Intracranial infections – 4 – Immunocompromise

Immunocompromised patients have special considerations for infection of the brain, including HIV encephalopathy, toxoplasmosis, cryptococcus, and progressive multifocal leukoencephalopathy (PML).

This lecture is the fourth in a series of 5 about imaging intracranial infection and covers special considerations in immunocompromised patients. The series of videos will cover:
1) General considerations
2) Diffuse infections
3) Focal infection
4) Immunocompromised patients
5) Other considerations

HIV encephalopathy is a result of direct infection of the white matter in the brain by the HIV virus. It is bilateral, symmetric, and tends to progress over time. There is usually no enhancement.

Toxoplasmosis is the most common opportunistic infection of the brain in HIV patients. Common imaging findings include multifocal masses and enhancement, often involving the basal ganglia. The “target sign”, or bullseye like appearance of the enhancing lesions, is common. The imaging appearance of toxoplasmosis overlaps a great deal with lymphoma, which tends to be more solidly enhancing and involves the periventricular white matter more. Often a treatment trial for toxoplasmosis is begun with short term follow-up imaging to see if the patient improves.

Cryptococcus is also a common brain infection in immunocompromised patients. It’s most common manifestation is enlargement of the perivascular spaces of the basal ganglia, or gelatinous pseudocysts. This typically does not have much, if any, postcontrast enhancement.

Finally, progressive multifocal leukoencephalopathy (PML) is an infection caused by reactivation of a virus (JC virus) in the brain parenchyma in the setting of immune suppression, either because of HIV or immunosuppressive medications. It is manifested by bilateral, subcortical, asymmetric white matter abnormalities without enhancement. Treatment is restoration of the immune system, but outcomes are poor.

The level of this lecture is appropriate for radiology residents, radiology fellows, and trainees in other specialties who have an interest in neuroradiology or may see patients with CNS infections.

Other videos on the intracranial infection playlist are found here.

Intracranial infections – 3 – Focal Infections

Focal infections are those infections of the brain which are walled off either within the brain parenchyma or in the extra-axial space, such as subdural or epidural abscess.

This lecture is the third in a series of 5 about imaging intracranial infection and covers focal brain infections. The series of videos will cover:
1) General considerations
2) Diffuse infections
3) Focal infection
4) Immunocompromised patients
5) Other considerations

Abscess in the brain, regardless of location, is characterized by hyperintensity on diffusion weighted imaging. There is often mass effect, surrounding edema, and a peripherally enhancing fluid collection. Abscess can arise from local infection, such as a surgery or sinusitis, or can occur from hematogenous spread.

Ventriculitis is a highly morbid complication of intracranial abscess, as can be found when pus spills into the ventricle. Sinus thrombosis is also a potential complication of intracranial abscess.

One key feature which separates the diffusion restriction of abscess from lymphoma is that the abnormal DWI is in the center of a peripherally enhancing collection, whereas in lymphoma it is the enhancing portion itself which is restricted.

The level of this lecture is appropriate for radiology residents, radiology fellows, and trainees in other specialties who have an interest in neuroradiology or may see patients with CNS infections.

Other videos on the intracranial infection playlist are found here.

Intracranial infections – 2 – Diffuse infections

Diffuse infections are those infections of the brain which affect large regions of the brain or affect the brain diffusely. This includes meningitis, encephalitis, and ventriculitis.

This lecture is the second in a series of 5 about imaging intracranial infection and covers diffuse brain infections. The series of videos will cover:
1) General considerations
2) Diffuse infections
3) Focal infection
4) Immunocompromised patients
5) Other considerations

Meningitis is infection centered in the surfaces of the brain, particularly the pia and subarachnoid space. This can be caused by bacteria, viruses, or other unusual pathogens like tuberculosis or fungi. Imaging findings include incomplete FLAIR suppression and leptomeningeal enhancement. Basilar meningitis is a special subset of meningitis affecting the spaces around the brainstem and skull base. It is special because it is more likely to be an unusual pathogen. Sarcoidosis and leptomeningeal metastases can also mimic an infectious meningitis.

Encephalitis is similar to meningitis, although it is centered in the brain parenchyma. There is a great deal of overlap between these conditions and they can often be seen together (meningoencephalitis). Compared to meningitis, encephalitis is even more likely to be viral. The medial temporal lobes are commonly involved, and when they are a diagnosis of herpes encephalitis should be considered. This encephalitis caused by HSV can be rapidly debilitating or fatal. Encephalitis can also be autoimmune or inflammatory, mimicking infection.

Finally, ventriculitis is infection within the CSF of the ventricles themselves. This is often seen by abnormal FLAIR or diffusion in the ventricles, sometimes with periventricular enhancement. This can be from a primary pathogen with sparing of the parenchyma or as a complication of meningitis or abscess. Ventriculitis also has somewhat poor prognosis.

The level of this lecture is appropriate for radiology residents, radiology fellows, and trainees in other specialties who have an interest in neuroradiology or may see patients with CNS infections.

Other videos on the intracranial infection playlist are found here.

Intracranial infections – 1 – General principles

Intracranial infections are a common consideration in neuroradiology. Patients often present with altered mental status, often with a fever or systemic symptoms. The radiologist needs to know how to proceed in those instances, both in terms of what types of imaging to perform and how imaging findings might relate to the systemic findings.

This lecture is the first in a series of 5 about intracranial infection and covers general principles behind imaging the patient with suspected intracranial infection. The subsequent 4 videos cover additional more specific considerations.

The two main imaging techniques used in imaging intracranial infection are CT and MRI. CT is a rapid screening test which can detect edema, hemorrhage, midline shift, and prominent masses. It is available in the vast majority of hospitals (at least in the US, but also many abroad), and doesn’t require much compliance from the patient. It also requires no screening for metallic implants and other MRI considerations.

MRI is much better at evaluating intracranial infection however. Key sequences include DWI, FLAIR, T2, and T1 postcontrast. DWI is ideal for seeing pus and infarct. FLAIR is ideal for imaging edema. T2 can give you some unique clues, as a few diseases such as lymphoma, tuberculosis, and abscess can be T2 dark. Many infections also cause breakdown of the blood brain barrier, which shows up as enhancement on postcontrast MRI.

Lumbar puncture and echocardiogram are the two main systemic tests which may help you determine if an intracranial infection is present. LP directly assesses for infections agents and inflammatory cells in the CSF, while echocardiogram indirectly assesses for bacterial endocarditis, which can spread to the CSF through emboli.

The level of this lecture is appropriate for radiology residents, radiology fellows, and trainees in other specialties who have an interest in neuroradiology or may see patients with CNS infections.

Other videos on the intracranial infection playlist are found here.