Aphasia is a term to describe a disturbance of language function that results from brain injury. Most commonly, clinicians characterize aphasia as either a Broca's aphasia/expressive aphasia (with decreased verbal fluency) or a Wernicke's aphasia/receptive aphasia (with decreased comprehension) depending on the location of the brain lesion. Conduction aphasia is a rare form of aphasia were both expression and comprehension remain intact, but the patient shows an isolated impairment in the ability to repeat simple phrases. Carl Wernicke first proposed that a disconnection between the two speech systems (Broca's area and Wernicke's area) leads to this unique condition. Ludwig Lichtheim and Norman Geschwind expanded on Wernicke’s work. This model became the common explanation for conduction aphasia. In this model, there is a lesion in the arcuate fasciculus which connects the two para-Sylvian speech centers. Recent studies have suggested that this is an incomplete model of conduction aphasia in that lesions outside of the arcuate fasciculus can also lead to the clinical presentation of this phenomenon.
Lesions in Broca's area in the inferior frontal lobe can lead to loss of verbal fluency. Whereas, lesions in the Wernicke's area lead to impaired comprehension. Patients with lesions in either Broca's or Wernicke's area will have impaired repetition. However, there is a group of patients with relatively intact comprehension and verbal fluency but will display severely impaired repetition. A lesion such as a stroke or a tumor affecting the arcuate fasciculus which connects Broca's and Wernicke's areas can lead to this condition. More recent evidence suggests that lesions of the left superior temporal gyrus, the left supramarginal gyrus, the left inferior parietal lobe (Brodmann's area 40), the left primary auditory cortices (Brodmann's area 41 and 42), and the insula, can all lead to this condition. Thus, lesions in multiple areas surrounding the Sylvian fissure may impair repetition. This likely represents the interconnectivity of the language areas of the brain.
Broca's and Wernicke's aphasia are relatively common in middle cerebral artery strokes. A pure conduction aphasia affecting only repetition is uncommon.
The area that mediates language in the brain is in the dominant hemisphere. In most people, this is the left hemisphere the brain. Broca's areas reside in the inferior frontal lobe in Brodmann's area 44 and 45. Wernicke's area is in the region of the superior temporal gyrus in Brodmann's area 22. The arcuate fasciculus connects these two regions. The classical explanation for conduction aphasia is that damage to the arcuate fasciculus impairs the transmission of information between Wernicke’s area and Broca’s area. This injury leads to impaired repetition. Thus, the patient can comprehend the speech but cannot transmit the information the speech production centers in Broca's area to allow repetition to occur. Recent research based on anatomically distributed modular networks model shows that patients with conduction aphasia clinically often have lesions in the supramarginal gyrus or deep parietal matter, which suggests that damage to anatomically related structures may also lead to a disconnection between Broca's and Wernicke's areas.
The findings in conduction aphasia may be subtle because of the lack of neurological findings. The patient may complain of trouble coming up with words, or making errors when he/she tries to speak. During the assessment of aphasia, the clinician should examine the patient's verbal fluency, comprehension, repetition, reading, writing, and naming. A patient with relatively well-preserved auditory comprehension, fluent speech production, reading, writing, but poor speech repetition may have a conduction aphasia. Patients may display well-articulated responses similar to the target word and continue to repeat words or phrases to correct the error (conduit d’approache). The rest of the neurological exam (cranial nerves, motor, sensory, reflexes, gait, coordination) is typically normal.
The finding of conduction aphasia at the bedside suggests the possibility of a lesion in the dominant hemisphere of the brain, specifically the areas that connect Wernicke's and Broca's area. The clinician should perform neuro-imaging to look for a stroke, tumor, infection, or another pathology in the setting of conduction aphasia. CT or MRI of the brain is the first imaging modality of choice. Depending on the results, further testing may be necessary to determine the precise etiology.
There is no standard treatment for aphasia. Speech and language therapy is the core mainstay of care for patients with aphasia. The therapist should tailor the treatment to the individual needs of the patient. Conduction aphasia is a relatively mild language impairment, and most patients can return to day-to-day life. No medical or surgical treatment improves the outcome in patients with aphasia. However, if the underlying lesion is an infection or tumor, then removing the offending lesion may prevent worsening of the deficit.
Aphasia differs from dysarthria which results from impaired articulation. Dysarthria is a motor dysfunction due to disrupted innervation to the face, tongue or soft palate that results in slurred speech. Conduction aphasia differs from Wernicke's and Broca's aphasia in that there is an isolated inability to repeat. Neurodegenerative disorders such as primary progressive aphasia lead to a gradual loss of language function. However, this condition affects all aspects of language. Trans-cortical motor and sensory aphasias do not affect repetition. Anomic aphasia affects only the ability to name objects and does not affect repetition.
The prognosis for conduction aphasia depends on the underlying cause. If the cause is a stroke, patients can make a good recovery but may have persistent deficits. Speech therapy may be useful to help remediate the effects of the aphasia and restore normal function.
Conduction aphasia is less disabling that other types of aphasia. However, it still may cause issues with communication. Education regarding the patient's condition should involve the patient, family, and caregivers.
When patients complain of stumbling over words, it is important for clinicians to ask the patient to repeat simple phrases to test for a conduction aphasia. Impaired repetition suggests a lesion in the para-Sylvian region of the dominant hemisphere. Therefore, the clinician should get neuro-imaging to look for pathology affecting this region of the brain.
Patients with conduction aphasia may take weeks to months to recover. The patient, family, and caregivers, and the treatment team need to know of the patient's deficits and be supportive of the recovery efforts to maximize recovery. Patients should continue to engage in social and leisure activities to avoid social isolation. Importantly, patients with conduction aphasia rarely have intellectual deficits, but only an isolated difficulty in repetition.