Minamata Disease

Minamata disease is the neurological syndrome of chronic methylmercury poisoning first recognised in Minamata Bay, Japan, in 1956, where a chemical plant discharged mercury waste into the sea, and fish and shellfish eaten by the local population accumulated methylmercury. A second outbreak occurred in Niigata. Similar methylmercury poisoning arises where mercury contaminates aquatic food chains, including artisanal gold mining areas in Africa and fish eating communities around contaminated lakes.

The disease is defined by the clinical and pathological picture: damage to the cerebellar granular layer, the calcarine cortex, the postcentral and precentral cortex, and peripheral sensory nerves. The classical syndrome is the Hunter-Russell triad of sensory disturbance, ataxia and constriction of visual fields, with dysarthria and hearing impairment.

Congenital Minamata disease occurs when fetal exposure through the placenta is severe, even when the mother has mild or no symptoms. Mortality in the early outbreak was about 30 percent of severe cases; survivors have lasting disability. Prognosis depends on dose and age, and neurological loss is largely irreversible.

• Sensory disturbance: stocking glove and perioral paraesthesia, impaired touch, pain and two point discrimination.
• Cerebellar ataxia: broad based gait, dysmetria, dysdiadochokinesia, intention tremor, nystagmus, dysarthria (scanning speech).
• Concentric visual field constriction (tunnel vision), sometimes with blindness.
• Hearing loss, olfactory and taste disturbance.
• Cognitive: memory loss, mood change, in severe cases dementia, mania, coma, convulsions.
• Congenital: cerebral palsy features, microcephaly, intellectual disability, primitive reflexes persisting, seizures, dysarthria, hypersalivation.
• Late: progression, or worsening with age (chronic Minamata disease).
• Red flags: acute encephalopathy, convulsions, pregnancy or lactation exposure, children with developmental delay.

• Exposure history: duration of consumption of contaminated fish or shellfish, residence in the area, family cases.
• Neurological examination documenting the sensory, ataxia, visual field and hearing features.
• Whole blood mercury (normal below 10 micrograms/L), hair mercury (above 50 micrograms per gram in the acute outbreak), urinary mercury (mild elevation), cord blood or placental mercury in neonates.
• Perimetry (Goldmann): concentric constriction; pure tone audiometry; nerve conduction studies show sensory axonal neuropathy.
• MRI brain: atrophy of cerebellum, calcarine cortex, and occipital lobe; PET and SPECT show reduced occipital and cerebellar perfusion.
• Differential diagnosis: multiple sclerosis, spinocerebellar ataxia, alcoholic cerebellar degeneration, paraneoplastic cerebellar degeneration, other metal poisoning (lead, thallium, arsenic), cerebral palsy of other cause, and functional gait disorders.

• Stop exposure: prohibit contaminated fish consumption, test household members, and mothers who are pregnant or breastfeeding.
• Chelation has limited benefit in chronic established disease but lowers body burden: succimer (DMSA) 10 mg/kg orally every 8 hours for 5 days then every 12 hours for 14 days; avoid dimercaprol. Monitor liver enzymes, full blood count, and zinc.
• Symptomatic treatment of tremor and spasticity: baclofen 5 mg orally three times daily, titrated every 3 days to a maximum of 80 mg per day; botulinum toxin injection for focal spasticity (onabotulinumtoxinA 100 to 300 units per session, no more often than every 12 weeks).
• Paraesthesia and neuropathic pain: amitriptyline 10 mg orally at night, increased by 10 mg weekly to 75 mg, or gabapentin 300 mg orally at night increased to 300 mg three times daily and then to 600 mg three times daily as tolerated.
• Rehabilitation: physiotherapy, occupational therapy, speech and language therapy, hearing aids, visual aids, orientation and mobility training, and special education for congenital cases.
• Congenital disease: early developmental assessment, seizure control (sodium valproate 10 to 15 mg/kg/day in 2 divided doses, increased to 30 mg/kg/day; avoid in pregnancy), nutritional and swallowing support.
• Screening: blood and hair mercury in the community; antenatal advice on fish intake; neonatal assessment of exposed mothers.
• Follow up: neurological and functional review every 6 months; audiology and ophthalmology yearly.
• Prevention: stop industrial and mining mercury release, monitor fish mercury, and give community warnings.