Epidemiology of Language Disorders

Language is one of the most complex of all human functions. Due to this complexity, there are many mechanisms involved in the development and maintenance of language. It follows, therefore, that there may be many reasons why language may fail to develop at or near the expected age. We expect to find language delay in children with impairments involving hearing and speech such as hearing loss, malformation of the vocal apparatus (as in cleft palate), or paralysis of the speech musculature. Similarly, we expect to find language disabilities associated with higher- level cognitive dysfunctions such as general mental retardation or pervasive developmental disorders such as infantile autism. Language dysfunction also can result from acquired postnatal injuries to the areas of the brain that subserve language. Language dysfunction resulting from acquired brain lesions is referred to as acquired aphasia.

However, there are children with developmental language delay that cannot be attributed to any of these more common causes of language disorders. Such children are characterized by a failure to develop normal language functions in the absence of those factors that often provide the general setting in which failure of language is usually observed: deafness, mental deficiency, motor disability, or severe emotional disturbance. The failure can manifest itself either as a disability in expressive (productive) language only with near-normal receptive language (comprehension), or as a disability in both receptive and expressive language. The relationship between the language disabilities and the speech articulation (pronunciation) disorders is inconsistent in this population. That is, some language-impaired children also demonstrate impaired development of speech articulation, whereas others do not. Furthermore, speech articulation disorders may or may not be accompanied by language deficits.

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