Anosmia and Hyposmia

Loss or decreased olfactory function is estimated to be present in approximately 1 percent of the American population under the age of 60 and in more than half of the population over that age.y The causes of loss or decreased olfactory function are variable, and simply establishing the presence of olfactory loss per se provides little insight into the cause. Nearly two thirds of patients with chronic anosmia or hyposmia (i.e., those that are presumably permanent) are due to prior upper respiratory infections, head trauma, and nasal and paranasal sinus disease, and most reflect damage to the olfactory neuroepithelium. [1 Other causes include iatrogenic interventions (e.g., septoplasty, rhinoplasty, turbinectomy, radiation therapy), intranasal neoplasms (e.g., papillomas, hemangiomas, and ameloblastomas), intracranial tumors or lesions (e.g., Foster Kennedy syndrome, olfactory groove meningiomas, frontal lobe gliomas), epilepsy, psychiatric disorders, exposure to environmental chemicals, hypothyroidism, renal disease, and kidney disease. According to Finelli and Mair, y the single most egregious error of neurologists in dealing with olfactory disturbances is the failure to recognize the symptom of anosmia as the principal or sole feature of an olfactory groove meningioma. Five to 10 percent of head trauma patients have olfactory dysfunction, and the majority of these have anosmia. y Of particular interest to the neurologist is the observation that olfactory dysfunction may be the first sign of Alzheimer's disease and idiopathic Parkinson's disease. y Patients with intractable epilepsy who are candidates for temporal lobe resection have a hyposmic condition prior to surgery. In most of these cases the hyposmia is bilateral, although asymmetry in function can be present, the greatest decrement being observed on the side of the focal damage. Interestingly, after surgical removal of diseased tissue in the right hemisphere, some measures of olfactory function improve significantly on the left side of the nose.

In most patients with congenital anosmia, MRI reveals a lack of or marked hypoplasia of the olfactory bulbs and stalks bilaterally. For example, in a study of 25 patients with congenital anosmia from the University of Pennsylvania Smell and Taste Center, MRI revealed an absence or hypoplasia of olfactory bulbs and tracts in all instances.y Verification of a physical cause of this disability proved to be quite therapeutic for some of these individuals.

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