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What is Central pontine myelinolysis and explain in detail?
Central pontine myelinolysis is a neurological condition involving severe damage to the myelin sheath of nerve cells in the pons (an area of the brainstem). It is predominately iatrogenic (treatment-induced), and is characterized by acute paralysis,  dysphagia (difficulty swallowing), dysarthria (difficulty speaking), and other neurological symptoms.
Central pontine myelinolysis was first described as a disorder in 1959. The original paper described four cases with fatal outcomes, and the findings on autopsy. The disease was described as a disease of alcoholics and malnutrition. Central pontine indicated the site of the lesion and myelinolysis was used to emphasise that myelin was affected. The authors intentionally avoided the term demyelination to describe the condition, in order to differentiate this condition from multiple sclerosis and other neuroinflammatory disorders.Since this original description, demyelination in other areas of the central nervous system associated with osmotic stress has been described outside the pons (extrapontine). Osmotic demyelination syndrome is the term used for both central pontine myelinolysis and extrapontine myelinolysis.Central pontine myelinolysis, and osmotic demyelination syndrome, present most commonly as a complication of treatment of patients with profound hyponatremia (low sodium), which can result from a varied spectrum of conditions, based on different mechanisms. It occurs as a consequence of a rapid rise in serum tonicity following treatment in individuals with chronic, severe hyponatremia who have made intracellular adaptations to the prevailing hypotonicity.

Signs and symptoms
Symptoms depend on the regions of the brain involved. Prior to its onset, patients may present with the neurological signs and symptoms of hyponatraemic encephalopathy such as nausea and vomiting, confusion, headache and seizures. These symptoms may resolve with normalisation of the serum sodium concentration. Three to five days later, a second phase of neurological manifestations occurs correlating with the onset of myelinolysis. Observable immediate precursors may include seizures, disturbed consciousness, gait changes, and decrease or cessation of respiratory function.The classical clinical presentation is the progressive development of spastic quadriparesis, pseudobulbar palsy, and emotional lability (pseudobulbar affect), with other more variable neurological features associated with brainstem damage. These result from a rapid myelinolysis of the corticobulbar and corticospinal tracts in the brainstem.In about ten per cent of people with central pontine myelinolysis, extrapontine myelinolysis is also found. In these cases symptoms of Parkinsons disease may be generated.

Causes
The most common cause is overly-rapid correction of low blood sodium levels (hyponatremia). Apart from rapid correction of hyponatraemia, there are case reports of central pontine myelinolysis in association with hypokalaemia, anorexia nervosa when feeding is started, patients undergoing dialysis and burn victims. There is a case report of central pontine myelinolysis occurring in the context of refeeding syndrome, in the absence of hyponatremia.It has also been known to occur in patients suffering withdrawal symptoms of chronic alcoholism.  In these instances, occurrence may be entirely unrelated to hyponatremia or rapid correction of hyponatremia. It could affect patients who take some prescription medicines that are able to cross the blood-brain barrier and cause abnormal thirst reception - in this scenario the central pontine myelinolysis is caused by polydipsia leading to low blood sodium levels (hyponatremia).In schizophrenic patients with psychogenic polydipsia, inadequate thirst reception leads to excessive water intake, severely diluting serum sodium. With this excessive thirst combined with psychotic symptoms, brain damage such as central pontine myelinolysis may result from hyperosmolarity caused by excess intake of fluids, (primary polydipsia) although this is difficult to determine because such patients are often institutionalised and have a long history of mental health conditions.It has been observed following hematopoietic stem cell transplantation.Central pontine myelinolysis may also occur in patients prone to hyponatremia affected by:

Severe liver disease (e.g., cirrhosis)
Liver transplant
Alcoholism
Hypokalemia
People with serum sodium <105 mEq/L
Severe burns
Malnutrition
Anorexia nervosa
Severe electrolyte disorders
HIV/AIDS
hyperemesis gravidarum
Hyponatremia due to peritoneal dialysis
Wernicke encephalopathy

Pathophysiology
The currently accepted theory states that the brain cells adjust their osmolarities by changing levels of certain osmolytes like inositol, betaine, and glutamine in response to varying serum osmolality. In the context of chronic low plasma sodium (hyponatremia), the brain compensates by decreasing the levels of these osmolytes within the cells, so that they can remain relatively isotonic with their surroundings and not absorb too much fluid. The reverse is true in hypernatremia, in which the cells increase their intracellular osmolytes so as not to lose too much fluid to the extracellular space.With correction of the hyponatremia with intravenous fluids, the extracellular tonicity increases, followed by an increase in intracellular tonicity. When the correction is too rapid, not enough time is allowed for the brains cells to adjust to the new tonicity, namely by increasing the intracellular osmoles mentioned earlier. If the serum sodium levels rise too rapidly, the increased extracellular tonicity will continue to drive water out of the brains cells. This can lead to cellular dysfunction and central pontine myelinolysis.

Diagnosis
It can be diagnosed clinically in the appropriate context, but may be difficult to confirm radiologically using conventional imaging techniques.  Changes are more prominent on MRI than on CT, but often take days or weeks after acute symptom onset to develop