Dyslexia and impaired eye movements

Pubblicato il

Monday, 22 January 2018

Argomento

Oculistica Pediatrica e Vizi di Refrazione

Causes and correlations in the latest studies.

The correlation between dyslexia and altered eye movements has found further confirmation in the study by the research group of the ophthalmology clinic of the University of Genoa, directed by Prof. Carlo Emilio Traverso, published in No. 3 2017 of IROO (Italian Review of Ophthalmology).

The dyslexia is a specific learning difficulty, also known as "reading disorder'which can also occur in people with absolutely normal intelligence and intact sensory abilities. Individuals with developmental dyslexia have difficulty recognising and spelling words accurately and/or fluently. In the case of total impairment of reading skills, one speaks of alessia.

These reading difficulties are often noticed in the child's first years of life or at the beginning of the school experience, although dyslexia can also begin in adulthood as a result of traumatic brain injury, stroke or dementia.

The Visual Dyslexia Study

Among the causes of dyslexia, some environmental factors have been identified, but above all a genetic predisposition: studies in the literature show that 40% of children of dyslexic parents have a high probability of developing the same disorder.

In the study from the eye clinic in Genoa, 22 Italian-speaking children, aged between 8 and 13 years, were recruited, of whom 11 were healthy children and 11 had been diagnosed with dyslexia, based on DSM-IV recommendations (see ADHD Guidelines). The parameters included in the diagnostic procedure included: a normal level of general intelligence, reading disorder, no neurological or educational dysfunction that could explain the reading disability. All participants underwent orthoptic and ophthalmological examination, visual acuity testing with a single 3 m optotype and slit-lamp examination.

The study confirmed the correlation between visual dyslexia e alteration of eye movements and identified the cause in the inability to process linguistic material. An important difference between the group of healthy children and the dyslexic group in the stability of fixation of a stationary target was highlighted, with a higher rate of loss of the same in patients with dyslexia. Analysis of the reading parameters also revealed a significant difference between the two groups.

In particular, children with visual dyslexia performed more eye movements when reading a text and the number of fixations was significantly increased, especially for long or less common words, as had already been reported in other studies in the literature. This difficulty can mainly be explained by the increased number of progressive saccades. The saccade are rapid eye movements performed to bring the area of visual interest to coincide with the foveathe central area of the retina, characterised by maximum visual acuity.

Other studies in the literature had already shown that the voluntary component of the saccade is specifically impaired in dyslexic patients, compared to normal controls of the same age; furthermore, the development of fixation stability is impaired in dyslexic patients, mainly due to deficiency in the magnocellular pathway.

The study published in IROO confirms that dyslexic patients exhibit more frequent regressive saccades and pauses during fixation to return to words more frequently and attempt to decode and understand them, thus altering reading fluency compared to subjects in the control group. These results support the thesis that the altered eye movements are mainly due to the reduced visual processing of linguistic material typical of dyslexic patients.

Among the news to be reported are the results of a French research at the University of Renneswhich seems to have identified a potential physiological cause of dyslexia. According to the research team, this neurodevelopmental disorder is linked to specific abnormalities of the eye, particularly in the arrangement of photoreceptors.

Retinal photoreceptor arrangement and visual dyslexia

The French study, published in the journal Proceedings of the Royal Societyexamined the eyes of 30 dyslexics and 30 healthy people. The results showed that there were differences in the arrangement of retinal photoreceptors, particularly the cones, responsible for colour vision. There are, in fact, cones for red, for green and for blue, and in non-dyslexic people it was found that the area without cones for blue in one eye was oblong or irregular, while in the other eye it was round and the latter eye tended to take on the function of the dominant eye. In dyslexic people, both eyes had areas without cones for blue that were round in shape and, therefore, it could be hypothesised that this anatomical abnormality results in dyslexics not having a dominant eye and the brain going into confusion because it receives two identical images that it cannot decide between. This situation could be one of the causes of blurred vision and confusion while reading.

Professor John Stein, an expert on dyslexia and emeritus professor in neuroscience at Oxford University, commented that the French research is interesting mainly because it emphasises the importance of eye dominance in reading, but that it is not an explanation applicable to all cases of dyslexia.