The evolution of alexia and simultanagnosia in posterior cortical atrophy

Early alexia and higher visual impairments characterize Posterior cortical atrophy (PCA), a progressive dementing syndrome most often caused by Alzheimer disease. Posterior cortical atrophy is rare, and the nature of the visual impairments in PCA are unclear. The authors observed two patients who had an insidiously progressive reading difficulty characterized by letter-by-letter reading and otherwise intact cognitive functions. Over time, these patients developed "ventral simultanagnosia" with preserved detection of multiple stimuli but inability to interpret whole scenes. Subsequently, they progressed to Balint syndrome with "dorsal simultanagnosia," optic ataxia, and oculomotor apraxia. Structural imaging was normal, but functional imaging revealed posterior cortical dysfunction. On a letter reading task, both patients had a word superiority effect, and on a whole word reading task, they could not read most words with missing or crosshatched letters. An inability to assess whole scenes progressed to an inability to detect more than one stimulus in an array. These findings suggest an evolution of PCA with progressive difficulty in visual integration beginning with letters, progressing to whole scenes, and culminating in Balint syndrome. These changes may reflect an extension of the pathophysiology of PCA from the extrastriate visual cortex to its occipitotemporal and occipitoparietal connections.