User:Marcetk/Transcortical sensory aphasia draft

Transcortical sensory aphasia is caused by lesions in the inferior left temporal lobe of the brain located near Wernicke’s area, and is usually due to minor hemorrhages or contusions in the temporal lobe, or infarcts of the left posterior cerebral artery (PCA). This type of aphasia is very similar to Wernicke’s aphasia in that patients exhibit poor comprehension and naming, have fluent spontaneous speech, and exhibit paraphasia. However, Transcortical sensory aphasia differs from Wernicke’s aphasia in that patients still have intact repetition and exhibit echolalia, or the compulsive repetition of words. Transcortical sensory aphasia cannot be diagnosed through brain imaging techniques, and the results of functional magnetic resonance imaging (fMRI) are difficult to interpret. Therefore, clinicians rely on assessment and observation to determine if a patient presents with the characteristics of TSA.

Imaging
Sensory aphasia cannot be diagnosed through the use of imaging techniques. Differences in cognition between asymptomatic subjects and affected patients can be observed via functional magnetic resonance imaging (fMRI). However; these results only reveal temporal differences in cognition between control and diagnosed subjects. The degree of progression during therapy can also be surveyed through cognition tests monitored by fMRI. Many patients’ progress is assessed over time via repeated testing and corresponding cerebral imaging by fMRI.

Sensory aphasias are typically diagnosed by non-invasive evaluations. Neurologists or speech pathologists will administer oral evaluations to determine the extent of a patient’s comprehension and speech capability. The following standardized testing techniques are most commonly used in the diagnosis of transcortical sensory aphasia.

Clinical Assessment
Initial assessment will determine if the cause of linguistic deficiency is aphasia. If the diagnosis is then confirmed, testing will next address the type of aphasia and its severity. The Boston Diagnostic Aphasia Examination specializes in determining the severity of a sensory aphasia through the observation of conversational behaviors. Several modalities of perception and response are observed in conjunction with the subject’s ability to process sensory information. The location of the brain lesion and type of the aphasia can then be inferred from the observed symptoms. The Minnesota Test for Differential Diagnosis is the most lengthy and thorough assessment. It pinpoints the subject’s strengths and weaknesses in the areas of audio, vision, reading comprehension, speech, and writing. From this differential diagnosis, a patient’s course of treatment can be determined. After treatment planning, the Porch Index of Communicative Ability is the best test of prognosis and the degree of recovery.

Characteristics
Transcortical sensory aphasia is characterized as a fluent aphasia. Fluency is determined by direct qualitative observation of the patient’s speech to determine the length of spoken phrases, and is usually characterized by a normal or rapid rate; normal phrase length, rhythm, melody, and articulatory agility; and normal or paragramatic speech. Transcortical sensory aphasia is a disorder in which there is a discrepancy between phonological processing, which remains intact, and lexical-semantic processing, which is impaired. This disconnect occurs since Wernike’s area is not damaged in patients with TSA, therefore repetition is spared while comprehension is affected. In addition to problems in comprehension, Transcortical sensory aphasia is further characterized based on deficits in naming and paraphasia.

Comprehension
When clinically examined, patients with TSA will exhibit poor comprehension of verbal commands. Based on the extent of the comprehension deficiency, patients will have difficulty following simple commands such as “close your eyes.” Depending on the extent of affected brain area, patients are able to follow simple commands but may not be able to comprehend more difficult, multistep commands such as, “point to the ceiling, then touch your left ear with your right hand." When increasing the complexity of verbal commands comprehension is often tested by varying the grammatical structure of the command to determine whether or not the patient understands different grammatical variations of the same sentence.

Naming
Naming involves the ability to recall an object. Patients with TSA, as well as patients with all other aphasia subtypes, exhibit poor naming. Clinical assessment of naming involves the observer first asking the patient to name high frequency objects such as clock, door, and chair. TSA patients who name common objects with ease generally have difficulty naming both uncommon objects and specific parts of objects such as lapel, or the dial on a watch.

Paraphasia
Patients with TSA typically exhibit paraphasia; their speech is fluent but often error-prone. Their speech is often unintelligible as they tend to use the wrong words, e.g. tree instead of train or uses words in senseless and incorrect combinations.

Wernicke-Lichtheim Connectionist Model
In 1874, Carl Wernicke claimed that thought and language were supported by two distinct regions in the brain. He believed that disturbances in language due to brain damage resulted from damage to psycholinguistic functions that were represented by these specific areas. These areas are the center for acoustic images, found in the temporal lobe cortex, and the center for motor images, located in the inferior frontal region, which are connected by subcortical fiber tracts. According to Wernicke, sounds were sent to the center for acoustic images via the acoustic nerve. Wernicke also proposed the idea that these two centers, along with the commissure linking the acoustic nerve with the center for motor images, were the first structures used when a child is beginning to acquire language abilities through imitation of what he hears. Ludwig Lichtheim, another German physician, was influenced by the work of Wernicke and Broca and is responsible for developing the localizationalist concept of aphasia. He had a great interest in aphasia, especially those that he believed could not be explained by Wernicke’s model. He proposed that these types of aphasia were due to disturbances in the pathways that connect major speech centers instead of the speech centers themselves. According to Lichtheim, there were specific centers in the brain for auditory images and motor images, and these centers were connected by a commissure that passes through the insula, similar to Wernicke’s proposal. However, Lichtheim postulates the existence of a third center where concepts are elaborated because he believed that other parts of the brain had to be accessed when less automatic characteristics of language, such as comprehension, were involved. Pathways that link the auditory center to non-language areas where concepts are determined accomplish this. Lichtheim also presents the idea that the phonological information that is used in verbal output is controlled by not only the direct connections linking the center for auditory images and the center of motor images, but also by an indirect connection linking these two centers with the non-language concept center. According to his model, the commissural pathways that link the concept center with the sensory and motor speech centers were include two separate commissures and a set of converging fiber tracts that come from various regions of the cerebral cortex to the sensory and motor centers. Lichtheim suggested that a lesion interrupting the commissures between the auditory image center and the concept center would cause a sensory aphasia with fluent paraphasic speech and difficulty in understanding both spoken and written language, similar to Wernicke’s aphasia. However, unlike Wernicke’s aphasia, the lesion would not disrupt the capacity to repeat, read aloud, or write to dictation, although there is a loss of intelligence regarding what the patient comprehends. Lichtheim discusses a patient in an 1885 article who could repeat what was spoken to him and had fluent expression of language but had poor comprehension of what was spoken to him. Lichtheim first refers to this as inner-commissural word-deafness. In 1908, Wernicke recognizes aphasias in which repetition remains intact and refers to them as transcortial aphasias, using transcortical sensory aphasia to refer to Lichtheim’s “inner-commissural word-deafness.”