By Pat Anson, Editor

A tingling, sometimes painful sensation in the hands and feet – the early stages of small fiber neuropathy -- may be more damaging to the peripheral nervous system than previously thought, according to new research published in JAMA Neurology.

A 3-year study by Johns Hopkins neurologists found that patients with small fiber neuropathy showed unexpected deterioration over the entire length of sensory nerve fibers, not just nerve fibers at the surface of the skin.

“I liken small fiber neuropathy to the canary in the coal mine,” says senior author Michael Polydefkis, MD, professor of neurology at the Johns Hopkins University School of Medicine and director of the Cutaneous Nerve Lab. “It signals the beginning of nerve deterioration that with time involves other types of nerve fibers and becomes more apparent and dramatically affects people’s quality of life. The results of this new study add urgency to the need for more screening of those with the condition and faster intervention.”

Nearly 26 million people in the United States have diabetes and about half have some form of neuropathy, according to the American Diabetes Association.  Small fiber neuropathy can also be caused by lupus, HIV, Lyme disease, celiac disease or alcoholism.

Diabetic peripheral neuropathy causes nerves to send out abnormal signals. Patients feel pain or loss of feeling in their toes, feet, legs, hands and arms. It may also include a persistent burning, tingling or prickling sensation. The condition can eventually lead to injuries, chronic foot ulcers and even amputations.

Polydefkis and his colleagues found that small fiber nerve damage occurs even in patients with prediabetes, and the early symptoms of burning pain may be less benign than most clinicians think. Routine nerve tests, like nerve conduction, often fail to identify nerve damage because they mostly assess injury to large diameter nerve fibers.

In an effort to measure nerve damage more accurately, Johns Hopkins researchers took small samples of skin — the size of a large freckle — from 52 patients diagnosed with small fiber neuropathy and from 10 healthy controls. Skin samples were taken from the ankle, the lower thigh near the knee and the upper thigh. Three years later, samples from the same area in the same patients were taken for comparison.

Microscopic analysis of the skin samples showed that patients with small fiber neuropathy initially had fewer nerve fibers on the ankle compared to the upper thigh, demonstrating the most nerve damage was further down the leg. But after three years, researchers found that longer nerve fibers were also lost from the lower and upper thighs, something that was not expected.

“We are all taught in medical school that the longest nerves degrade first, and we show that this isn’t always the case,” says lead author Mohammad Khoshnoodi, MD, assistant professor of neurology at Johns Hopkins,

Patients with prediabetes or diabetes had at least 50 percent fewer small nerve fibers in their ankles initially than those participants with an unknown cause for their small fiber neuropathy, indicating these patients started the study with more damage to their small nerve fibers.

The patients with prediabetes continued to have worsening damage to their small nerve fibers over the course of the study, losing about 10 percent of their nerve fiber density each year at all sites tested along the leg. Patients with diabetes also lost similar rates of nerve fibers along the three sites of the leg.

“I expected that people with diabetes would do worse, but I didn’t really expect people with prediabetes to experience a similar rate of degradation of their small nerve fibers,” says Polydefkis.

Researchers caution that their study was small, and that other factors such as high blood sugar, smoking, high blood pressure and high cholesterol, may also have contributed to the decline in nerve fibers.