arcuate fasciculus

**Anatomy:** The arcuate fasciculus is a long association fiber bundle in the dominant (usually left) hemisphere. It arches around the posterior end of the Sylvian fissure, linking the posterior superior temporal region (Wernicke's area) to the inferior frontal region (Broca's area). It is part of the superior longitudinal fasciculus complex. Classically, it runs deep to the insula and supramarginal gyrus region. Tractography (DTI) shows both direct and indirect (via inferior parietal lobule) pathways.

**Function:** It supports phonological processing and the transfer of auditory-language information to motor-speech areas. This is essential for repeating heard words and for phonemic monitoring of speech.

**Clinical significance - Conduction aphasia:** - Fluent speech, but with frequent phonemic (literal) paraphasias - Intact comprehension (Wernicke's area intact) - Intact motor speech production (Broca's area intact) - Impaired repetition (hallmark) - Patients are aware of their errors and often make self-correction attempts (conduction d'approche), which causes frustration - Naming is often impaired

**Aphasia comparison (high-yield):** - Broca: nonfluent, comprehension intact, repetition impaired. Lesion in the inferior frontal gyrus. Supplied by the superior division of the left MCA. - Wernicke: fluent, comprehension impaired, repetition impaired. Lesion in the superior temporal gyrus. Supplied by the inferior division of the left MCA. - Conduction: fluent, comprehension intact, repetition impaired. Lesion in the arcuate fasciculus (and/or supramarginal gyrus/insula). Usually from an MCA branch (often the inferior division) stroke. - Global: nonfluent, comprehension impaired, repetition impaired. Large MCA infarct. - Transcortical motor: nonfluent, comprehension intact, repetition intact. Often involves the ACA-MCA watershed zone. - Transcortical sensory: fluent, comprehension impaired, repetition intact. Posterior watershed zone (MCA-PCA). - Transcortical mixed (isolation aphasia): nonfluent, comprehension impaired, repetition intact. - Anomic: fluent, comprehension intact, repetition intact, with word-finding difficulty only. Nonspecific and can occur with many lesions.

**Mnemonic:** Conduction = "Can't repeat" with the "connection" cut between Wernicke and Broca.

**Test-taking pearls:** A patient who speaks fluently with paraphasic errors, understands commands, and cannot repeat a phrase such as "no ifs, ands, or buts" points to an arcuate fasciculus lesion. Language dominance is on the left in about 95% of right-handed people and about 70% of left-handed people. The most common etiologies are ischemic stroke (left MCA, posterior branches), tumor, and trauma.