Morphometric Comparisons of Optic Nerve Axon Loss in Acquired Immunodeficiency Syndrome

Axonal degeneration and diminution of the axonal population in the optic nerve have been documented in aging and in various neuro-ophthalmic conditions. We applied morphometric techniques to the postmortem examination of optic nerves obtained from patients with acquired immunodeficiency syndrome. Twelve optic nerves (eight from patients with AIDS and four from age-matched control eyes) were stained with paraphenylenediamine and morphometrically analyzed with a computer-assisted image and measurement system. Degeneration was often severe and was scattered throughout all of the AIDS-affected optic nerves. In the AIDS-affected optic nerves, the mean axonal population was markedly lower than the mean obtained from normal optic nerves (880,000 vs 1,507,000). Despite the approximate 40% loss of axons, mean axonal diameters were not markedly different, suggesting that no particular class of axon was especially susceptible to AIDS-associated degeneration. The extent and pattern of axonal loss in optic nerves of patients with AIDS suggest that the changes may not only be secondary to damage at the retina, but may reflect an AIDS-associated primary optic neuropathy.

Tumor necrosis factor-alpha (TNF-alpha)-induced optic neuropathy in rabbits

Both in vitro and in vivo studies have implicated a role for tumor necrosis factor-alpha (TNF alpha) in various demyelinating diseases, including HIV-related encephalopathy. To investigate whether intravitreal TNF alpha can induce optic nerve axonal damage in a rabbit eye model, fifteen rabbit eyes were initially injected with TNF alpha (final concentrations: 2U, 20U, and 200U respectively) and studied at varying time intervals for up to 24 weeks post-injection, using light and electron microscopy. Control optic nerves (no injection or diluent injection only) had normal myelinated axons and glia; the myelinated regions, neural retina, retinal glia and vasculature of control retinas were normal. In TNF alpha-exposed optic nerves, intact, degenerating and demyelinated axons were interspersed. Astrogliosis was present, particularly from 8 weeks p.i. and was noted up to 24 weeks. Oligodendrocytes were not severely affected in TNF alpha-exposed optic nerves, and activated macrophages or microglia were not obvious. Axonal degeneration was visible among the more superficial myelinated fibers in TNF alpha-exposed retinas however the neural retina glia were unaffected. These observations suggest that the axonal degeneration induced in TNF alpha-exposed rabbit optic nerves over a 24 week period was most likely related to direct effects of TNF alpha on optic nerve axons, and not primarily due to anterograde degeneration from retinal lesions. In-so-far as neurological pathology in general, and optic nerve degeneration in particular, has been described in AIDS, and TNF alpha levels may be elevated in this disease, it is of great clinical significance that TNF alpha has the capacity to mediate neuronal or axonal injury. If so, strategies to block or inhibit TNF alpha can be pursued for treatment for the neurological symptoms of AIDS.

Parallel thalamic activation of the first and second somatosensory areas in prosimian primates and tree shrews

In Tupaia belangeri and Galago senegalensis, microelectrode recordings immediately after ablation of the representation of the forelimb in the midportion of the first somatosensory area, S-I, revealed that all parts of the second somatosensory area, S-II, remained highly responsive to cutaneous stimuli. In this way, prosimian primates, close relatives of simian primates, and tree shrews differ markedly from monkeys in which S-II is deactivated by comparable ablations, and resemble such mammals as cats and rabbits in which S-II also remains highly responsive following ablations in S-I. Thus, it appears that the generalized mammalian condition is that S-I and S-II are independently activated via parallel thalamocortical pathways. A dependence of S-II on serial connections from the thalamus to the S-I region and then to S-II apparently evolved with the advent of anthropoid primates, and may be present only in monkeys and perhaps other higher primates.

AIDS-related optic neuropathy: a histological, virological and ultrastructural study

BACKGROUND

Clinical and histopathological evidence of optic nerve axonal loss has been reported in AIDS patients without retinitis. The study was carried out to investigate the possible involvement of HIV-infected cells in the development of optic nerve degeneration.

METHODS

Optic nerves were obtained from eight AIDS patients and four normal controls. These nerves were morphologically and immunohistochemically analyzed. Additionally, using PCR amplification techniques, the retina and optic nerve samples obtained from three HIV-seropositive patients and one control were examined for the presence of HIV and cytomegalovirus antigens.

RESULTS

We noted various stages of axonal degeneration in the optic nerves obtained from AIDS patients in whom there was an absence of retinal findings. Characteristic glial changes involving hypertrophic astrocytes, vacuolated oligodendrocytes, and mononuclear phagocyte series cells were also seen in the AIDS optic nerves. HIV DNA was present in at least four of five optic nerves but in only one of five retinas. Control specimens were each negative for all cytomegalovirus and HIV antigens.

CONCLUSIONS

Degeneration in the optic nerve may be mediated by HIV-infected macrophages rather than by direct viral infection of neurons. Axonal degeneration due to AIDS at the level of the optic nerve can occur independently of retinal infection.

Identifying simulators of cognitive deficit through combined use of neuropsychological test performance and event-related potentials

Event-related potential (ERP) methods for identifying malingering of cognitive deficit assume that elements of brain/cognitive functioning are not under a person's direct control, whereas neuropsychological methods assume that malingered cognitive deficit will present differently than true impairment in terms of level or pattern of errors on tests of cognitive function. Two studies were conducted to examine the combined use of neuropsychological and ERP methods for identifying malingering because of the potential independence of these approaches. All normal control participants performing at their best level (n = 25) were correctly classified by both ERP and neuropsychological methods. All participants simulating cognitive deficit (n = 35) were correctly classified on the basis of a positive outcome on either the neuropsychological or the ERP method. Results suggest that the neuropsychological and psychophysiological measures of malingering that were studied contribute non-redundant information in the classification of simulators.

Preliminary morphometric study of tumor necrosis factor-alpha (TNF alpha)-induced rabbit optic neuropathy

Intravitreal injections of tumor necrosis factor-alpha (TNF alpha) induce morphological changes in the optic nerve suggesting a primary TNF alpha-induced optic neuropathy. A preliminary morphometric analysis of axonal loss and degeneration in optic nerves of a TNF alpha rabbit eye model was conducted. Fifteen rabbits were injected in one eye only with TNF alpha (final doses: 2 U, 20 U and 200 U respectively), and optic nerve cross-sections were examined using a semi-automated image analysis system, at varying time intervals for up to 24 weeks post-injection. Control optic nerves (uninjected or injected with diluent only) were also assessed. Intact, degenerating and demyelinated axons were visible in TNF alpha-exposed optic nerves. Considerable variation was observed between TNF alpha-exposed optic nerves, however the intact axon density consistently decreased in the 200 U TNF alpha group, although not in the 2 U and 20 U groups. Overall, TNF alpha-exposed optic nerves had significantly more degenerating profiles compared to control optic nerves. These observations suggest a dose-dependent axonal loss and degeneration associated with the morphological changes induced in rabbit optic nerves following intravitreal injection of TNF alpha.

Optic nerve morphometry following axonal degeneration from SAIDS in rhesus monkeys

In order to further study AIDS (acquired immunodeficiency syndrome) related neuropathologic findings, specifically progressive diffuse leuko-encephalopathy (PDL) and optic neuropathy, ten macaque monkeys (Macaca mulatta) were infected with simian immunodeficiency virus (SIV), observed for varying periods, and then sacrificed and tissue prepared for analysis. A marked difference from human AIDS pathology was found: in all monkeys, there was significant peripheral axonal dropout, as opposed to relatively little dropout in the central areas of the nerves (t stat. = -5.9, p < .001). In those monkeys infected with SIV for over one year, the axonal loss in the periphery was also greater than in the central optic nerve (t stat. = -5.03, p < .001); monkeys infected with SIV for less than one year had slightly less peripheral than central loss (t stat. = -4.5, p = .001). When compared with humans, however, it was found that the overall loss of axons was less (15% in monkeys vs. up to 45% in humans). There was also a lack of discernible retinal pathology, such as cotton wool spots, in the monkey tissue.