The brains of patients who’ve received double hand transplants can recreate lost neurological control systems, according to new brain data from a French surgical team.
The doctors extensively analyzed two patients, of the six total who had the surgery, for evidence of how they learn to reuse their hands.
It’s well-known that after a limb has been amputated, a body’s nervous system reorganizes the neurons that formerly controlled the region to take on other tasks. But the reverse process — what happens when hands get reattached — can only be studied by, well, reattaching limbs.
“It’s not as complex as it might seem,” Jon Kaas, a neuroscientist at Vanderbilt University who reviewed the study for the Proceedings of the National Academy of Sciences. “The main thing is to try to reconnect the severed nerves that go into the hand.”
Transplantation of complex structures like hands and faces is far from old hat for surgeons. It’s only in the last decades that transplanting “composite” body parts made of multiple types of tissue has become possible. When a surgeon transplant a liver, it’s mainly liver cells that are transplanted. Transplanting a hand requires reconnecting bone, muscle, tendon and — most importantly — nerves.
The French doctors measured the regions of the brain known to control the movement of muscles and found that to a certain extent, the patients’ brains had detected the hands and could use them years after the transplants.
“Our findings show that newly transplanted muscles can be recognized and integrated into the patient’s motor cortex,” they write.
That’s because after surgeons carefully find all the tiny nerves they can from the former stump and attach them to the donated hands, the nervous system does a remarkable thing. It begins to regenerate nerves at the rate of about 1 millimeter a day. Over a period of months, control and sensation returns to the patients and the hands begin to act and feel like their own appendages.
The nervous system, even without hands, retains some of the structures that receive and send messages to the hands. In fact, it’s that residual knowledge that leads to the phenomenon of “phantom limbs,” in which the brain mistakenly tries to use the hand or limb. But when a hand is reattached, the brain’s ability to control and feel the hands kicks back in.
“When the hand is transplanted and the original connections, to some extent, are being re-established, you’re reversing that process of plasticity and going back to something closer to the original organization of the brain,” said Kaas.
Though transplanting composite tissue makes for a far more complex and difficult surgery, the worst risk remains the same as for any transplant — that the body will reject the foreign tissue. For that reason, all hand-transplant recipients will have to take immunosuppresants for the rest of their lives, and cope with the increased risk of infection that entails.
The two hand transplantees who were studied for the new paper have not had transplant or life-endangering complications, but they have experienced differing rates of both sensory and motor control. For reasons that are unknown and might be purely coincidental, both patients were right-handed but experienced better control of the transplanted left hands.
Kaas said not to make too much of that, though, until a larger sample set is available to study.
“It could depend on just technical things,” he said. “It could depend on how well the hand is connected or how much damage there was done to the nerves and muscles.”
On the other hand, there could be some unknown reason for what the French researchers found.