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Aphasia - The pathology behind speech defects

 Courtesy: Harrison’s principles of Internal medicine

Courtesy: Harrison’s principles of Internal medicine

You are probably thinking “Whoa! That does not look like ‘medicine made simple’! Relax. We are going to break down every column and row of that table with the knowledge of our previous article, the circuit of communication.

Assessment of speech is an integral part of neurological examination. Disorders of speech are like pieces of a bigger jigsaw puzzle. Since distinct parts of the brain control different aspects of speech, disorders of speech can give us valuable clues about the part of the brain affected. That brings us to the question:

What are the different aspects of speech to be assessed?

Well, firstly, there’s comprehension – the ability to make sense of what we hear and read. 

Secondly there’s repetition – our ability to repeat words or sentences even if they are in a foreign language that we don’t understand.

Then there’s naming – our ability to find words to denote an object, person or place.

And last but not the least; we have to assess the fluency – an aspect which deals with the rate, flow and continuity of speech.

Disability in any one of these aspects can lead to language impairments, commonly known as aphasia.

 
brain-input-output-connections
 

So, here’s a quick recap of the circuit. The written word is perceived by the primary visual cortex and the spoken word is perceived by the primary auditory cortex. These signals are sent to the Wernicke’s area which is the comprehension hub of the brain. With the help of some association areas in the cortex, the Wernicke’s area makes sense of these signals. The essence of it is conveyed to the motor hub of the brain – the Broca’s area via a bunch of neurons known as Arcuate fasciculus. The brain formulates a response here and executes it with the help of motor cortex. You see, repetition is possible even without the help of cortical association areas.

 
aphasia-flow-chart
 

Auditory signals can simply be relayed from the ear to the Broca’s area, bypassing the cortical association areas. This explains why we are able to repeat a foreign language without much understanding.

For unknown reasons, difficulty in word-finding or in naming an object, known as Anomia is a common feature in all types of Aphasias. When the patient is asked to name an object, he will fail to come up with the appropriate word. Instead he may describe the object (Example: “the thing used for travelling”) or may come up with the wrong word (example – “bus” for “boat”).

Now that we’ve got the basics figured, from here on it is simple mathematics!

Pathology in area A = Aphasia type A

The pathology could be anything – stroke, tumor or an abscess. But with different permutations and combinations of impairments of the 4 aspects of speech - comprehension, repetition, naming and fluency, this equation just gets more interesting and intriguing.

Lets deal with the simpler ones first. An infarct of the Wernicke’s area will obviously impair comprehension. The patient will not able to respond to a simple question like “What is your name?” However, surprisingly, a patient with Wernicke’s aphasia will be able to follow simple commands.  He will lift his hand if you ask him to because this does not involve a verbal response. This helps us to differentiate Wernicke’s aphasia from deafness and malingering. Since Broca’s area is intact in this situation, language output will be normal in terms of fluency.  Since signals from the auditory cortex cannot be relayed through a damaged Wernicke’s area, repetition is also impaired.

Wernicke's Aphasia:
Comprehension, naming and repetition impaired.
Fluency normal.
 

In a patient with Broca’s aphasia, the motor output of the circuit in terms of fluency, repetition and naming is impaired. Comprehension is preserved. However, Broca’s aphasia has to be differentiated from dysarthria, which is a difficulty in articulation. A patient with dysarthia will have slurring of speech, but his speech will otherwise be meaningful.

Broca's aphasia:
Fluency, repetition and naming impaired.
Comprehension normal.
 

If pathology spares the functionality of Wernicke’s and Broca’s area, but selectively involves the conduction fibers between the two areas, then comprehension and fluency of speech will be preserved, but repetition will be impaired. This is known as Conduction aphasia. On the other hand, if a patient suffers a sever stroke involving all the areas of the communication circuit, he will have a Global aphasia. The patient will neither be able to comprehend, nor be able to repeat or name objects.

Conduction aphasia:
Repetition and naming impaired.
Fluency and comprehension normal
 
Global aphasia:
All aspects impaired.
 

Another combination of symptoms could be impaired comprehension with intact repetition and fluency. These are features similar to Wernicke’s aphasia, but with intact repetition. This indicates that the lesion is in the cortical sensory association areas, disconnecting it from the main language circuit. Similarly, a patient may present with features similar to Broca’s aphasia, but with intact repetition. This occurs when the lesion disconnects the main language circuit from the main motor areas of the cortex. Some patients may have a combination of the two – inability to comprehend and to talk fluently, but able to repeat. The patient will merely be able to echo or repeat whatever the examiner says, but will be unable to comprehend or frame sentences on his own. This is known as Isolation aphasia.

Isolation aphasia:
Comprehension, fluency and naming impaired.
Repetition (Echolalia) + 

Strangely, some patients might be unable to comprehend the spoken word, but have no difficulty understanding the written word. These patients are not deaf either. They will be able to appreciate sounds as they have intact auditory cortex. Such patients are said to have "Pure word deafness". In time, patients with Pure word deafness learn lip reading and have been found to benefit from it.

The visual counterpart of Pure word deafness is Pure alexia. The patient will behave like an illiterate when asked to read from a book. But his response will be absolutely normal while having a conversation.

That brings us back to where we started!

 Courtesy: Harrison’s principles of Internal medicine

Courtesy: Harrison’s principles of Internal medicine

Seems simpler now, doesn’t it?

Author: Soundarya V (Facebook)

Sources and citations

Hall, John E., and Arthur C. Guyton. "Unit XI : Chapter 57 - Cerebral Cortex, Intellectual Functions of the Brain, Learning and Memory." Guyton and Hall Textbook of Medical Physiology. 12th ed. 698-705. Print.
M. Marsel Mesulam. "Section 3: Chapter 36: Aphasia, Memory Loss, and Other Focal Cerebral Disorders." Harrison's Principles of Internal Medicine. 19th ed. Vol. 1. 176-78. Print.