Erethism, also known as erethismus mercurialis, mad hatter disease, or mad hatter syndrome, is a neurological disorder which affects the whole central nervous system, as well as a symptom complex, derived from mercury poisoning. Erethism is characterized by behavioral changes such as irritability, low self-confidence, depression, apathy, shyness and timidity, and in some extreme cases with prolonged exposure to mercury vapors, by delirium, personality changes and memory loss. People with erethism often have difficulty with social interactions. Associated physical problems may include a decrease in physical strength, headaches, general pain, and tremors, as well as an irregular heartbeat. Mercury is an element that is found worldwide in soil, rocks, and water. People who get erethism are often exposed to mercury through their jobs. Some of the higher risk jobs that can lead to occupational exposure of workers to mercury are working in a chlor-alkali, thermometer, glassblowing, or fluorescent light bulb factory, and working in construction, dental clinics, or in gold and silver mines. In factories, workers are exposed to mercury primarily through the base products and processes involved in making the final end consumer product. In dental clinics it is primarily through their interaction and installation of dental amalgams to treat dental caries. In the case of mining, mercury is used in the process to purify and completely extract the precious metals. Some elemental and chemical forms of mercury (vapor, methylmercury, inorganic mercury) are more toxic than other forms. The human fetus and medically compromised people (for example, patients with lung or kidney problems) are the most susceptible to the toxic effects of mercury. Mercury poisoning can also occur outside of occupational exposures including in the home. Inhalation of mercury vapor may stem from cultural and religious rituals where mercury is sprinkled on the floor of a home or car, burned in a candle, or mixed with perfume. Due to widespread use and popular concern, the risk of toxicity from dental amalgam has been exhaustively investigated. It has conclusively been shown to be safe although in 2020 the FDA issued new guidance for at-risk populations who should avoid mercury amalgam. Historically, this was common among felt hat makers in England, who were exposed to mercury vapours they used when stabilising wool during the felting process. The industrial workers were exposed to the mercury vapours, giving rise to the expression "mad as a hatter". Some believe that the character the Mad Hatter in Lewis Carroll's Alice in Wonderland is an example of someone with erethism, but the origin of this account is unclear. The character was almost certainly based on Theophilus Carter, an eccentric furniture dealer who was well known to Carroll.
Signs and symptoms Acute mercury exposure has given rise to psychotic reactions such as delirium, hallucinations, and suicidal tendencies. Occupational exposure has resulted in erethism, with irritability, excitability, excessive shyness, and insomnia as the principal features of a broad-ranging functional disturbance. With continuing exposure, a fine tremor develops, initially involving the hands and later spreading to the eyelids, lips, and tongue, causing violent muscular spasms in the most severe cases. The tremor is reflected in the handwriting which has a characteristic appearance. In milder cases, erethism and tremor regress slowly over a period of years following removal from exposure. Decreased nerve conduction velocity in mercury-exposed workers has been demonstrated. Long-term, low-level exposure has been found to be associated with less pronounced symptoms of erethism, characterized by fatigue, irritability, loss of memory, vivid dreams, and depression (WHO, 1976).
Effects of chronic occupational exposure to mercury, such as that commonly experienced by affected hatters, include mental confusion, emotional disturbances, and muscular weakness. Severe neurological damage and kidney damage can also occur. Signs and symptoms can include red fingers, red toes, red cheeks, sweating, loss of hearing, bleeding from the ears and mouth, loss of appendages such as teeth, hair, and nails, lack of coordination, poor memory, shyness, insomnia, nervousness, tremors, and dizziness. A survey of exposed U.S. hatters revealed predominantly neurological symptomatology, including intention tremor. After chronic exposure to the mercury vapours, hatters tended to develop characteristic psychological traits, such as pathological shyness and marked irritability (see box). Such manifestations among hatters prompted several popular names for erethism, including "mad hatter disease", "mad hatter syndrome", "hatter's shakes" and "Danbury shakes".
Biomarkers of exposure While hatters in the past were diagnosed with erethism through their symptoms, it was sometimes harder to prove that erethism was the result of mercury exposure, as seen in the case of the hatters of New Jersey below. Today, although erethism from the hat making industry is no longer an issue, it persists in other high-risk occupations. As a result, methods have been established to measure the mercury exposure of workers more accurately. They include the collection and testing of mercury levels in blood, hair, nails, and urine. Most of these biomarkers have a shorter half-life for mercury (e.g. in blood the half-life is usually only around 2–4 days), which makes some of them better for testing acute, high doses of mercury exposure. However, mercury in urine has a much longer half-life (measured in weeks to months), and unlike the other biomarkers is more representative of the total body burden of inorganic and elemental mercury. This makes it the ideal biomarker for measuring occupational exposure to mercury because it is suitable to measuring low, chronic exposure, and specifically exposure to inorganic and elemental mercury (i.e. mercury vapor), which are the two types most likely to be encountered in a higher risk occupation.
History among hatters
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