A boy who researchers called F.S. had a severed nerve in his arm. They did not mention his age, and his case is buried deep in a study published in 1936. But that severed nerve exposed an unusual phenomenon that has been a curiosity for many scientists since.
The nerve, called the median, is connected to the thumb, index and middle fingers. So, for F.S., those three fingers were numb. Scientists Thomas Lewis and George Pickering watched F.S. closely, hoping to learn more about the nerve’s function, and they found something bizarre and unexpected. When immersed in water, the fingers with feeling wrinkled, but the three numb fingers remained smooth. In fact, Lewis and Pickering wrote, wrinkling in F.S.’s entire palm was “almost sharply separated by a line” from his wrist to the base of his middle finger. After a few months, the boy healed naturally and full feeling returned to his hand.
The study, published in 1936 in the journal Clinical Science, brought a new mystery to the surface. Common perception is that wrinkling is a local effect on the skin, unconnected to rest of the body. But if that was the case, why would a nerve that stretches down the length of the arm be involved? Decades of research have since provided some answers, and some scientists believe these answers could help doctors diagnose diseases that disrupt the nervous system.
Einar Wilder-Smith, a neurologist at the National University of Singapore, has done research looking into the wrinkling effect over the past decade. His research suggests that finger wrinkling relies on nerve endings that entangle sweat glands and blood vessels in our fingers. “We always think of nerve fibers as motor function or sensory function,” says Wilder-Smith. “But there are many in the background that go unnoticed, and possibly only get noticed by wrinkling.” Many nerve fibers sit on key crossroads for blood, small arteries called arterioles which lead to tinier blood vessels. These nerves either tell blood vessels to constrict or allow them to relax depending on signals like temperature.
Palms are also riddled with sweat glands, especially in fingers. These sweat glands can act as a two-way street; not only does sweat come out, but water can go in. When water intrudes on a sweat gland, the water comes in contact with nerve fibers. The nerve fibers are collectively jostled from their sleep and begin to fire all at once, telling the arterioles to constrict. The arterioles constrict as a group and for a stretch of time, leaving an empty space between the blood vessels and the skin. The skin on the finger tips then slowly sinks inward, creating a set of folds like a collapsed tent. The end result is the prune-like fingertips that fascinate children and scientists alike.
So, unwrinkled fingers could mean that a major nerve has been entirely disabled, as was the case with F.S. “Cutting nerve is like cutting a power supply to your socket,” said Wilder-Smith. But it could also signify a less direct kind of nerve damage, such as the corrupting effect of a larger disease such as leprosy or diabetes. Because of this, some doctors are already using the wrinkle-effect as a diagnostic tool.
Andre van Rij, a surgeon with the University of Otago in New Zealand, sometimes uses the test took look for nerve damage from diabetes in patients’ legs and feet. Since sweating is also reliant on working nerves, he said, the classical way to look for general nerve damage is to see if feet sweat. The method is called the iodine and starch test; doctors swab iodine on the patient’s skin, let it dry, sprinkle on some white starch powder, then put the foot in a plastic bag. If the foot sweats, the iodine turns the starch dark blue, and doctors know there is nerve damage. But van Rij says it’s easier to look for wrinkling on feet. “It’s easy just putting someone’s foot in warm water, and you see if they don’t wrinkle,” he said.
Another study shows some correlation between finger wrinkling and the central nervous system — the brain and spinal cord. A 2001 study out of Tel Aviv University in Israel compared finger wrinkling in 18 Parkinson’s disease patients with nine healthy patients. The researchers counted the wrinkles on their research subjects’ fingers one at a time, and found that while those suffering from Parkinson’s still wrinkled, they wrinkled less than people without the disease. Wilder-Smith said the Parkinson’s study isn’t necessarily useful as a diagnostic tool. “The differences are much more subtle,” he said. But the study does demonstrate the connectivity of nerves throughout the body, he said, because when part of the central nervous system breaks down, still-working networks of nerves are more likely to try to compensate for what was lost.
Wilder-Smith has been looking for ways to use wrinkling as a diagnostic tool while avoiding water altogether. Soaking a hand in warm water can be inconvenient, he said, because it takes about half an hour of soaking for skin to wrinkle, which is about as long as a conventional neurological exam. He’s taken to using an anesthetic cream that causes blood vessel constriction called EMLA. Wilder-Smith says the cream produces wrinkles faster than water does and can be applied directly to the fingertips. “We’re not sticking hands in buckets anymore because it’s much more practical to put EMLA on the skin,” he said.