Agnosia is a rare disorder whereby a patient is unable to recognize and identify objects, persons, or sounds using one or more of their senses despite otherwise normally functioning senses. The deficit cannot be explained by memory, attention, language problems, or unfamiliarity to the stimuli. Usually, one of the sensory modalities is affected. For example, a patient with agnosia may not be able to identify a cup by sight, although they may be able to tell its color and identify it by touch by its shape and texture. It is not same as anomia. Anomia is a naming disorder in which patients cannot name an object despite using their other sensory modalities like touch and smell.
Classically, there are 2 forms of agnosia: apperceptive and associative.
Types of Agnosia
There are 3 main types of agnosia, based on the type of sensation involved.
Visual agnosia refers to an impairment in recognizing visually presented objects, despite otherwise normal visual field, acuity, color vision, brightness discrimination, language, and memory. Patients can recognize objects using other sensory modalities. Sometimes impairment is worse for certain types of objects, so a variety of objects should be tested to diagnose precisely. Visual agnosia is the most common and better-understood agnosia.
Agnosia is further divided in 2 subtypes: apperceptive visual agnosia and associative visual agnosia.
Types of Visual Agnosia
Types of Auditory Agnosia
Tactile agnosia refers to the inability to recognize objects by touch. They can name objects by sight.
Agnosia can result from various neurological conditions like strokes, tumors, infections, dementia, hypoxia, toxins such as carbon monoxide poisoning, head injury, developmental disorders, or other neurological conditions. It can present suddenly, for example, in stroke or head injury, or gradually, for example, in tumor, dementia. Symptoms depend on the area involved. People with agnosia usually retain their other cognitive abilities.
Agnosia happens when there is brain damage along pathways that connect primary sensory processing areas. These areas typically include the posterior parietal cortex and occipitotemporal regions.
Pure forms of agnosia are very rare. Less than 1% of all neurological patients have agnosia. Visual agnosia is the most common and better-described type of agnosia.
Careful patient history is very important in cases of suspected agnosia. Clues to diagnosis and etiology often involve a detailed interview with the patient, family members, and caregivers. It is harder to diagnose when the patient lacks the insight of his or her deficits. History will reveal clues pointing to the type of agnosia. For example, a patient with visual agnosia will specify problems in day-to-day life, for example, getting dressed or inability to differentiate fork and a knife because of inability to recognize certain types of objects visually. Patients with topographical agnosia will present with getting often lost, findings directions. Patients with akinetopsia often bump in things, get confused how things got moved around, have difficulty in crossing roads and driving and are accident prone. Patients with simultagnosia will often have trouble reading and writing and interpreting multiple things at a time.
Other important points to elicit in history can confirm that other sensory modalities, memory, and cognition are not affected.
Initial assessment involves asking patients to identify common objects through sight, touch, and smell. A thorough physical examination is done to detect for any primary deficits in individual senses (visual fields, acuity, reading, drawing, hearing tests, among others) or in the ability to communicate that may interfere with identifying the objects (memory, cognition, attention, aphasia).
An important aspect of diagnosing agnosia is ruling out alternative explanations. Recognition disturbance can occur in many other neurological conditions, but agnosia can be diagnosed only in the absence of aphasia, generalized dementia, delirium, or any deficit that may impair some or all of the information-processing steps involved in object recognition.
It is very important to rule out genuine unfamiliarity with stimuli. It is important to consider cultural and life experience and other factors that may lead to unfamiliarity. Testing of common objects and constructing a balanced set of items that are determined individually should be done.
For example, to assess for visual agnosia, show the patient several familiar items like keys, a pen, and a banana and ask to name them. Differentiating visual agnosia and aphasia is important. For prosopagnosia, show the patient pictures of famous people, their family members, and themselves. For color agnosia, show solid color cards and ask them to name the colors. For simultagnosia, present pictures with several objects or people and ask them to identify what they see. For tactile agnosia, make sure patient is unable to see, then put common things in each hand one by one and ask the patient to identify by touch alone. Inability to correctly identify at least 50% of stimuli tested will suggest the diagnosis of agnosia.
Diagnosis is mainly clinical, based on clinical history and physical examination including a thorough neurologic exam, psychological exam, and certain standardized tests of brain function. It is important to make sure that the abnormality cannot be explained in another way, for example, genuine unfamiliarity with the stimuli, poor instructions, overlooked disease like neuropathy and cataracts, or other neurologic conditions like dyspraxia or dysphasia.
Referrals to ophthalmology, speech pathology, audiology, psychiatry, and other professionals may be needed for detailed evaluation of sensory and neurobehavioral status. Once a tentative diagnosis of agnosia is made, a thorough neuropsychological examination is done to test intelligence, memory function, linguistic competence, and assess sensory-perceptual processing.
Various standardized tests are available to test for memory and cognition (MMSE, Montreal Cognitive assessment, ADAS-cog) and aphasia (Boston naming test, western aphasia battery).
Diagnosis is supplemented by neuroimaging tests such as head CT and brain MRI to look for etiology. More testing may be needed based on clinical judgment and suspected etiology. Auditory or visual evoked potentials may be used to determine whether a deficit exists in the sensory projection area as opposed to primary sensory or association cortex.
Agnosia can considerably limit the everyday functioning of the patients. It can also significantly affect the lives of family and caretakers. There is no direct cure. Treat the underlying cause, when possible. For example, treating and preventing stroke, antibiotics and/or surgery for brain abscess, and surgery and/or radiation for brain tumors.
Agnosia is managed supportively. Rehabilitation, speech, and occupational therapy play an important role in the treatment of agnosia and mainly focus on teaching patients to use the intact sensory modalities to compensate. Restorative training has limited utility. Interventions tend to focus on helping these patients and their family and caregivers cope and adapt to the condition, and furthermore, to help patients function independently within their context. Counseling of family members and helping them with behavioral changes is equally important.
Rehabilitation approaches need to be individualized and should focus on the specific deficit by developing compensatory strategies.
Alternative Cues and Strategies
Some general strategies that can be used are alternative cues, for example, teaching people with prosopagnosia learn to identify any facial scars or hairstyles. For patients with visual agnosia, teaching alternative strategies like learning to identify everything by touch; in patients with prosopagnosia, teaching voice recognition for people; and teaching patients with auditory agnosia to lip read and write everything.
It is important to remember that pure forms of agnosia are very rare. It is important to evaluate and rule out dementia, aphasia, acute confusional states (delirium), disturbance of attention, and unfamiliarity with stimuli. Also, clinicians need to confirm make sure no actual sensory impairment such as color blindness, cataracts, hearing loss, neuropathy, among others.
Few patients with agnosia regain their sensory function. Most recovery occurs within the first three months and to a variable degree may continue up to a year. Prognosis depends on - patient's age, etiology, type, size and location of the area affected, the extent of impairment, effectiveness of therapy.