Congenital Minamata Disease

                                    Affected Development            
        Methyl mercury poisoning in Congenital Minamata disease can cause detrimental effects to the nervous system, specifically the Central Nervous System (CNS).  The exact levels or dosages of mercury and how it impacts child development in the CNS are still in its incipient stages, however the specific regions that it affects is widely recognized. The diagram below pinpoints specifically what lesions of the brain and spinal cord were the most affected. The red sections indicate predominantly affected regions of the brain, whereas the blue sections indicate less severely affected regions, such as the spinal cord and other nerve fibers. Each of the regions corresponds to a developmental defect, but we will focus on the primary regions that are affected (Myers et al. 2000).  

Here are the 6 primary regions with its associated defects: 
1. Cerebellum- This is the most impacted region of the brain that leads to a myriad of problems.  The cerebellum controls coordination of movement and balance. Some of  the defects include loss of balance, speech impairment, and improper walking, also known as gait disturbance. 
2. Precentral cortex (precentral gyrus)-This region is responsible for movement, which is why it is also referred to as the primary motor cortex. The cerebellum interacts closely with the precentral cortex to coordinate balance. When these regions are malfunctioning, it leads to muscle cramps, weak muscles, and jerky movements.  
3.Postcentral cortex (postcentral  gyrus) -This region is responsible for sensation, which is why this can also be called the somatosensory cortex.  When this region is not functioning properly, it leads to "stereo anesthesia", or lack of sensation.  
4. Occipital lobe- The occipital lobe is located in the cerebral cortex. Lesions in this area cause blindness and visual defects.  
5. Temporal lobe- The temporal lobe is also located in the cerebral cortexLesions in this area cause hearing loss and deafness. 
6. Sensory nerves- The ganglion, sciatic nerve, and the sural nerve are all  responsible for sensation of touch, pain and temperature.  When these nerves are dysfunctional, it can lead to numbness or even paralysis (Eto et al. 1997).  

Picture
Affected Regions of the CNS due to Congenital Minamata Poisoning(National Institute of Minamata Disease).



        
What are the "predicted" affected processes?

      Due to the complexity of the brain and the processes involved in normal neurogenesis, there is still research being conducted on child development of Congenital disease patients.  Therefore, it is predicted that the mercury levels cause "complete loss of neurons and hyperplasia of the astrocytes" found in the cerebral cortex and cerebellum. (Barkir et al. 1980). Neurodegeneration, or complete loss of neurons, can be caused by a multitude of factors: the neurons didn't proliferate properly, incomplete cell migration due to faulty microtubules, and as a result nerve cells are poorly developed. (Grandjean et al. 2010).  It is then predicted that the astrocytes and microglial cells proliferate to compensate for the loss of neurons.  The process where astrocytes grow in number to assist CNS lesions is called astrogliosis.  The exact processes of how they operate for Congenital Minamata disease are still being developed and understood, but some proposed explanations are elaborated in the Disease Etiology section (Grandjean et al. 2010).  


Varying levels of Mercury leads to varying defects

    There have been studies conducted on affected development of Congenital Minamata disease that suggest higher levels of methylmercury could cause deleterious effects to the cerebellum, however the exact dose of mercury is unknown since many variables are involved, such as means of exposure to mercury, the duration and time of exposure, the amount of methylmercury ingested, the general health of the mother, and other unknown factors (Davidson et al. 2004).  Since no conclusive data about levels of mercury exists, it is assumed high dosages would cause toxic effects to the brain. Mercury continues to be an elusive element(Davidson et al) and more studies need to be conducted to confirm low to medium dosages have significant impact on child development (Dalgard et al. 1994).  
       Now that we established that the levels of mercury is still unknown, this can also account for the varying developmental defects that can be associated with Congenital Minamata disease.  Many studies indicate primary defects in the CNS would lead to varying effects in infants such as blindness to muscle failure, as well as the defects listed above. This makes sense because the cerebellum controls coordination of movement and the cerebral cortex controls visual centers (Davidson et al. 2004).  


 Affected Development section completed by Arpana Neelmegh