ventral regions through the lower midbrain to the pons. Because pulse corticosteroid treatment was again effective, we continued the oral prednisolone treatment. This case presented none of the characteristic symptoms of anti-NMDA receptor antibody encephalitis during the clinical course other than repeated demyelinating encephalitis and severe syndrome of inappropriate antidiuretic hormone secretion (SIADH). Additional clinical observations are needed to better understand the underlying pathology of the NMDA receptor antibodies in the cerebrospinal fluid in this case.
