Mad hatter’s disease is a form of mercury poisoning that affects the brain and nervous system. People can develop mercury poisoning by inhaling mercury vapors.

Mad hatter’s disease is caused by chronic mercury poisoning. It is characterized by emotional, mental, and behavioral changes, among other symptoms.

A doctor may describe the neurological changes as erethism or mercurial erethism.

In this article, we explore the causes, symptoms, and treatments of mercury poisoning. We also briefly look into the history of the saying “mad as a hatter.”

A woman with sleep deprivation coughs into her arm, both of which are symptoms of mad hatter disease.Share on Pinterest
Early symptoms of mercury poisoning may include sleep disturbances, a wet cough, and muscle pain.

Mercury is a metal that can turn to vapor at room temperatures. The lungs can easily absorb this vapor, and once mercury is in the body, it can pass through cell membranes and the blood-brain barrier.

Mercury is also a neurotoxin, and it can cause neurological damage that leads to hallucinations and psychosis.

As well as affecting the brain and the lungs, mercury can damage various other areas, including the:

  • nervous system
  • stomach
  • kidneys
  • skin
  • thyroid
  • breasts
  • muscles
  • liver
  • adrenal glands
  • testes and prostate

When chronic mercury poisoning affects the brain and nervous system, a person might be said to have mad hatter’s disease. The doctor may instead refer to the neurological changes as erethism.

In medieval Europe, mercury was used in medicine and manufacturing. Later, hatmakers commonly cured felt using a form of mercury called mercurous nitrate.

As the hatmakers inhaled mercury vapors over time, many experienced neurological symptoms of mercury poisoning. By 1837, “mad as a hatter” was a common saying.

Nearly 30 years later, Lewis Carroll published Alice in Wonderland, which contained the now-famous Mad Hatter character.

In the United States, hatmakers continued to use mercury until 1941.

There are early and late symptoms of mercury poisoning, depending on the level of exposure.

The neurological changes that characterize Mad hatter’s disease occur after long-term exposure.

Early symptoms of mercury poisoning may include:

  • a rash
  • skin itchiness
  • muscle pain
  • a metallic taste in the mouth
  • sores or inflammation in the mouth
  • vomiting
  • stomach pain
  • diarrhea
  • sleep disturbances
  • a wet cough

Later symptoms of mercury poisoning may include:

  • irritability and a lack of patience
  • shyness and the desire to avoid people
  • anxiety
  • insomnia
  • a tremor that begins in the hands and affects the face and head
  • trouble thinking or concentrating
  • memory loss
  • changes in movement, which may become coarse or jerky

The World Health Organization (WHO) explain that exposure to mercury may be:

  • Inorganic: A person may be exposed through their job or through contact with mercury in dental fillings or cosmetics, for example.
  • Organic: A person can be exposed to mercury in their diet.

The three most common sources of exposure to mercury are:

  • certain dental fillings
  • contaminated fish
  • workplaces

Inorganic exposure

If a person has a cavity, a dentist may fill it with amalgam — a mixture of metals that contains mercury.

According to a 2012 review, each filling releases up to 28 micrograms of mercury a day, and the body absorbs 80% of this.

However, the Food and Drug Administration (FDA) state that these types of fillings are safe, starting at 6 years of age.

Also, a person might be exposed to mercury through broken thermometers or blood pressure monitors.

A person may also be exposed to mercury at work, such as in factories or workshops that produce batteries, lamps, or light bulbs.

Organic exposure

Mercury is a naturally occurring element in the Earth’s crust.

Volcanic activity can cause mercury to enter the water, where it becomes methylmercury, and contaminate fish.

It is possible to get mercury poisoning by eating contaminated fish. Tuna, swordfish, and shark are more likely than other varieties to contain high concentrations of mercury.

To prevent mercury poisoning, fishing has been prohibited in more than 3,000 lakes in the U.S.

While, according to the WHO, everyone experiences some exposure to mercury, children may have an increased risk of inhaling mercury vapors.

This is because crawling or playing on the floor can put them in close contact with mercury that has spilled from thermometers or other instruments.

A child’s smaller lung capacity can increase the risk associated with inhaling mercury vapors.

Also, a fetus may be exposed to mercury in the womb if the woman has consumed contaminated fish or shellfish.

The exposure can change the development of the baby’s brain and nervous system, going on to affect:

  • the ability to think
  • memory
  • attention
  • language
  • movement
  • spatial skills

In addition, anyone who works in an environment that contains mercury has an increased risk of poisoning.

The main treatment for mercury poisoning involves preventing any further exposure and administering:

  • oxygen
  • bronchodilators, drugs that increase airflow in the lungs
  • fluids

If there is a high amount of mercury in the blood and urine, the doctor may also administer drugs called chelating agents, which help the body pass more of the metal in the urine.

The most common chelating agents are:

  • dimercaprol
  • penicillamine
  • unithiol
  • succimer

According to a 2018 article, the neurological symptoms of mercury poisoning are reversible once the metal has been cleared from the body.

If a person does not receive this treatment, however, they are at risk of:

  • hypoxia, a condition in which the body’s tissues are starved of oxygen
  • permanent lung damage
  • death

Anyone who believes that they may have been exposed to mercury, including as a vapor, should contact a doctor.

It is especially important to receive medical care if any symptoms of mercury poisoning are present.

The doctor may diagnose mercury poisoning using blood tests, toxicology analyses, and X-rays.

Mad hatter’s disease refers to neurological symptoms of mercury poisoning over a long period of exposure.

Its name stems from the fact that hatmakers used to use a mercury compound to cure felt.

Mercury is still present in workplaces, such as factories that produce batteries or lamps. A person can also be exposed to it from a broken thermometer or other broken equipment.

In addition, mercury is present in some types of seafood and certain dental fillings. However, the FDA consider dental fillings that contain mercury to be safe for anyone over the age of 6.

Anyone who comes into contact with liquid mercury should consult a doctor. Seek medical attention for any symptoms of mercury poisoning.

Mercury poisoning is reversible if a person receives treatment in time. If a person does not receive medical care, the poisoning can be fatal.