1. VASCULAR - OCCLUSION OF THE POSTERIOR CEREBRAL ARTERY
A Sample Case: Upon awakening a young school teacher went to her class and was astonished to find that she could not read the materials on her desk. She tried to read small printed items then large printed items but could not comprehend them.
At her neurologic exam she was intelligent, alert, and well oriented. She demonstrated a right homonymous hemianopsia but other cranial nerves were normal as was her muscular strength and sensory systems. She understood information presented orally or auditorily but could not name letters of the alphabet shown to her visually though she could identify and name them by touch. Her response to letters presented visually was to describe their shape but she could not name what they were. When asked to write her insurance information she did so accurately but couldn’t read back what she wrote.
An homonymous hemianopsia is due to injury to the visual pathway behind the optic chiasm - a visual defect in the right visual field is due to injury on the left side. Such a visual defect would not in itself cause an inability to read, the left visual field would be used instead. Language functions are usually a property of the left hemisphere and injury could result in language difficulties of motor or receptive types. She shows that her motor speech, whether spoken or written, is normal and information presented orally or by touch is handled well. She demonstrates a pure alexia - an inability to read.
Her post-chiasmal lesion on the left has deprived her left calcarine cortex, which ordinarily would deal with language, of direct visual input. The left visual field and its visual pathway to the right hemisphere is intact (otherwise she would be blind). Visual information from the right visual cortex would ordinarily be transferred to the left hemisphere through the corpus callosum for integration of visual field information. However, here the callosal fibers are damaged. The language dominant left hemisphere gets neither direct input from the right visual field nor indirect input from the left visual field via the corpus callosum hence she cannot comprehend written words.
The suddenness of the loss suggests a vascular etiology and the symptoms are referable to branches of the posterior cerebral artery. Vascular studies demonstrated an occlusion in branches of the left posterior cerebral artery supplying the calcarine cortex and adjacent splenium of the corpus callosum.