Introduction

Definition of agnosia

Agnosia is a rare condition where an individual has normally functioning senses such as vision or hearing but their brain is unable to process the information from one or more of the senses to recognise objects, locations, sounds or people¹. This condition can be caused by disorders that damage the brain, not problems with memory, attention, language problems or new experiences or environments¹. The nature of the disorder can also affect how agnosia develops in individuals, for example, a head injury can cause it to appear suddenly whereas dementia may lead to agnosia developing over time.¹

Examples of disorders that can damage the brain and lead to agnosia are as follows:¹

• Stroke
• Tumours (such as brain tumours)²
• Dementia
• Hypoxia - low levels of oxygen are available to tissues in the body which prevents them from functioning properly³
• Infections (such as herpes simplex encephalitis) ⁴
• Head injury
• Developmental disorders (such as autism) ⁵ 
• Toxins (such as carbon monoxide poisoning^)1,2

There are many different types of agnosia which depend on the parts of the brain, that are damaged¹ and can be classified based on the senses involved. Some types of agnosia can also take two forms which are apperceptive or associative. The apperceptive form is where the individual can not interpret the features of a stimulus and connect it to its meaning whereas, individuals with the associative form of an agnosia can perceive or describe the stimulus but can not identify it.^1,6

Figure 1: The image is of the lateral (side) view of the brain created with BioRender.com and licensed.⁷

In order to diagnose an individual with agnosia and the type, an extensive patient history is needed from both the patient and those who are familiar with them.¹ The more information is given to the healthcare professional, the easier it is to determine what type the individual has. Before agnosia can be diagnosed, assessments will be completed of the patient's senses, and brain function and additional exams to ensure that their symptoms are not due to anything else such as memory loss, impairments or unfamiliarity with the objects or location.¹ Therefore, understanding the characteristics of each type can aid in diagnosing and managing the condition. 

Visual agnosia 

Visual agnosia is described as the most common type of agnosia where an individual is unable to recognise what object they are seeing despite functional vision and it can not be explained to be due to cognitive functions or cognitive decline.^1,7 However, when other senses are used, the individual would be able to say what the object is.

Many types of visual agnosia have been documented and the different types are described in table 1 below. 

Subtypes of visual agnosia

Table 1: A list describing the different types of visual agnosia currently documented^{1,2,5, 8-10}.

Auditory agnosia

Auditory agnosia is when an individual is unable to recognise or react to sounds which can not be attributed to problems with hearing.^{1, 11} This condition has a number of subtypes and can range in severity for patients.¹¹

Subtypes of auditory agnosia

Table 2: A list describing the different types of auditory agnosia.^1,6,11-13

Tactile agnosia

Tactile agnosia is when an individual is unable to identify objects through touch but they may be able to with other senses .¹

Subtypes of tactile agnosia

Amorphognosia

When an individual is unable to identify an object by touching its shape or feeling its size.¹ 

Ahylognosia

When an individual can recognise the object only from feeling its shape or size but not from other physical characteristics such as texture, weight or density.^1,14 

Additional agnosias

There are also additional types of agnosia that have been documented outside of the main groups being visual, auditory and tactile. 

• Olfactory agnosia is the inability to identify familiar scents¹⁵
• Gustatory agnosia is where an individual has difficulty recognising familiar tastes ¹⁵

Lastly, there is another type of agnosia which is different from the others where individuals diagnosed with anosognosia are not aware of their health conditions and this unawareness is involuntary and not due to denial.¹⁶ 

FAQs

Can agnosia be managed?

The main ways agnosia is managed is to help the individual and those important to them adapt to the condition and make changes that will improve the individual’s quality of life. For example, the individual can learn how to use their other senses to compensate for those involved in agnosia or rearrange the individual’s environment to make it easier for them to locate objects and avoid hazards.

Depending on the cause of agnosia, treatments of the cause can be offered such as if it is caused by an infection,¹ antibiotic can be given, however, please note that this is not always possible.

Summary

Overall, agnosia is a rare condition which can be caused by damage to the brain and results in an individual’s inability to process information from one or more of their functional senses. There are many agnosias that can be divided into two forms in some types and characterised by the senses involved. Each type is unique and may be known by different names but knowing the differences can inform the management of conditions. If you are experiencing any of the symptoms described for the types of agnosia, consult a Healthcare Practitioner for further advice.