Feb. 15, 2011 -- Many cases of mysterious nerve damage turn out to be caused by overuse of popular denture products, an increasing number of reports suggests.
The culprit: zinc in Fixodent, from Procter & Gamble, and -- until it became zinc-free last May -- Poligrip from GlaxoSmithKline.
The body needs zinc. But because the body balances zinc and copper, people who get way too much zinc have dangerously low levels of copper. Moreover, zinc overdose itself may be toxic.
The result is bone marrow suppression and degeneration of the spinal cord, usually resulting in crippling nerve damage. It's been called "human swayback disease."
Some patients have ended up in wheelchairs. At least one patient died. Lawsuits have blossomed, with a bellwether case scheduled to be heard this summer in Miami. GlaxoSmithKline already has settled a large number of cases; Procter & Gamble has not.
For years, doctors have reported mysterious cases of people suffering a spastic gait, limb weakness and numbness, and difficulty walking. Only around the year 2000 did researchers find that these patients have very low levels of copper in the blood. Copper deficiency can be caused by an overdose of zinc, but most of these patients had no obvious exposure to zinc.
In 2007, Italian researcher Marco Spinazzi, MD, suggested a link to denture cream. And in 2008, Sharon Nations, MD, and colleagues at the University of Texas Southwestern Medical Center identified four patients for whom this was their main source of zinc.
Since then, other researchers have reported similar findings. Of special note: the death of a Georgia patient from zinc overdose linked to denture cream.
Vanderbilt researcher Peter Hedera, MD, had long wondered about unexplained cases of low blood-copper levels in which patients developed crippling disabilities. After hearing of the Nations study, he investigated 11 of these patients. Sure enough, all 11 were using excessive amounts of denture cream.
"Poorly fitting dentures requiring large amounts of denture cream to provide a sufficient seal appear to be the common characteristics of patients," Hedera and colleagues reported in the journal Neurotoxicology.
Linking Zinc Poisoning to Denture Cream
Why did it take researchers so long to figure this out? In an explosive report, ABC news recently reported that publication of the Nations study in the journal Neurology was delayed by a reviewer, Kenneth Shay, DDS, with undisclosed financial ties to Procter & Gamble. Shay, it turned out, violated his confidentiality agreement by sharing the unpublished manuscript with P&G.
"The American Academy of Neurology considers any violation of these ethics policies to be egregious misconduct," the organization says in a statement provided to WebMD.
Shay says he was critical of the Nations report because he felt the researchers' method of measuring zinc in dental cream had "fundamental flaws." After the Nations team refused to accept his revisions, he says, he sent the manuscript to P&G.
"To fact-check my revisions, I shared it with a colleague at P&G," Shay tells WebMD. "I said, 'Don't share this around, but I'm interested in if the following facts are true or not. ... There was no sense of 'Squash this,' or anything. It was entirely my way of trying to make sure my review was accurate."
The ABC report implied that Shay managed to delay publication of the Nations report by about two years. In fact, it appeared some 15 months after it was submitted, "in line with standard time frames," according to the American Academy of Neurology.
And regardless of how the Nations team measured zinc, Shay -- a geriatric dentist at the Ann Arbor VA who has studied dental-cream use -- admits that many patients with poorly fitting dentures use wildly excessive amounts of denture cream to make them fit.
The irony, he says, is that the cost of using dental cream in this way quickly exceeds the cost of new, better-fitting dentures.
Is Fixodent Safe? Is Poligrip Safe?
Before May 2010, Poligrip contained a relatively small amount of zinc -- about 34 milligrams per gram in the original formulation and about 27.5 milligrams per gram in the Polyseal formulation, Nations and colleagues found. In February 2010, GlaxoSmithKline sent a "Dear Doctor" letter warning that excessive use of the product by patients could be dangerous.
"These patients should be advised to consult their dentist for advice," wrote GlaxoSmithKline chief medical officer Howard Marsh, MD.
Fixodent contains smaller amounts of zinc -- about 17 milligrams per gram, according to the Nations study.
According to P&G spokeswoman Tricia Gottlieb, the amount of zinc "an average denture adhesive user would ingest from daily usage of Fixodent" is less than the zinc in most daily multivitamins, less than the zinc in six oysters, and comparable to the zinc in 6 ounces of ground beef.
Gottlieb notes that when used correctly, a 2.4-ounce tube of Fixodent should last six to eight weeks. Patients suffering nerve damage tended to use far more: two or even three tubes a week.
What isn't known is how many people have been using such large amounts of dental cream or how long they've been doing it. Those who continue to use Fixodent in this way are clearly at risk.
Symptoms of Zinc Overdose
Hedera and colleagues examined 11 patients with nerve damage linked to dental cream. Seven had been using Poligrip, three had been using both Poligrip and Fixodent, and one had used only Fixodent.
Symptoms began with numbness and movement difficulty affecting the feet and legs. This rapidly progressed to the arms, and patients began losing their sense of balance.
The patients' bone marrow suppression -- resulting in low counts of both red and white blood cells and platelets -- got better when they stopped using dental cream.
Some of the patients' neurologic symptoms improved, but most remained impaired. Three remain in wheelchairs, five need walkers, and one uses a cane. One no longer needs a cane, and another patient, who never lost the ability to walk, made an "almost complete recovery."