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 speech and occupational therapy may help patients compensate.
Agnosias are uncommon.
Discrete brain lesions can cause different forms of agnosia, which may involve any sense. Typically, only one sense is affected:
Other forms of agnosia involve very specific and complex processes within one sense.
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. Prosopagnosia often accompanies damage to the inferotemporal lobe—often bilateral small lesions, especially in the fusiform gyrus.
Anosognosia is lack of awareness that a deficit exists or lack of insight into an existing deficit. It often accompanies damage to the right, nondominant parietal lobe (which is usually due to an acute stroke or traumatic brain injury). Patients with multiple impairments can be unaware of one impairment but fully aware of others. Patients with anosognosia may deny their motor 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.
Somatosensory agnosia can also result from lesions in the parietal lobe. Patients with somatosensory agnosia have difficulty identifying a familiar object (eg, key, safety pin) that is placed in the hand on the side of the body opposite the damage. However, when they look at the object, they immediately recognize and can identify it.
Occipitotemporal lesions may cause
Right-sided temporal lesions may cause
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 in the ability to communicate 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.
When possible, the cause of agnosia is treated (eg, surgery and/or antibiotics for cerebral abscess, surgery and/or radiation for brain tumor).
Rehabilitation with speech or occupational therapists can help patients learn to compensate for their deficits.
Agnosias are uncommon but may affect any sense.
Diagnose agnosias by asking patients to identify objects or, for subtle agnosias, by doing neuropsychologic tests.
Do brain imaging to characterize the causative lesion.
Recommend rehabilitation with speech or occupational therapy to help patients compensate for deficits.