Osmotic demyelination syndrome refers to acute demyelination seen in the setting of osmotic changes, typically with the rapid correction of hyponatraemia. It is the more recent term replacing central pontine myelinolysis, recognising that extrapontine structures can also be affected, previously known as extrapontine myelinolysis.
The initial description of central pontine myelinolysis by Adams et al. in 1959 2 was entirely in a population of chronic alcoholics, and certainly, this scenario is common. Since then it has been increasingly recognised in other patient groups, but usually in the setting of rapidly corrected electrolyte disturbance:
- chronic alcoholics
- chronically debilitated patients
- transplant recipients
Clinically osmotic demyelination syndrome presents in a biphasic pattern. The first phase is usually attributable not to the demyelination but rather to the inciting electrolyte abnormality, with patients being acutely encephalopathic. Following rapid reversal of this abnormality, the patient transiently improves before progressing onto the classic osmotic demyelination syndrome features 2-3 days later. When pontine involvement is prominent, clinical features consist of:
- spastic quadriparesis
- pseudobulbar palsy
- changes in levels of consciousness
Although the exact mechanism is still uncertain, it is known that oligodendroglial cells are most susceptible to osmotic stresses, leading to their demise. It is not surprising that the distribution of osmotic myelinolysis, therefore, parallels the distribution of these cells.
Histologically, osmotic demyelination syndrome is characterised by intramyelinitic splitting, vacuolisation and myelin sheath rupture 3. After many days, macrophages can be identified.
CT may demonstrate low attenuation crossing the midline in the lower pons. However, CT assessment of the skull base can be difficult due to beam hardening artifact and, if available, MRI is preferred.
The earliest change is seen on DWI with restriction in the lower pons. This is seen within 24 hours of the onset of quadriplegia 3. This same region demonstrates eventual high T2 signal and later low T1 signal. The T1 and T2 changes may take up to two weeks to develop. This region has a classic trident shaped appearance. Occasionally gadolinium enhancement is also demonstrated, just as in the acute phase of multiple sclerosis (MS) plaque. The peripheral fibres (ventrolateral longitudinal fibres), as well as the periventricular and subpial regions, are typically spared.
Similar appearances are seen in other parts of the brain: basal ganglia, midbrain and subcortical white matter.
Signal characteristics of affected region include:
- T1: mildly or moderately hypointense
- T2: hyperintense, sparing the periphery and corticospinal tracts
- PD: hyperintense
- FLAIR: hyperintense
- DWI: hyperintense
- ADC: signal low or signal loss
- T1 C+ (Gd): usually there is no enhancement, but some authors reported that it may occur 5-6
Affected regions may demonstrate initial high uptake followed by subsequent low uptake with 18-FDG.
General imaging differential considerations include:
- 1. Miller GM, Baker HL, Okazaki H et-al. Central pontine myelinolysis and its imitators: MR findings. Radiology. 1988;168 (3): 795-802. Radiology (abstract) - Pubmed citation
- 2. Adams RD, Victor M, Mancall EL. Central pontine myelinolysis: a hitherto undescribed disease occurring in alcoholic and malnourished patients. AMA Arch Neurol Psychiatry. 2000;81 (2): 154-72. Pubmed citation
- 3. Ruzek KA, Campeau NG, Miller GM. Early diagnosis of central pontine myelinolysis with diffusion-weighted imaging. AJNR Am J Neuroradiol. 2004;25 (2): 210-3. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 4. Venkatanarasimha N, Mukonoweshuro W, Jones J. AJR teaching file: symmetric demyelination. AJR Am J Roentgenol. 2008;191 (3_supplement): S34-6. doi:10.2214/AJR.07.7052 - Pubmed citation
- 5. Juergenson I, Zappini F, Fiaschi A, Tonin P, Bonetti B. Teaching neuroimages: neuroradiologic findings in pontine and extrapontine myelinolysis: clue for the pathogenesis? Neurology. 2012 Jan 3;78(1):e1-2. doi: 10.1212/WNL.0b013e31823ed0b5. Pubmed citation
- 6. Jonathan Graff-Radford, Jennifer E. Fugate, Timothy J. Kaufmann, Jay N. Mandrekar, and Alejandro A. Rabinstein. Mayo Clin Proc. Nov 2011; 86(11): 1063–1067. Pubmed citation