www.lesswrong.com/posts/dtTpKiRa87cgasJG8/i-can-t-believe-it-both-is-and-is-not-...
2 corrections found
As the CT scan should theoretically have shown something in the case of encephalitis
A normal CT does not rule out encephalitis. In herpes simplex encephalitis especially, CT is less sensitive than MRI and is often normal early in the illness.
Full reasoning
A normal head CT is not enough to conclude that encephalitis would have been visible.
- The NCBI StatPearls review on Herpes Simplex Encephalitis says CT is "less sensitive than MRI" and "often shows no abnormalities in early infection." It adds that CT changes may take 3 to 4 days to appear.
- The IDSA encephalitis guidelines likewise note that even MRI findings may initially be normal or remain normal during illness. If MRI can be normal early, a normal CT certainly does not exclude encephalitis.
So the inference that encephalitis "should" have shown up on CT is incorrect; early imaging can be normal, and diagnosis typically relies on a broader workup such as CSF testing and MRI rather than a normal CT alone.
2 sources
- Herpes Simplex Encephalitis - StatPearls - NCBI Bookshelf
A computerized tomography (CT) scan is often the first test performed... It is less sensitive than MRI and often shows no abnormalities in early infection. It may take 3 to 4 days to manifest the changes in the temporal or frontal lobes.
- Management of Encephalitis: Clinical Practice Guidelines by the Infectious Diseases Society of America
Although MRI is useful for detection of early changes in encephalitis, it does not necessarily assist in differentiation of a specific etiology of encephalitis, and the findings may initially be normal or remain normal during the course of illness.
surely encephalitis would have shown up on the scan.
That is not how encephalitis imaging works: a normal CT, especially early on, does not exclude encephalitis.
Full reasoning
This statement is incorrect because early encephalitis can have normal imaging, especially on CT.
- The NCBI StatPearls review on Herpes Simplex Encephalitis states that CT is less sensitive than MRI and often shows no abnormalities in early infection.
- The IDSA encephalitis guidelines add that even MRI findings may initially be normal or remain normal during illness.
So a normal scan does not mean encephalitis would necessarily have been visible.
2 sources
- Herpes Simplex Encephalitis - StatPearls - NCBI Bookshelf
A computerized tomography (CT) scan is often the first test performed... It is less sensitive than MRI and often shows no abnormalities in early infection.
- Management of Encephalitis: Clinical Practice Guidelines by the Infectious Diseases Society of America
Although MRI is useful for detection of early changes in encephalitis... the findings may initially be normal or remain normal during the course of illness.