This patient has limbic encephalitis, or inflammation of the emotional and memory structures of the brain. The subacute progression of this patient's symptoms of personality change, psychosis, and seizures is most consistent with a paraneoplastic antibody syndrome, which is usually mediated by autoantibodies. Paraneoplastic limbic encephalitis is most commonly associated with lung tumors (usually small cell lung cancer), breast cancer, thymoma, germ cell tumors, and Hodgkin lymphoma. Patients frequently have the neurologic symptoms before discovery of the causative tumor. In this patient, the possible presence of the concomitant syndrome of inappropriate antidiuretic hormone secretion (SIADH), as suggested by the low serum sodium level, makes the particular diagnosis of anti-LG1 (formerly anti–voltage-gated potassium channel) antibody syndrome most likely because it is associated with both limbic encephalitis and SIADH. A definitive diagnosis is usually made after laboratory testing of serum and/or cerebrospinal fluid (CSF) for paraneoplastic antibodies in the appropriate clinical context. Treatment involves immunotherapy and treatment of the underlying tumor, if available. Although these syndromes traditionally have been characterized as “paraneoplastic” because they often are associated with an underlying cancer, many, including most anti-LG1 syndromes, are primarily autoimmune.

Although paranoia and seizures can be seen in Alzheimer disease, they typically occur late in the disease course. Alzheimer disease also is not generally associated with mesial temporal MRI changes.

Reactivation of human herpesvirus 1 also can cause an encephalitis with cognitive problems, psychiatric disturbance, and bitemporal seizures. However, the progression of this type of encephalitis is acute, occurring over days, not weeks. If not treated emergently, herpes encephalitis is lethal. Patients suspected of having human herpesvirus 6 encephalitis will need a lumbar puncture to rule out other viral causes of limbic encephalitis, such as human herpesvirus 3 (varicella-zoster virus), human herpesvirus 8, and cytomegalovirus.

The course of Lewy body dementia is typically longer than that described in this patient. Also, the seizures and imaging findings would be atypical.