ORLANDO, FLA. — Neuroimaging can make a big difference in the care of AIDS patients, who are vulnerable to several opportunistic diseases, one expert said at the annual meeting of the American Society of Neuroimaging.
James G. Smirniotopoulos, M.D., chairman of radiology at the Uniformed Services University of the Health Sciences in Bethesda, Md., noted that AIDS patients are vulnerable to both infectious and neoplastic opportunistic diseases. Neuroimaging is indicated in any AIDS patients who manifest:
▸ Mental status changes.
▸ Neurologic deficits.
▸ Seizures (focal or generalized).
▸ Headaches.
▸ Meningeal signs.
There are some cautions, though. AIDS patients typically have depression and other psychological conditions as a result of their situation; these should be separated out from genuinely neurologic causes.
In addition, in a substance-abuse population, seizures can be the result of substance withdrawal. Lastly, when AIDS patients complain of headaches, their immune status can determine the type of imaging used. For patients with very suppressed CD4 counts (less than 200 cells/μL), get a CT scan. If the count is mildly suppressed (greater than 200 cells/μL), get an MRI.
Once AIDS patients have been imaged, Dr. Smirniotopoulos and his colleagues triage them based on whether they have normal imaging results, atrophy, lesions without mass effect, or mass lesions.
He warned that several conditions can spuriously give an appearance of atrophy, including malnutrition, dehydration, steroid use, and long-term renal dialysis.
AIDS encephalopathy can also appear as atrophy. On images, this condition typically appears as bilateral white matter volume loss that can be symmetrical or not. “This is a disease process that is destructive of the parenchyma, but there's a lot of debate about what's really going on,” Dr. Smirniotopoulos said.
Progressive multifocal leukoencephalopathy (PML) is a lesion that has geographic signal and density abnormalities but without a mass effect. This lesion usually does not show any effect when enhanced using gadolinium. PML is a demyelinating white matter disease. On images, look for big geographic lesions that come right up to the gray matter and stop, Dr. Smirniotopoulos said.
The lesions are the result of infection with the ubiquitous JC papovavirus. As many as 70% of adults have antibodies to this virus, and almost 20% of patients with AIDS express antigens. PML is responsible for about 4% of AIDS deaths. Mortality is high in these patients. In the past, most patients with PML died within 4–6 months of diagnosis. Zidovudine and other antiretroviral drugs have improved survival only somewhat.
The two most common mass lesions seen on images in patients with AIDS are from primary infections and CNS lymphomas—with toxoplasmosis being the most common of the infections. “Toxoplasmosis is still probably what we think about first and foremost when an AIDS patient has a mass lesion,” Dr. Smirniotopoulos said.
If toxoplasmosis is suspected, try empiric therapy for 3 weeks. If any of the lesions fail to respond, it's time to get a biopsy, he said. The infection results primarily in paracentral brain abscesses. “Abscesses in toxoplasmosis tend to be relatively deep rather than being peripheral,” he said. The abscesses can be in gray or white matter. Abscesses are round, uniformly convex with smooth, thin walls and are often multifocal.
It can be difficult to distinguish between a toxoplasmosis infection and lymphoma. “Lesions that involve the deep white matter and the deep gray matter at the same time might be CNS lymphoma or toxoplasmosis, and the problem is that both of these diseases occur in immunosuppressed patients,” Dr. Smirniotopoulos said.
The good news is that in most cases—roughly five out of six—primary CNS lymphoma has distinguishing features on imaging that allow diagnosis. Lymphoma is a small round tumor with densely packed cells that result in hyperattenuation on a noncontrast scan.
CNS lymphomas are usually primary, non-Hodgkin's, and B cell. Primary lymphoma is usually paracentral, hugging the ventricles. “Very commonly, [CNS lymphomas are] going to be hypointense to gray matter on multiple pulse sequences,” Dr. Smirniotopoulos said.