History -  50y/M Alcoholic with altered behaviour
Findings - Hyperintense signal involving Globus Pallidus on T1W images.  Hyperintense signal involving the midbrain on T2W and FLAIR images. Generalized cerebral atrophy
Diagnosis - osmotic myelinolysis

OSMOTIC MYELINOLYSIS

Abnormal T2 hyperintense signal is observed in the central pons with sparing of peripheral pontine parenchyma are suggestive osmotic myelinolysis. Abnormal T1 hyperintense signal in the globus pallidi is consistent with hepatocerebral degeneration.

Central pontine myelinolysis (CPM) is an acute demyelinating disease caused by rapidly fluctuating serum osmolality, resulting in symmetric demyelination within the central basis pontis.

Pontine lesions may be associated with extrapontine myelinolysis (EPM), which can symmetrically affect the cerebellar peduncles, caudate nucleus, putamen, frontal and temporal white matter, fornix, external and extreme capsules, claustrum, thalamus, subthalamic nucleus, internal capsule, amygdaloid nucleus, lateral geniculate nucleus, deep layers of the cerebral cortex, hippocampus and corpus callosum.

It is commonly associated with alcoholism, chronic nutritional deficiency and many other systemic diseases with electrolyte abnormalities. Some disorders seemingly predispose to this condition, such as liver failure, liver transplantation, pituitary tumor resection, severe burns, chronic renal failure, hemodialysis, lymphoma, carcinoma, malnutrition, severe bacterial infections, dehydration, electrolytic disorders (hyponatremia, hypernatremia,7 hyperglycemia, hypokalemia,6 diabetes), acute hemorrhagic pancreatitis and chronic alcoholism, among others

The term Osmotic Demyelination syndrome has been proposed because of the common association with rapidly corrected hyponatremia.

Clinical features: The symptoms of CPM include spastic quadriparesis, pseudobulbar palsy, changing levels of conciousness, and coma.

Pathological findings: The classical pontine demyelinating lesions appear as triangular or butterfly like areas of symmetric gray discoloration along the midline of basis pontis. A rim of normal white matter always surrounds the area of demyelination , and the tegmentum is usually not involved. There is  sparing of axons and the nerve cells of pontine nuclei without evidence of inflammation.

MRI : The regions of demyelination within the brain demonstrate high signal intensity on the long TR sequences, most prominently seen in the upper and middle pons. The pontine lesion is central, with surrounding rim of normal appearing pontine parenchyma and characteristic sparing of the corticospinal tracts.

Gadolinium contrast enhancement is occasionally seen in the periphery of the signal abnormality .

Extrapontine myelinolysis may occur even without pontine abnormality and characteristically involve the deep gray  matter.

In subacute and chronic phases, pontine lesions, mainly extrapontine ones, may become smaller in size and more clearly-defined.

Differential diagnosis include ischemia , Multiple sclerosis , encephalitis, toxic exposures and radiation therapy effects especially when extrapontine lesions are seen.

 

Apparently, there is no correlation between clinical signs and symptoms and the persistence of lesions on neuroimaging exams, as well as between extent of lesions on MRI and clinical severity of subsequent presentation or clinical outcome. Abnormal radiological results may be found despite evident clinical improvement or after resolution of clinical symptoms, or else, they may resolve despite the presence of clinical neurological symptoms.

 

Wernickes encephalopathy refers to an acute or subacute syndrome characterised by disorientation, gaze paralysis, ataxia and nystagmus. Lesions are typically found in the mammillary bodies, periaqueductal gray matter and hypothalamus. In the acute stages of the disease, there is hemorrhage, necrosis and edema, whereas in later stages atrophy of these regions particularly the mammillary bodies may be the sole manifestation. Dorsal median involvement of the thalamus with or without enhancement, is characteristic.

Marchiafava Bignami disease is a rare condition in which demyelination and necrosis primarily affect the corpus callosum and sometimes also involve the extracallosal regions. In the acute stage of the disease, the corpus callosum demonstrates diffuse swelling  and high signal intensity on the long TR images.