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Scientists used a new imaging technique to learn how the human brain heals itself in just a few weeks after a tumor was removed.

People with pituitary tumors suffer vision loss as the tumor grows. When the tumor is removed, they sometimes regain vision. The images showed the integrity of myelin in the optic nerves can predict vision recovery.

Myelin is the insulation that wraps around connections between neurons.

“Before the study, we weren’t able to tell patients how much, if at all, they would recover their vision after surgery,” says David Paul, an MD candidate at the University of Rochester and first author of the study published in the journal Science Translational Medicine.

When pituitary tumors grow large, they can compress the optic chiasm, the intersection of the nerves that connect visual input from the eyes to the brain. Nerve compression can lead to vision loss, which usually improves after these tumors are surgically removed.

Leaky insulation

Paul and his colleagues used a technique called diffusion tensor imaging (DTI) to show how changes in a particular bundle of nerve fibers relate to vision changes in these patients.

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“DTI measures how water spreads in tissue,” explains Bradford Mahon, the study’s senior author and an assistant professor in brain and cognitive sciences and in neurosurgery. “The myelin insulation normally prevents water from spreading within the nerves, which would cause the nerves to malfunction.”

Paul describes myelin damage by analogy to an insulated copper cable. In the human brain, DTI can measure the “leakiness of the insulation,” or how well myelin constrains the flow of water in brain tissue.

One DTI-based measurement, called radial diffusivity, can be used as an indicator of myelin insulation; an increase in this measure means there is less insulation to restrict the movement of water within a nerve.

In their study, the researchers found that inadequate insulation resulted in poorer visual ability in patients.

‘Unique opportunity’

Paul says this particular patient population is unique because unlike other diseases such as stroke, trauma, or multiple sclerosis, these patients have a problem that can be treated by surgery and the effect of the tumor on the brain is the same every time.

Every pituitary tumor that grows large enough will compress the optic chiasm in more or less the same place, and removal of the tumor is often followed by a recovery of visual abilities.

“These patients grant us a unique opportunity to understand human brain repair because the surgery is minimally invasive and patients recover very quickly after surgery,” says Edward Vates, director of the Pituitary Program in the neurosurgery department and co-author of the study.

Other types of brain injury

The measurements established in the study provide a new way to measure the structural integrity of nerve fibers, and may ultimately be applicable across the full range of brain diseases and injuries.

“There’s a lot of variability in how people recover from brain injuries,” says Mahon. “Anything we can learn about patients who go on to make a good recovery may help us to promote recovery from brain injury of any cause.”

He adds that the visual system is the best understood circuitry in the human brain, and his lab has developed very precise ways of studying vision before and after surgery.

“If we can develop our prognostic methods in the context of the early visual pathway, then we can apply the same types of models to more complex systems in the brain, like language recovery after a stroke,” says Mahon.

Additional researchers from the University of Rochester and the University of Michigan Medical Center collaborated on the project, which was funded by the National Institute of Neurological Disorders and Stroke and the National Eye Institute.

Source: University of Rochester