Long-term Exposure to Mercury Linked to ALS in Older Patients, Case Report Suggests

Long-term mercury exposure over a period of years may increase the risk of amyotrophic lateral sclerosis later (ALS) in life, a case report suggests.

The study, “Heavy metal intoxication and amyotrophic lateral sclerosis: causal or casual relationship?” was published in the journal Aging Clinical and Experimental Research.

Amyotrophic lateral sclerosis is a neurodegenerative disease that affects adults. It is characterized by a loss of motor neurons, leading to muscle weakening or atrophy, respiratory failure and dementia.

The prevalence rate of ALS is estimated to be only 1–2.6 patients per 100,000 people and is mostly identified as a motor neuron disease linked to aging — its incidence is known to increase around age 70.

The cause of ALS is still not fully understood, but scientists think both genetic and environmental factors are involved.

Now, a team of researchers reports the case of a 87-year-old woman complaining of walking disturbances, with easy fatiguability and a tendency to fall because of muscle weakness. The patient also had high blood pressure and cholesterol levels, but cardiac tests were normal.

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Symptoms worsened over the years and the patient was admitted to the hospital, where clinicians observed muscle twitches in both arms and legs, leading them to suspect ALS. Tests measuring muscle response and electrical activity in response to a nerve’s stimulation of the muscle confirmed an ALS diagnosis. However, the patient was negative for known ALS-associated gene mutations.

Clinicians performed a thorough examination of the patient’s life habits and discovered that she had eaten swordfish once a day for the last 10 years. Because fish are known to carry high levels of mercury, they decided to measure the mercury levels in the patient’s blood.

Mercury levels were elevated — 135 micrograms per liter (the normal value is less than 10 micrograms per liter), although the patient had never experienced a mercury exposure event in her life.

They began treatment with a metal chelant (an agent that binds to metals) called meso 2–3 dimercaptosuccinic acid (marketed as Chemet) for four weeks at a dose of 30 micrograms per kilogram a day. However, after one month, mercury levels were still elevated.

ALS progressed and the patient developed swallowing difficulties (dysphagia) and respiratory failure after one year.

Scientists believe that the “environmental toxin plus aging” hypothesis proposed for neurodegenerative diseases is a potential explanation for this case of ALS. This theory argues that a toxin can accumulate in neurons for years and only damage the cells when defenses decrease because of aging.

Overall, “a proper toxicological screening should be performed in neurodegenerative diseases, such as ALS, to confirm heavy metal role in the disease [development], thus trying to harness the chelants as a possible add-on treatment in such disorders.”