Aphasia

Excerpt

Aphasia is an acquired language disorder caused by damage to the brain's language centers, characterized by difficulties in verbal or written expression, comprehension, or both. Most cases of aphasia involve a combination of these impairments, affecting multiple language functions. Common clinical types include Broca and Wernicke aphasia, conduction aphasia, transcortical motor or sensory aphasia, and alexia, with or without agraphia.

Although the primary cause of aphasia is stroke, particularly ischemic stroke, other causes include traumatic brain injury (TBI), brain tumors, and neurodegenerative diseases. Patients may present with symptoms such as difficulty articulating words, forming sentences, comprehension deficits, or a combination of these. Aphasia symptoms can range from mild impairment to a complete loss of fundamental language components, including semantics, grammar, phonology, morphology, and syntax. They may affect verbal communication, written language, or, more commonly, both.

The classical model of aphasia was developed by Wernicke and Lichtheim in the 19th century and was further refined neuroanatomically by Geschwind in the 1960s. This model provides the foundation for understanding aphasia's clinical features and the related neuroanatomical lesions. The brain's language centers are typically located in the dominant hemisphere, most often the left, within the peri-Sylvian region. Spoken language is received by the primary auditory cortices in the Heschl gyrus (transverse temporal gyrus) and processed in the Wernicke area in the posterior superior temporal gyrus. Written language is transmitted from the primary visual cortex in the occipital lobe to the angular gyrus and then to the Wernicke area.

The Broca area, located in the inferior frontal region, is responsible for the motor execution of speech and sentence formation. The arcuate fasciculus is the neural pathway that connects the Wernicke area to the Broca area (see Image. The Broca and Wernicke Areas of the Brain). According to the classical aphasia model, specific aphasia syndromes correspond to the location of the brain lesion. A posterior lesion involving the Wernicke area results in fluent aphasia, characterized by impaired comprehension and severe paraphasia. In contrast, an anterior lesion affecting the Broca area leads to nonfluent aphasia, in which patients have normal comprehension but produce speech that is telegraphic, effortful, and dysprosodic, without paraphasic errors. A lesion in the arcuate fasciculus or the white matter tract connecting the Wernicke and Broca areas results in conduction aphasia, characterized by impaired repetition and phonemic paraphasia.

Global aphasia is the most common type of aphasia, impacting both language comprehension and expression to varying extents. Transcortical motor aphasia is a nonfluent type, similar to Broca aphasia, but with preserved repetition. Transcortical sensory aphasia is a fluent aphasia with impaired comprehension, resembling Wernicke aphasia but with intact repetition. Patients with transcortical motor or sensory aphasia often display excessive repetition, such as perseveration or echolalia. Anomia is a milder form of aphasia resulting from a small lesion in the dominant peri-Sylvian region.

The contemporary language model, or the dual-stream model, was developed by Hickok and Poeppel, and is supported by modern neuroimaging studies, including functional magnetic resonance imaging (MRI), diffusion tensor imaging, and MRI tractography. This dual-stream model outlines 2 main language processing streams involving cortical and subcortical structures, as mentioned below.

1. The dorsal stream: This is located in the dominant hemisphere region and processes auditory-to-articulation information, connecting the frontal speech areas and the temporoparietal junction. This stream is crucial in fluent speech production. Lesional analysis indicates that the dorsal stream primarily involves the gray matter of the frontoparietal regions.

2. The ventral stream: This is located in both temporal lobes and processes auditory-to-meaning information, which is essential for auditory comprehension. This stream encompasses much of the gray matter in the lateral temporal lobe. Conduction aphasia results from lesions in gray matter, particularly in the area Spt (Sylvian fissure, parietal-temporal junction), a posterior region that is part of the dorsal stream, rather than from involvement of the white matter tract of the arcuate fasciculus.

In addition to cortical language areas, subcortical structural lesions can also lead to aphasia by disrupting the connections within the cortical-subcortical language networks. However, these causes are generally rare. Lesions in the basal ganglia, thalamus, and cerebellum may occasionally result in aphasia. Typically, aphasia resulting from basal ganglia lesions is mild, characterized by impaired language expression, such as word fluency, while comprehension and repetition remain intact.

Thalamic aphasia occurs when the left-sided ventral anterior or paramedian nuclei are affected and can be either fluent or nonfluent. This type of aphasia primarily results in lexical-semantic deficits, with relative preservation of repetition. Rarely, cerebellar lesions on either side may lead to aphasia, typically characterized by deficits in word retrieval, semantics, and syntax. Overall, subcortical aphasia tends to be milder and associated with a better prognosis.