So let’s move on to the juicy part: the practical problems. Remember when you were a child and you’d swap Barbie and Ken’s heads just for fun? Maybe threw in some extra glue to make sure it held? Well, transplanting a human head onto a new body is absolutely nothing like that.
Brain blood supply
One major difference between the human head and that of a doll is the extensive circulatory system which transports oxygenated blood from the lungs to the brain and the other tissues in the area. Now, here’s a boring anatomical fact that didn’t make the TED talk: the lungs are in the chest. And to make matters even worse, the brain can only go without oxygen for about five minutes. Afterwards, the lack of oxygen will permanently damage it.
But Canavero is a genius, so of course he thought of this. His plan is to cool down the body (and the head) to ~10°C. To be fair, lowering body temperature slows down the metabolism and allows the brain to resist for a longer time without oxygen. This is a technique called therapeutic hypothermia and it’s actually used during some cardiac surgeries. Of course, it carries quite a few risks: it increases the chance of infection, it affects the way medication works, it induces changes in the cardiovascular system etc. But carefully considering the risks affecting your patient and not subjecting them to unnecessary ones are minor details when you’re revolutionizing medicine.
The bigger problem is that it’s not magic, so it doesn’t slow down time by as much as you want. Studies estimating the safe duration of oxygen deprivation during deep therapeutic hypothermia (i.e. between 10°-20°C) based on mathematical calculations have concluded that anything above 30-40 minutes would lead to severe and irreversible neurological damage. In other words, one would have at most 40 minutes to detach the head from its original blood supply, maybe do something about the spinal cord while they’re there, and reattach it to the new blood supply. This alone would be enough to discourage any reasonable person from claiming that head transplants are a year or two away. But Sergio Canavero is an optimist.
In his “paper”, he writes that 45 minutes under deep hypothermia leads to no neurological damage, “with a slight increase on approaching the hour”. More than that, once the body has been cooled down, it needs to be rewarmed. Studies suggest that one should aim for an increase of 0.2°-0.25°C per hour in order to avoid complications such as swelling of the brain and seizures. Yet Canavero’s optimism triumphs over this as well: he doesn’t plan to lower the temperature of the donor body and instead he hopes this body would rewarm the head “in minutes”. Granted, we don’t know what the effects on the brain would be, because no one has studied anything similar before, but hey, if we cross our fingers hard enough, it’s definitely going to work, no?
But wait, there’s more. You know how once the oxygenated blood has reached your brain and dropped off the oxygen there, it also needs to go back to the lungs to pick up more oxygen? Well, the one head transplant attempt in monkeys found a small problem here: even though the veins in the neck were sutured back together, the blood couldn’t flow out of the head anymore because there was a big scar which developed there. Canavero’s solution? Throw in an irrelevant sentence after mentioning the problem, then conclude he will use the exact same method that didn’t work in the monkey for reconnecting the veins in the human transplant. Told you he was an optimist.
Spinal cord reattachment
There are probably other problems regarding the head blood supply, but I’m no vascular surgeon (and neither is Canavero), so let’s move on to his area of expertise (you know, with him being a neurosurgeon and all): reattaching the spinal cord. You would think that, given the millions of nerves in the spinal cord, as well as its importance for movement, the plan would be incredibly sophisticated. But that would mean you have not been paying attention. Canavero is all about simplicity. In his own words: “You cut the spaghetto, you apply PEG, and boom.”
Confused? He probably is too.
Let’s try to unpack. PEG stands for polyethylene glycol. PEG is a versatile substance with many uses, including as the basis for many laxatives, as an excipient in certain medications, as part of cosmetics, and even as a preservative for historical artifacts. As great as PEG is, however, it does not magically glue spinal cords back together. There are some animal studies which highlight the potential of this substance for promoting axonal regeneration in the spinal cord (and thus fusion of the severed parts), but, as usual, the full story is more complicated.
For one, PEG needs to be optimized for this task. It’s not enough to take the substance by itself, throw a handful at the spinal cord, and hope for the best. One has to consider how this substance interacts with the biochemical environment in which it is placed and adjust its physical and mechanical properties accordingly. As you can imagine, no such studies have been conducted in humans and it is unclear whether findings from animal studies would translate well, in particular because there are quite a few differences in terms of axonal regeneration capabilities between humans and animals.
Additionally, even in animal studies, the spinal cord fusion is not complete, with only some axons fusing together. Of course, this means that the animals do not regain full range of motion. Although Canavero casually bypasses the problem, there is something even more concerning: it is unclear whether this partial fusion could lead to long-lasting pain or any other side effects. So while PEG might prove to be useful in spinal cord fusion, we are looking at years, if not decades, of research including both animal studies and human clinical trials.
Seemingly randomly, Canavero is also intent on using electrical stimulation of the spinal cord. I must admit that I am not entirely certain why, but based on current legitimate literature, two possibilities are apparent: to promote axonal fusion or to improve locomotion.
Regarding the first one (i.e. promoting axonal fusion), there are some animal studies which suggest that applying PEG together with an electric field at the site of spinal cord injury yields better results in terms of fusion. Again, this needs further research and validation in humans.
The second possibility, that of improving locomotion, is actually a very active and promising area of research and deserves a post by itself. But to keep it brief: this field focuses on developing electrical stimulators which can be implanted in the spinal cord, below the injury, and which can restore movement for people with spinal cord injuries. One prominent laboratory is that of Grégoire Courtine, a French neuroscientist at the Swiss Federal Institute of Technology Lausanne (EPFL). In the last decade, him and his team have made several majors breakthroughs. They have discovered that successful stimulation delivered by such implants needs to preserve proprioceptive information (= information about limb posture in space). Additionally, they have made improvements regarding the positioning of the electrodes in the spine. And last, but not least, they have integrated this knowledge into designing implants which were subsequently tested clinically. The results speak for themselves.
While brilliant, it is unclear how this method fits into Canavero’s approach and why he needs it in the first place. If his spinal cord fusion works, patients shouldn’t remain paralyzed. If it doesn’t work, then what’s the point of the head transplant?
The brain death problem
And the issues don’t stop there. Before the procedure even begins, there is a “small” hurdle: where does the donor body come from? While there is an ethical aspect to this question, which we will address below, here we are still focused on the technical part. Canavero claims it will “probably” come from brain dead people. But “brain dead” does not equal “patiently waiting for a new head”.
As grim as it sounds, brain dead means dead. And when the brain dies, even though blood oxygenation is maintained through mechanical ventilation, the body gets the “not alive anymore” signal and begins to pack up. That translates into things such as: immune system hyperactivation, increased blood pressure, altered hormone levels, changes in body temperature, and altered lung function. And if that person was an organ donor, all of these changes are problematic and need to be managed through difficult and critical medical management protocols which ensure organ viability. Some of the organs might not survive enough to be transplanted. Some might not have been viable to begin with, because people don’t usually die from being healthy. To give you an idea of what doctors are working with: lungs are only viable in 10-20% of the cases.
But if you want to transplant the entire body, then everything needs to work well. As we’ve already outlined above, the chances of that happening aren’t too great to begin with. Add to that the chronic shortage of organ donors, the fact that the donor and recipient have to match with respect to skin tone and body size, as well as the fact that transplant organs tend to go to people who actually need them and not to fringe neurosurgeons trying to enact their favourite book, and you’ll have another hit on the “why head transplantation is stupid” Bingo card.
Even if everything discussed above is resolved, there are a couple of “minor” issues remaining. Things such as immunorejection, potential mismatches between the brain and the body in terms of fine movement control, and psychological issues related to seeing one’s head on a foreign body. To be clear, there is nothing minor about these problems, but given the magnitude of those above, they really seem like the least of Canavero’s worries.
In terms of immunorejection, it is a known fact that transplanted organs are attacked by the immune system, so patients have to be on life-long medication which suppresses immune system function. Not only does this have a number of adverse effects, but sometimes, the organ is slowly rejected anyway and has to be replaced. Based on intention, the organ is the body, but based on size, what if the immune system of the body ends up rejecting the head?
Regarding control mismatch issues, it is also a known fact that the body and the brain develop in tune with each other. For example, a violinist and a software engineer will have different groups of highly developed muscles, and this will be reflected in their brains. Can a brain that has been trained a certain way its entire life take over a completely differently developed body? As everything else in Canavero’s plan, unclear. And what happens if it can’t take over? What would the functional and psychological impact of that be? Also unclear.
Finally, limb transplants have made it clear that adapting to a new body part is psychologically challenging. Even patients who underwent extensive psychological screening and were offered professional support could sometimes not get used to the foreign limb. So there is no reason to doubt that this will happen in the case of head transplants. The solution when it comes to transplanted limbs is fairly straightforward: remove them. But what can one do in the case of a body?