Olfaction in neurodegenerative disease Hyman and colleagues have emphasized the many earlier observations of an early neuronal degeneration in the olfactory region of the hippocampus in cases of Alzheimer, Lewy body, and Parkinson disease. Moreover, a large proportion of patients with other degenerative diseases of the brain have anosmia or hyposmia. A number of theories have been proposed to explain the initial loss of smell, the most relevant of which is based on the finding that the earliest neuropathologic changes of many neurodegenerative processes begin in olfactory structures and then appears serially in neighboring structures, only later reaching the parts of the brain that produce the characteristic neurologic features of these diseases. The implication from these findings, originating with Braak and Braak, has been that Lewy bodies in particular are caused by a pathogen that enters through the peripheral olfactory system and proceeds centrally through the medial temporal lobe (see Chap. 38 for further details). Prions have been suggested as a candidate agent because of their ability to alter protein folding and to transfer this property in a sequentially topographic manner. The studies relating to olfaction in Parkinson disease have been reviewed by Doty, Braak and colleagues, Quinn et al, and Benarroch. It should be emphasized to patients, however, that the reverse is not the case; that is, the majority of individuals with hyposmia do not have a generalized neurodegenerative disease.
These terms refer to distortions of odor perception where an odor is present. Parosmia may occur with local nasopharyngeal conditions such as infection of the nasal sinuses and upper respiratory infections. In some instances, the abnormal tissue itself may be the source of unpleasant odors; in others, where partial injuries of the olfactory bulbs have occurred, parosmia is in the nature of an olfactory illusion. Parosmia may also be a troublesome symptom in persons with depressive and psychotic illnesses, who may report that every article of food has an extremely unpleasant odor (cacosmia). Sensations of disagreeable taste are often associated (cacogeusia). Nothing is known of the basis of this state; there is usually no loss of discriminative sensation.
The treatment of parosmia is difficult. The use of neuroleptic or antiepileptic drugs has yielded unpredictable results. Claims for the efficacy of zinc and vitamins have not been verified (and there is a risk that zinc administration may interfere with the absorption of copper). Some reports indicate that repeated anesthetization of the nasal mucosa reduces or abolishes the parosmic disturbance. In many cases, the disorder subsides spontaneously. Minor degrees of parosmia are not necessarily abnormal, for unpleasant odors have a way of lingering for several hours and of being reawakened by other olfactory stimuli, as every pathologist knows.
The report of an odor without stimulus, olfactory hallucination, is always of central origin. The patient perceives an odor that no one else can detect (phantosmia). Most often this a manifestation of temporal lobe seizures (“uncinate fits”), in which circumstances the olfactory hallucinations are brief and accompanied or followed by an alteration of consciousness and other manifestations of epilepsy (see Chap. 15 on epilepsy).