Ординатура / Офтальмология / Английские материалы / Clinical Neuro-ophthalmology A Practical Guide_Schiefer, Wilhelm , Hart_2007

Fig. 22.21. Visualizing the optic radiation by MR diffusion tensor imaging (DTI)-based tractography

• Pearl

Functional magnetic resonance (fMRI) and DTI are recent and innovative modalities that allow visualization of both the cortical and the subcortical structures of the posterior part of the visual system in detail. In particular, the optic radiation can now be localized, when planning the surgical approach to hemispheric, periventricularly located lesions (■ Fig. 22.21).

Conclusion

Optimal management of intraand extra-axial lesions of the visual pathway can be provided when an interdisciplinary team cooperates closely during the diagnostic and treatment process. The ophthalmologist, who is confronted first with these patients plays an important role initiating the proper course of action based on an intimate understanding of the treatment philosophy and management algorithms of the multi-specialist team.

Further Reading

Kaye AH, Black McL P (eds) (2000) Operative Neurosurgery, Volume 1, Churchill Livingstone, Edinburgh, London, New York

Winn HR (ed) (2004) Youmans Neurological Surgery, 5th edition, Elsevier Saunders, Edinburgh, London, New York

Rengachary SS, Ellenbogen RG (2005) Principles of Neurosurgery, 2nd ed, Elsevier Mosby, Edinburgh, London, New York

Side Effects of Radiation Therapy

Radiation therapy of the anterior visual pathway has long been used with a certain degree of reluctance, since the side effects are potentially disastrous. For many years, it was believed that radiation therapy of optic nerve tumors was of little use, since the healthy portions of the nerve had practically the same radiosensitivity as the tumor. The risk of bilateral radiation optic neuropathy and total blindness prevented even consideration of radiotherapy as a valid choice for treatment. In addition, if the globe lay within the field of treatment, the risk of a radiation retinopathy and/or cataract were additional possibilities. If the tumor was located close the pituitary gland, endocrinopathies were also possible. ■ Table 23.1 lists the critical radiation doses for the various structures in and around the anterior visual pathway.

Table 23.1. Critical dosage thresholds

Three-Dimensional Planning

of Radiotherapy

and Conformational Irradiation

Three-dimensional planning of treatment has led to a significant reduction in the side effects of radiotherapy, as compared with the classic two-dimensional method. The advent of computed tomography and magnetic resonance imaging has also allowed improvements to be made in the use of radiation as a therapeutic medium. The basis for calculation of volumetric dosing of radiation has paralleled the development of stereotactic methods of neurosurgery. The computational process is similar, but the tumor, rather than being imaged, is exposed to gamma or X-rays. The tumor is computationally located, and then the “virtual” tumor is irradiated by an instrument that uses the same pattern. This process is referred to as conformational dosing, a method that spares surrounding normal tissues to a maximal extent; the contralateral optic nerve is now largely spared. Focused stereotactic targeting allows a highly precise delivery of radiation. For this method a mask is prepared, so that no significant inaccuracies can result from inadvertent movements or changes in body position (■ Fig. 23.1). The accuracy of the method has a precision of 1 to 2 mm.

Fig. 23.1. Patient fitted with mask for application of stereotactically guided conformational irradiation

Fig. 23.2. Regression of optic disc edema in a patient with an optic nerve sheath meningioma after fractionated, stereotactically guided, conformational irradiation

Radiosurgery and Fractionated Irradiation

There are two primary methods for the use of radiation therapy, radiosurgery, in which the total dose of radiation is administered in a single session, and fractionated irradiation, in which the total dose is given over a period of several sessions. In the case of a meningioma, this may be as many as 30 separate sessions. Radiosurgery has the inherent risk of a high single dose causing damage to healthy tissues, even though the total dose of radiation is within an acceptable range. Experience has taught us that for the production of radiation optic neuropathy, single dosing often plays an important role. If the total single dose remains less than 2 Gy, optic nerve damage is very unlikely.

• Pearl

As a general rule, when seeking to retain function, fractionated application is preferred.

This is radiobiologically more favorable, since the tumor cells can partially recover between the single sessions and begin to divide again, increasing their susceptibility to the radiation effect.

Indications for Radiation Therapy

Radiation therapy is indicated when surgical removal of a tumor is incomplete or when growth of the neoplasm recurs following surgical excision. This applies not just to pituitary adenomas, but also to meningiomas and gliomas (■ Fig. 23.2). Primary tumors of the optic nerve or its meningeal sheath cannot be surgically removed without also sacrificing visual function. Radiation therapy for meningiomas can usually be expected to stop further growth, and will often improve visual function, even though the tumor shows no reduction in size on CT or MRI images. It is likely that the number of useful applications for radiation oncology will continue to expand in coming years.

Conclusion

Radiation therapy plays an increasingly important role in the treatment of benign tumors of the anterior afferent visual pathway, due primarily to the evolution of stereotactic methods that allow some degree of protection for healthy tissues while either shrinking the size or preventing further growth of neoplasm. It can be expected that the indications for this type of therapy will continue to expand.

Further Reading

Pitz S, Becker G, Schiefer U, Wilhelm H, Jeremic B, Bamberg M, Zrenner E (2002) Stereotactic irradiation of optic nerve sheath meningioma: a new treatment alternative. Br J Ophthalmol 86: 1265–1268

Physiologic Foundation of Reading

The optical tissues of the eye ensure the formation of a wellfocused and complete image of text onto the retina. Refractive errors and loss of accommodation are easily compensated 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, however, 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.

• 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) visible at a single glance (■ Fig. 24.1). This area of the central visual field has been called the perceptual span for reading.

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 perceptual span during a moment of fixation (shown in green) can be extended up to 5° in the direction of 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.

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). Additional 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 following ocular sources of reading problems should be either identified or ruled out:

■Optical problems: uncorrected refractive errors, in particular 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-ophthal- mology, these are discussed below in more detail.

■Motor disturbances of reading: both infraand supranuclear disturbances of ocular motility and gaze control.

Specific and Isolated Reading

and Spelling Problems, also Referred to as Developmental Dyslexia

: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 particular poor grades in English, while the remaining subjects 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 multiple 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 disorder 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, neurological 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 tests are best administered by psychologists or pediatric psychiatrists that are already familiar with the details of such clinical tests.

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, neurological diseases (aphasia, alexia, cerebral palsy), and/or primary psychiatric disorders (psychoses and severe neurotic disorders).

• Pearl

Such problems are usually unearthed during very careful 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 the physiologic blind spot (the latter is not depicted in ■ Fig. 24.2). The blind spot serves as a reference point, showing the extent of displacement of the scotoma.

The lower resolving power of the eccentric retinal locus can be compensated to a degree by the use of angular mag-

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 variable 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. Reading becomes a task that is no longer enjoyable.

In the case of a left-sided hemianopia, the patient will be able to trace along the line of print with near-normal speed, but will be unable to locate quickly the first word of the next line. This degree of impairment is much less disabling than is the case with right-sided hemianopias.

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 scotoma 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 island 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. Without 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). Hemianopic 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)

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 saccadic 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 used (see Hahn et al. 2006, under “Further Reading” and

Chapter 24 S. Trauzettel-Klosinski

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

Solution

Magnification

Training in the use of eccentric fixation

Contrast enhancement

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 finger), 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. Restoration 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, Featherston S, Dietz K, Trauzettel-Klosinski S (2006) New standardised texts for assessing reading performance in four European languages. 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 attention: 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 indication of cortical plasticity. Neuroophthalmology 18: 117–131

Trauzettel-Klosinski S (2002) Reading disorders due to visual field defects – 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 parameters. 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) Cortical activation during word reading and picture naming in dyslexic and non-reading-impaired children. Clin Neurophysiol 117: 1085– 1097

A

acute zonal occult outer retinopathy

AION, see anterior ischemic optic

altitudinal visual field defect/loss 53 amacrine cells 225

anterior ischemic optic neuropathy

– striata (see also [primary] visual cortex) 27, 96

–V1 (see [primary] visual cortex) 27

autosomal dominant optic neuropathy (ADOA) 123

AZOOR, acute zonal occult outer retinopathy 16

B

Bassen-Kornzweig syndrome 234 Behçet’s disease 94, 200