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 DysfunctionTransport LossesNeural LossesAllergic...
Nội dung trích xuất từ tài liệu:
Chapter 030. Disorders of Smell, Taste, and Hearing (Part 2) 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 odorantto the olfactory neuroepithelium (transport loss), injure the receptor region(sensory loss), or damage central olfactory pathways (neural loss). Currently noclinical tests exist to differentiate these different types of olfactory losses.Fortunately, the history of the disease provides important clues to the cause. Theleading causes of olfactory disorders are summarized in Table 30-1; the mostcommon etiologies are head trauma in children and young adults, and viralinfections in older adults. Table 30-1 Causes of Olfactory DysfunctionTransport Losses Neural Losses Allergic rhinitis AIDS Bacterial rhinitis and sinusitis Alcoholism Congenital abnormalities Alzheimers disease Nasal neoplasms Cigarette smoke Nasal polyps Depression Nasal septal deviation Diabetes mellitus Nasal surgery Drugs/toxins Viral infections Huntingtons choreaSensory Losses Hypothyroidism Drugs Kallmann syndrome Neoplasms Malnutrition Radiation therapy Neoplasms Toxin exposure Neurosurgery Viral infections Parkinsons disease Trauma Vitamin B12 deficiency Zinc deficiency Head trauma is followed by unilateral or bilateral impairment of smell in upto 15% of cases; anosmia is more common than hyposmia. Olfactory dysfunctionis more common when trauma is associated with loss of consciousness,moderately severe head injury (grades II–V), and skull fracture. Frontal injuriesand fractures disrupt the cribriform plate and olfactory axons that perforate it.Sometimes there is an associated cerebrospinal fluid (CSF) rhinorrhea resultingfrom 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. Perversionof the sense of smell may occur as a transient phase in the recovery process. Viral infections can destroy the olfactory neuroepithelium, which is thenreplaced by respiratory epithelium. Parainfluenza virus type 3 appears to beespecially detrimental to human olfaction. HIV infection is associated withsubjective distortion of taste and smell, which may become more severe as thedisease progresses. The loss of taste and smell may play an important role in thedevelopment and progression of HIV-associated wasting. Congenital anosmias arerare but important. Kallmann syndrome is an X-linked disorder characterized bycongenital anosmia and hypogonadotropic hypogonadism resulting from a failureof migration from the olfactory placode of olfactory receptor neurons and neuronssynthesizing gonadotropin-releasing hormone (Chap. 340). Anosmia can alsooccur in albinos. The receptor cells are present but are hypoplastic, lack cilia, anddo not project above the surrounding supporting cells. Meningiomas of the inferior frontal region are the most frequent neoplasticcause of anosmia; loss of smell may be the only neurologic abnormality. Rarely,anosmia can occur with gliomas of the frontal lobe. Occasionally, pituitaryadenomas, craniopharyngiomas, suprasellar meningiomas, and aneurysms of theanterior part of the circle of Willis extend forward and damage olfactorystructures. These tumors and hamartomas may also induce seizures with olfactoryhallucinations, indicating involvement of the uncus of the temporal lobe. Olfactory dysfunction is common in a variety of neurologic diseases,including Alzheimers disease, Parkinsons disease, amyotrophic lateral sclerosis,and multiple sclerosis. In Alzheimers and Parkinsons, olfactory loss may be thefirst clinical sign of the disease. In Parkinsons disease, bilateral olfactory deficitsoccur more commonly than the cardinal signs of the disorder such as tremor. Inmultiple sclerosis, olfactory loss is related to lesions visible by MRI, in olfactoryprocessing areas in the temporal and frontal lobes. Dysosmia, subjective dis ...