[A case of chronic inorganic mercury poisoning with progressive intentional tremor and remarkably prolonged latency of P300].

A 59-year-old man showed slowly progressive intentional tremor for 40 years prior to first visit to us in 1996. He was exposed to mercury vapor for about 3 years (1956-1959) and the diagnosis of chronic inorganic mercury poisoning was made. Hasegawa dementia scale-revised (HDS-R), mini-mental state (MMS) examination and P300 examination were performed. HDS-R and MMS were within normal range but the latency of P300 was remarkably prolonged. His tremor was considered to be due to chronic inorganic poisoning because there were no other causes and the frequency of his tremor was 3-4 Hz. which was lower than that in essential tremor. The prolonged P300 latency was also considered to be due to the same cause because there were no other causes and the head MRI were normal. Chronic inorganic mercury poisoning has been reported to produce organic changes in the brain and P300 is considered to be useful to detect these changes.