Mercury toxicity

Question 1

Regarding mercury, state:

• Where it is derived from
• Its melting point

Answer 1

Where it is derived from:
· Hydrargyrum: liquid silver

Melting point:
· -39 degrees
· It is liquid at room temperature

Question 2

State the 3 different forms of mercury, and their biological effects

Answer 2

Elemental/ metallic:
• Lipid soluble (therefore it can cross cells membranes)
• Vapour is readily absorbed by the lung and quickly diffused into blood
• Has the longest half life

Non elemental, inorganic:
• Poorly absorbed via GI tract
• Not really lipid soluble and thus, not very toxic

Organic form:
• Also lipid soluble
• Intestinal and dermal absorption
• Hair has highest concentration

Question 3

Explain how mercury causes neurotoxicity and list the 5 potential ways it does so

Answer 3

Because mercury is lipid soluble, it can easily cross the blood brain barrier. There is no way to excrete the mercury, and thus it will keep accumulating. Mercury eventually leads to apoptosis of cells.

This can happen the following ways:
· Protein inhibition
· Disruption of mitochondria function
· Direct affect on ion exchange in a neuron
· Disruption of neurotransmitters
· Destruction of the structural framework of neurons

Question 4

Describe the two ways mercury passes the blood brain barrier, and mention the forms of mercury

Answer 4

Inhalation:
• Form: metallic form
• Mercury associates with erythrocytes
• Via blood circulation, it will eventually reach the brain
• Once exposed to proteins in the neurones, it will bind to them

Ingestion:
• Form: organic form
• Associates with cysteine proteins which will then travel to the brain and pass the BBB

Question 5

Discuss the regions of central nervous systems affected by mercury using Minamata disease as an example

Answer 5

· Cerebellum and Precentral gyrus: Loss of balance, tremors, jerky movements
· Postcentral gyrus: Lack of sensation (touch)
· Occipital lobe: vision
· Temporal lobe: hearing
· Dorsal root ganglion of spinal cord

Question 6

List the clinical presentations of chronic (11) mercury toxicity

Answer 6

• Neurodevelopmental toxicity because it crosses the placenta (foetus)
• Neurocognitive defects
• Neuromotor disabilities: cerebral palsy, movement abnormalities, abnormal reflexes
• Nephrotoxicity, targeting the proximal tubules
• Kidney failure
• Teratogenicity
• Cardiovascular toxicity
• Carcinogenicity
• Genotoxicity, mutagenesis
• Reproductive toxicity
• Immunotoxicity (mainly in animals, not common in humans)

Question 7

List the clinical presentations of acute (6) mercury toxicity

Answer 7

• Respiratory system: irritation, corrosive bronchitis, interstitial pneumonitis
• Gastrointestinal track: colitis
• Oral cavity: stomatitis, glossitis, ulcerative gingivitis
• Central nervous system
• Renal tubular necrosis
• Skin reaction: Acrodynia, Pink’s disease (pinkish discoloration and desquamation, itchiness or pain in the extremities)

Question 8

Describe the mercury cycle

Answer 8

· Mercury is released into the atmosphere naturally from volcanoes, or through mining or burning fossil fuels
· The mercury (in its organic form) will subsequently be deposited into the ocean
· Methyl mercury is converted into its inorganic form, but then algae will process it to its organic form
· Then fish may eat algae containing mercury, and then bigger fish will eat the fish
· Humans will eat the fish, and the process of bioaccumulation repeats itself

Question 9

Discuss the dental amalgam controversy and apply the evidence based practice to address patients concerns

Answer 9

· “Where possible exposure to mercury from dental amalgams be reduced where a safe and practical alternative exists. This becomes more prudent in special populations, including children, women in pregnancy and persons with existing kidney disease”
· Old study showed that amalgam used in Class I restorations was very minimal
· Human body can tolerate a concentration of 0.2 µg/m3 for long-term inhalation exposure to elemental mercury vapour, and an intake of total mercury of 2 µg/kg body weight per day
· Evidence from studies is questionable; they cannot fully explore the side effects of mercury
· Some studies suggest that susceptibility to mercury toxicity differs among individuals based on multiple genes, not all of which have been identified.
· These studies further suggest that the levels of exposure to mercury vapor from dental amalgams may be unsafe for certain subpopulation