Why do my fingers wrinkle when I take a bath?
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Did you ever emerge from a bath or pool as a kid, grab a towel, notice it felt funny in your hands, and look down to see that your fingertips were as wrinkled as prunes? Maybe it frightened you. Maybe it thrilled you, so much so that you’d stay in the pool long after your parents told you to get out, just so you get properly prune-y. Even as adults, it fascinates us, which is why we’re devoting a whole episode of The Curious Clinicians to what is known in medicine as “aquagenic wrinkling.”
When a plum is exposed to heat and dry air, its water content evaporates, transforming it into a characteristically shriveled prune. Is that what is happening to your fingertips? Is aquagenic wrinkling simply due to loss of water? It’s actually the opposite. The osmolarity of blood plasma is ~285 mOsm/L. The osmolarity of most bath water is around 0 mOsm/L, since it contains very little solute. Since the body is hypertonic compared to the bath water, prolonged submersion should result in water flow from the bath into the fingers, making them swell. The more hypotonic the bath is, the more water should flow into the fingers.
One 2005 study tested this framing by submerging the hands of 14 volunteers in solutions of varying tonicity. In distilled water (osmolality of 0 mOsm/L), wrinkling occurred in less than 12 minutes. In a 10% salt solution (osmolality 1700 mosm/L), wrinkling took more than double that amount of time, and with a 20% salt solution, no wrinkling occurred after more than an hour. As the tonicity of water increased, wrinkling decreased, implying that water moving into the fingertips (which have an abundance of sweat glands that water can enter) is a driver of wrinkling.
This doesn’t make much sense though. Anyone who has had prune-fingers knows that they feel thin and dried-out, not swollen and water-logged. There must be something else going on.
Almost 90 years ago, researchers noted that in patients with a palsied median nerve (the large nerve that controls movement and sensation in much of the forearm and hand), no skin wrinkling occurred. This indicated that there was a nervous system component to this phenomenon. The activation of the nervous system most likely happens due to vasoconstriction. One 2003 study analyzed blood flow in the fingers of subjects whose fingers were wrinkled through immersion in 40°C water (hotter than body temperature) and then exposed to hot air from a blow dryer. As predicted, the hot air caused vasodilation and increased blood flow, allowing the body to release excess heat. Paradoxically, the hot water actually caused vasoconstriction and a decrease in blood flow.
Combining all the studies we’ve discussed thus far, this seems to be the basic mechanism of wrinkling: Water enters sweat glands in the skin → vasoconstriction → skin wrinkling. But how does sweat-gland swelling cause vasoconstriction? And how does the vasoconstriction cause wrinkling? The most reasonable hypothesis is that as water moves into sweat glands, the cell membranes are destabilized by the rapid change in electrolyte concentrations. This causes adjacent sympathetic nerves to fire and induce vasoconstriction. Fingertips have high concentrations of blood vessels and arterio-venous (AV) shunts called glomus bodies. During vasoconstriction, the glomus bodies shrink, which creates a negative pressure force on the overlying skin and pulls it down. As the epidermis is more tightly anchored to the dermis in certain spots (and therefore more stable), only some sections of skin retract, creating that irregular, wrinkled appearance.
That’s the physiological “why.” But is there an evolutionary “why” that explains aquagenic wrinkling? One researcher, Nick Davis, tested the hypothesis that wrinkling helps to improve grip strength in wet environments. He had more than 500 volunteers grasp a metal load cell, first with dry fingers, then fingers briefly dipped in water, and then finally wrinkled fingers. The load cell measured the amount of force the participants were applying to maintain their hold. Dry fingers required the least force. However, compared to wet fingers, wrinkled fingers had to grip the cell less tightly. It’s not unreasonable to wonder if wrinkling evolved to help our primitive ancestors perform tasks like foraging among wet rocks or catching fish by hand.
Although most of us are no longer hunter-gatherers anymore, that doesn’t mean aquagenic wrinkling is useless. In advanced diabetes, peripheral nerve function is affected, most notably in the extremities. One low-tech, but quite sensitive, way of assessing for peripheral neuropathy is to soak someone’s hands in water for 20 minutes and see how much (if at all) their fingers wrinkle!
Take Home Points
- Aquagenic wrinkling starts with the movement of water across sweat glands and into fingertips.
- This leads to sympathetic-mediated vasoconstriction within the dermal pulp.
- When the overlying skin is pulled down in an irregular manner, wrinkling occurs.
- Whether the increased grip efficiency offered explains why it may have developed is unknown
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Credits & Suggested Citation
◾️Episode written by Tony Breu
◾️ Show notes written by Giancarlo Buonomo and Tony Breu
◾️Audio edited by Clair Morgan of nodderly.com
Breu AC, Abrams HR, Cooper AZ. Aquagenic Wrinkling. The Curious Clinicians Podcast. April 17, 2024.
Image Credit: Flickr/Sharron Drummond
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