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.

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 ).

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

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.

Temporal Evolution: Overview

Variable imaging features may be evident in osmotic demyelination, depending on when the process is imaged. In particular, the acute, subacute, and chronic imaging characteristics differ. Furthermore, some variable imaging features, including the diffusion-weighted imaging (DWI) and postcontrast imaging characteristics, may be present or absent according to phase.

Acute Phase

Several reports have suggested that DWI images might facilitate the early diagnosis of CPM. However, the exact frequency and onset in time of the appearance of these abnormal findings in relation to symptoms is still unclear.

When present, DWI signal hyperintensity may begin to appear within 24 to 72 hours after onset of symptoms. ADC signal varies from hypointensity (restricted diffusion) when intramyelinolytic cytotoxic edema predominates to hyperintensity if vasogenic edema related to myelin destruction predominates. These competing processes result in variable DWI and ADC signal profiles that may differ between patients and may differ during the course of the disease in an individual patient as the balance of cytotoxic and vasogenic processes shifts.

Several case reports, such as that of Ruzek et al., have shown that early DWI changes are a common finding in CPM/EPM. However, others have reported that these signal changes may not regularly precede tissue changes described on conventional MRI sequences. One report suggests that DWI can be normal in the acute stage of CPM, even within 1 week after the onset of symptoms. Graff-Radford et al. reported that 21% of their patients showed no abnormalities on early MRI; however, all patients had characteristic pontine signal abnormality on T2-weighted images on repeat imaging. This finding is in agreement with a more recent report that early MRIs were normal in 25% of cases.

Early in the diagnosis of CPM/EPM, faint hyperintensity on T2-weighted and fluid-attenuated inversion recovery (FLAIR) imaging in characteristic locations may be the only finding. T1-weighted images usually show normal to hypointense T1 signal. Postcontrast enhancement due to blood-brain barrier disruption may also occur at this stage.

Subacute Phase

Multiple case reports, such as that of Cramer et al., show development of increasing T2 hyperintensity within the central pontine region a week after the development of symptoms, with variable expansion of the pons. DWI images may still show increased signal with corresponding decreased signal on the ADC map. Alternatively, follow-up imaging may show normalized or elevated ADC values, suggesting the disappearance of cytotoxic edema in the later phases despite persistent increased signal on T2-weighted images due to vasogenic edema and/or gliosis. Similarly, enhancement should resolve in the subacute period.

Chronic Phase

Usually, the degree and extent of pontine hyperintensity on T2-weighted images decrease a few weeks after the onset of symptoms. Case reports have shown that the milder the signal on T2-weighted images beyond the subacute phase, the better the long-term outcome.

The classic configuration of “trident” or “bat wing” pontine T2 hyperintense signal abnormality with sparing of the corticospinal tracts and peripheral pons persists, although the lesion is smaller in size compared with initial imaging due to volume loss. Corresponding T1 hypointensity without enhancement typically also persists.

These findings are summarized in Table 6.1 , as well as in two MRI cases of CPM ( Figs. 6.4 and 6.5 ).

TABLE 6.1

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