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We describe MR findings in patients with Minamata disease who have been followed for a long time. All patients examined were affected after daily eating of a large quantity of methylmercury-contaminated seafood, from 1955 to 1958, and showed typical neurological findings. On MR images, the visual cortex, the cerebellar vermis and hemispheres, and the postcentral cortex are significantly atrophic in Minamata disease. The visual cortex is slightly hypointense on T1-weighted images and hyperintense on T2-weighted images, probably representing the pathologic changes of status spongiosus. MRI can demonstrate the lesions located in the calcarine area, cerebellum, and postcentral gyri, which are probably related to three of the characteristic manifestations of this disease: the constriction of the visual fields, ataxia, and sensory disturbance, respectively.

To better understand the neurologic events related to chronic Minamata disease (organic mercury poisoning), we studied data from 77 patients with Minamata disease as certified at autopsies performed from 1976 to 1994 (mean age: 72.3 years). Major neurologic findings included: sensory impairment in 80.5% of the patients which was limited to the extremities in 42.9%. Impairment of lower extremity coordination was present in 35.8% of the patients , constriction of the visual fields in 28.8%, and retrocochlear hearing loss in 15.3%. There was no correlation between the degree of cerebellar incoordination and the methylmercury concentration in the cerebellum. Comparedwith the classic type ofMinamatadisease, the incidence of major neurologic findings was markedly decreased. In light of these findings, supplemental examinations including brain computed tomography (CT), magnetic resonance imaging (MRI), short latency somatosensory evoked potential (SSEP), or tremogram may be necessary to clinically diagnose Minamata disease, especially in atypical or mild cases.

To elucidate the neurologic features of chronic Minamata disease, and the incidence of complications with aging, we studied 80 patients with documented Minamata disease (organic mercury poisoning) from 1986 to 1994 (mean age: 63 years). Of the cardinal neurologic findings, sensory impairment was seen with highest frequency in 98.8% of patients limited to the extremities in 86.3%. Impairment of lower extremity coordination was observed in 60%, constriction of the visual field in 51.9%, and retrocochlear hearing loss in 41%. To assess age-related complications, patients were separated into three groups by age: Group I (10 to 39 years); Group II (40 to 69 years); Group III (> or = 70 years). The incidences of hypertension and cerebrovascular diseases, organic ophthalmologic disorders (including cataracts), presbyacusis, and cervical spondylosis deformans increased significantly with age. Compared with a preceding survey (1981 to 1985, 171 patients, mean age: 63.5 years), the incidences of complicated hypertension and cataracts had decreased, whereas those of cerebrovascular disease and retinitis pigmentosa remained unchanged. The incidences of abnormal brain computed tomography (CT), presbyacusis, cervical spondylosis deformans, and positive tests for urine sugar also increased. The incidences of these complications other than retinitis pigmentosa were similar to those in the general population. These results accurately reflect the recent epidemiological disease tendencies in Japan toward a decreased incidence of hypertension and an increased incidence of diabetes.

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