76 Olfactory disturbances can be classified as either quantitative (anosmia, hyposmia, hyper-osmia) or qualitative (parosmia, cacosmia). Con genital olfactory disturbances manifest themselves as partial anosmia ("olfactory blindness"). The perceived intensity of a persistent odor decreases or disappears with time (olfactory adaptation). External factors such as an arid environment, cold, or cigarette smoke impair the ability to smell; diseases affecting the na-sopharyngeal cavity impair both smell and taste. Odors and emotions are closely linked and can influence each other. The perception of smell may be qualitatively changed (parosmia) because of autonomic (hunger, stress) and hormonal changes (pregnancy) or disturbances such as ozena, depression, traumatic lesions, or nasopharyngeal empyema. Olfactory hallucinations can be caused by mediobasal and temporal tumors (focal epilepsy), drug or alcohol withdrawal, and psychiatric illnesses such as schizophrenia or depression.
Tests of smell. One nostril is held closed, and a bottle containing a test substance is held in front of the other. The patient is then asked to inhale and report any odor perceived. In this subjective test, odor perception per se is more important than odor recognition. Odor perception indicates that the peripheral part of the olfactory tract is intact; odor recognition indicates that the cortical portion of the olfactory pathway is also intact. More sophisticated tests may be required in some cases. Because there is bilateral innervation, unilateral lesions proximal to the anterior commissure and cortical lesions may not cause anosmia.
Anosmia/hyposmia. Unilateral anosmia may be caused by a tumor (meningioma). Korsakoff syndrome can render the patient unable to identify odors. Viral infections (influenza), heavy smoking, and toxic substances can damage the olfactory epithelium; trauma (disruption of olfactory nerves, frontal hemorrhage), tumors, meningitis, or radiotherapy may damage the olfactory pathway. Parkinson disease, multiple sclerosis, Kallmann syndrome (congenital anosmia with hypogonadism), meningoen-cephalocele, albinism, hepatic cirrhosis, and renal failure can also cause olfactory disturbances.
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