The term neurotoxicity refers to damage to the brain or peripheral nervous system caused by exposure to natural or man-made toxic substances.
These toxins can alter the activity of the nervous system in ways that can disrupt or kill nerves. Nerves are essential for transmitting and processing information in the brain, as well as other areas of the nervous system.
Due to their high metabolic rate, neurons are at the greatest risk of damage caused by neurotoxins. This is followed, in order of risk, by oligodendrocytes, astroocytes, microglia and capillary endothelium cells.
Depending on a neurotoxin’s chemical profile, it will cause damage to certain parts or particular cellular elements of the nervous system. Non-polar substances are more soluble in lipids and can therefore access the nervous tissue more easily than polar compounds, which are less soluble in lipids. The body’s response to neurotoxins is influenced by factors such as the neurotransmitter affected, cellular membrane integrity and the presence of detoxifying mechanisms.
Some examples of substances that can be neurotoxic to humans include:
- Chemotherapy drugs that are used to kill fast growing cells
- Drug therapies or drugs of abuse
- Heavy metals such as mercury and lead
- Certain foods and food additives
- Industrial and cleaning solvents
Some examples of neurotoxic substances our environment has become polluted with and that it is difficult for people to avoid exposure to include:
- Car exhaust
Effects of neurotoxicity
Some of the effects of neurotoxicity may appear immediately, while others can take months or years to manifest.
The effects of neurotoxicity depends on various different factors such as the characteristics of the neurotoxin, the dose a person has been exposed to, ability to metabolise and excrete the toxin, the ability of affected mechanism and structures to recover and how vulnerable a cellular target is.
Some of the symptoms of neurotoxicity include:
- Paralysis or weakness in the limbs
- Altered sensation, tingling and numbness in the limbs
- Vision loss
- Loss of memory and cognitive function
- Uncontrollable obsessive and/or compulsive behavior
- Behavioral problems
- Sexual dysfunction
- Loss of circulation
- Flu-like symptoms
Other conditions that may develop as a result of neurotoxicity include chronic fatigue syndrome, attention deficit hyperactivity disorder, chronic sinusitis and asthma that does not respond to therapies. Symptoms may also resemble those seen in some autoimmune conditions such as irritable bowel syndrome or rheumatoid arthritis.
Some examples of toxins that occur naturally in the brain and can lead to neurotoxicity include oxygen radicals, beta amyloid and glutamate. Aside from causing movement disorders, cognitive deterioration and dysfunction of the autonomic nervous system, neurotoxicity has also been shown to be a major contributor to progressive neurological disorders such as Alzheimer’s disease.
The best test to show whether the peripheral nervous system has been affected is the nerve conduction test. Tests used to detect damage to the brain include pupillography, computerized balance heart rate variability, brain imaging with the triple-camera SPECT system and neuropsychological testing .
The treatment approach to neurotoxicity is elimination or reduction of the toxic substance and therapy to relieve symptoms or provide support. Treatment may also involve avoiding air, food and water pollutants. Some examples of therapies used in the treatment of neurotoxicity include massage, exercise and immune modulaiton.
The outcome of neurotoxicity depends on the duration and extent of exposure to the toxic substance, as well as the degree of neural damage. Exposure to neurotoxins can be fatal in some cases, while in others, patients survive but may not completely recover. In other cases, patient do completely recover after receiving treatment.
Scientist are looking at whether occupational and environmental toxins may play a role in neurodegenerative diseases such as Alzheimer’s, Parkinson’s disease and multiple sclerosis. Another popular research topic in this area is the mechanisms behind neuroimmune responses that occur in the nervous system. Whether or not the interaction between environmental factors and genes contributes to brain disorders in children is another research question that is currently being investigated.