Agnosia is a rare condition in which an affected individual is unable to recognise and identify objects, persons, sounds, shapes or smells even though the affected sense is not defective and there is no significant amnesia (memory loss) or problems with attention.
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As such, affected individuals may not be able to identify an object, but will still be able to identify it by its other features such as colour (by sight) or using touch. Agnosia is different from anomia where patients cannot attribute a name to an object despite using other senses.
Types of Agnosia and Symptoms
There are 2 forms of agnosia: apperceptive (failure to recognise due to defects in early stages of perception processing) and associative (failure to recognise despite no defects to perception). Apperceptive patients cannot draw or copy objects whereas associative patients can.
There are 3 main types of agnosia:
- Visual agnosia
- Auditory agnosia
- Tactile agnosia
Visual agnosia is the most common form of agnosia and refers to impairments to the recognition of objects despite having a fully functioning vision, language and memory. If needed, patients can identify unrecognisable objects using touch, for example.
In apperceptive visual agnosia, patients cannot draw or copy a presented object because they cannot perceive the object through their knowledge of the object remains intact. Associative visual agnosia is when patients can indeed copy or draw the object but are unaware of what they have drawn.
Prosopagnosia refers to the specific inability to recognise familiar faces despite being able to identify other features of the person such as hair, gender, emotions etc.
Apperceptive prosopagnosia refers to the inability to recognise facial expressions but the ability to recognise non-facial features such as hair or clothing. Associative prosopagnosia is where patients can perceive the age and gender of a face.
Simultanagnosia is the specific inability to recognise objects when they are presented together, however, can identify each object when presented alone. In dorsal simultanagnosia, patients cannot see more than one object at a time and may also have difficulty reading and may bump into objects closely situated.
Patients with ventral simultanagnosia cannot identify more than one object at one time but are still able to see them and thus unable to add meaning to a full picture for example.
Other forms of visual agnosia include colour agnosia (inability to identify colours), topographical agnosia (inability to orient to surroundings due to a lack of spatial information perception) and finger agnosia (specific difficulty in naming a finger of both hands, though still able to identify fingers per se).
Rarer forms include akinetopsia (inability to perceive motion) and agnostic alexia (inability to recognise words visually despite still being able to write and talk).
Auditory agnosia is the inability to recognise specific sounds despite having fully functioning hearing. Phonagnosia is the inability to recognise familiar voices despite being able to recognise the spoken words by other people.
Verbal auditory agnosia is a ‘word’ deafness and the inability to comprehend spoken words (but able to read, write and speak normally) whereas non-verbal auditory agnosia is the specific inability to comprehend noises but with a retained ability to comprehend speech. Amusia is the specific inability to comprehend music and unable to distinguish between other sounds and what would form music.
Tactile agnosia is the inability to recognise objects by touch when they can name objects by other senses, such as vision. Amorphognosia is the specific inability to identify the size or shape of an object, whereas anosognosia is the specific inability to identify textures and weight (i.e. the difference between cotton and metal).
Causes of Agnosia
Agnosia is usually caused by neurological conditions including stroke, tumours, infections, hypoxia, dementia, head injuries and some neurodevelopmental disorders. These conditions can affect various and multiple parts of the brain. If related to stroke or injury, agnosia onset can be sudden, whereas in conditions such as dementia and cancer the onset may be gradual.
Apperceptive visual agnosia is associated with lesions to the parietal and occipital cortices whereas associative visual agnosia is associated with lesions to the bilateral inferior occipitotemporal cortex.
Prosopagnosia is caused by damage to the fusiform face area located within the inferior temporal cortex (fusiform gyrus). Dorsal simultanagnosia is related to lesions to the bilateral occipitotemporal cortex whereas ventral simultanagnosia is related to lesions to the left inferior occipital area.
Auditory agnosia is typically related to damage to the right temporal cortical lesions. Phonagnosia is caused by damage to the sound association region.
Diagnosis and Treatment Strategies
Initial clinical assessments involve asking suspected patients to identify objects through sight, touch or smell, combined with thorough neurological and physical examinations e.g. vision tests to rule out vision problems, for example, in addition to sensory tests that enable clinicians to rule out sensory problems such as colour blindness or hearing loss.
Memory and cognitive assessments are also made to rule out the possibility of cognitive impairment and dementia to rule out neurological disease, as well as impairments to communication such as dyspraxia and dysphasia.
It is also helpful to have the help of a family member who can point out whether symptoms seem new or had already been present.
Once all alternative explanations have been ruled out, agnosia can be diagnosed, but only in the absence of dementia, aphasia, delirium or other cognitive dysfunction. Particularly tailored examinations may be then given to diagnose types of agnosia e.g. for simultanagnosia multiple objects to be presented together compared to prosopagnosia where photos of famous or familiar faces are shown. Inability to correctly identify 50% or more will be the basis of diagnosis, be it visual, auditory or tactile.
Agnosia can have a significant impact on the daily living on the affected individuals as well as to family, friends and colleagues. There is no cure for agnosia. If there is an underlying cause, such as infection or tumour, treating the underlying cause may improve symptoms.
Tailored strategies to help patients deal with the agnosia include training to help them look for alternative cues, such as scars or hairstyles, or vocal cues to identify individuals in the case of prosopagnosia.
People with auditory agnosia can be taught to lip read, and those with visual agnosia to identify everything by touch. Labelling everything, decluttering (only keeping essential items) and providing predictable environments and consistency can help.
The outlook for patients with agnosia is typically poor in terms of quality of life and the fact that full recovery may not be possible.
Thus, an interdisciplinary team of ophthalmologists, speech and language pathologists, neurologists, audiologists, psychiatrists and occupational therapist may be needed to assist and rehabilitate. Patient education and an interdisciplinary effort can improve outcomes with patients with agnosia.
Kumar & Wroten, 2019. Agnosia. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Bookshelf ID: NBK493156.
Burns, 2004. Clinical management of agnosia. Top Stroke Rehabil. 11(1):1-9. https://www.ncbi.nlm.nih.gov/pubmed/14872395