White matter disconnectivity fingerprints causally linked to dissociated forms of alexia.

For over 150 years, the study of patients with acquired alexia has fueled research aimed at disentangling the neural system critical for reading. An unreached goal, however, relates to the determination of the fiber pathways that root the different visual and linguistic processes needed for accurate word reading. In a unique series of neurosurgical patients with a tumor close to the visual word form area, we combine direct electrostimulation and population-based streamline tractography to map the disconnectivity fingerprints characterizing dissociated forms of alexia. Comprehensive analyses of disconnectivity matrices establish similarities and dissimilarities in the disconnection patterns associated with pure, phonological and lexical-semantic alexia. While disconnections of the inferior longitudinal and posterior arcuate fasciculi are common to all alexia subtypes, disconnections of the long arcuate and vertical occipital fasciculi are specific to phonological and pure alexia, respectively. These findings provide a strong anatomical background for cognitive and neurocomputational models of reading.

Pure alexia caused by separate lesions of the splenium and optic radiation.

Pure alexia is severe difficulty in reading and understanding written language but with normal oral language and writing abilities. We report a patient with pure alexia caused by two different infarct lesions in the left lateral thalamus and the left splenium of the corpus callosum. A 56-year-old right-handed man was admitted to hospital with right homonymous hemianopia associated with pure alexia. He could write kana characters but not kanji. His cranial magnetic resonance imaging revealed two different infarct lesions in the left optic radiation and the left splenium of the corpus callosum. Magnetic resonance angiography showed mild stenosis at the origin of the right vertebral artery and stenosis of the left distal posterior cerebral artery. The mechanism of developing pure alexia can be simply explained by disconnection. We assumed that agraphia of kanji was caused by the effect of ischemia and edema following transient obstruction in branches from the distal posterior cerebral artery.

Alexia without agraphia following biopsy of a left thalamic tumor.

Alexia without agraphia is a rare disconnection syndrome characterized by the loss of reading ability with retention of writing and verbal comprehension. We report a patient who developed alexia without agraphia after undergoing a biopsy for a malignant glioma involving the left thalamus. A 15-year-old right-handed male presented with 3 days of severe headache, and vomiting, and 1 month of blurry vision in his right visual field. Magnetic resonance imaging of the brain disclosed a large exophytic mass originating in the left thalamus, with mass effect and hydrocephalus. The patient underwent biopsy of the left thalamic mass via a transcallosal approach. Postoperatively, the patient complained of inability to read or identify letters. Examination revealed alexia without agraphia. The syndrome of alexia without agraphia can be rarely caused after surgery. A transcallosal procedure through the splenium of the corpus callosum may disrupt the visual association fibers traveling from the right occipital cortex to the left angular gyrus. In our case the syndrome occurred because of a preexisting right homonymous hemianopia resulting from a left thalamic tumor.