Adnkronos (Adnkronos) – One in four people is at risk of suffering a stroke during their lifetime. In Italy, well over 100 people experience a new event each year, and over a million live with its consequences; stroke sequelae are the leading cause of permanent disability worldwide.
Among those who survive, three out of four have some form of disability. These numbers have returned to the forefront in recent days, on the occasion of World Stroke Day, celebrated on October 29th, and they raise a crucial question even after the emergency phase: how much recovery will be possible? Why do some people recover so well, while others, despite starting from seemingly similar injuries, recover much less? A possible answer comes from a study by the IRCCS San Raffaele in Rome, published in the journal 'Stroke,' which proposes examining a parameter previously considered primarily a factor of "disruption": the variability with which the brain responds to identical stimuli.
The study opens a new perspective: what until now may have seemed simply 'noise' in the brain's responses could become a signal of its ability to reorganize.
The research, conducted at the Department of Neuroscience and Neurorehabilitation and the Brain Connectivity Laboratory at San Raffaele, in collaboration with eCampus University and the San Raffaele Institute in Sulmona, was led by engineer Lorenzo Nucci, a researcher at the Roman institute, and was scientifically coordinated by Paolo Maria Rossini, director of the Department of Neuroscience and Neurorehabilitation, and Domenica Le Pera, head of the Neuromotor Rehabilitation Unit at the San Raffaele IRCCS.
The researchers studied 77 people with their first stroke, assessing them between the seventh and fourteenth day after the event and again after two months of intensive daily rehabilitation. For a baseline, the same was done on 10 healthy volunteers of the same age and gender. Using transcranial magnetic stimulation, a painless and non-invasive technique, they observed the response of the nerve pathways that control the hand muscles from the brain (if the nerve pathway is functioning, the hand responds with a small twitch, known as an evoked motor potential). Traditionally, the amplitude of the motor response is primarily considered; greater amplitude is generally associated with greater integrity of the nerve pathways. However, the San Raffaele team also analyzed another element: how much that response changes in amplitude and shape from one stimulus to the next.
People who exhibited more variable brain responses in the first few weeks after a stroke subsequently recovered a greater portion of the theoretically recoverable movement. Conversely, more uniform and "monotonous" responses were associated with a lower capacity for recovery. "For many years, variability between one response and the next was treated as something to be reduced or eliminated from analysis," explains Lorenzo Nucci. "Our results suggest, however, that precisely those oscillations may contain important biological information. A brain that can produce different responses to identical, successive stimuli appears to retain a broader repertoire of neural circuits that can be used to reorganize itself after the damage caused by a stroke. Looking ahead, this means trying to understand not only how much damage has occurred, but how much recovery capacity is still available." Another finding concerns the cerebral hemisphere not directly affected by the injury. Despite being structurally intact, even the "healthy" hemisphere showed less extensive and less variable responses in stroke survivors than in healthy volunteers. This data reinforces the view that stroke is not simply a case of damage limited to a single area of the brain, but rather an event capable of altering the functioning of the entire organization of the brain's neural networks.
"Today, we are increasingly better able to intervene in the acute phase of stroke, but the next major challenge is understanding what will happen next," Rossini emphasizes. "Two people may present seemingly similar clinical conditions, yet follow very different recovery trajectories. Being able to identify not only the damage but also the brain's functional reserve in the first few weeks after a stroke could profoundly change the way we plan rehabilitation. The goal is not to predict the future with a mathematical formula, but to have increasingly reliable tools to offer each individual the most appropriate path. Knowing early on who has the greatest potential for recovery is one of the main challenges in neurorehabilitation."
The study, which received support from the Ministry of Health, does not yet offer a test ready for routine clinical use. The authors emphasize the need to confirm the results in larger populations, with longer observation periods and evaluating the evolution of brain variability even in the later stages of rehabilitation.
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