Central pontine myelinolysis: A pictorial review of MRI findings

A male patient, with known history of alcohol excess, was brought to the accident and emergency department in a drowsy and lethargic state. On admission, patient was hyponatraemic. Despite correction of serum sodium, the patient continued to deteriorate neurologically.

MRI brain showed diffuse pontine signal abnormality consistent with central pontine myelinolysis. The MRI brain showed hyperintense T2 signal changes in both sides of the pons and fairly symmetrical patchy high T2 signal changes in both thalami on axial FLAIR (Fig 1) and T2 (Fig 2). The piglet sign refers to the axial images resembling the face of a piglet (Fig. 3) with the temporal lobes representing ears, the carotid arteries as the eyes, the pontine signal as snout and the fourth ventricle as the mouth.

Central pontine myelinolysis (CPM) was first described as a disease affecting the alcoholics and the malnourished by Adams and colleagues in 1959[1]. Main characteristic features include loss of myelin and oligodendroglia in the central pons, with no inflammatory changes and normal vasculature. With extrapontine involvement, it is known as osmotic demyelination syndrome. Many studies have proven that rapid correction of hyponatraemia is a major risk factor associated with CPM.

The initial symptoms of CPM include reduced level of awareness, dysarthria and dysphagia, which begin to appear two to three days after rapid correction of hyponatraemia. Over the course of two weeks, patients may develop impaired cognition, impaired sensation, weakness and/or stiffness of limbs and difficulty in coordination. With non-specific clinical findings, a history of serum sodium derangement should raise the clinical suspicion.

The trigger for demyelination usually involves rapid correction of chronic hyponatraemia which causes the shift of water molecules, hence leading to destruction of the myelin sheath. It is a recognised complication of the treatment of patients with chronic hyponatraemia (hyponatraemia >48 h), particularly in those with alcohol excess.

Although many patients improve, some may develop a permanent disability, behavioural or intellectual disability or movement disorders such as parkinsonism.  No specific therapy has been shown to improve outcome of CPM.

Learning points
Recognition of the radiological piglet sign aids in the diagnosis of CPM. It is also important to note that radiological findings lag behind and may take up to 24 hours after the onset of clinical symptoms. Early imaging studies may not show any findings and hence repeat imaging may sometimes be required.

It is important to identify individuals at risk and follow guidelines for correction of hyponatraemia, to ensure that it is not too rapidly corrected.  The rate of rising of sodium should be kept below 10mmol/L per 24 hours if they have had hyponatraemia for more than 2 days [2].