E Mulroy¹, S Murphy², T Lynch^1
1Dublin Neurological institute and ²Department of Old Age Medicine, Mater Misericordiae University Hospital, Eccles St, Dublin 7


Abstract
Alexia without agraphia (also called pure alexia or word blindness) was the first of the disconnection syndromes (syndromes caused by disconnection of the right from the left cerebral hemisphere through interruption of the communication pathways between them) to be described. Déjerine in 1892 reported a patient who developed this syndrome after an infarct of the left occipital lobe and splenium of the corpus callosum. We describe a patient who developed alexia without agraphia due to an embolic left occipital lobe infarct extending to the posterior commissure and splenium of the corpus callosum.


Case Report
A 78-year old man presented with 2 episodes of transient visual loss. He had a past medical history of COPD, ischaemic heart disease and a recent diagnosis of atrial fibrillation. On careful questioning, he admitted to transiently losing vision in the right half of the visual field of both eyes. His symptoms resolved completely over 1 hour. He noted that although he could read the actual letters on his paper, he had difficulty putting them together to form words and was unable to comprehend what he was reading. Interestingly, he had no problem with comprehension of words read aloud. He had no problems with either spontaneous writing or writing in response to dictation.


Figure 1: T2 Axial FLAIR sequence showing a large left occipital lobe infarct extending to the splenium of the corpus callosum and posterior commissure 



Neurological examination was reported as normal but formal visual field testing using a Humphrey visual field analyser showed a right superior quadrantopia. CT Brain showed a large area of hypoattenuation in the left occipital lobe suggestive of infarction. This was confirmed by MRI Brain which showed a subacute infarct in the left occipital lobe extending rostrally to the corpus callosum (Figure 1). A diagnosis of alexia without agraphia due to a cardioembolic stroke was made and he was started on Warfarin therapy and discharged home.



Figure 2: Diagrmatic representation of the impaired trans-callosal tranfer of information from the single functioning right occipital cortex to the language centres in the left hemisphere.(VWF=Visual Word Form system)





Discussion
We report a patient with alexia without agraphia, the first well described callosal disconnection syndrome. It was first reported by Déjerine in two papers in 1891 and 1892, wherein he described two stroke patients, one of whom developed agraphia and alexia, the other who developed alexia without agraphia after the ischaemic event^1,7. Post-mortem analysis of infarct location in these two patients allowed Déjerine to propose a pathophysiological cause for this disconnection syndrome. Geschwind confirmed these theories in his series on disconnection syndromes in 1965² and these disorders of visual pattern recognition were further detailed in the 1980’s by Damasio^8,9, one of Geschwind’s students.



Each occipital hemisphere receives information from the opposite half of the visual world (Figure 2: blue lines). For this visual information to be interpreted it must then be transferred to the visual word form system (VWF) located in the left inferior occipitotemporal white matter (the Angular Gyrus; Figure 2: red circle), which is specifically dedicated to the processing of letter strings. Information from the left side of the visual field which is received by the right visual cortex must therefore be transferred to the word form system in the left hemisphere. This transfer occurs through the posterior commissure and splenium of the corpus callosum. This information transfer is disrupted in alexia without agraphia (Figure 2: yellow lines). This pathway has been confirmed by functional Brain imaging studies³. Spontaneous writing and writing in response to dictation are usually initiated in Wernicke’s area, while lesions of the left posterior frontal lobe and left superior parietal lobe lesions can cause pure agraphia without alexia^4-6.


Our patient with alexia without agraphia confirms the classic location of left occipital lobe infarcts involving the posterior commissure and posterior corpus callosum. In these patients, damage to the posterior commissure and splenium of the corpus callosum interrupts the connections between the remaining functioning right occipital cortex and the language centres in the dominant left hemisphere resulting in alexia. Agraphia is not present as the connections involving the frontal and parietal lobes involved in writing are not interrupted.



Correspondence: E Mulroy
Dublin Neurological Institute at the Mater Misericordiae University Hospital, Eccles Street, Dublin 7
Email: eoinmulroy@gmail.com




References
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