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Agnosia is a disorder that results in a lack of recognition. An affected person does not recognize objects or is unable to name them. Usually affecting only one sense, agnosia may be visual, auditory (the affected person does not recognize sounds), or tactile. Several factors can be at the root of agnosia even if the main cause is stroke. Infection, tumor, vascular problems, dementia such as Alzheimer's disease or a severe psychiatric disorder may also be at the source of agnosia. Other terminology is derived from agnosia: anosognosia is when an individual is incapable of understanding a prognosis, hemiasomatognosia is the inability to recognize half of one's body and one's own, which is very frequent in cases of hemiplegia after stroke.


The disease may manifest itself in various manners. In visual agnosia, a person may not recognize the geometric shapes of objects or cannot make the connection between an object and the information stored in his memory about it: the objects are sometimes recognized, but the name is not remembered. Spatial agnosia concerns the inability to place an object in space. Auditory agnosia is manifested as a disorder of speech comprehension.


The diagnosis of agnosia is generally easy. For example, visual agnosia is diagnosed by questioning the patient about the names of objects or sometimes with further neuropsychiatric testing. Various medical examinations are then used to identify the cause of the agnosia. Often, when considered outside of an obvious cause such as a stroke, the diagnosis of agnosia is difficult to establish, and the discovery of its origin is done by eliminating potential causes.


Treatment depends on the original cause of the agnosia. Different rehabilitation exercises can be used, but the methods are not yet well defined. In addition, some forms of agnosia are orphan diseases; there is therefore, at this time, no treatment.