Enlarged cisterna magna - within normal variant

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My son has had several MRIs and recently had a MRA (brain and neck

arteries). The recent report mentions a large cisterna magna but notes that

it is within 'normal variance'. This has been noted on his prior brain scans

as well. Enlarged cisterna magna can be caused by genetic abnormalities as

well as types of infection. We have had the CDs (total & percent) done

recently but do not have the results back yet. If there is an infection in

the brain, will this be evident with the CD amounts (4, 8, 16, 19, 56)? His

viral testing has come back negative for everything including

Cytomegalovirus which has been shown to influence the cisterna magna.