Multiple chemical sensitivity

Multiple chemical sensitivity (MCS) is an unrecognized and controversial diagnosis characterized by chronic symptoms attributed to exposure to low levels of commonly used chemicals. Symptoms are typically vague and non-specific. They may include fatigue, headaches, nausea, shortness of breath, cognitive difficulties and dizziness. Recent imaging studies have shown that it is likely a neurological condition. MCS is a chronic disease that requires ongoing management. In the long term, about half of people with MCS get better and about half continue to be affected, sometimes severely.

Classification In nosological terms, MCS may be more than one disease. It is generally considered a subtype of non-allergic chemical intolerance (also called chemical sensitivity). MCS is considered an acquired disorder, meaning that it was not present from birth but instead developed later. Compared to other conditions with medically unexplained physical symptoms, such as Myalgic encephalomyelitis/chronic fatigue syndrome, fibromyalgia or Gulf War syndrome, MCS symptoms are only present in response to environmental triggers.

Name The name multiple chemical sensitivity has been criticized, partly because MCS is not a sensitivity in the allergic or immunological meaning of that word. Being more sensitive than average to some chemical exposures (e.g., secondhand smoke) is fairly common. MCS is generally used to refer to more significant disability. The name has also been criticized because it suggests that chemical exposure is the actual cause or etiology, which has not been proven. The word chemical in the name is used loosely and includes natural substances (e.g., the aromas produced by citrus or other fragrant plants). Other names, such as idiopathic environmental intolerance, have been recommended as more accurate alternatives.

Definitions Different researchers and proponents use different definitions, which complicates research and can affect diagnosis. For example, the 1987 definition that requires symptoms to begin suddenly after an identifiable, documented exposure to a chemical, but the 1996 definition by the WHO/ICPS says that the cause can be anything, including other medical conditions or psychological factors. In 1996, an expert panel at WHO/ICPS was set up to examine MCS. The panel accepted the existence of "a disease of unclear pathogenesis", rejected the claim that MCS was caused by chemical exposure, and proposed these three diagnostic requirements for a group of conditions that includes MCS, which they called idiopathic environmental intolerances (IEI):

the disease was acquired (not present from birth) and must produce multiple relapsing symptoms; the symptoms must be closely related to "multiple environmental influences, which are well tolerated by the majority of the population"; and it could not be explained by any other medical condition. In Japan, MCS is called chemical hypersensitivity or chemical intolerance (化学物質過敏症; kagaku bushitsu kabinsho), and the 1999 Japanese definition requires one or more of four major symptoms – headaches; malaise and fatigue; muscle pain; joint pain – combined with laboratory findings and/or some minor symptoms, such as mental effects or skin conditions. The defined lab findings are abnormalities in parasympathetic nerves, cerebral cortical dysfunction diagnosed by SPECT testing, visuospatial abnormalities, abnormalities of eye movement, or a positive provocation test. Another definition requires a known precipitating event (e.g., an injury or an illness) followed by the appearance of multi-organ symptoms that predictably wax and wane in response to a variety of exposures that do not bother other people. For example, this could describe someone who always feels sick after using ordinary household cleaning products, when exposed to new car smell, or having a live Christmas tree in the house, and then feels better again when not around these things. Additionally, the symptoms cannot be explainable through other conditions that can be identified with ordinary medical tests, such as an allergic reaction.

Symptoms Symptoms are typically vague and non-specific, such as fatigue or headaches. These symptoms, although they can be disabling, are called non-specific because they are not associated with any single specific medical condition. Symptoms affect a variety of different organ systems. Different people have different symptoms and different affected systems, but cognitive and neurologic symptoms (e.g., headache and brain fog) are common, as are systemic symptoms (e.g., fatigue). Other people have symptoms affecting the eyes, ears, and nose (e.g., stuffy nose), the respiratory system (e.g., shortness of breath), gastrointestinal system (e.g., nausea), musculoskeletal system (e.g., joint pain), or dermatological system (e.g., itching). A 2010 review of MCS literature said that the following symptoms, in this order, were the most reported in the condition: headache, fatigue, confusion, depression, shortness of breath, arthralgia, myalgia, nausea, dizziness, memory problems, gastrointestinal symptoms, respiratory symptoms. Symptoms mainly arise from the autonomic nervous system (such as nausea or dizziness) or have psychiatric or psychological aspects (such as difficulty concentrating).

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