Ophthalmic abnormalities in children with dyslexia: A look at current research

Dyslexia (or “specific reading disorder”) is a neurodevelopmental disorder which manifests as persistent and significant difficulties in learning to read efficiently, despite normal intelligence, intact hearing and vision, conventional schooling, adequate motivation, and sociocultural opportunity.[1] Dyslexia afflicts almost 5%–15% of school-going children.[1] Dyslexic children present with academic problems such as reading slowly and incorrectly, skipping lines while reading aloud and making repeated spelling mistakes.[1] If the disability remains undetected, these children fail to achieve school grades at a level that is matching with their intelligence. Eventually, their poor school performance adversely impacts their self-image, peer and family relationships, and health-related quality of life.[2] Although its exact etiology remains unknown, this invisible disability is believed to be a result of functional disruption in neural systems and is genetically inherited.[1]

What Causes Dyslexia Remains an Enigma!

There are at least two theories that have tried to explain its etiology:

The phonologic-deficit hypothesis

The functional unit of the phonologic module is the “phoneme”, defined as the smallest discernible segment of speech; for example, the word “bat” consists of three phonemes:/b//ae//t/(buh, aah, tuh). Dyslexic children have deficits in “phonologic awareness.”[1,3] According to this most-accepted “phonologic-deficit hypothesis,” dyslexic children have difficulty developing an awareness that words, both written and spoken, can be broken down into smaller units of sound and that, in fact, the letters constituting the printed word represent the sounds heard in the spoken word.[1,3] Their decoding difficulty results in these children having difficulty in mastering the relationships between the spelling patterns of words and their pronunciations.[1,3]

The magnocellular-deficit hypothesis

This hypothesis contends that the magnocellular visual system (which spreads from the magno cells in the retina to the magnocellular layers of the thalamus) is abnormal in dyslexic children, causing difficulties in some aspects of visual perception and in binocular control, which further adds to the reading impairment.[4]

Remedial Education: Cornerstone of Treatment of Dyslexia

Worldwide, remedial education is the established treatment modality used for ameliorating the reading difficulties of dyslexic children, and it should begin early when the child is in primary school.[1] Under the individual supervision of the remedial teacher, the dyslexic child undergoes systematic and highly structured training exercises to learn that words can be segmented into smaller units of sound (“phoneme awareness”).[1] Only after phonemic awareness gets developed “phonics instruction” begins. The child is taught that these sounds (“phonemes”) are linked with specific letters and letter patterns (“phonics”). The goal of teaching phonics is to link the individual sounds to letters and to make that process fluent and automatic, for both reading and spelling.[1] The child also practices reading stories, both to apply newly acquired decoding skills to reading words in context and to experience reading for meaning.[1] The dyslexic child has to undergo 1 hourly remedial education sessions twice or thrice weekly for a few years to achieve academic competence.[1] A recent meta-analysis has reported the effectiveness of remedial education in significantly ameliorating the reading impairment of dyslexic children.[5]

Visual Stress in Dyslexic Children

About one-third of dyslexic children complain of eye strain and/or visual distortions (“visual stress”) while reading.[6,7] These unpleasant subjective symptoms (sometimes termed as Meares-Irlen syndrome) include: seeing letters moving on the page, blurring or forming strange patterns, spatial distortions, frequent blinking, losing one's place on the page, and headache which generally get worse the longer child tries to read.[6,8] Bright lights, fluorescent lights, or glossy paper is described as often making the symptoms worse while reading.[6] Some optometrists have claimed that these symptoms can be alleviated either using colored overlays (transparencies) or tinted spectacle lenses as they reduce the pattern glare of letters.[8] The afflicted child is asked to read text (black ink written on white paper) after placing on the text a colored sheet of transparent plastic (colored overlay).[8] A wide array of colored overlays, singly and in combination, is employed to find the most suitable color as per the child's specific subjective preference.[8] If the child wishes to wear tinted spectacle lenses, the testing has to be repeated as the precise tint for an overlay is not necessarily the same as that for tinted spectacles.[8] It is important to remember that although dyslexic children are more likely to complain of visual stress while reading, these symptoms can also occur in regular non dyslexic children.[8]

In recent years, colored overlays and tinted lenses are being increasingly used by dyslexics in the UK not only to ameliorate visual stress but also to improve fluency and speed of reading.[9] Six of the eight UK charities supporting parents of dyslexic children have included information on their websites endorsing these expensive treatments (average cost of a colored overlay is £45 and tinted lenses is £200).[9]

Ophthalmic Abnormalities in Dyslexic Children: Recent Evidence

Creavin et al. (UK)[10] have reported the first large prospective population-based study which has simultaneously assessed a broad spectrum of ophthalmic abnormalities using reproducible orthoptic tests in a cohort of children aged 7 to 9 years. They assessed 5822 children, of whom 651 (11%) were dyslexic. They have reported that there was no association between dyslexia and: (i) strabismus (squint), (ii) motor fusion (coordination of the eyes to deal with two images), (ii) sensory fusion at a distance (fusion of two slightly dissimilar images at 6 m), (iii) refractive error, (iv) amblyopia, (v) convergence (simultaneous inward movement of both eyes toward each other to maintain single binocular vision), (vi) accommodation (ability of the ocular structures to change shape to focus on objects at different distances), or (vii) contrast sensitivity (ability to perceive differences between an object and its background). In fact, 80% of these dyslexic children had normal ophthalmic function in each test employed. The only two ophthalmic abnormalities found to be significantly associated with dyslexia were: (i) poor stereoacuity (defined as depth perception worse than 60 s/arc in the study) and (ii) poor sensory near fusion (fusion of two slightly dissimilar images at 33 cm) (odds ratio [OR] 1.58; 95% confidence interval [CI]: 1.01–2.47; P = 0.044 and OR 1.63; 95% CI: 1.02–2.60; P = 0.041, respectively).

Creavin et al.[10] have concluded that these two abnormalities may be the result of their reading difficulties or may be unrelated. Current research evidence indicates that the use of colored lenses or overlays to ameliorate reading difficulties cannot be endorsed and that any benefits reported by dyslexic children in clinical settings are likely to be the result of placebo, practice, or Hawthorne effects (tendency of some individuals to work harder and perform better when they are participants in an experiment).[10,11,12]

Quercia et al. (France)[13] have reported that dyslexic children have a distinctive low-level (<1 prism diopter) labile vertical heterophoria (latent strabismus) combined with torsion, probably due to imbalance in tonicity of the oblique muscles of the eyes. However, there was no statistical relationship between the amplitude or level of lability of vertical heterophoria and the reading skills of dyslexic children.[13] It is important to remember that vertical heterophoria is a common phenomenon present in 20% of the general population with no specific clinical signs.[13]

Kapoula et al. (France)[14] have reported that the use of convergent prisms to improve vergence (simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision) and spherical lenses to improve accommodation to improve the so-called oculomotor instability in dyslexics is not effective in improving their reading skills. Two recent studies by Kirkby et al. (UK)[15] and Wahlberg-Ramsay et al. (Sweden)[16] have reported that poor binocular coordination is unlikely to play a causal role in these children's reading difficulties. It is not the poor binocular coordination, but the reading task itself, namely, the dyslexic child's difficulties with linguistically processing printed text that induces deficits in their eye movement control.[15,16]

Conclusion

Dyslexia is a linguistic (and not a visual) disorder. There are no clinically significant ophthalmic abnormalities present in dyslexic children which can differentiate them from their regular peers. The cornerstone of treatment of dyslexia is remedial education along with the option of availing provisions (e.g., exemption from spelling mistakes, availing extra 30 min for all written tests, dropping a language, etc.) which help the child continue education in a regular mainstream school.[1,5,17] Every dyslexic child (like any regular peer) should undergo routine vision testing and if any refractive error is detected should use the regular prescribed spectacles/lenses. To conclude, there is no solid scientific evidence that dyslexic children who participate in any of the so-called vision therapies which use colored overlays, tinted lenses, convergent prism, spherical lens, and filters are more responsive to educational instruction than those who do not.[18]