There's more to reading than meets the eye. Heterogeneity of dyslexia in Posterior Cortical Atrophy

Objective. Posterior Cortical Atrophy (PCA) is known to affect reading ability more often and earlier than the classical, amnesic, form of Alzheimer's Disease (aAD). Reports about dyslexia in PCA have focused on the role of visual deficits, especially visual crowding. In the present study we investigated other possible causes of reading impairment in this disorder, with reference to the dual route model of reading, comparing PCA cases with patients with aAD. Materials. We assessed the ability to read words and nonwords (W_nW) and three-syllable words with unpredictable stress position (WUSP), the major ambiguity in reading Italian, in 19 PCA cases, 24 aAD cases and 25 age- and education-matched healthy participants. PCA patients were significantly younger than aAD patients, as expected, while education and MMSE scores were comparable. Qualitative analysis of reading errors was also performed for the W-nWtask. Methods. At the group level, we compared reading performance across the three study groups with ANOVA, and performed linear regression analysis within the PCA group to identify general cognitive predictors of dyslexia. At the single-case level, we defined PCA and aAD patients as having dyslexia based on cut-offs derived from the healthy control group, and then classified dyslexia subtype based on operational criteria inspired by the dual route model. Results. The PCA group showed poorer average reading scores than control and aAD groups. The only significant predictor of their reading performance was letter fluency; no effect was found for visual measures. In the PCA group there was also a higher prevalence of dyslexic patients than in the aAD group (8 /19, 42.0%, vs 2 /24, 8.3%). As to types of dyslexia, both aAD patients had central dyslexia (one had phonological dyslexia, the other one had mixed central features), while in the PCA group there were four patients with peripheral dyslexia (three with neglect dyslexia and one with crowding + neglect reading deficits), two patients with a mixed peripheral/central form of dyslexia, and two patients with central, namely phonological, dyslexia. Discussion. Our findings confirm literature evidence that documented worse reading difficulties in PCA than in aAD, but, in addition to the known peripheral dyslexias related to visual defects, they identify some form of central dyslexia. Moreover, reading performance in PCA was predicted by phonological, and not visual, ability. Conclusions. Our study highlights greater heterogeneity than expected of dyslexia in PCA.