CENTRAL PONTINE MYELINOLYSIS

Osmotic Demyelination Syndrome

Central Pontine Myelinolysis (CPM) is a devastating brainstem injury that’s caused by doctors. Let’s break the name down. 

Central Pontine = in the middle of the Pons

Myelino = myelin

Lysis = destruction

So CPM is the destruction of myelinated fibers in the middle of the Pons. 

Pathophysiology?

CPM is caused by the quick onset of hypernatremia. But actually, it would be more precise to say that CPM is caused by the quick onset of a hyperosmolar state. Sodium just happens to be the most important osmolar substance. The MOA is kinda tricky, but basically, oligodendrocytes in the brainstem are very sensitive to the osmolarity of their surroundings. They have to make new proteins every time the osmolarity rises (and break them down when it drops). It takes them a long time to build those proteins, and in the event of a rapid correction from hypo-osmolar to hyper-osmolar, the myelin doesn’t have time to build proteins, and huge swathes of myelin burst. 

What’s the most common cause in real life?

CPM is seen in basically only one circumstance -- rapid overcorrection of hyponatremia using IV saline. Hyponatremia is a common scenario though -- the sodium levels are drawn in almost every hospitalized patient at least once a day! Hyponatremia is so, so common! Mild hyponatremia can mostly be ignored, but severe hyponatremia is a medical emergency because it causes cerebral edema. IV saline is the treatment, but the key here is that you have to correct the hyponatremia slowly. If you blast them with 3% saline, you could precipitate CPM. 

Symptoms?

Locked-in Syndrome. Every upper motor neuron that leaves from the pons or below dies. That’s a lot of neurons! Can you name which motor neurons are excluded? The answer is CN3 and CN4. Check out the book/movie The Diving Bell and the Butterfly to better understand Locked-in Syndrome. The author wrote the entire book by communicating only using blinks and eye movements!