Agnosia

# Overview
Agnosia (a-gnosis, "non-knowledge", or loss of knowledge) is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is not defective nor is there any significant memory loss. It is usually associated with brain injury or neurological illness, particularly after damage to the temporal lobe.

# Historical Perspective

# Classification

## Types
- Visual agnosia is associated with lesions of the left occipital lobe and temporal lobes. Many are the inability to recognize objects. Subtypes:
Form agnosia: Patients perceive only parts of details, not the whole object.
Finger agnosia is the inability to distinguish the fingers on the hand.  It is present in lesions of the dominant parietal lobe, and is a component of Gerstmann syndrome.
Simultanagnosia: Patients can recognize objects or details in their visual field, but only one at a time. They cannot make out the scene they belong to or make out a whole image out of the details. They literally cannot see the forest for the trees.  Simultanagnosia is a common symptom of Balint's syndrome.
Associative agnosia: Patients can describe visual scenes and classes of objects but still fail to recognize them. He may, for example, know that a fork is something you eat with but may mistake it for a spoon. Patients suffering from associative agnosia are able to reproduce an image through copying.
Apperceptive agnosia: Patients are unable to distinguish visual shapes and so have trouble recognizing, copying, or discriminating between different visual stimuli. Unlike patients suffering from associative agnosia, those with apperceptive agnosia are unable to copy images.
Mirror agnosia: Patients cannot recognize objects or activity on either their left or right field of view. Impairment can vary from mild inattention to complete inability to perform spatial reasoning with regard to the afflicted side. The disorder takes its name from an experiment in which a patient was shown objects reflected in a mirror and saw them, but was unable to find them when prompted.
Semantic agnosia:
Agnosic alexia: Inability to recognize text.
Color agnosia: There is a distinction between color perception versus color recognition. Central achromatopsia and color blindness refer to deficiency in color perception.
Prosopagnosia also known as faceblindness and facial agnosia: Patients cannot consciously recognize familiar faces, sometimes even including their own. This is often misperceived as an inability to remember names. People with PA frequently (but not always) also suffer from topographical agnosia (poor sense of direction, get lost easily), automobilia agnosia (an inability to recognize cars), expressive agnosia (an inability to perceive people's moods from their facial expressions), verbal agnosia (see below) and possibly dyscalculia (poor with numbers).
- Form agnosia: Patients perceive only parts of details, not the whole object.
- Finger agnosia is the inability to distinguish the fingers on the hand.  It is present in lesions of the dominant parietal lobe, and is a component of Gerstmann syndrome.
- Simultanagnosia: Patients can recognize objects or details in their visual field, but only one at a time. They cannot make out the scene they belong to or make out a whole image out of the details. They literally cannot see the forest for the trees.  Simultanagnosia is a common symptom of Balint's syndrome.
- Associative agnosia: Patients can describe visual scenes and classes of objects but still fail to recognize them. He may, for example, know that a fork is something you eat with but may mistake it for a spoon. Patients suffering from associative agnosia are able to reproduce an image through copying.
- Apperceptive agnosia: Patients are unable to distinguish visual shapes and so have trouble recognizing, copying, or discriminating between different visual stimuli. Unlike patients suffering from associative agnosia, those with apperceptive agnosia are unable to copy images.
- Mirror agnosia: Patients cannot recognize objects or activity on either their left or right field of view. Impairment can vary from mild inattention to complete inability to perform spatial reasoning with regard to the afflicted side. The disorder takes its name from an experiment in which a patient was shown objects reflected in a mirror and saw them, but was unable to find them when prompted.
- Semantic agnosia:
- Agnosic alexia: Inability to recognize text.
- Color agnosia: There is a distinction between color perception versus color recognition. Central achromatopsia and color blindness refer to deficiency in color perception.
- Prosopagnosia also known as faceblindness and facial agnosia: Patients cannot consciously recognize familiar faces, sometimes even including their own. This is often misperceived as an inability to remember names. People with PA frequently (but not always) also suffer from topographical agnosia (poor sense of direction, get lost easily), automobilia agnosia (an inability to recognize cars), expressive agnosia (an inability to perceive people's moods from their facial expressions), verbal agnosia (see below) and possibly dyscalculia (poor with numbers).
- Auditory agnosia refers to similar symptoms for sounds. This term is sometimes used with the limited reference to environmental, nonverbal auditory cues, as separate from verbal agnosia There is a weakly defined link between these terms and CAPD/APD (originally Central Auditory Processing Disorder, the term Auditory Processing Disorder is now preferred).
Verbal auditory agnosia or Pure word deafness or simply word deafness: agnosia connected to verbal information. Patients may hear the sounds of the words, but they don't understand what they mean or can't recognize them as words.
Amusia or Receptive amusia is agnosia for music. It involves loss of the ability to recognize musical notes, rhythms, and intervals and the inability to experience music as musical.
Cortical deafness refers to people who do not respond to any auditory information but whose hearing is intact.
Phonagnosia is the inability to recognize familiar voices, even though the hearer can understand the words used.
- Verbal auditory agnosia or Pure word deafness or simply word deafness: agnosia connected to verbal information. Patients may hear the sounds of the words, but they don't understand what they mean or can't recognize them as words.
- Amusia or Receptive amusia is agnosia for music. It involves loss of the ability to recognize musical notes, rhythms, and intervals and the inability to experience music as musical.
- Cortical deafness refers to people who do not respond to any auditory information but whose hearing is intact.
- Phonagnosia is the inability to recognize familiar voices, even though the hearer can understand the words used.
- Somatosensory agnosia or Astereognosia is connected to tactile sense - that is, touch. Patient finds it difficult to recognize objects by touch based on its texture, size and weight. However, they may be able to describe it verbally or recognize same kind of objects from pictures or draw pictures of them. Thought to be connected to lesions or damage in somatosensory cortex.

# Pathophysiology

# Causes
Agnosia can result from strokes, dementia, or other neurological disorders. It may also be trauma-induced by a head injury, or hereditary. Until the 1990s, it was not believed to be genetic, but that view has changed.

# Differentiating Agnosia from Other Diseases

# Epidemiology and Demographics

# Risk Factors

# Screening

# Natural History, Complications, and Prognosis

# Diagnosis

## Diagnostic Criteria

## History and Symptoms

## Physical Examination

## Laboratory Findings

## Imaging Findings

## Other Diagnostic Studies

# Treatment
For all practical purposes, there is no direct cure. Patients may improve if information is presented in other modalities than the damaged one.  In some cases, occupational therapy or speech therapy can improve agnosia, depending on its etiology.

## Medical Therapy

## Surgery

## Prevention