An interdisciplinary team of researchers from the University of Rochester has used a new imaging technique to show how the human brain heals itself in just a few weeks following surgical removal of a brain tumour. The team found that recovery of vision in patients with pituitary tumours is predicted by the integrity of myelin, the insulation that wraps around connections between neurons, in the optic nerves. The study is published in the journal Science Translational Medicine.
Before the current study the medical community weren’t able to tell patients how much, if at all, they would recover their vision after surgery.
When pituitary tumours 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 tumours are surgically removed through the nose.
The team 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.
DTI measures how water spreads in tissue. The myelin insulation normally prevents water from spreading within the nerves, which would cause the nerves to malfunction. The researchers describe 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.
This particular patient population was 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 tumour on the brain is the same every time. Every pituitary tumour that grows large enough will compress the optic chiasm in more or less the same place, and removal of the tumour is often followed by a recovery of visual abilities.
The team state that these patients granted them a unique opportunity to understand human brain repair because the surgery is minimally invasive and patients recover very quickly after surgery. 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. Anything the medical community can learn about patients who go on to make a good recovery may help researchers to promote recovery from brain injury of any cause. At this time the visual system is the best understood circuitry in the human brain, and the lab has developed very precise ways of studying vision before and after surgery.
The team plan to develop prognostic methods in the context of the early visual pathway, and apply the same types of models to more complex systems in the brain, like language recovery after a stroke.
The team state that this research will create novel treatments to fix broken nervous systems. Harnessing new technologies to help the medical community understand how the brain repairs itself and restores function, and how researchers can accelerate that process will be one of the keys to restoring neurological function in a wide range of conditions, such as multiple sclerosis, stroke, and traumatic brain injury.
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