The opportunistic infections that are associated with HIV do not cause ADC. Instead, HIV causes ADC. However, researchers do not know how the virus damages the brain cells.

HIV may affect the brain through several mechanisms. The HIV proteins may damage nerve cells directly or indirectly.

Many researchers suggest that HIV damages the neurons indirectly. According to some researchers, HIV either infects or activates macrophages and microglia. These cells then produce toxins that can stimulate a series of reactions, which instruct neurons to kill themselves (abnormal neuronal apoptosis). The infected macrophages and microglia also appear to produce chemokines and cytokines (proteins that mediate and regulate immunity, inflammation and hematopoiesis), which can also affect neurons, as well as astrocytes. The affected astrocytes, which normally protect and nurture neurons, may now end up damaging them.

While the progression of dysfunction is variable, it is regarded as a serious complication and, if left untreated, can be fatal.

The condition causes cognitive impairment, motor dysfunction, speech problems and behavioral changes. The main characteristic of ADC is cognitive impairment, which results in mental slowness, trouble with memory and poor concentration. Symptoms of motor dysfunction include a loss of fine motor control, poor balance and clumsiness. Behavioral changes may include apathy (absence of emotion or enthusiasm), lethargy and diminished emotional responses and spontaneity.

In addition, many patients may also experience agitation, anxiety, fatigue, depression or other psychiatric manifestations. Mania and psychosis have been described as presenting symptoms or complications of ADC.

General: Patients who experience ADC symptoms should consult a qualified neurologist who can make a proper diagnosis. The neurologist will first rule out alternative diagnoses. This requires a neurological examination, brain scans (magnetic resonance imaging or computerized tomography scan) and lumbar puncture to evaluate the cerebrospinal fluid.

No single test is available to diagnose patients with ADC. Patients are diagnosed with ADC based on the following criteria: marked impairment of at least two ability domains of cognitive function (such as memory, concentration or mental slowness); the cognitive impairments interfere significantly with daily activities, and they present for at least one month and do not meet the criteria for delirium; and there is no evidence of another pre-existing disease that is associated with dementia (like a CNS infection or cerebrovascular disease).

Computerized tomography (CT) or magnetic resonance imaging (MRI) scan: A CT scan or MRI of the brain provides a detailed, three-dimensional picture of the brain. These tests can detect brain atrophy (shrinkage) that occurs in ADC patients.

Position emission tomography (PET): Position emission tomography (PET) or single-photon emission computed tomography (SPECT) scans can detect abnormalities in metabolism in the brain that may be associated with ADC.

Lumbar puncture: A lumbar puncture (spinal tap) may be performed to detect abnormalities in the cerebrospinal fluid (CSF). During the procedure, a needle is inserted in the lower back (usually between the third and fourth lumbar vertebrae) and a sample of CSF is removed from the spinal canal. This clear fluid is produced in normal cavities in the brain called ventricles (which are visible on CT scan and MRI). The fluid surrounds the brain and spinal cord, serving as a cushion to protect these structures. The CSF can be tested for various abnormalities that are related to dementia symptoms.

Electroencephalography (EEG): During an electroencephalography (EEG), several electrodes are attached to the scalp. The electrical activity of the brain can be measured. In the later stages of ADC, the electrical activity (which appears as waves) is slower than normal.

Neuropsychological testing: Neuropsychological testing is the most accurate way to assess a patient's cognitive abilities. During the test, the patient answers questions and performs tasks that have been prepared for this purpose. A neurologist, psychologist or other trained professional administers the test. It provides an accurate assessment of cognitive abilities such as memory, attention, orientation to time and place, speech and abilities to follow instructions. Reasoning, abstract thinking and problem solving also are tested.