Human immunodeficiency virus (HIV) infection is associated with various neurological manifestations in 40 to 70% of acquired immune deficiency syndrome (AIDS) patients, including primary effects of HIV, opportunistic infection, neoplasia, and vascular disease. Infections can be focal or diffuse. Cerebral toxoplasmosis is the most common mass lesion, HIV encephalopathy is the most common cause of neurological disease, and cytomegalovirus encephalitis (CMV) is the most common opportunistic viral infection found in AIDS patients. Other common infections include progressive multifocal leukoencephalopathy (PML), cryptococcal meningitis, and neurosyphilis. CMV generally has an appearance similar to primary HIV on magnetic resonance imaging (MRI) (diffuse white matter hyperintensity on T2-weighted MR sequences). However, it rarely presents as a peripherally enhancing mass, or as a necrotizing meningoencephalitis with associated subependymal enhancement, a finding that can also be seen in advanced herpes zoster, and less commonly with other herpes viruses. Another common central nervous system (CNS) lesion is primary CNS lymphoma, which has been shown to be associated with Epstein-Barr virus in almost 100% of cases.

Based on MRI appearance, the lesions may be divided into

• Diffuse white matter disease HIV encephalopathy (HIVE) and diffuse CMV encephalitis
  
• Patchy white matter disease HIVE, PML, herpes encephalitis
  
• Focal mass with enhancement Toxoplasmosis, lymphoma, Cryptococcus, mycobacteria, CMV, neurosyphilis, bacterial and fungal abscesses (Nocardia, Bartonella, Rhodococcus)
  
• Focal mass without enhancement Cryptococcus (gelatinous pseudocysts, cryptococcomas, enlarged perivascular spaces), toxoplasmosis, and atypical primary CNS lymphoma. The lack of enhancement may be due to severely depressed cell-mediated immunity.
  
• Focal enhancement without mass Toxoplasmosis, cerebral infarct, viral encephalitis, bacterial cerebritis, PML (generally nonenhancing, but occasional faint peripheral enhancement; enhancement may be related to inflammatory reaction or may occur due to improved immune response following therapy [IRIS-immune reconstitution inflammatory syndrome] may occasionally be diffuse or speckled).
  
• Focal lesion, without mass effect or enhancement PML (no mass effect) and diffusely infiltrating primary CNS lymphoma (subtle mass effect)
  
• Meningitis/meningeal disease HIV meningoencephalitis, Cryptococcus, mycobacterial infection (basal meningitis), neurosyphilis, metastatic systemic lymphoma
  
• Ventriculitis CMV, herpes virus (especially advanced herpes zoster virus), Cryptococcus
  
• Cerebrovascular disease Tuberculosis (TB)/ cryptococcal meningitis can result in arteritis/ thrombosis and cerebral infarcts, neurosyphilis (vasculitis), CMV, Varicella zoster virus