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Posterior or pure alexia is an uncommon acquired reading disturbance in which the loss of the ability to read is not associated with other language deficits.1 It has been reported almost without exception after left (dominant) hemispheric lesions involving visual temporo-occipital pathways and splenium of the corpus callosum, and is considered to be a strong lateralising feature. The explanations of this deficit are on the basis of: (a) a disconnection syndrome—that is, the lesion prevents visual information from reaching the language area, or (b) the presence of a lateralised visual language association area residing in the dominant posterior inferior temporal lobe. We describe a patient with an atypical interhemispheric organisation who developed pure alexia and features of apperceptive agnosia after a right occipitotemporal infarction.
The patient is a 71 year old man, with a college degree education. He writes, uses a fork, spoon, or knife with the right hand, but prefers to use his left hand or leg for every other activity. He has no antecedents of sinistrality in first degree relatives. At the age of 68 he had a myocardial infarction complicated with an interventricular communication and heart failure. He underwent cardiac surgery and recovered. Immediately after surgery his examination showed a left visual hemianopia and he complained of inability to read. However, no neurological assessment was required at that time and no further studies were conducted.
Three years later he was referred to the neurology clinic because of a syncopal episode and further attention was then paid to his cognitive complaints. He referred that during the first year after his surgery letters were just unmeaning lines for him and he had some confusion with numbers. Later, he learnt gradually to discriminate numbers and to recognise many letters but was still unable to put them together into syllables or meaningful words.
He was given a cognitive examination (quantified data are summarised in table 1). He was cooperative and fully oriented. On a line bisection task he scanned from left to right and upside down bisecting all lines. He had no visuospatial disorientation according to his performance in tests of the visual object and space perception battery.2Visuospatial perception and visuoconstructive tasks disclosed features consistent with apperceptive agnosia. He was able to recognise and copy simple shapes and figures (circle, cube...) but was impaired in recognising schematic drawings (for example, from the Boston naming test or cookie theft scene) or when he tried to copy more complex figures such as a house or Rey's figure. The main deficit pertained to piecing together separate visual stimuli, especially line drawings, into a whole percept. Recognition of real objects and pictures of famous people was appropriate. Colour matching was normal but he committed errors when asked to name a colour on confrontation or to point to a certain colour. Because he reported to have always had some difficulty with colour discrimination it was not certain the interpretation of this deficit was achromatopsia.
Spontaneous speech, comprehension, and repetition were normal. Confrontation naming with the Boston naming test was in the lower range disclosing very mild word finding problems (in addition to several failures clearly due to perceptual difficulties). Handwriting was meaningful and syntactically correct both spontaneously and to dictation. He was perfectly able to copy single letters and words. When trying to read, the patient was able to recognise many letters but he committed frequent errors such as between M and V, R and A, or H and R. He was unable to read syllables or simple words. On the contrary, he recognised with ease three syllable words spelled aloud by the examiner. He was able to read and to recognise numbers. Calculation was normal, with isolated errors. He had no problems distinguishing between right and left sides.
Brain CT showed an old infarction in the territory of the right posterior cerebral artery involving the occipital visual cortex and periventricular white matter extending to the forceps major of the corpus callosum (figure). No lesions were found in the left hemisphere.
The development of posterior alexia and apperceptive agnosia after a right occipito-temporal infarction is exceptional—only one case has been reported, in a left handed person, by Gloninget al,3 and discloses a rather atypical interhemispheric organisation in this patient. Firstly, he had a right hemispheric dominance for language, which is uncommon even in left handed patients. The neuropsychological profile of his reading disorder showed the typical features of a posterior alexia due to dominant hemispheric damage. His alexia could not be explained on the basis of neglect, visuospatial disorientation, simultanagnosia, or a global visual agnosia. Neither could it be due to his difficulties with complex visual perception, as he was perfectly able to copy words that afterwards he was unable to read. It is noteworthy that he was not able to recover any capability to read relying on his intact left temporo-occipital cortex.
The second remarkable deficit was consistent with the descriptions of Lissauer (1889) and of Bauer and Rubens (1985) of apperceptive agnosia.4 He had clear difficulties in perceiving the overall configuration of complex line figures or schematic drawings, which impaired both their copy and their recognition. This deficit could not be attributed to primary visual deficit, deficit in visual scanning, visuospatial disorientation, or just to constructional apraxia. Apperceptive agnosia is classically related to right temporo-occipital lesions whereas associative agnosia occurs after similar left lesions. All previously reported cases of posterior alexia, when accompanied by agnosic features, have been of the associative type consistent with left damage. On the contrary, this patient encompassed a striking combination of agnosic features—that is, a selective associative agnosia for words plus features of apperceptive agnosia for other complex visual stimuli. This combination of deficits suggests that reverse hemispheric dominance for language was not accompanied by reverse dominance for other functions.
Finally, the clinicoanatomical relation in this case mirrors the posterior alexia after left hemispheric lesions,5suggesting a similar linguistic organisation when the right hemisphere is dominant. In this patient damage to the posterior transcallosal fibres on their way to the left dominant angular gyrus and/or to the dominant occipitotemporal gyrus was also the crucial anatomical substrate in producing the syndrome of pure alexia.
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