L3 Acquired Neurogenic Language & Communication Disorders

Acquired Neurogenic Language & Communication Disorder: An Introduction

Introduction to Acquired Neurogenic Communication Disorders

  • Acquired: Indicates that the disorder was not present at birth.

  • Neurogenic: Refers to involvement of the central nervous system (CNS), including the brain and spinal cord.

  • Communication: Defined as the act of giving, receiving, and sharing information.

  • Disorder: Signifies a functional abnormality.

  • Categories of Acquired Neurogenic Communication Disorders:

    • Aphasia

    • Dysarthria

    • Apraxia of speech

    • Cognitive communication disorder

What is Aphasia?

  • Definition: An impairment in the comprehension or formulation of language, resulting from damage to the cortical centers responsible for language.

  • Common Cause: While many different brain diseases and disorders can cause aphasia, a cerebrovascular accident (CVA), also known as a stroke, is the most common reason.

  • Symptoms: Symptoms can range significantly from mild impairment to the complete loss of any fundamental components of language, including:

    • Semantic (meaning of words)

    • Grammar

    • Phonology (sound system)

    • Morphology (word structure)

    • Syntax (sentence structure)

  • Impacted Language Modalities: Aphasia affects verbal comprehension, verbal expression, reading, and writing.

Etiology of Aphasia

  • Most Common Cause: Cerebrovascular Accident (CVA).

  • Other Causes: Aphasia can also be observed in various neurodegenerative diseases and other conditions:

    • Alzheimer disease

    • Frontotemporal lobar degeneration

    • Vascular dementia

    • Some forms of Parkinson disease

    • Brain tumor

    • Traumatic brain injury (TBI)

    • Infections (can lead to mass effects from brain tumors or direct injury)

Prevalence of Aphasia

  • United States:

    • Approximately 180,000 new cases of aphasia are reported each year (National Institute on Deafness and Other Communication Disorders (NIDCD)).

    • 1 out of every 272 Americans is affected by aphasia.

  • Hong Kong:

    • Over 20,000 new cases of stroke occur annually.

    • Up to 38% of stroke patients in Hong Kong are affected by aphasia (Kong, et al., 2018).

Language Coding & Decoding: Brain Structures and Processes

  • Language Dominance: The language area of the brain is typically located in the dominant hemisphere, which is usually the left side for the majority of the population (Lahiri, et al., 2019).

  • Key Brain Structures:

    • Wernicke Area:

      • Location: Posterior end of the superior temporal gyrus (Temporal lobe).

      • Function: Processes visual and auditory information, converting spoken or read words into meaningful ideas. It is the center for comprehension and the planning of words, where words are selected to convey ideas.

    • Broca Area:

      • Location: Inferior frontal area (Frontal lobe).

      • Function: Center for the motor execution of speech and sentence formation. It converts thoughts into a program of muscle movements for the mouth and throat to produce speech (Ochfeld, et al., 2010).

    • Arcuate Fasciculus:

      • Definition: A neural pathway that directly connects the Wernicke area to the Broca area.

The Connectionist Model of Language

  • Metaphor: This model describes how the language-dominant hemisphere interprets incoming verbal messages and then formulates, plans, and executes verbal and gestural responses (Brookshire & McNeil, 2014).

  • Spontaneous Speech Process:

    1. Wernicke's Area Role (Meaning Formulation): Retrieves necessary words from the central lexicon and constructs a sentence adhering to phonologic, syntactic, and semantic rules.

    2. Transmission to Broca's Area: Sends the neurally coded sentence to Broca's area via the arcuate fasciculus.

    3. Broca's Area Role (Action Plan): Translates the code into an action plan for speech production.

    4. Primary Motor Cortex Role: Puts finishing touches on the message and sends it down through the pyramidal system to the cranial nerves.

    5. Speech Muscle Activation: Cranial nerves set the speech muscles into motion.

    6. Self-Monitoring and Repair: Wernicke's area continuously monitors the produced speech to ensure it matches the intended message. Any mismatch triggers a repair mode.

  • Comprehension of Speech Process:

    1. Auditory Input: Message travels from the ears via ascending fibers to the primary auditory cortex in the temporal lobes.

    2. Encoding and Transmission: Auditory cortices encode the acoustic information and send the encoded message to Broca's area.

    3. Semantic Processing in Wernicke's Area: Upon recognizing the message as speech with a known phonologic form, Wernicke's area sorts through semantic representations to find word meanings.

    4. Syntactic Analysis in Wernicke's Area: Consults syntactic rules to determine relationships between words.

    5. Meaning Deduction and Response Planning: Once the sentence's meaning is deduced, Wernicke's area determines if it should be interpreted literally or figuratively and sends instructions to other brain areas for the appropriate response (e.g., talk, write, gesture).

  • Writing Process:

    1. Message Formulation in Wernicke's Area: Formulates a message with appropriate words, correct syntactic order, and spelling.

    2. Transmission to Premotor Cortex: Sends the message via the arcuate fasciculus to the premotor cortex for hand and arm movements, setting up movement plans.

    3. Transmission to Motor Cortex: Arcuate fasciculus relays plans to the motor cortex.

    4. Monitoring: Vision and Wernicke's area collaborate to monitor the written output.

    5. Repair: If Wernicke's area is not satisfied, corrections are made.

  • Oral Reading Process:

    1. Visual Perception and Encoding: The text is perceived and translated into a neural code in the visual cortex that Wernicke's area can understand.

    2. Meaning Extraction in Wernicke's Area: The coded message is sent to Wernicke's area, where its meaning is extracted.

    3. Recoding for Broca's Area: Wernicke's area recodes the sentence into a form suitable for Broca's area and transmits it via the arcuate fasciculus.

    4. Articulatory Plan in Broca's Area: Broca's area recodes the phrase into an articulatory plan for the speech muscles and sends it to the primary motor cortex.

    5. Speech Muscle Activation: The primary motor cortex sends the message down pyramidal fibers to the cranial nerves, which move the speech muscles.

Pathophysiology of Aphasia

  • Primary Cause: Lesions to the language areas of the brain, predominantly in the dominant (typically left) hemisphere, specifically involving the Wernicke area, Broca area, and arcuate fasciculus.

  • Most Common Cause - CVA:

    • Occurs due to diminished or stopped blood supply to a part of the brain.

    • Ischemic Event: Embolism (blood clot travels) or thrombosis (blood clot forms locally).

    • Hemorrhagic Event: Intracerebral hemorrhage (bleeding within the brain) or subarachnoid hemorrhage (bleeding in the space surrounding the brain).

    • Subcortical Damage: In some instances, aphasia can arise from damage to subcortical structures deep within the left hemisphere, including the internal and external capsules, the thalamus, and the caudate nucleus.

  • Other Causes:

    • Progressive Deterioration of Brain Tissue: Seen in neurodegenerative diseases like Alzheimer disease, Pick disease, vascular dementia, and some forms of Parkinson disease.

    • Direct Physical Injuries: Traumatic brain injury (TBI).

    • Other Injuries: Infections or mass effects from brain tumors.

Different Types of Aphasia

  • Classification: Aphasia syndromes are categorized based on their fluency of speech, comprehension abilities, and repetition abilities, which are dependent on the lesion's specific location (Grossman & Irwin, 2018).

    • Fluent Aphasia: Patients speak in sentences that sound normal in rhythm and intonation but may contain made-up words (neologisms) or incorrect sounds (paraphasias). Types include Wernicke, transcortical sensory, conduction, and anomic aphasia.

    • Non-fluent Aphasia: Patients struggle to produce words, omit words, and speak in very short, effortful sentences. Types include Broca, transcortical motor, mixed transcortical, and global aphasia.

Specific Aphasia Syndromes

  • Wernicke Aphasia (Receptive Aphasia):

    • Lesion: Wernicke area (center for comprehension and word planning).

    • Comprehension: Markedly impaired; patients are unable to understand written and/or spoken words.

    • Expression: Speech is fluent but often meaningless, characterized by paraphasias (phonemic and semantic), neologisms, and jargon. Naming abilities are typically poor.

  • Broca Aphasia (Expressive Aphasia):

    • Lesion: Broca area (center for motor speech production and sentence formation).

    • Comprehension: Generally relatively well-preserved, and patients may be able to read.

    • Expression: Non-fluent. Patients struggle to formulate grammatically correct spoken and written language, often producing