Causes
Pure alexia almost always involves an infarct to the left posterior cerebral artery (which perfuses the splenium of the corpus callosum and left visual cortex, among other things). The resulting deficit will be pure alexia - i.e., the patient can write but cannot read (even what they have just written). This is because the left visual cortex has been damaged, leaving only the right visual cortex (occipital lobe) able to process visual information, but it is unable to send this information to the language areas (Broca's area, Wernicke's area, etc.) in the left brain because of the damage to the splenium of the corpus callosum. The patient can still write because the pathways connecting the left-sided language areas to the motor areas are intact.
Read more about this topic: Pure Alexia