Burning Mouth Syndrome
Burning Mouth Syndrome
Yale University School of Medicine Taste laboratory
Burning mouth syndrome (BMS): an intra-oral disorder most prevalent in post-menopausal women.
BMS can affect any area of the mouth. It is characterized by the sudden onset of pain and burning, sensations that may build in intensity over time. The tip of the tongue is usually affected, and the pain is often accompanied by taste phantoms. Taste phantoms are taste sensations that occur in the absence of stimuli: a patient will report a bitter taste in her mouth when there's nothing there. In fact, one of the reasons BMS is so difficult to diagnose is the absence of visible abnormality in the mouth.
The difficulty of diagnosing BMS is one of the reasons its rates of prevalence are difficult to determine. BMS may strike 2.6% of the general population (Grushka & Sessle, 1987), but, among women seeking treatment for symptoms of menopause, the incidence of oral burning and related disorders is higher--over 40% are affected (Wardrop, Hailes, Burger & Reade, 1989). One study of BMS found that over 60% of diagnosed BMS patients also suffered from taste phantoms (Grushka, Sessle & Howley, 1986); in our lab, the most common taste phantom BMS sufferers report is a "metallic" taste.
Sensory testing has revealed deficits in taste (Bartoshuk et al., 1999) and heat pain tolerance (Grushka et al., 1986) among BMS patients. Touch and sensations of warmth and cold are, apparently, unaffected. The nature of BMS complaints makes topical anesthesia an attractive angle to pursue for pain control, but it is ineffective. One of the diagnostic tools for BMS, in fact, is applying topical anesthesia to the affected area and waiting to see if the pain gets worse: it will (Ship et al., 1995).
Though many attempts have been made to explain the causes of BMS, none was ever able to explain the full constellation of symptoms or account for the spontaneous remissions known to occur in about half of all patients. We have proposed a model, based on our recent studies, for the etiology and pathogenesis of BMS.
Our research
Our laboratory work (Bartoshuk et al., 1999) suggests that BMS may be a sensory phantom, that the burning sensations are created centrally in the nervous system: this is why the tongues of patients with BMS are not visibly different from those without complaints. Sensory phantoms are relatively common; many people are familiar with the phantom limb sensations that can occur in the wake of amputations but are unaware that disorders such as tinnitus (ringing in the ears) are also sensory phantoms.
We believe BMS and other oral pain phantoms result from damage to the taste system. Our research suggests that taste normally inhibits oral pain; many sufferers of BMS know their symptoms ease when they eat, chew gum or sip cold water. We believe there is a basis for this sort of phenomenon as, during evolution, animals with damaged tongues (whether from disease or fighting), would have continued to be able to eat if tasting food inhibited the discomfort in their tongue. When the taste system is damaged, it can no longer inhibit oral pain and certain individuals (especially genetic supertasters) are more likely to experience pain phantoms (like BMS) as a result.
Viral infections are the major source of taste system damage. The nerve that carries taste sensations from the front of the tongue to the brain passes through the middle ear, a site quite vulnerable to damage by cold and flu viruses. Fortunately, the body can repair this damage, and this may be why burning mouth syndrome often disappears by itself. Head trauma can also cause the symptoms we associate with BMS, as do a variety of medications. Some medications, for example, have been implicated in taste loss: this is the sort of trigger that may cause BMS in someone genetically susceptible.
Genetic variation in taste was discovered accidentally. In 1931, a chemist named Fox synthesized phenylthiocarbamide (PTC) and, when some blew into the air, a colleague commented on how bitter it tasted; Fox tasted nothing. Fox pursued this quirk with a geneticist, and they called their discovery "taste blindness." Family studies demonstrated that tasting is the dominant trait; scientists of the time believed that nontasters lacked the kind of bitter taste receptors that would respond to PTC.
Our own work with this genetic variation began in 1975. We use PROP in our research as it's chemically related to PTC and shows the same pattern with regard to genetic taste sensitivity. PROP is also used in large quantities as a drug to treat a thryoid disorder, so we know the small amounts we use to test patients are harmless. We've found that the differences between nontasters and tasters are much more extensive than initially thought, and that some tasters are much more responsive to PROP than others. We believe these "supertasters" carry two copies of the dominant allele. About 25% of the people in the United States are supertasters, and 25% are nontasters; the remaining 50% are medium tasters, more sensitive to certain oral sensations than nontasters, but less sensitive than supertasters. Women are more likely to be supertasters than men, and Asians are more likely to be supertasters than are Caucasians.
Dr. Inglis Miller and Dr. Mark Whitehead are anatomists who made discoveries that have led to our current understanding of BMS. They studied fungiform papillae, small structures on the tongueUs surface that hold taste buds. These fungiform papillae are easy to see: they do not hold onto dye the way the rest of the tongue does, and swabbing food coloring across the front of the tongue leaves them pink circles against a colored background. Supertasters have the most fungiform papillae and nontasters the fewest. Each contains about six taste buds, with each bud surrounded by pain neurons. This association between taste buds and pain neurons means supertasters not only perceive the most intense tastes but also the most intense oral pain and irritation (chili peppers or alcohol, for example). Incidentally, supertasters also perceive the most intense oral touch sensations from food; since fat in food produces touch sensations (oily, viscous, creamy, thick), supertasters perceive fat in food more keenly than nontasters or medium tasters.
Our research suggests that individuals who suffer from BMS are likely to be supertasters.
Why is burning mouth syndrome related to menopause?
BMS is most likely to afflict post-menopausal women for two reasons. First, as noted above, women are more likely to be supertasters than men, and BMS affects supertasters. Second, hormones affect taste (particularly bitter) in women. Sensitivity to bitter tastes varies with the menstrual cycle, reaching a maximum early in pregnancy. As many poisons taste bitter, we suspect this hormonal shift is one way nature helps protect a developing fetus from harm. At menopause, sensitivity to bitterness diminishes for hormonal reasons, and can have the same result as a cold or the flu acting as damage to the taste system.
Clonazepam: Potential treatment
Dr. Miriam Grushka discovered that Clonazepam (Klonopin) treats burning mouth pain in about 70% of patients (Grushka, Epstein & Mott, 1998). Dr. Grushka's research suggests a beginning dose of 0.25 mg per day. If this is ineffective after one week, she usually recommends an increase to 0.50 mg per day. If the new dose is ineffective after another week, an increase to 0.75 mg per day is suggested. Higher doses are usually ineffective and may produce unwanted side effects (drowsiness, etc.). In these cases, small doses (e.g., 50 to 300 mg per day) of gabapentin may be helpful. Clonazepam, like all prescription drugs, should only be used under the direction and care of a physician. For more information about treatment, contact Dr. Grushka at 416-484-6235.
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