Introduction

Reading skills are defined as a proficiency in reading, developed through practice and influenced by ability. This includes word recognition, pronunciation and comprehension. Visual requirements accepted as necessary for reading are a clearly focused image on the retina maintained by accommodation (focusing), convergence (moving both eyes inwards) and pupil miosis (decrease in pupil size). There is also a need for an adequate saccadic system, the visual system which enables fast conjugate eye movements to fixate an object of interest on the fovea. This enables eye movements across the page and back to fixate on words or start a new line of print.

Specific reading disorder (SRD), also termed as specific developmental dyslexia or specific learning difficulties (SpLD), is a sub‐type of dyslexia and defined as a disorder manifested by difficulty in learning to read despite conventional instruction, adequate intelligence and socio‐cultural opportunity (Chritchley 1970). There is no universal definition of dyslexia but the former is from the World Federation of Neurology and is the most widely used. An alternative definition describes SRD as present when the automatisation of word identification (reading) and/or word spelling does not develop, or does so very incompletely or with great difficulty (Gersons 1997). This definition does not exclude those children with an intelligence quotient (IQ) at the lower end of normal where a reduction in reading ability may simply be viewed as in line with their intelligence. Specific reading disorder is often defined on the basis of a discrepancy between IQ and standard reading scores (Newsham 2001). It can exist in normal children where there is a significant difference between reading age and chronological age.

Specific reading disorder is a complex learning difficulty because of the number of characteristics associated with it. These vary from individual to individual and not all will exhibit signs or symptoms of ocular defects. It is the purpose of this review to examine only those people with SRD who have been identified as having ocular signs and symptoms.

Epidemiology

Dyslexia affects 5 to 10% of school populations, according to the British Dyslexia Association. The incidence varies according to the threshold adopted for identification of the condition (Lennerstrand 1992). It is a multicultural problem although it is more prevalent in English speaking countries due to the complexities of language construction and left to right reading. A study of English, French and Italian dyslexics concluded that there is a universal neurocognitive basis for dyslexia and that differences in reading performance among dyslexics of different countries are due to different orthographies (systems of spelling) (Paulesu 2001). However a Chinese study stated that biological abnormalities of impaired reading are constrained by culture (Siok 2004) Dyslexia is often familial (Chase 1996) and is thought to be more prevalent in males (Katusic 2001). It is accepted that SRD with associated ocular signs and symptoms occurs in a small percentage of the dyslexic population but the precise incidence is unknown.
The causes of SRD are still debated. Some believe that eye defects are a consequence of and not a cause of reading disability. However others have suggested mechanisms for primary ocular defects as outlined below:

• anatomical and physiological abnormalities in both language and visual areas of the brain (Demb 1998);

• small cerebral haemorrhages; increased risk in premature babies (Newsham 2001);

• defects of motor perception often associated with other motor defects i.e. dyspraxia (poor co‐ordination and balance) (Nicolson 1999);

• defects in processing of retinal and extra‐retinal signals (magnocellular and parvocellular pathways) (Stein 1997);

• saccadic dysmetria (impairment of the saccadic system) (Biscaldi 1998);

• a disturbance of the smooth pursuit system preventing steady fixation on words when reading (Raymond 1988);

• genetic factors, although no specific dyslexia gene has been found (Nopola‐Hemmi 2001).

Although SRD can exist without any other developmental problems it has now been linked with other pervasive disorders, in particular attention deficit hyperactivity disorder (ADHD) and dyspraxia (Richardson 2001). Finally some believe retention of primitive survival reflexes inhibit the development of reading through maintaining the correct posture (McPhillips 2000).

Presentation and diagnosis

Specific reading disorder with ocular signs and symptoms will present as difficulty in learning to read with additional symptoms of headaches, glare from print or awareness of print movement. Frequently sufferers will have difficulty in following a line of print or may lose their place when beginning a new line. There may be other developmental or behavioral problems.

Ocular signs can include one or all of the following:

• poor convergence ‐ the ability to move the eyes disjugately to maintain binocular fixation on a near object;

• poor binocular control ‐ the ability to use two eyes together to maintain a single image with fusion and stereopsis;

• poor tracking ‐ ability to follow a slow moving target, known as smooth pursuit. This helps to maintain fixation on a single target;

• incomplete saccades ‐ voluntary movements of the eyes to change fixation from one object, or word, to another;

• a lack of a reference eye when reading. This is not the same as a dominant eye used in preference when using monocular viewing;

• poor ability to focus (accommodation).

Symptoms may include:

• pattern glare or severe discomfort when reading often associated with headaches, also known as Meares‐Irlen syndrome. The cause of this is controversial but is said to be a defect of the colour receptors in the retina;

• movement and blurring of letters. Awareness of moving print can also be caused by defects in motion linked with travel sickness;

• diplopia (double vision);

• blurred vision;

• headaches, particularly of the migraine type.

A diagnosis of SRD is made by comparing verbal IQ with reading ability and comprehension based on the averages for chronological age. SRD is unlikely to be diagnosed before eight years of age as many normal children struggle with reading in their formative years. Poor reading may be considered to be in line with IQ. Detection may be hindered as optometrists, orthoptists and ophthalmologists are not always trained to look for ocular defects specifically associated with reading difficulties and the detection of ocular motility disorders does not form part of the educational assessment of a child with reading difficulties. As a result concerned parents may have problems finding a suitably qualified specialist, for which cost can be prohibitive. Specialist clinics do exist and referral is usually made to an orthoptist or behavioural optometrist via school, paediatricians or general practitioners. However, there is no standard practice for referral of dyslexics for specialised ocular examinations.

Treatment options

Specific reading disorder is thought normally to persist into adult life. However, there are many therapies used to cope with this disorder. Because impaired eye movements are frequently noted in this group, claims are made that ocular exercises should be used in treatment. Treatment aims to improve reading speed by improving ocular stability and comfort therefore assisting in comprehension through better ability to concentrate for prolonged periods.

There are several ocular interventions used in the treatment of SRD. The treatment used will depend on the ocular findings during history taking and examination. The most common are listed below.

• Exercises to improve convergence and/or accommodation, helping to reduce fatigue and maintain binocular viewing.

• Plus lenses to aid accommodation (focusing).

• Prisms and physiological diplopia exercises to improve positive and negative fusional vergences i.e. motor fusion (the ability to maintain a single fused image in binocular viewing).

• Tracking exercises (eye gym) to improve the ability to fixate on an object and follow an object across the field of vision. This also improves saccadic movement aiding in moving eyes to a new word or line of print.

• Occlusion therapy to establish a reference eye in binocular viewing to eliminate confusion resulting in jumping of print when reading.

• Coloured overlays placed directly on reading print or in tinted spectacle correction to eliminate glare (Meares‐Irlen syndrome).

• Filters to aid motion defects, usually yellow or blue. These defects are often associated with dyspraxia.

Some treatments may become permanent as in coloured overlays or tinted spectacles; other treatments may only be used temporarily.

Rationale for a systematic review

There is significant controversy about the effectiveness of ocular interventions in SRD. Many treatments are expensive and are not universally available. A systematic review will clarify the evidence on ocular interventions and their effectiveness on reading. Being able to clarify whether particular interventions work, and on which particular ocular signs and symptoms, will help to focus treatment on those likely to benefit from ocular treatment and prevent unnecessary and often costly treatment of those unlikely to benefit.