Structural anatomy of pure and hemianopic alexia

Background

The two most common types of acquired reading disorder resulting from damage to the territory of the dominant posterior cerebral artery are hemianopic and pure alexia. Patients with pronounced hemianopic alexia have a right homonymous hemianopia that encroaches into central or parafoveal vision; they read individual words well, but generate inefficient reading saccades when reading along a line of text. Patients with pure alexia also often have a hemianopia but are more disabled, making frequent errors on individual words; they have sustained damage to a brain region that supports efficient word identification.

Objective

To investigate the differences in lesion site between hemianopic alexia and pure alexia groups, as rehabilitative techniques differ between the two conditions.

Methods

High‐resolution magnetic resonance images were obtained from seven patients with hemianopic alexia and from six patients with pure alexia caused by a left occipital stroke. The boundary of each lesion was defined and lesion volumes were then transformed into a standard stereotactic space so that regional comparisons could be made.

Results

The two patient groups did not differ in terms of damage to the medial left occipital lobe, but those with pure alexia had additional lateral damage to the posterior fusiform gyrus and adjacent tissue.

Conclusions

Clinicians will be able to predict the type of reading disorder patients with left occipital lesions have from simple tests of reading speed and the distribution of damage to the left occipital lobe on brain imaging. This information will aid management decisions, including recommendations for reading rehabilitation.Although an acquired reading disorder (alexia) is usually part of a more generalised language disorder (aphasia), it can occur as an isolated deficit, usually as a consequence of damage to the brain within the distribution of the left posterior cerebral artery. The two common forms of isolated alexia are hemianopic alexia and pure alexia. The first is the result of a right homonymous hemianopia (RHH), which impairs text reading more than single‐word reading. This is because, in left‐to‐right readers, visual information to the right of fixation is needed to plan rightward reading saccades.^1,2,3 Scanning along a line of text is affected if the RHH encroaches to within 5° of fixation, in right foveal or parafoveal vision.⁴ However, the relationship between text‐reading speed and the number of degrees of sparing of central vision is not linear, and most symptomatic patients have defects encroaching to within 2–3° of fixation or less. By contrast, patients with pure alexia have a severe impairment of single‐word recognition. Although they often have an associated RHH, their syndrome is not a consequence of this deficit, and there have been a few patients with pure alexia without an accompanying field defect.¹ Rather, their impairment is the consequence of damage to a whole‐word recognition system that allows a skilled reader to recognise seven‐letter words as quickly as words of three letters. Patients with pure alexia have damage in the whole‐word recognition system, its connections to primary visual areas or, occasionally, its connections to “higher” language areas. Although these patients can still read, they rely on a more‐or‐less intact letter recognition system and a laborious reversed‐spelling procedure, covert or overt, to arrive at the word''s identity, so‐called “letter‐by‐letter reading”. Thus, the word “dog” is not recognised, but explicitly spelling out “d”, “o”, “g” permits essential reading.It is important to differentiate between the two conditions, as there is specific rehabilitation for hemianopic alexia.⁵ Although occasionally some success has been reported in retraining patients with pure alexia,⁶ most regard pure alexia as an irremediable condition. Pure alexia is more disabling, but many patients with hemianopic alexia find text reading so laborious that they give up recreational reading, and if reading speed is an important skill in their job, their continuing employment may be at risk.The aim of this study was to determine whether pure alexia and hemianopic alexia can be differentiated by the limits of their left occipital lesion on magnetic resonance images (MRIs). Although much has been written about the pathological anatomy of pure alexia,^7,8,9,10 this is the first study directly comparing lesion site and size between patients with pure alexia and hemianopic alexia. The outcome is practical, in terms of diagnosis and referral for appropriate rehabilitation. Further, it provides additional data that identify the region responsible for rapid whole‐word recognition, by excluding areas that may be damaged by posterior cerebral artery (PCA) territory stroke but do not contribute to the syndrome of pure alexia.