• Reading Disorders

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The ability to read is a basic necessity of modern societal organization. Whether printed or on a monitor, our most important source of information is the printed word. Reading is one of the most important skills of daily living, and loss of reading ability results in increased dependency, poor communication, poor employability, decay of mental agility, and reduced quality of life. Reading is a complex sensorimotor cognitive function that can be damaged at many different levels.

Reading Disorders

S. Trauzettel-Klosinski

Physiologic Foundation of Reading

The optical tissues of the eye ensure the formation of a well- focused and complete image of text onto the retina. Refrac- tive errors and loss of accommodation are easily compen- sated for by use of appropriate spectacle corrections. For the legibility of newsprint at a distance of 25 cm (10 in.), an acuity of about 20/50 or better is required.

The resolving power of the retina drops rapidly with increasing eccentricity (^■ Fig. 24.1). Visual acuity, how- ever, is not an adequate measure for the ability to read, since acuity tests measure the ability to recognize correctly individual alphanumeric characters, one at a time.

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^Pearl

During fixation at some point in a line of print, a group of adjacent characters are seen, an ability that requires a minimum breadth of visual field of about 2° to either side of the locus of fixation and 1° above and below.

The total area of perception during fixation can actually extend significantly beyond these limits, up to about 5° in the direction of reading (i.e., to the right). This ability allows for a type of parafoveal sensory input that makes several words (up to 15 characters in reading direction) vis- ible at a single glance (^■ Fig. 24.1). This area of the central visual field has been called the perceptual span for reading.

To recognize correctly the next group of letters, a precise saccade in their direction is required. During fluent reading, a regular sequence of saccades and fixational pauses is regularly interrupted by a return sweep back to the start of the next line of print. Recordings of eye movements during reading tasks produce typical staircase patterns.

Fig. 24.1. Acuity as a function of eccentricity, minimum size of the reading visual field, and total perceptual span, as mapped onto a passage of text: Acuity falls sharply with increasing eccentricity. To read newsprint at a distance of 10 in., an acuity of 0.4 (20/50) is required; the minimum size of reading visual field (shown in blue) amounts to 2° to the left and right of fixation and 1° above and below fixation. Text is clearly legible in this region only. Total per- ceptual span during a moment of fixation (shown in green) can be extended up to 5° in the direction of reading

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Chapter 24 S. Trauzettel-Klosinski

The total area of retina used for reading is contained within a few square millimeters, but is represented at the visual cortex by a much larger proportion of neural tissue (neurosensory magnification). The central 10° of visual field, which correspond to about 2% of the entire visual field, is processed by a contiguous volume of more than half of the primary visual cortex. The perceived portion of text is transposed in the angular gyrus (the “reading center”) into a phonetic form that is then further processed within a “sensory speech center” (the Wernicke region). Addition- al centers of brain cortex participate in the processing of the linguistic input, depending on the precise nature of the task, but that subject is beyond the scope of this book.

Reading Disturbances:

Overview and Differential Diagnosis

The ophthalmologist is often the first professional sought out by patients with reading problems. An initial ophthalmic evaluation should include a detailed history of the patient’s family, personal, ophthalmic, and scholastic history.

This information allows for a correct classification of the reading problem and indicates the appropriate path for its further investigation (see flow diagram).

Ocular Sources of Reading Problems

If a child complains of asthenopic problems (headache, tearing, blurring, etc., during close work) or tells an adult that he/she has recently had problems with reading, the fol- lowing ocular sources of reading problems should be either identified or ruled out:

■ Optical problems: uncorrected refractive errors, in par- ticular hyperopia, poor accommodative amplitude (presbyopia in midlife, poor accommodation in children), and clouding of the ocular media

■ Sensory disturbances of reading: caused by problems of binocularity (heterophoria, strabismus, convergence insufficiency) or visual field defects within 5° of eccentricity (in this context, ocular sources of reading problems include lesions of the afferent visual pathways).

Given the great importance of reading problems, caused by visual field defects, particularly in neuro-ophthalmology, these are discussed below in more detail.

Motor disturbances of reading: both infra- and supra-

Specific and Isolated Reading and Spelling Problems, also Referred to as Developmental Dyslexia

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^Definition

Developmental dyslexia is an isolated disturbance of reading and writing in someone of normal intelligence and adequate schooling, with no accompanying deficits (whether sensory, motor, or psychological).

The patient’s history can yield valuable clues: The problem has been present since the beginning of study of reading and writing. The parents report school problems, in par- ticular poor grades in English, while the remaining sub- jects of school study are initially unaffected.

Characteristically, there are problems with reading (both in the understanding and fluency of reading) and writing, including the improper spelling of words in mul- tiple forms in a single passage of text, transposition of sequences of letters, confusion with symmetrical letters (q with p, and d with b), and confusing similar sounding consonants (e.g., d with t). It is not so much the type of error that is important, but rather the frequency with which errors are made.

Occasionally, a family history of similar problems is reported, and there often are secondary psychological problems consequently.

Developmental dyslexia has been attributed to a disor- der of language processing, which is often characterized by an impairment of the conversion of alphabetical characters into the phonetic elements of language, and by a deficit in phonological awareness. In addition, there are indications of deficits in the magnocellular auditory and visual pathways, resulting in poor processing of rapidly flowing sequences of stimuli.

The responsibility of the ophthalmologist is to rule out a primary ocular source of the reading problem, to treat any associated disorders, such as refractive anomalies (particularly high hyperopia and high corneal astigmatism) and strabismus (which is itself never a cause of developmental dyslexia), and to lay the foundation for a complete diagnostic study of possible causes: poor cognitive abilities, inadequate schooling, visual and auditory deficits, neuro- logical diseases, and primary psychiatric disorders.

In addition, the ophthalmologist should arrange for a specific diagnostic evaluation of developmental dyslexia, based on standardized testing procedures. As a rule, these

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Flow diagram.Diagnostic procedures for reading disorders

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A more extensive description of developmental dyslexia and its differential diagnosis by the ophthalmologist can be found in other works (see Trauzettel-Klosinski et al.

2002, 2003, under “Further Reading”).

Reading and Writing Deficits of Other Causes When taking a history, the physician may find that the problem is not limited to reading and writing. There can be other sources of trouble: poor aptitude, inadequate schooling, up to and including illiteracy, auditory deficits, neuro- logical diseases (aphasia, alexia, cerebral palsy), and/or primary psychiatric disorders (psychoses and severe neu- rotic disorders).

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^Pearl

Such problems are usually unearthed during very care- ful history taking, and their discovery will generally lead to an appropriate plan for management.

Reading Disorders Caused by Visual Field Defects

Defects in the central visual field can limit the extrafoveal vision that is critical to the task of reading. This region, referred to as the reading visual field, gives a sufficient size of perceptive span to allow further scanning of the text in an organized fashion. Defects in this region of the visual field, depending on their location and shape, lead to a variety of reading disturbances and the adaptive strategies for dealing with them (for a detailed discussion, see under “Further Reading” Trauzettel-Klosinski, Neuro-Ophthalmology 27 [1–3]: 79–90, 2002).

Central Scotoma (^■ Fig. 24.2, 1 b–d)

An absolute central scotoma with central fixation results in an obscuration of the reading field (^■ Fig. 24.2, 1 b). This results in a marked loss of reading ability. Many patients with this problem develop a useful adaptive mechanism, eccentric viewing, or eccentric fixation. A healthy locus of retina at the border of the scotoma becomes the new center of the visual field (^■ Fig. 24.2, 1 c). As a result, the scotoma is displaced away from the locus of fixation, and with it

nification, i.e., enlargement of the size of printed text (24.2, 1 d). This is the basis for the effectiveness of magnifying visual aids.

Ring Scotoma (^■ Fig. 24.2, 2 a, b)

With preservation of a central island of foveal function surrounded by an absolute central scotoma, the tiny area of retained vision may be too small for effective reading.

The insufficient size of the reading visual field results in a marked discrepancy between the acuity measures for single optotypes on the one hand and the loss of reading ability in central fixation on the other (^■ Fig. 24.2, 2 a), Reading ability is retained with enlarged type only in the event that the patient is capable of eccentric fixation (^■ Fig. 24.2, 2 b).

Concentric Constriction of the Visual Field (^■ Fig. 24.2, 2 c)

Here too, the central island of vision may be too small to allow for reading, although some degree of reading ability can be achieved with the use of higher contrasts and very small type sizes, allowing more of a printed sequence of letters to fall within the functioning central island of remaining vision.

Homonymous Hemianopia (^■ Fig. 24.2, 3)

Hemianopic defects obscure half of the reading field (^■ Fig. 24.2, 3 a). In this instance, the most important vari- able is the distance from the center of the field to the closest margin of the defect. In instances of macular sparing, which is the case with occipital lobe disease and sparing of the occipital pole, reading may not be significantly impaired (^■ Fig. 24.2, 3 b). A small paracentral homonymous defect will cause a severe problem with reading (^■ Fig. 24.2, 3 c).

In the event that a homonymous defect is located on the right side, the patient will not be able to generate targeted saccades to the next group of letters. Instead, numerous small and inaccurate saccades are substituted, numerous backtracking movements are made within the same line of print, and the speed of reading is markedly slowed. Read- ing becomes a task that is no longer enjoyable.

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A number of adaptive strategies that can help with this class of reading problem. Despite the retention of intact foveal function, many patients without macular sparing can learn to use a form of eccentric fixation. This strategy allows the patient to displace the field defect toward the hemianopic side, allowing the intact hemifield to overlap the vertical meridian (^■ Fig. 24.2, 3 d). Thus, they sacrifice some of their acuity in favor of a small area of perception that has been shifted into the affected hemifield, allowing as much as 1 to 1.5 ° of overlap. For the task of reading, this is a very significant improvement. This ability is thought to be the result of a form of functional plasticity of cortical processing, an acquired skill that requires much practice to master.

Another adaptive strategy is the learning of predictive saccades, in which the foveal fixation point is jumped toward the blind side by estimating the distance to the next line of print (in left hemianopias) or the next group of letters (in right hemianopias).

Another important compensating strategy for homonymous hemianopias that improves spatial orientation (though not reading) is the use of regularly initiated sac- cadic shifts of fixation toward the blind side. This strategy, just like that of eccentric fixation, shifts the blind portions of the visual field toward the hemianopic side and can lead to a faulty interpretation of conventional perimetric results as indicating preservation or recovery of some vision on the hemianopic side.

Heteronymous hemianopias, as in chiasmal disease, can cause problems with reading that are a result of the hemifield slide phenomenon (see Chaps. 2 and 15).

Rehabilitation

The goal of rehabilitation is to regain and optimize the reading ability. To assess the residual function as a basis for rehabilitation, a standardized low vision test battery can be

Fig. 24.2. The effect of various forms of visual field loss on the minimum reading visual field (shown in blue) 1a Normal state.

1b–d Absolute central scotoma. With central fixation the reading visual field is useless (1b), with eccentric fixation the central scoto- ma is displaced peripherally, and the eccentric locus of fixation has a lower spatial resolution (1c). With magnification of text, reading ability can be restored (1d). 2a and b Ring scotoma. The central is- land is too small to be useful for reading (2a). Reading will require both eccentric fixation and text magnification (2b). 2c Concentric

constriction of the visual field. The central island of vision is too small to permit reading. 3a–d Hemianopic loss of visual field. With- out macular sparing the reading visual field is half-destroyed (3a);

with macular sparing of adequate size the reading visual field may be unimpaired (3b). A small paracentral homonymous defect in the visual field can produce profound loss of reading ability (3c). Hemi- anopic visual field loss can be displaced toward the blind side by shifting fixation to an eccentric locus in the seeing half of the field, restoring a small band of perception to the hemianopic side (3d)

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www.amd-read.net). The most effective measures include visual aids (magnification and contrast enhancement) as well as training in the proper use of visual aids, establishing an optimal (i.e., eccentric) fixation locus, and specific training for the improvement of reading. For hemianopias, magnification is generally not helpful. Instead, tactile clues (ruler, cylindric ruler with a red guide line, or index fin- ger), rotation of reading material by 90° to read in an up or down direction, and/or special training are more likely to be of help. The success rate for the use of visual aids is fortunately quite good: 90% for patients treated at the Low Vision Clinic in the University Eye Hospital Tübingen.

Attempts at rehabilitation, even in the worst instances of visual loss, are always worth trying.

Conclusion

The prerequisites for retention or recovery of reading ability in patients with visual field defects are summarized in

■ Table 24.1:

■ If foveal function has been destroyed and fixation is eccentric, the problem is one of inadequate retinal resolution. This type of problem can often be helped with magnification of reading material.

■ If foveal function is intact and fixation is central, the size of the reading visual field is the problem. Restora- tion of reading ability requires development of or training in the use of eccentric fixation.

■ If foveal function is reduced and fixation remains central, a composite deficit partly of poor resolution and partly of small size of the reading visual field occurs.

In this situation, an increase in the contrast of reading material is often helpful.

Further Reading

Aulhorn E (1953) Über Fixationsbreite und Fixationsfrequenz beim Lesen gerichteter Konturen. Pfluegers Arch 257: 318–328 Hahn GA, Penka D Gehrlich C, Messias A, Weismann M, Hyvärinen L,

Leinonen M, Feely M, Rubin G, Dauxerre C, Vital-Durand F, Feather- ston S, Dietz K, Trauzettel-Klosinski S (2006) New standardised texts for assessing reading performance in four European lan- guages. Brit J Ophthalmol 90: 480–484

Horton JH, Hoyt WF (1991) The representation of the visual field in human striate cortex: a revision of the classic Holmes map. Arch Ophthalmol 109: 816–824

Legge GE, Ahn SJ, Klitz TS, Luebker A (1997) Psychophysics of reading XVI. The visual span in normal and low vision. Vision Res 37: 1999–

2010

MacKeben M, Trauzettel-Klosinski S, Reinhard J, Dürrwächter U, Adler M, Klosinski G (2004) Eye movement control during single-word reading in dyslexics. J Vision 4: 388–402

McConkie GW, Rayner K (1975) The span of the effective stimulus during a fixation in reading. Percept Psychophys 17: 578–586 Pilz K, Braun C, Altpeter E, MacKeben M, Trauzettel-Klosinski S (2006)

Modulation of visual stimulus discrimination by sustained focal at- tention: an MEG study. Invest Ophthalmol Vis Sci 47: 1225–1229 Trauzettel-Klosinski S (1997) Eccentric fixation with hemianopic field

defects. A valuable strategy to improve reading ability and an indi- cation of cortical plasticity. Neuroophthalmology 18: 117–131 Trauzettel-Klosinski S (2002) Reading disorders due to visual field de-

fects – a neuro-ophthalmological view. Neuroophthalmology 27(1–3): 79–90

Trauzettel-Klosinski S, Brendler K (1998) Eye movements in reading with hemianopic field defects: the significance of clinical parame- ters. Graefes Arch Clin Exp Ophthalmol 236: 91–102

Trauzettel-Klosinski S, Reinhard J (1998) The vertical field border in hemianopia and its significance for fixation and reading. Invest Ophthalmol Vis Sci 39: 2177–2186

Trauzettel-Klosinski S, Schäfer WD, Klosinski G (2002) Legasthenie.

Ophthalmologe 99: 208–229

Trauzettel-Klosinski S, MacKeben M, Reinhard J, Feucht A, Dürrwächter U, Klosinski G (2002) Pictogram naming in dyslexic and normally reading children assessed by SLO. Vision Res 42: 789–799 Trauzettel-Klosinski S, Dürrwächter U, Klosinski G, Braun C (2006) Corti-

cal activation during word reading and picture naming in dyslexic and non-reading-impaired children. Clin Neurophysiol 117: 1085–

1097 Table 24.1. The effect of visual field defects on reading ability

Foveal function ➞ Fixation ➞ Problem Solution

Extinguished Eccentric Low retinal resolution Magnification

Intact Central Reduced size of reading visual field Training in the use of eccentric fixation

Reduced Central Combination of poor resolution

and small visual field Contrast enhancement

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