Central pontine myelinosis

Central pontine myelinolysis (CPM) is neurological disease caused by severe damage of the myelin sheath of nerve cells in the brainstem, more precisely in the area termed the pons, predominately of iatrogenic etiology. It is characterized by acute paralysis, dysphagia (difficulty swallowing), and dysarthria (difficulty speaking), and other neurological symptoms.

It can also occur outside the pons. The term "osmotic demyelination syndrome" is similar to "central pontine myelinolysis", but also includes areas outside the pons.

Central pontine myelinolysis presents most commonly as a complication of treatment of patients with profound, life-threatening hyponatremia (low sodium). It occurs as a consequence of a rapid rise in serum tonicity following treatment in individuals with chronic, severe hyponatraemia who have made intracellular adaptations to the prevailing hypotonicity. Hyponatremia should be corrected at a rate of no more than 8-10 mmol/L of sodium per day to prevent central pontine myelinolysis.

Although less common, it may also present in patients with a history of chronic alcoholism or other conditions related to decreased liver function. In these cases, the condition is often unrelated to correction of sodium or electrolyte imbalance.

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 hyponatremia the levels of these osmolytes fall, causing the cells to absorb free-water. The reverse is true for hypernatremia, in which cells will shrink to dilute the hypernatremic fluid. So rapid correction of sodium in hyponatremia would cause the extracellular fluid to be relatively hypertonic. Free water would then move out of the cells to decrease this relative hypertonicity. This leads to a central pontine myelinolysis, manifesting as the paralysis. This would look like the brain is shrinking.

The demyelination of the axons (nerve fibers in the brain) damages them.

In the context of chronic low plasma sodium, the brain's cells (neurons and glia) adapt by taking in a small amount of water; the net effect is to move water out of the interstitium and equilibrate (or nearly so) the intracellular and extracellular tonicities. The chronic hyponatremia is thus compensated.

With correction of the hyponatremia with intravenous fluids, the intra- and extra-cellular tonicities are again changed, this time in the opposite direction. With the use of intravenous hypertonic saline, the correction can be too quick, not allowing enough time for the brain's cells to adjust to the new tonicity. With a rise in extracellular tonicity, the cells compensate by losing a small amount of water. This loss will continue until the intra- and extra-cellular tonicities are equal. If hypertonic therapy continues or is too rapid, the extracellular tonicity will continue to drive water out of the brain's cells, leading to cellular dysfunction and the condition of central pontine myelinolysis.

Rapid correction of hypernatremia causes water to move into cells, leading to multiple cerebral hemorrhages, equally catastrophic as osmotic demyelination.

Causes
The most common cause is the too rapid correction of low blood sodium levels (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 has been observed following hematopoietic stem cell transplantation.

CPM may also occur in patients affected by
 * severe liver disease
 * liver transplant
 * alcoholism
 * severe burns
 * malnutrition
 * anorexia
 * severe electrolyte disorders
 * AIDS
 * hyperemesis gravidarum.

Diagnosis
It can be difficult to identify using conventional imaging techniques. It presents more prominently on MRI than on CT, often taking several weeks after acute onset of symptoms before it becomes identifiable. Imaging by MRI demonstrates an area of high signal return on T2 weighted images.

Symptoms


Clinical presentation of CPM is heterogeneous and depend on the regions of the brain involved. Observable immediate precursors may include seizures, disturbed consciousness, gait changes, and decrease or cessation of respiratory function.

Frequently observed symptoms in this disorder are acute para- or quadraparesis, dysphagia, dysarthria, diplopia, loss of consciousness, and other neurological symptoms associated with brainstem damage. The patient may experience locked-in syndrome where cognitive function is intact, but all muscles are paralyzed with the exception of eye blinking. These result from a rapid myelinolysis of the corticobulbar and corticospinal tracts in the brainstem.

Prevention and treatment
To prevent CPM from its most common cause, overly rapid reversal of hyponatremia, the hyponatremia should be corrected at a rate not exceeding 10 mmol/L/24hr or 0.5 mEq/L/Hr; thus avoiding hypernatremia. Details concerning the etiology and correction of electrolyte disorders are discussed extensively in general medicine texts. Alcoholic patients should receive vitamin supplementation and a formal evaluation of their nutritional status.

Once demyelination of the pons has begun, there is no cure or specific treatment. Care is supportive, with the goal of preventing complications like aspiration pneumonia or deep vein thrombosis. Alcoholics are usually given vitamins to correct for other deficiencies.

Research has led to improved outcomes. Animal studies suggest that inositol reduces the severity of osmotic demyelination syndrome if given before attempting to correct chronic hyponatraemia. Further study is required before using inositol in humans for this purpose.

Prognosis
The prognosis is overall poor. While some patients die, most survive and of the survivors, approximately one-third recover; one-third are disabled but are able to live independently; one-third are severely disabled. Permanent disabilities range from minor tremors and ataxia to signs of severe brain damage, such as spastic quadriparesis and locked-in syndrome. Some improvements may be seen over the course of the first several months after the condition stabilizes.

The extent of recovery depends on how many axons were damaged.