Central Pontine and Extra Pontine Myelinolysis

A 72-year-old alcoholic patient was brought to the ED with an altered state of consciousness. On arrival, serum electrolyte levels were as follows: sodium (Na): 110 mmol/l (135-145 mmol/l). The patient was diagnosed with hyponatremia and was put on sodium corrective measures. MRI brain was performed due to drowsiness, confusion, and raised Na: 145 mmol/l after 2 days.

On MRI, there is omega shaped T2/FLAIR hyperintensity involving the central tegmentum of pons with characteristic sparing of the periphery. This is suggestive of pontine myelinolysis (Figure 3). In addition, there is symmetrical hyperintensity involving the caudate and lentiform nucleus and also partly the thalamus (Figure 1 & 2). These findings are consistent with extra pontine myelinolysis. All the above lesions showed diffusion (DW) hyperintensity (Figure 4) but no restriction on ADC (Figure 5), which is an early finding of myelinolysis.

Pontine myelinolysis described historically is now part of a syndrome termed "osmotic demyelination syndrome" and it has two distinct entities: Central pontine myelinolysis (CPM) where only pons is involved and extra pontine myelinolysis (EPM), where structures other than the pons are involved. Alcoholism and malnutrition are the most common predisposing factors associated with this condition. Other causes include cirrhosis of liver, post liver transplantation, electrolyte imbalance, psycogenic polydipsia and burns [1].
Classical presentation of pontine myelinolysis is with confusion, pseudo bulbar palsy and locked-in syndrome. However, this presentation is rare and initial symptoms include weakness, confusion and dysarthria which can be confused with stroke. Over a period, it may progress to quadriparesis and pseudo bulbar palsy. Co-existence of extra pontine myelinolysis (EPM) produces more complicated symptoms like a variety of psychiatric, behavioural and movement disorders [2].
MRI is the imaging modality of choice. MRI findings include hyperintensity on T2/FLAIR images in the pons in CPM & similar findings involving the basal ganglia, thalamus, lateral geniculate body and cerebellum in EPM. In severe cases the pontine lesions may show diffusion restriction, which is secondary to cell necrosis. When such a finding is present it can be confused with pontine infarct. With appropriate clinical details, this finding can be used for early prediction on CPM [3].
Pathologically, loss of myelin is seen starting in the pons and spreading centrifugally. This is believed to be secondary to osmotic gradient effects. In the initial period of hyponatremia, brain cells try to lose electrolytes swiftly and make themselves isotonic to the extravascular compartment. In the event of rapid influx of electrolytes, the brain is not able to cope with the rapid influx which results in cell dehydration and myelin lysis [4].
Mortality rate of pontine myelinolysis is said to be as high as 40-50%. The current recommended treatment guidelines mention an increase of Na level of not more than 8 mmol/day. No specific treatment has been advocated apart from conservative treatment for established cases of CPM/EPM [5]. In this patient despite all possible measures, the patient could not be revived after 7 days of conservative treatment.
Conclusion: Pontine myelinolysis is a unique type of demyelination with high mortality, which should always be thought of in chronic alcoholics presenting with conflicting symptoms. This condition can be totally avoided by following the guidelines of correction of hyponatremia. However, MRI of the brain should always be performed if there are doubts.