[A case of left dorsomedial thalamic infarction with unilateral schizophrenia-like auditory hallucinations]

We report a case of a right-handed, 73-year-old woman with auditory hallucinations lateralized to the right ear. A brain MRI revealed a small infarction in the left dorsomedial nucleus (DM) of the thalamus. The patient did not have either psychiatric or neurological prior history, and had otherwise been treated for ischemic heart disease, hypertension, and hyperlipidemia for 10 years. Two months prior to admission, she had become forgetful, and had lost her wallet several times. She concurrently began to experience auditory hallucinations in which she heard the voices of her acquaintances, or "the gods". She frequently monologized and wandered about outside following the contents of the hallucinations. Therefore, she was admitted to our institution. On admission, no apparent abnormalities were revealed by physical examinations or blood analyses. She was alert and had no aphasic symptoms. Except for memory disturbances, no neurological symptoms, including no hearing loss, were found. A brain MRI showed a small localized infarction in the left DM, but EEG findings were normal. The patient had prominent anterograde memory deficits: she hardly remembered what she had done the very same day, or the names of the doctor and hospital. She also demonstrated a retrograde amnesia of the past decade or two: she showed difficulty recalling either personal history or social events that occurred during this era. Wechsler Adult Intelligence Scale-Revised (WAIS-R) revealed a total IQ of 75 (verbal IQ 77; performance IQ 77). The verbal hallucinations continued with frequent occurrence even after admission. They included voices telling her about misfortunes, such as death or sickness, of her relatives. These turned into threats and commands, such as "I'm gonna kill ya. I attack you from behind. You, do not eat!" In addition, she occasionally experienced "third person auditory hallucination", in which several men were discussing the plan to kill her. As is characteristic of this type of case, the hallucinations always appeared in only her right ear. They did not occur in the other modalities (e.g. as a visual one). She was convinced that the hallucinations were real and looked frightened while they were happening. Whereas the anterograde amnesia continued for 6 months after admission, the retrograde amnesia gradually improved within 2 or 3 months after admission, although a partial amnesia on the past decade eventually turned out to persistent. On the other hand, the hallucinations did not ameliorate satisfactorily with risperidone (3-6 mg/day), but on augmentation with olanzapine (5-20 mg/day), they lessened gradually and almost disappeared within 6 months. She also slowly developed symptoms similar to those of frontal lobe syndrome, i.e., aspontaneity and apathy. In conclusion, our case indicates the importance of DM on memory function. It is noteworthy that schizophrenia-like hallucinations developed in the case. Localized neuronal deficits evoked by infarction in the left DM probably caused the schizophrenia-like hallucinations; the lateralization phenomenon further indicates the involvement of specific neuronal mechanisms in the mediation of the hallucinations. According to the knowledge of the functional anatomy of the DM and the lateralization phenomenon of auditory hallucinations, it is possible that the neuronal loop, comprised of the prefrontal cortex and thalamus, designated as "basal ganglia-thalamocortical circuits", in addition to the left temporal cortex, plays an important role in the development of the hallucinations in this case. This possibility might also shed light on the neurological basis of schizophrenia.