Dyslexia, Acquired

Pure Alexia

Pure alexia, also known as alexia without agraphia (dysgraphia), or letter-by-letter reading, is the only form of acquired dyslexia in which spelling can be preserved (hence, the “pure” and the “without agraphia”). When the patient is unable to read a word promptly, it is typically spelled aloud letter by letter from left to right (hence, the third name for this disorder); if all the letters are named correctly, the patient can usually then produce the whole word. It is not clear how this acquired dyslexia arises, but a promising current theory is that it is due to brain damage specifically harming the process of letter identification. Letters such as L or Z, which are not very confusable with other letters, can still be read easily, but letters such as E or Q, which are highly confusable with other letters, are very difficult to distinguish from the letters they are confusable with. To guarantee correct letter identification here, the patient must focus all attention on the letter rather than, as normal readers do, distribute attention across all the letters in the word. So attention has to be devoted to single letters, letter by letter, and that's what causes the letter-by-letter reading behavior.

Surface Dyslexia

Irregular words are those that disobey standard letter-sound rules, words such as yacht or blood. Patients with surface dyslexia frequently misread such words, and, what is more, they make “regularization errors”: reading the irregular words according to the rules (so yacht is read as if it rhymed with matched and blood as if it rhymed with mood). Accuracy of reading of regular words and of nonwords can be normal. This is typically interpreted in terms of a dual-route model of reading aloud. In such a model of reading, one route from print to speech uses letter-sound rules: This route (the “nonlexical route”) correctly reads regular words and nonwords but makes regularization errors with irregular words. The other reading route, the “lexical route,” operates by recognizing real words as familiar wholes: It correctly reads regular and irregular words but cannot read nonwords (such as VIB or SINT) since nonwords are not familiar wholes. Given such a model of reading aloud, surface dyslexia is ascribed to an impairment of the lexical route for reading aloud; the nonlexical route may be entirely intact.

Phonological Dyslexia

Here, there is a selective impairment in the reading aloud of nonwords. Accuracy of reading aloud regular and irregular words is much better and may even be intact. This form of acquired dyslexia is also usually interpreted in terms of the dual-route model of reading aloud: The nonlexical reading route is selectively damaged, with the lexical reading route relatively spared or even intact.

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