The central nervous system consists of the brain and spinal cord, surrounded by cerebrospinal fluid. The brain is protected by a ‘blood-brain barrier’ that controls which substances can pass in and out. HIV-infected cells in the brain called microglia have very long lives, and as long as virus remains in these cells, there is potential for it to escape and spread throughout the body. Further, HIV infection in the brain can lead to cognitive impairment and psychiatric disorders.

The only way to determine the amount of HIV in the brain is to measure viral load in the cerebrospinal fluid, which requires a lumbar puncture or ‘spinal tap’. People with a high blood viral load in the blood do not always have high levels in the cerebrospinal fluid, and vice versa.

HIV in the central nervous system may have a number of sources, including continued viral replication within the brain itself, virus that crosses the blood-brain barrier from elsewhere in the body and virus carried in infected immune cells. If HIV is coming from elsewhere, reducing the overall level of virus in the body should minimise the amount entering the brain. However, if new HIV is being produced in the brain, then treatments that cross the blood-brain barrier will be necessary to prevent ongoing viral replication.