AssessmentsCentral pontine myelinolysis
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 52-year old female, with a longstanding history of Hashimoto thyroiditis, comes to the emergency department because of drowsiness after an acute episode of pneumonia. Her temperature is 37.0°C (98.6°F), pulse is 68/min, respirations are 18/min, and blood pressure is 118/78 mm Hg. Her medications include levothyroxine, prednisone, acetaminophen, and sertraline. After beginning intravenous maintenance fluid administration, the first-year resident on duty notices her severe hyponatremia (100mmol/L) and gives her a rapid infusion of hypertonic saline. The patient is discharged but returns three days later in an acutely altered condition. Physical examination shows that the patient is unable to speak, is disoriented, and cannot move her legs. Which of the following most likely explains this series of events?
In central pontine myelinolysis, pontine refers to the pons of the brainstem, myelin refers to the fatty layer of insulation that wraps around neurons, and -lysis refers to destruction.
So, central pontine myelinolysis is the destruction of the myelin sheath around nerve cells that are in the pons.
The main cause of destruction is rapid osmotic changes, meaning that a lot of water leaves the cells, and dries them out, causing them to die.
So the other name for central pontine myelinolysis is osmotic demyelination syndrome.
Taking a look at the brain, the pons is part of the brainstem and it’s nestled between the midbrain and the medulla oblongata.
The pons itself has control centers that help manage the respiration rate and the depth of breathing while we’re awake and when we sleep. So if you try to take a deep breath right now - that’s your pons in action!
Neuron clusters or nuclei for cranial nerves V: trigeminal, VI: abducens, VII: facial, and VIII: vestibulocochlear are also housed in the pons.
Cranial nerve V allows you to feel things on your face and controls the muscles that help you chew, bite, and swallow.
Cranial nerve VI allows your eyes to move side to side.
Cranial nerve VII helps with facial expressions - like making a weird face, and cranial nerve VIII helps with hearing.
All of these nerves are made up of lots of individual neurons which capture signals from their dendrites, and pass those signals along through their axons.
In addition to the neurons, there are also supporting cells called oligodendrocytes and astrocytes.
Oligodendrocytes physically wrap their fatty myelin-rich cell membranes around neuronal axons that are nearby to help action potentials move more quickly through them.
And astrocytes help repair damaged neurons.
Neurons and oligodendrocytes are very sensitive to changes in the amount of water and electrolytes in themselves and their environment.In other words, in the intracellular and extracellular compartment.
And the pons is a part of the brain that’s particularly sensitive.
The cell membrane is permeable to water, but some substances called osmolytes cannot cross over as easily and are called semi-permeable.
Examples of osmolytes are electrolytes like potassium, sodium, and chloride, as well as organic substances like phosphorylated glucose - which makes the molecule more polar and prevents it from slipping through the membrane easily.
Normally, extracellular osmolality matches intracellular osmolality, meaning they are in equal balance of water and solute concentration.
When there’s a difference between the intracellular and extracellular osmolality, water flows towards the compartment with a higher osmolality to balance things out - and that’s osmosis - the process, not the company!