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Central Pontine Myelinolysis


Introduction

Central pontine myelinolysis (CPM) was originally described by . He first detailed the entity in a group of malnourished and alcoholic patients. Further studies and advancement in medicine have shown that CPM most commonly results from the rapid correction of serum sodium in hyponatremic patients. The pathophysiology of CPM is currently not fully understood. However, it has been shown that CPM results from the physiologic imbalance of osmoles within the brain. Many other conditions associated with disorders of solute metabolism, including inappropriate antidiuretic hormone secretion syndrome, malnutrition, psychogenic polydipsia, liver transplantation, and dialysis disequilibrium syndrome, share the common finding of alterations in cellular volume control. The imaging findings of CPM correspond to locations within the brain (in this case the pons) that are most susceptible to osmotic stress, as do the findings of extrapontine myelinolysis (EPM). Together, CPM and EPM constitute the osmotic demyelination syndromes (ODSs).

The central pons is the most commonly identified site of involvement in ODSs. A necropsy series of 58 cases identified isolated central pontine involvement in 50% of cases. The other 50% of cases had either central pontine with extrapontine (30%) or isolated extrapontine (20%) involvement ( Fig. 6.1 ). Histologic sites of EPM have been described within the cerebellum, lateral geniculate body, external capsule, extreme capsule, hippocampus, putamen, cerebral cortex/subcortex, thalamus, and caudate, in descending order of frequency. Importantly, extrapontine involvement is usually symmetric.

Figure 6.1, Relative proportions of central pontine myelinolysis (CPM) , extrapontine myelinolysis (EPM) , and CMP with EPM.

Myelinolysis results in preservation of local neurons and axons in the effected sites without an inflammatory reaction, as evident by paucity of lymphocytes on histologic specimens. These findings help to differentiate myelinolysis from multiple sclerosis or infarction. Histologic specimens have also demonstrated splitting and vacuolization of myelin sheaths, which are subsequently taken up by macrophages.

Imaging Pattern

Classically, CPM demonstrates T2 hyperintensity within the central pons, with peripheral pontine sparing, as well as sparing of the corticospinal tracts. This results in a “trident” or “bat wing” appearance on axial images ( Fig. 6.2 ).

Figure 6.2, Central pontine myelinolysis—axial and coronal T2-weighted images show T2 hyperintense signal involving the central pons with peripheral pontine sparing (red circle) and sparing of the corticospinal tracts (blue arrows) . This pattern of involvement results in a T2 “bat wing” or “trident” appearance on axial imaging.

EPM also causes T2 hyperintensity but in typically symmetric extrapontine locations (listed previously) ( Fig. 6.3 ).

Figure 6.3, Central pontine and extrapontine myelinolysis—axial FLAIR images at the level of the pons (A) and basal ganglia (B and C) demonstrating classic central pontine pattern of involvement, as well as multiple symmetric sites of extrapontine involvement including the basal ganglia, thalami, and external capsules in a patient with history of alcohol abuse presenting with hyponatremia.

Conventional CT and MR imaging findings typically lag behind the clinical manifestations of CPM. Although CT may show late low-attenuation changes in the central pons in some cases, serial MR imaging is the most appropriate method to evaluate patients with clinically suspected CPM. One case series of two patients proposes that T2 hyperintensity in extrapontine locations may predate central pontine T2 hyperintensity in some patients.

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