Agnosia is inability to identify an object using one or more of the senses. Diagnosis is clinical, often including neuropsychologic testing, with brain imaging (eg, CT, MRI) to identify the cause. Prognosis depends on the nature and extent of damage and patient age. There is no specific treatment, but occupational therapy may help patients compensate.
Agnosias are uncommon. They result from damage to (eg, by infarct, tumor, trauma) or degeneration of areas of the brain that integrate perception, memory, and identification.
Discrete brain lesions can cause different forms of agnosia, which may involve any sense. Typically, only one sense is affected. Examples are hearing (auditory agnosia, the inability to identify objects through sound such as a ringing telephone), taste (gustatory agnosia), smell (olfactory agnosia), touch (tactile agnosia), and sight (visual agnosia).
Other forms of agnosia involve very specific and complex processes within one sense. For example, prosopagnosia is inability to identify well-known faces, including those of close friends, or to otherwise distinguish individual objects among a class of objects, despite the ability to identify generic facial features and objects.
Anosognosia often accompanies damage to the right, nondominant parietal lobe. Patients deny their deficit, insisting that nothing is wrong even when one side of their body is completely paralyzed. When shown the paralyzed body part, patients may deny that it is theirs. In an often related phenomenon, patients ignore the paralyzed or desensitized body parts (hemi-inattention) or the space around them (hemineglect). Hemineglect most often involves the left side of the body.
Occipitotemporal lesions may cause an inability to recognize familiar places (environmental agnosia), visual disturbances (visual agnosia), or color blindness (achromatopsia). Right-sided temporal lesions may cause an inability to interpret sounds (auditory agnosia) or impaired music perception (amusia).
At bedside, patients are asked to identify common objects through sight, touch, or another sense. If hemineglect is suspected, patients are asked to identify the paralyzed parts of their body or objects in their hemivisual fields. Physical examination is done to detect primary deficits in individual senses or communication that may interfere with testing for agnosias. For example, if light touch is defective, patients may not sense an object even when cortical function is intact. Also, aphasias may interfere with patient's expression. Neuropsychologic testing may help identify more subtle agnosias.
Brain imaging (eg, CT or MRI with or without angiographic protocols) is required to characterize a central lesion (eg, infarct, hemorrhage, mass) and to check for atrophy suggesting a degenerative disorder.
Recovery may be influenced by size and location of lesions, degree of impairment, and patient age. Most recovery occurs within the first 3 mo but may continue to a variable degree up to a year.
There is no specific treatment. Rehabilitation with speech or occupational therapists can help patients learn to compensate for their deficits.
Last full review/revision September 2008 by Alexander Auchus, MD