The Helping Brain -- Visual Brain Areas Assist Inflamed Optic Nerves

Newswise — When the optic nerve is attacked by an inflammatory disorder called optic neuritis, certain brain areas reorganize themselves in response to the faulty visual information transmitted by the nerve, according to a study presented October 5, 2004, at the 129th annual meeting of the American Neurological Association in Toronto.

"We found that certain areas of the brain normally associated with more specialized higher visual processing are particularly active early after the onset of optic neuritis and are probably contributing to the recovery process," said lead author Ahmed Toosy, MD, of the Institute of Neurology in London.

Optic neuritis is an advantageous condition in which to study reorganization of brain pathways after injury because the condition is usually temporary. Symptoms ranging from mild blurring to complete loss of vision appear rapidly, within a few hours or days, often accompanied by pain. About half the time these represent an early stage of multiple sclerosis.

Vision typically begins to return about a week later, and most people regain normal vision. Detectable abnormalities remain, however, particularly in the way the optic nerve conducts visual information from the retina into the visual processing areas of the brain. This suggests that other parts of the nervous system have adapted to compensate for changes in the optic nerve.

Indeed, in an earlier study, Toosy and his colleagues found that optic neuritis had produced lasting changes in the visual part of the cortex, the outer layers of the brain that integrate information. Since this preliminary study examined patients after they had recovered, it was not clear whether the changes were a direct response to the inflammation or something that appeared later.

In the present study, the researchers have conducted a larger prospective study, using functional magnetic resonance imaging (fMRI) to examine how the brain behaves in the early stages of optic neuritis. Toosy and colleagues studied 20 optic neuritis patients during the period of inflammation and visual disturbance, as well as 1, 3, 6, and 12 months later.

By comparing activity in the higher visual centers of the brain with data from the optic nerve and information on improvements in vision, the researchers determined that already at the first examination the visual areas of cortex were adapting to altered input coming from the retina via the optic nerve.

"These results strongly suggest a genuine adaptive role for cortical reorganization early after optic neuritis," said Toosy. "We now need to examine in greater detail the roles of these areas."

The techniques used in this study could also be applied to other conditions such as stroke in which adaptive cortical reorganization is strongly suspected, said Toosey. The researchers also intend to investigate why it is that some patients tend to recover better than others after their optic nerves have been damaged.

"These and other studies employing modern imaging techniques are leading to an increase in our understanding of how the different parts of the brain adapt to injury to other parts of the central and peripheral nervous systems," said Robert P. Lisak, MD, of Wayne State University in Detroit, Michigan, and chair of the ANA's public information committee.

[abstract]
267. Adaptive Cortical Reorganization in Acute Optic Neuritis: A Longitudinal Functional MRI Study

Ahmed T. Toosy, Simon J. Hickman, Gordon T. Plant, Daniel Altmann, Gareth J. Barker, David H. Miller, and Alan J. Thompson; London, United Kingdom

Cortical reorganization associated with neurological recovery may be adaptive (contribute to clinical function) or non-adaptive. Distinguishing between these is important because adaptive plasticity may be more amenable to future therapeutic strategies. We investigated cortical plasticity by using visual fMRI and optic nerve MRI to study 20 acute optic neuritis patients at baseline, 1, 3, 6 and 12 months. We performed correlation analyses to investigate the relationships between fMRI activity, clinical function and optic nerve structure. Co-existing correlations between structure and clinical function may confound inferences about the adaptive nature of cortical reorganization, an issue not previously addressed in studies of cortical plasticity. We dealt with this by using a novel technique that modeled the fMRI response and optic nerve structure together with clinical function. This helped to determine the contribution fMRI made to clinical function after accounting for the relationship between optic nerve structure and clinical function. We found significant effects at baseline, bilaterally in the lateral occipital complexes, which are normally involved in higher order visual processing. These results strongly suggest a genuine adaptive role for cortical reorganization early after optic neuritis.