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A52 year-old man was noted to have shoulder and hip weakness, more pronounced on the right following aortic valve repair.
MRI diffusion weighted images: Note the area of diffusion abnormality between the distributions of the middle and anterior cerebral arteries bilaterally. The area in between two vascular territories is known as a watershed. Watershed infarcts occur following reduced perfusion pressure, as often occurs during cardiac events or severe bleeding. The watershed territory between the MCA and ACA corresponds to the shoulder and hip girdle muscles on the motor homunculus, leading to the characteristic clinical deficits (patients' weakness is often referred to the man in the barrel distribution. Looking at the image on the right, the watershed on the left is much more affected than the right. In some cases, this may be secondary to superimposed carotid artery stenosis. Also, looking closely, there is an area of abnormality in the right MCA - PCA watershed.
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