ear, nose, and throat disorders
Overview of Smell and Taste Abnormalities
Smell
The inability to detect certain odors, such as gas or smoke, may be dangerous, and several systemic and intracranial disorders should be excluded before dismissing symptoms as harmless. Whether brain stem disease (involvement of the nucleus solitarius) can cause disorders of smell and taste is uncertain because other neurologic manifestations usually take precedence. Anosmia (complete loss of the sense of smell) is probably the most common abnormality. Hyperosmia (increased sensitivity to odors) usually reflects a neurotic or histrionic personality but can occur intermittently with seizure disorders. Dysosmia (disagreeable or distorted sense of smell) may occur in patients with infection of the nasal sinuses, partial damage to the olfactory bulbs, or mental depression. Some cases, accompanied by a disagreeable taste, result from poor dental hygiene. Seizure activity that originates in the uncus of the temporal lobe can cause brief, vivid, unpleasant olfactory hallucination; rarely herpes encephalitis causes olfactory hallucinations. Hyposmia (partial loss of smell) and hypogeusia (diminished sense of taste) can follow acute influenza, usually temporarily. Sudden loss of smell may be an early symptom of To distinguish most flavors, the brain needs information about both smell and taste. These sensations are communicated to various areas of the brain from receptors in the nose and mouth. The olfactory epithelium is an area of the nasal mucosa in the upper part of the nasal cavity. The smell receptors in this epithelium are specialized nerve cells with cilia that detect odors. Airborne molecules entering the nasal passage stimulate the cilia, triggering a nerve impulse that is transmitted upward through the cribriform plate and across a synapse within the olfactory bulbs (the distal ends of the 1st cranial (olfactory) nerves. The olfactory nerves transmit the impulse to the brain, which interprets the impulse as a distinct odor. Information is also sent to the middle part of the temporal lobe (the smell and taste center), in which memories of odors are stored. Thousands of tiny taste buds cover most of the tongue’s surface. A taste bud contains several types of ciliated taste receptors. Each type detects one of the five basic tastes: sweet, salty, sour, bitter, or savory (also called umami, the taste of monosodium glutamate). The sensitivity of specific areas of the tongue may vary for each taste (eg, the tip of the tongue for sweetness, the front sides for saltiness, along the sides for sourness, and the back one-third for bitterness); however, these tastes can be detected all over the tongue, and the contribution of regional differences to taste sensation is probably minor. Nerve impulses from the taste buds are transmitted to the brain through the facial and glossopharyngeal nerves (cranial nerves VII and IX). The brain interprets the combination of impulses from the olfactory and taste receptors along with other sensory information (eg, the food’s texture and temperature) to produce a distinct flavor when food enters the mouth and is chewed. 