INTRODUCTION TO HIV-ASSOCIATED NEUROCOGNITIVE DISORDERS (HAND)

HIV infection is associated with both cognitive impairment and mood disorders. Whilst HIV infection itself imposes a significant psychological burden, injury to the central nervous system (CNS) can lead to neuropsychiatric complications, similarly to other neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases. For example, mania may be the presenting symptom of HIV infection in patients without known mental illness. On the other hand, depressive mood affects more than 30% of HIV-infected individuals even in the current era of combination antiretroviral therapy (cART) ^{1 2}.

The driver of CNS injury is a combination of HIV-mediated toxicity and persistent host immune response [3]. Viral proteins including tat, gp120, Nef and host pro-inflammatory products including TNF-α, platelet-activating factor, IL-1, IL-6, CD40L and monocyte chemoattractant protein-1 create a vicious cycle that can be maintained with minimal viral replication. The introduction of cART addressed effectively the viral burden, thus reducing viral-associated neurotoxicity and in part also reduced host immune activation. Altogether, these effects of cART have altered the clinical landscape of HIV-associated neurocognitive disorders (HAND) but with little effect on the coexistence of mood disorders ^{1 2}.

PERSISTENCE OF HAND IN CART ERA

As Ferrara et al. report in this chapter, the cART era has ushered in a significant decline in the incidence of HIV-associated dementia (HAD) (the more severe end of the disease spectrum), but milder forms of HAND, such as mild and asymptomatic cognitive impairment, continue to persist and affect about 50% of infected individuals. One possible explanation for the persistence of mild cognitive impairment is that delayed treatment of HIV infection may cause some irreversible neuronal damage. Brain imaging studies are in part consistent with this hypothesis. Chang et al. [4] showed improvement of cognitive performance and brain metabolites in HIV patients with cognitive impairment after starting cART, but the metabolites did not fully normalize. Furthermore, more recent brain imaging studies have shown that nadir CD4 count is one of the best predictors of brain atrophy [5]. However, the full effects of earlier versus later cART initiation on neurocognitive impairment and recovery have yet to be determined. In response, studies such as START (Strategic Timing of Antiretroviral Treatment) aim to assess the timing of cART initiation in relation to neurocognitive performance. Another possibility is that ongoing minimal viremia may be sufficient for chronic immune activation, leading to CNS injury. In this regard, the clinical trial CIT2 investigating whether CNS-targeted cART could improve neurocognitive functioning was terminated early after an interim analysis showed that completion of the study would have had a low probability of finding a significant difference between cART CNS-targeted and non-CNS targeted interventions [6].