Comment

‘Dyslexia’ is commonly used to refer to a developmental reading disability. In contrast, acquired dyslexia, sometimes termed ‘alexia’ to highlight the distinction, indicates a difficulty in reading in someone who previously read normally.

Acquired dyslexias can be divided into:

• peripheral dyslexia, where there is a defect in the visual processing of written material with intact language centres

• deep/central dyslexia, where there is a breakdown of the normal linguistic processing.

The deep dyslexias are seen in semantic dementia and with lesions (strokes and tumours) affecting the dominant hemisphere temporal and parietal lobes. A non-dominant hemisphere lesion can cause alexia by two mechanisms.

• A lesion in the right parietal lobe can cause ‘neglect alexia’ due to a failure to read the left half of words correctly.

• A lesion in the non-dominant splenium can prevent transfer of visual information to the primary language centres in the temporal and parietal lobes. Classically these patients can write but cannot read (alexia sine agraphia). They can recognise letters but not words and hence are very slow to read long words.