An international team of scientists has discovered that the human body has an entirely unique and separate sensory system that is largely imperceptible in most people and is based on blood vessels and sweat glands and not on nerves. They found that people lacking known nerve receptors can still touch and feel, and the discovery of how this is possible may help us understand unexplained pain such as fibromyalgia.
An article on the discovery appears in the the the December 15 issue of the journal PAIN, and describes the work of researchers at Albany Medical College, New York, USA, the University of Liverpool and Cambridge University in the UK, and other research centres.
Senior author Dr Frank Rice, a Neuroscience Professor at Albany Medical College (AMC) told the media that the discovery was:
"Almost like hearing the subtle sound of a single instrument in the midst of a symphony."
"It is only when we shift focus away from the nerve endings associated with normal skin sensation that we can appreciate the sensation hidden in the background," added Rice, who is a leading authority on the nerve supply to the skin.
Rice and colleagues got the opportunity to discover this hidden sensory system when lead author Dr David Bowsher, Honorary Senior Research Fellow at the University of Liverpool's Pain Research Institute, diagnosed two unrelated adult patients with a previously unknown abnormality consisting of "congenital absence of pain with hyperhidrosis (CAPH)".
The condition meant that both patients were born with very little ability to feel pain. The condition is rare, and people who have it also have excessively dry skin, often have severe mental handicaps and are prone to harming themselves by accident.
Bowsher explained that:
"Although they had a few accidents over their lifetimes, what made these two patients unique was that they led normal lives. Excessive sweating brought them to the clinic, where we discovered their severe lack of pain sensation."
He said he and his colleagues became more curious when they did tests with sensitive instruments and found that all the skin sensation on both the patients was severely impaired, including their ability to sense temperature and mechanical contact.
The authors wrote that "sural nerve biopsy showed that all types of myelinated and unmyelinated fibers were severely reduced".
"But, for all intents and purposes, they had adequate sensation for daily living and could tell what is warm and cold, what is touching them, and what is rough and smooth," said Bowsher.
Bowsher sent skin biopsies to Rice's lab in the US, where they specialize in examining the nerve endings of skin at the multimolecular level to help analyze chronic pain linked to conditions like nerve injuries, diabetes and shingles. Rice's lab developed the method in collaboration with the Karolinska Institute in Stockholm, Sweden.
Rice explained that under normal conditions:
"The skin contains many different types of nerve endings that distinguish between different temperatures, different types of mechanical contact such as vibrations from a cell phone and movement of hairs, and, importantly, painful stimuli."
But to their surprise, the samples that Bowsher sent them:
"Lacked all the nerve endings that we normally associated with skin sensation."
Rice and colleagues were puzzled: "how were these individuals feeling anything?"
They suggest the answer lies in the presence of sensory nerve endings in small blood vessels and sweat glands in the skin, which they had been aware of for some time.
"For many years, my colleagues and I have detected different types of nerve endings on tiny blood vessels and sweat glands, which we assumed were simply regulating blood flow and sweating," said Rice, but they didn't think they contributed anything to "conscious sensation".
But in the samples from England, they found that while the other sensory endings were missing, there were still normal types of nerve endings in the blood vessels and sweat glands.
"Apparently, these unique individuals are able to 'feel things' through these remaining nerve endings," said Rice.
"What we learned from these unusual individuals is that there's another level of sensory feedback that can give us conscious tactile information," he added.
Rice suggested that perhaps problems in this hidden sensory system's nerve endings may help explain "mysterious pain conditions such as migraine headaches and fibromyalgia, the sources of which are still unknown, making them very difficult to treat".
"Absence of pain with hyperhidrosis: A new syndrome where vascular afferents may mediate cutaneous sensation."
David Bowsher, C. Geoffrey Woods, Adeline K. Nicholas, Ofelia M. Carvalho, Carol E. Haggett, Brian Tedman, James M. Mackenzie, Daniel Crooks, Nasir Mahmood, J. Aidan Twomey, Samantha Hann, Dilwyn Jones, James P. Wymer, Phillip J. Albrecht, Charles E. Argoff, Frank L. Rice.
PAIN - 15 December 2009 (Vol. 147, Issue 1, Pages 287-298).