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AJR Teaching File: Symmetric Demyelination

¹ All authors: Department of Radiology, Derriford Hospital, Level 6, Derriford Rd., Plymouth PL6 8DH, United Kingdom.

Received November 1, 2007; accepted after revision December 21, 2007.

 
Address correspondence to N. Venkatanarasimha (nandashettykv{at}yahoo.com).

Keywords: central pontine myelinolysis • CT • demyelination • MRI

Clinical History

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A 40-year-old man presented with acute onset walking difficulty, slurred speech, and slight blurring of vision. Other relevant clinical history included chronic alcoholism and poor nutrition. Clinical examination revealed mild lower limb incoordination, dysarthria, and bilateral partial abducent nerve palsy. The blood tests for full blood count, renal functions (sodium, 142 mmol/L; potassium, 4 mmol/L; urea, 4.6 mg/dL; creatinine, 85 µmol/L), blood glucose (6.1 mmol/L), serum osmolality (285 mosm/kg), and liver function tests (albumin, 41 g/L; globulin, 25 g/L; bilirubin, 12 µmol/L; aspartate aminotransferase, 30 U/L; -gluta myltransferase, 45 U/L; and alkaline phosphate, 142 U/L) were within normal limits. The findings on the admission CT of the head prompted further assessment with MRI of the brain.

Radiographic Description

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Unenhanced cranial CT showed a well-defined triangular low-density focus in the central pons (Fig. 1A). Cranial MRI was performed. Standard axial T1-weighted, T2-weighted (Fig. 1B), and fluid-attenuated inversion recovery (Fig. 1C) sequences; coronal T1-weighted and fluid-attenuated inversion recovery sequences; and gadolinium-enhanced axial (Fig. 1D) and coronal (Fig. 1E) T1-weighted sequences were obtained. These images revealed a triangular symmetric pontine signal abnormality on the axial images and prolongation of T1 and T2 relaxation times. After the administration of gadolinium, there was no enhancement of the pontine abnormality. In addition, sparing of the ventrolateral pons and the tegmentum was seen. No extrapontine abnormality was shown.

View larger version (138K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —40-year-old man with chronic alcohol abuse and acute onset walking difficulty, mild dysarthria, and blurring of vision. Unenhanced CT scan of head shows well-defined low-density focus in central pons.

View larger version (188K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —40-year-old man with chronic alcohol abuse and acute onset walking difficulty, mild dysarthria, and blurring of vision. Cranial T2-weighted MR image (TR/TE, 4,480/99) shows triangular area of high signal intensity in central pons and sparing of tegmentum and ventrolateral pons.

View larger version (171K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C —40-year-old man with chronic alcohol abuse and acute onset walking difficulty, mild dysarthria, and blurring of vision. Axial fluid-attenuated inversion recovery image (8,630/111) shows trident-shaped pontine high-signal-intensity abnormality.

View larger version (161K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D —40-year-old man with chronic alcohol abuse and acute onset walking difficulty, mild dysarthria, and blurring of vision. Axial T1-weighted gadolinium-enhanced image (533/8.1) shows triangular central pontine hypointensity with no enhancement.

View larger version (162K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1E —40-year-old man with chronic alcohol abuse and acute onset walking difficulty, mild dysarthria, and blurring of vision. Coronal T1-weighted gadolinium-enhanced image (553/8.1) shows nonenhancing central pontine low-signal abnormality.

Top Clinical History Radiographic Description Differential Diagnosis Diagnosis Commentary Objective Conclusion References

Diagnosis

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The diagnosis in this patient was central pontine myelinolysis.

Commentary

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Central pontine myelinolysis is a syndrome of symmetric osmotic demyelination involving the pons. The classic appearance is a trident- or triangle-shaped central pontine hyperintensity on T2-weighted axial images, as seen in our patient. Similar histologically symmetric lesions in extrapontine locations, including the white matter of the cerebellum, thalamus, globus pallidus, putamen, and lateral geniculate body, have been described and are termed "extrapontine myelinolysis" [1].

After the original description in alcoholic patients by Adams et al. [2], central pontine myelinolysis has been reported to occur most commonly in patients with electrolyte abnormalities, in particular hyponatremia that is rapidly corrected or overcorrected. Focal symmetric demyelination in the central nervous system may also be precipitated by aggressive correction of a hyper- or hypoosmolar state and has been associated with high rates of morbidity and mortality. In addition, malnutrition, hepatic failure, liver transplantation, sepsis, malignancy, chronic renal failure with dialysis, and severe burns have all been described in association with acute osmotic demyelination syndrome [3].

The clinical manifestations of myelinolysis vary considerably depending on the degree of pontine involvement and the presence of extrapontine lesions; they range from asymptomatic or spastic quadriparesis and pseudobulbar palsy or complete locked-in syndrome to a rapidly fatal outcome. The condition was originally thought to be uniformly fatal, but now survival accompanied by varying degrees of neurologic residuum to complete recovery is well documented. In our patient, no significant improvement in neurologic signs was seen during his 3-week hospitalization, and he was discharged to a rehabilitation hospital.

Pathologically, disruption of the blood–brain barrier after a rapid increase in serum sodium concentration is considered to play a critical role. The disruption of the blood–brain barrierhistologically shows loss of oligodendroglia and reactive astrocytosis. Oligodendroglial cells are most susceptible to central pontine myelinolysis-related osmotic stresses, with the distribution of changes paralleling the distribution of oligodendroglial cells that normally embed large neurons in the central pons, thalamus, putamen, lateral geniculate, and other extrapontine sites [1]. It has also been postulated that either the grid arrangement of the descending and crossing tracts in the pons or perhaps its vulnerable vascularity is responsible for the peculiar focal nature of the lesion. The transverse pontocerebellar fibers are most frequently involved, followed by long rostrocaudal tracts. Neurons and axons are relatively preserved, differentiating this process from a central pontine infarct [4]. Furthermore, usually no inflammatory reaction is associated with osmotic demyelination, differentiating this process from acute disseminated encephalomyelitis and multiple sclerosis, which are characterized by marked perivascular inflammation [3].

CT of the brain often fails to show the early changes of central pontine myelinolysis because it results from subtle alteration in the tissue water content, which is further obscured by artifact in this region of the brain [5]. However, sometimes CT shows low-attenuation changes in the pons [6], as seen in our patient. MRI of the brain has greater sensitivity to the early increase in the tissue water content [7] and to demyelination in general [8], and its multiplanar capability helps in better anatomic localization. The typically described MRI findings are of a homogeneous, well-defined region in the pons showing symmetric hypointensity on T1-weighted images, hyperintensity on T2-weighted and FLAIR images, and no associated mass effect. The extent and distribution of the abnormality may vary. It commonly involves the basis pontis and extends from the pontomedullary junction into the midbrain [9], with characteristic sparing of the tegmentum. In more severe disease, almost the entire central pons may be involved with only a thin rim of normal signal around it. Peripheral contrast enhancement may occur but is not a prominent imaging feature. It is this distribution in combination with the clinical features that provides the basis for the diagnosis because the signal characteristics we have described are not specific.

Similar signal intensity changes can be seen in conditions such as acute disseminated encephalomyelitis and multiple sclerosis, but the lesions in these conditions are asymmetric and may enhance with gadolinium. In addition, clinically the time course of these conditions differs. Vasculitis and infarction can have similar signal changes on MRI, but those conditions almost always involve the cerebral white matter and rarely are limited to the central pons. A brainstem neoplastic process such as glioma is unusual in a case like ours in the absence of a mass effect on imaging; it is also unlikely to have an acute presentation.

Conventional MRI findings may lag behind the clinical manifestation of central pontine myelinolysis. In such cases, diffusion-weighted imaging (DWI) has been shown to be useful in the early diagnosis of central pontine myelinolysis [10]. In the acute phase, restricted diffusion is shown on DWI with corresponding low apparent diffusion coefficient (ADC) values. This appearance on DWI and the ADC values suggest the presence of cytotoxic edema in acute central pontine myelinolysis. Follow-up imaging may show recovered DWI signals and ADC values, suggesting the disappearance of cytotoxic edema in the later phase.

Guo et al. [11] reported a case of central pontine myelinolysis after liver transplantation and described the appearances on MR spectroscopy and perfusion imaging. They showed that in the acute phase the N-acetyl acetate–creatinine ratio decreased and the choline–creatinine ratio increased. It has been postulated that the neuronal loss and reactive gliosis contribute to the decrease in N-acetyl acetate and increase in choline levels, respectively. In the later stages, a decrease in both the N-acetyl acetate and choline levels is suggestive of further neuronal loss. MR perfusion imaging may show increased cerebral blood volume, reflecting a high metabolic activity due to an increase in cell number and activity [11]. In the later stages of gliosis, perfusion decreases with increasing axonal injury.

Complete radiologic resolution of the pontine changes on follow-up MRI [12] has been described.

Objective

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The educational objective of this article is to show a bilaterally symmetric pontine demyelinating lesion with a classic appearance on MRI.

Conclusion

Top Clinical History Radiographic Description Differential Diagnosis Diagnosis Commentary Objective Conclusion References

 
Osmotic pontine myelinolysis is a neurologic disorder that commonly affects patients with chronic alcoholism and those with an electrolyte imbalance. The central pons is the most preferred site and the tegmentum is characteristically spared. MRI may classically show a trident-shaped central pontine signal intensity abnormality and helps in the early diagnosis of this potentially fatal condition.

References

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1. Lampl C, Yazdi K. Central pontine myelinolysis. Eur Neurol 2002; 47:3 –10[CrossRef][Medline]
2. Adams RD, Victor M, Mancall EL. Central pontine myelinolysis: a hitherto undescribed disease occurring in alcoholic and malnourished patients. Arch Neurol Psychiatry 1959;81 : 154–172[Abstract/Free Full Text]
3. Adams RD, Victor M. Principles of neurology, 5th ed. New York, NY: McGraw-Hill,1993
4. Graham DI, Lantos PL, eds. Greenfield's neuropathology, 7th ed. London, UK: Arnold/Oxford University Press, 2002
5. Sterns RH, Riggs JE, Schochet SS Jr. Osmotic demyelination syndrome following correction of hyponatremia. N Engl J Med1986; 314:1535 –1542[Abstract]
6. Miller GM, Baker HL, Okazaki H, Whisnant JP. Central pontine myelinolysis and its imitators: MR findings. Radiology1988; 168:795 –802[Abstract/Free Full Text]
7. Ormernd IEC, Miller DH, McDonald WI, et al. The role of NMR imaging in the assessment of multiple sclerosis and isolated neurological lesions: a quantitative study. Brain 1987;110 :1579 –1616[Abstract/Free Full Text]
8. Ragland RL, Duffis AW, Gendelman S, Song PM, Rabinowitz JG. Central pontine myelinolysis with clinical recovery: MR documentation. J Comput Assist Tomogr 1989;13 : 316–318[Medline]
9. Dana DeWitt LD, Ferdinando S, Buonanno J, et al. Central pontine myelinolysis: demonstration by nuclear magnetic resonance. Neurology 1984;34 : 570–576[Abstract/Free Full Text]
10. Ruzek KA, Campeau NG, Miller GM. Early diagnosis of central pontine myelinolysis with diffusion-weighted imaging. AJNR2004; 25:210 –213[Abstract/Free Full Text]
11. Guo Y, Hu J-H, Lin W, Zheng K-H. Central pontine myelinolysis after liver transplantation: MR diffusion, spectroscopy and perfusion findings. Magn Reson Imaging 2006;24 :1395 –1398[CrossRef][Medline]
12. Lilje CG, Heinen F, Laubenberger J, Krug I, Brandis M. Benign course of central pontine myelinolysis in a patient with anorexia nervosa. Pediatr Neurol 2002;27 : 132–135[CrossRef][Medline]

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