NEURAL SOURCES OF DYSGEUSIA

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Dysgeusia, an unwanted, chronic taste (akin to chronic pain), is a disorder that diminishes the quality of life. The evaluation of patients with dysgeusia has led to the development of a diagnostic protocol intended to highlight variations in the characteristics of dysgeusias which may provide clues concerning their causes. The protocol uses a spatial taste test designed to evaluate the integrity of the cranial nerves subserving taste and a topical anesthesia test designed to reveal inhibitory relations among those cranial nerves (first discovered by Halpern and Nelson in an animal model). Three groups of studies are proposed which concern dysgeusias (usually salty or metallic) that originate within the taste nervous system (in this case the dysgeusias are referred to as taste phantoms). The first group of studies are to be conducted on patients in whom cranial nerve VII is unavoidably sacrificed during surgery. Surgeries involving cranial nerve VII offer an experimental opportunity to study taste phantoms produced by cutting VII both central to and peripheral to the geniculate ganglion, which contains the cell bodies that keep the neurons in VII viable. When VII must be sacrificed during acoustic neuroma surgery, the cut is central to the geniculate ganglion. This causes Wallerian degeneration from the cut to the brain and thus denervates the central neurons. When VII must be sacrificed during stapedectomy or mastoidectomy, the cut is peripheral to the geniculate ganglion. This causes Wallerian degeneration from the cut to the tongue. Sprouting from the central side of the cut may lead to the formation of a neuroma. One goal of the proposed research is to quantify any differences between the phantoms from these two sorts of nerve cuts that might be attributable to the nature of the pathology induced by the cut. Anatomical evaluation of samples of VII from patients in whom it has been sacrificed may provide additional relevant information. The second group of studies will investigate a new type of taste phantom that has been produced in the laboratory. The chorda tympanic nerve (a branch of VII) innervates the anterior tongue. When this nerve is unilaterally anesthetized at the tympani membrane (via an injection in the ear canal), a taste phantom occurs (in about half of the subjects so anesthetized) contralateral to the side anesthetized on the rear of the tongue (innervated by the glossopharyngeal nerve, IX). The phantoms produced vary in quality across subjects and are abolished if anesthesia is placed directly on the area from which the phantom appears to arise. This kind of phantom may occur in patients who suffer localized damage to taste. Taste phantoms are subjective experiences, but a third group of studies using functional magnetic resonance imaging (fMRI) may provide an objectively measured correlate. Since the side of the brain activated by the anesthesia-induced taste phantoms can be varied, these phantoms offer important experimental advantages in fMRI studies.