The identifying feature of these language disturbances is a preservation of the ability to repeat. Destruction of the vascular border zones between anterior, middle, and posterior cerebral arteries, usually as a result of prolonged hypotension, carbon monoxide poisoning, or other forms of anoxic–ischemic injury, may effectively isolate the intact motor and sensory language areas, all or in part, from the rest of the cortex of the same hemisphere. In the case reported by Assal and colleagues, for example, multiple infarcts had isolated all of the language area.
In transcortical sensory aphasia the patient suffers a deficit of auditory and visual word comprehension, making writing and reading impossible, in every way conforming to Wernicke aphasia. Speech remains fluent, with marked paraphasia, anomia, and empty circumlocutions. However, unlike the deficit in Wernicke and conduction aphasias, the ability to repeat the spoken word is preserved. This facility in repetition may be of extreme degree, taking the form of echoing, parrot-like, word, phrases, and songs that are heard (echolalia). In a series of 15 such patients, CT and isotope scans have uniformly disclosed a lesion in the posterior parietooccipital region, according to Kertesz and colleagues. In general, this disorder has a good prognosis.
Presumably, in transcortical sensory aphasia, as in Wernicke aphasia, information cannot be transferred to the Wernicke area for conversion into word meaning. Paraphasia is thought to result from the reduced control of the motor language areas by the auditory and visual areas, though the direct connection between them, presumably the arcuate fasciculus, is preserved. Preservation of this direct connection is said to account for the ability to repeat.
In transcortical motor aphasia, the patient is unable to initiate conversational speech, producing only a few grunts or syllables as in Broca-type aphasia. Comprehension is relatively preserved, but repetition is strikingly intact, distinguishing this syndrome from pure word mutism. Transcortical motor aphasia occurs in two clinical contexts: (1) in a mild or partially recovered Broca aphasia in which repetition remains superior to conversational speech (repeating and reading aloud are generally easier than self-generated speech) and (2) in states of abulia and akinetic mutism with frontal lobe damage. Several cases under our observation have resulted from infarctions in the watershed zone between the anterior and middle cerebral arteries after cardiac arrest or shock.
This uncommon disorder, a derivative of Wernicke aphasia originally described by Lichtheim in 1885, is characterized by an impairment of auditory comprehension and repetition and an inability to write to dictation. Self-initiated utterances are usually correctly phrased but sometimes paraphasic; spontaneous writing and the ability to comprehend written language are preserved, thus distinguishing this disorder from Wernicke aphasia. Patients with pure word deafness may declare that they cannot hear, but shouting does not help, sometimes to their surprise. Audiometric testing and auditory evoked potentials disclose no hearing defect, and nonverbal sounds, such as a doorbell, can be heard without difficulty. The patient is forced to depend heavily on visual cues and frequently uses them well enough to understand most of what is said. However, tests that prevent the use of visual cues readily uncover the deficit. If able to describe the auditory experience, the patient says that words sound like a jumble of noises. As in the case of visual verbal agnosia (see the following text), the syndrome of pure auditory verbal agnosia is not pure, particularly at its onset, and paraphasic and other elements of Wernicke aphasia may be detected (Buchman et al). At times, this syndrome is the result of resolution of a more typical Wernicke aphasia, and it will be recognized that word deafness is an integral feature of all instances of Wernicke aphasia. Conceptually, it has been thought of as an exclusive injury of the auditory processing system therefore allowing relative preservation of internal language.