Agnosia

Article Author:
Anil Kumar
Article Editor:
Scott Dulebohn
Updated:
3/13/2018 2:58:11 PM
PubMed Link:
Agnosia

Introduction

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.

  • Apperceptive agnosia is a failure in recognition due to deficits in early stages of perceptual processing.
  • Associative agnosia is a failure in recognition despite no deficit in perception. Associative agnosia patients can typically draw, match or copy objects while apperceptive agnosia patients cannot.

Types of Agnosia

There are 3 main types of agnosia, based on the type of sensation involved.

  1. Visual (vision)
  2. Auditory (hearing)
  3. Tactile (touch)

Visual Agnosia 

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.

  • Apperceptive visual agnosia refers to an abnormality in visual perception and discriminative process, despite the absence of elementary visual deficits. These people are unable to recognize objects, draw, or copy a figure. They cannot perceive correct forms of the object, although knowledge of the object is intact. Apperceptive visual agnosia is typically associated with lesions to the parietal, occipital cortex
  • Associative visual agnosia refers to difficulty with understanding the meaning of what they are seeing. They can draw or copy but do not know what they have drawn. They correctly perceive the form and know the object when tested with verbal or tactile information, but cannot identify the object. They are unable to link the fully perceived visual stimulus to prior experience to help them recognize the stimulus. Associative visual agnosia is usually associated with damage to the bilateral inferior occipitotemporal cortex.

Types of Visual Agnosia

  • Prosopagnosia is the inability to recognize familiar faces. Patients can often identify other aspects like gender, hair, emotions. Prosopagnosia results from damage to fusiform face area (located in the inferior temporal cortex in fusiform gyrus). People with apperceptive prosopagnosia cannot perceive facial expression and cues but can recognize non-facial clues like hair and clothing. Associative prosopagnosia patients can derive some facial information like gender and age. Mr. P in Oliver Sack’s book The Man Who Mistook His Wife for a Hat presents a classic case of prosopagnosia.
  • Simultanagnosia is the inability to recognize and sort out objects when they appear together, but they can recognize them when they appear alone. Patients are unable to perceive the overall meaning of a picture or multiple things together, although they can describe isolated elements. Two forms of simultagnosia have been described.
  1. Dorsal simultagnosia: Patients cannot see more than one object at a time. For example, when presented with a picture with table, chair and flower vase, they may report only one thing at a time. When their attention is diverted to the other thing, they can then identify only that thing; other things disappear to them. They often have difficulty reading as it involves viewing more than one word at a time. They often bump into objects that are close together. Dorsal simultagnosia is typically associated with lesions in the bilateral occipitotemporal cortex.
  2. Ventral simultagnosia: These individuals also cannot identify more than one object or complex objects at one time, although they can see more than one object at a time. They are unable to perceive the whole picture as one and derive a meaning out of it. For example, in a night sky picture with stars and full moon, they may identify moon as a ball, unable to derive the meaning of the whole picture. Ventral Simultagnosia is associated with lesions in the left inferior occipital area.
  • Color agnosia is the inability to identify and distinguish colors, despite intact basic color vision and brightness discrimination mechanisms. It is very difficult to diagnose this type of agnosia as colors can only be appreciated visually. Usually, these patients have a lesion in the left occipitotemporal region of the brain.
  • Topographical agnosia is the inability to orient to surroundings because of inability to interpret the spatial information. These patients have a good memory of the layout and specifics of the places well known to them, but they are unable to navigate their way through. They are unable to use the visual cues to guide them in the right direction. Associated with a lesion in the right posterior cingulate area of the brain.
  • Finger agnosia is a difficulty in naming and differentiating among the fingers of either hand as well the hand of others. It does not refer to the inability to identify a finger as a finger, as the name may suggest. It is a part of the constellation of symptoms often referred to as Gerstmann syndrome which includes acalculia, agraphia, finger agnosia, and left-right disorientation.
  • Akinetopsia refers to the inability to perceive motion.
  • Agnostic alexia refers to the inability to recognize words visually (they can still write and talk without difficulty).

Auditory Agnosia

  • Auditory agnosia is an inability to recognize sounds despite intact hearing. Auditory agnosias are typically associated with right side temporal lesions.

Types of Auditory Agnosia

  • Phonagnosia is the inability to recognize familiar voices. They can recognize words spoken by others. It is caused by damage to certain parts of sound association region.
  • Verbal auditory agnosia or pure word deafness is the inability to comprehend spoken words but can read, write, and speak in a relatively normal manner.
  • Nonverbal auditory agnosia is the inability to comprehend nonverbal sounds and noises, with sparing of speech comprehension.
  • Amusia is the inability to recognize the music. They are unable to comprehend that certain types of sounds represent music and therefore cannot distinguish music from other sounds.

Tactile Agnosia

Tactile agnosia refers to the inability to recognize objects by touch. They can name objects by sight.

  • Amorphognosia is the inability to identify the size and shape of objects by touch, for example, a triangle or square.
  • Anosognosia is the inability to identify distinctive qualities like texture and weight, for example, a piece of wood, cotton or metal.
  • Tactile asymbolia is impaired recognition by touch in the absence of amorphognosia and ahylognosia.

Etiology

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.

Epidemiology

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.

History and Physical

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.

Evaluation

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.

Treatment / Management

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.

Organizational Strategies

  • Provide a predictable environment and consistency in performing a task, for example, placing things in the same place every day. Routines help develop consistency in performing a task.
  • Declutter, for example, things patients use every day should be kept out in one place. This may help patients with visual agnosia.
  • Label everything

Differential Diagnosis

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.

Prognosis

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.