Chapter 030. Disorders of Smell, Taste, and Hearing (Part 2)

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Disorders of the Sense of Smell These are caused by conditions that interfere with the access of the odorant to the olfactory neuroepithelium (transport loss), injure the receptor region (sensory loss), or damage central olfactory pathways (neural loss). Currently no clinical tests exist to differentiate these different types of olfactory losses. Fortunately, the history of the disease provides important clues to the cause. The leading causes of olfactory disorders are summarized in Table 30-1; the most common etiologies are head trauma in children and young adults, and viral infections in older adults. Table 30-1 Causes of Olfactory Dysfunction Transport Losses Neural Losses Allergic...

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Chapter 030. Disorders of Smell, Taste, and Hearing (Part 2) Disorders of the Sense of Smell These are caused by conditions that interfere with the access of the odorant to the olfactory neuroepithelium (transport loss), injure the receptor region (sensory loss), or damage central olfactory pathways (neural loss). Currently no clinical tests exist to differentiate these different types of olfactory losses. Fortunately, the history of the disease provides important clues to the cause. The leading causes of olfactory disorders are summarized in Table 30-1; the most common etiologies are head trauma in children and young adults, and viral infections in older adults. Table 30-1 Causes of Olfactory Dysfunction
Transport Losses Neural Losses Allergic rhinitis AIDS Bacterial rhinitis and sinusitis Alcoholism Congenital abnormalities Alzheimer's disease Nasal neoplasms Cigarette smoke Nasal polyps Depression Nasal septal deviation Diabetes mellitus Nasal surgery Drugs/toxins Viral infections Huntington's chorea Sensory Losses Hypothyroidism Drugs Kallmann syndrome Neoplasms Malnutrition Radiation therapy Neoplasms Toxin exposure Neurosurgery
Viral infections Parkinson's disease Trauma Vitamin B12 deficiency Zinc deficiency Head trauma is followed by unilateral or bilateral impairment of smell in up to 15% of cases; anosmia is more common than hyposmia. Olfactory dysfunction is more common when trauma is associated with loss of consciousness, moderately severe head injury (grades II–V), and skull fracture. Frontal injuries and fractures disrupt the cribriform plate and olfactory axons that perforate it. Sometimes there is an associated cerebrospinal fluid (CSF) rhinorrhea resulting from a tearing of the dura overlying the cribriform plate and paranasal sinuses. Anosmia may also follow blows to the occiput. Once traumatic anosmia develops, it is usually permanent; only 10% of patients ever improve or recover. Perversion of the sense of smell may occur as a transient phase in the recovery process. Viral infections can destroy the olfactory neuroepithelium, which is then replaced by respiratory epithelium. Parainfluenza virus type 3 appears to be especially detrimental to human olfaction. HIV infection is associated with subjective distortion of taste and smell, which may become more severe as the
disease progresses. The loss of taste and smell may play an important role in the development and progression of HIV-associated wasting. Congenital anosmias are rare but important. Kallmann syndrome is an X-linked disorder characterized by congenital anosmia and hypogonadotropic hypogonadism resulting from a failure of migration from the olfactory placode of olfactory receptor neurons and neurons synthesizing gonadotropin-releasing hormone (Chap. 340). Anosmia can also occur in albinos. The receptor cells are present but are hypoplastic, lack cilia, and do not project above the surrounding supporting cells. Meningiomas of the inferior frontal region are the most frequent neoplastic cause of anosmia; loss of smell may be the only neurologic abnormality. Rarely, anosmia can occur with gliomas of the frontal lobe. Occasionally, pituitary adenomas, craniopharyngiomas, suprasellar meningiomas, and aneurysms of the anterior part of the circle of Willis extend forward and damage olfactory structures. These tumors and hamartomas may also induce seizures with olfactory hallucinations, indicating involvement of the uncus of the temporal lobe. Olfactory dysfunction is common in a variety of neurologic diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. In Alzheimer's and Parkinson's, olfactory loss may be the first clinical sign of the disease. In Parkinson's disease, bilateral olfactory deficits occur more commonly than the cardinal signs of the disorder such as tremor. In
multiple sclerosis, olfactory loss is related to lesions visible by MRI, in olfactory processing areas in the temporal and frontal lobes. Dysosmia, subjective distortions of olfactory perception, may occur with intranasal diseases that partially impair smell or during recovery from a neurogenic anosmia. Most dysosmic disorders consist of disagreeable odors, sometimes accompanied by distortions of taste. Dysosmia also can occur with depression.