The pathologic basis of the dementia appears to be a diffuse and multifocal rarefaction of the cerebral white matter accompanied by scanty perivascular infiltrates of lymphocytes and clusters of a few foamy macrophages, microglial nodules, and multinucleated giant cells (Navia, Chos, Petito et al). Evidence of CMV infection may be added, but accumulating virologic evidence indicates that the HIV encephalopathy is a result of direct infection with HIV. Which of these changes corresponds most closely to the presence and severity of dementia has not been settled. The pathologic changes in HIV dementia are actually not as uniform as portrayed here. In one group of patients, there is a diffuse pallor of the cerebral white matter, most obvious with myelin stains, accompanied by reactive astrocytes and macrophages; the myelin pallor seems to reflect a breakdown of the blood–brain barrier.
In another form of this process, referred to in the past as “diffuse poliodystrophy,” there is widespread astrocytosis and microglial activation in the cerebral cortex, with little recognizable neuronal loss. In yet other patients, small or large perivascular foci of demyelination, like those of postinfectious encephalomyelitis, are observed; the nature of this diffuse white matter lesion is not understood. These forms of pathologic change may occur singly or together and all correlate poorly with the severity of the dementia. Progressive multifocal leukoencephalopathy also occurs in patients with AIDS and is simulated by the primary white matter encephalopathy. This important entity is discussed in the following text.
HIV Myelopathy, Peripheral Neuropathy,
A myelopathy, taking the form of a vacuolar degeneration that bears a marked pathologic resemblance to subacute combined degeneration (due to vitamin B12 deficiency), is sometimes associated with the AIDS dementia complex; or the myelopathy may occur in isolation, as the leading manifestation of the disease (Petito et al). This disorder of the spinal cord is discussed further in Chap. 42.
AIDS may also be complicated by several forms of peripheral neuropathy, as discussed in Chap. 43. A distal, symmetrical, axonal polyneuropathy, predominantly sensory and dysesthetic in type has been the most common neuropathic pattern. The HIV virus has been isolated from the peripheral nerves and ganglia. In fact, this stands as the first proven viral polyneuritis in humans (zoster being more a ganglionopathy). In other patients, a painful mononeuropathy multiplex occurs, seemingly related to a focal vasculitis, or there may be a subacute inflammatory cauda equina syndrome (a polyradiculitis) that is usually caused by an accompanying CMV infection (Eidelberg et al). Cornblath and colleagues have documented the occurrence of an inflammatory demyelinating peripheral neuropathy, of both the acute (Guillain-Barré) and chronic types, in otherwise asymptomatic patients with HIV infection. Most of these patients had a mild pleocytosis in addition to an elevated CSF protein content. Typically, patients with inflammatory demyelinating neuropathy have recovered—either spontaneously or in response to plasma exchange—suggesting an immunopathogenesis similar to that of the Guillain-Barré syndrome. Cornblath and associates suggest that all patients with inflammatory demyelinating polyneuropathies should now be tested for the presence of HIV infection. A ganglioneuronitis is also known to occur. Facial palsy is being reported with increasing frequency as a feature of HIV; its relationship to the generalized polyneuritis of AIDS is uncertain.