Optic Chiasm Field Defects

The optic chiasm has been a topic of much interest since the first century A.D., when Galen described the union of the optic nerves as a “shape…very much like the letter chi.” In the centuries that followed, many scientists and physicians studied the structural aspects of the optic chiasm, starting with Isaac Newton, who in 1706 first explained that the partial decussation of the optic nerve fibers was necessary for binocular vision. Abraham Vater and J.C. Heinicke provided the first clinical evidence of hemidecussation in 1723, when they described cases of transient “halved vision” (homonymous hemianopia), presumably of migrainous origin, and concluded that the optic nerves decussate before uniting into the optic tracts because “without decussation of fibers in these nerves divided vision can in no way be explained.” The first diagram of decussating fibers was published in 1750 by “Chevalier” John Taylor, an itinerant eye surgeon, notorious for his charlatan ways and a practice “deeply tainted with the dishonest arts of the quack.” In 1824, a century after Vater and Heinicke’s work on hemidecussation, William Wollaston reported experiencing two episodes of half vision in each eye. He concluded that this necessitated hemidecussation of the optic nerves at the chiasm. The growing body of knowledge of chiasmal anatomy and visual fields culminated in the work of Harvey Cushing on the diagnostic recognition and surgical management of pituitary tumors. In December 1901, a 16-year-old girl was referred to Cushing by Sir William Osler. She had headaches and loss of vision and was short, obese, and sexually underdeveloped, appearing as a child of 12. Cushing missed the possible connection of the patient’s symptoms and appearance to the chiasm and the pituitary. After the young girl developed papilledema, Cushing operated first to decompress one cerebral hemisphere and then the other. When both operations failed to restore her vision, he operated a third time on the cerebellum, but the patient died several days later. At autopsy, a large pituitary cyst was discovered.

Automated Perimetry in Glaucoma

Clinical experience and multiple prospective studies, such as the Collaborative Normal Tension Glaucoma Study and the Los Angeles Latino Eye Study, have demonstrated that the diagnosis of glaucoma is more complex than identifying elevated intraocular pressure. As a result, increased emphasis has been placed on measurements of the structural and functional abnormalities caused by glaucoma. The refinement and adoption of imaging technologies assist the clinician in the detection of glaucomatous damage and, increasingly, in identifying the progression of structural damage. Because visual field defects in glaucoma patients occur in patterns that correspond to the anatomy of the nerve fiber layer of the retina and its projections to the optic nerve, visual functional tests become a link between structural damage and functional vision loss. The identification of glaucomatous damage and management of glaucoma require appropriate, sequential measurements and interpretation of the visual field. Glaucomatous visual field defects usually are of the nerve fiber bundle type, corresponding to the anatomic arrangement of the retinal nerve fiber layer. It is helpful to consider the division of the nasal and temporal retina as the fovea, not the optic nerve head, because this is the location that determines the center of the visual field. The ganglion cell axon bundles that emanate from the nasal side of the retina generally approach the optic nerve head in a radial fashion. The majority of these fibers enter the nasal half of the optic disc, but fibers that represent the nasal half of the macula form the papillomacular bundle to enter the temporal-most aspect of the optic nerve. In contrast, the temporal retinal fibers, with respect to fixation, arc around the macula to enter the superotemporal and inferotemporal portions of the optic disc. The origin of these arcuate temporal retinal fibers strictly respects the horizontal retinal raphe, temporal to the fovea. As a consequence of this superior-inferior segregation of the temporal retinal fibers, lesions that affect the superotemporal and inferotemporal poles of the optic disc, such as glaucoma, tend to cause arcuateshaped visual field defects extending from the blind spot toward the nasal horizontal meridian.

Essentials of Automated Perimetry

Standard automated perimetry is a standard method of measuring peripheral visual function. Automated static perimetry gained wide acceptance among clinicians due to the test’s high reproducibility and standardization and ability to store, exchange, and statistically analyze digital data. Advances in the computerized visual field assessment have contributed to our understanding of the role that field of vision plays in clinical evaluation and management of patients. The Humphrey Visual Field Analyzer/HFA II-i is the most commonly used automated perimeter in the United States, and the examples in this chapter have been obtained with this instrument. Aubert and Förster in the 1860s developed the arc perimeter, which led to the mapping of peripheral neurologic visual field abnormalities and advanced glaucomatous field defects. Analysis of the central visual field was not seen as clinically important by most clinicians until 1889, when Bjerrum described a detected arcuate paracentral scotoma. Later, Traquair further contributed to kinetic perimetry on the tangent screen. In 1893, Groenouw proposed the term “isopter” for lines with the same sensitivity on a perimetry chart. Rønne further developed kinetic isopter perimetry in 1909 and described the nasal step in glaucoma. Although the first bowl perimeter was introduced in 1872 by Scherk, due to problems with achieving even illumination on the screen, it did not become popular. The version of the bowl perimeter introduced by Goldmann in 1945 became widely accepted and is a significant contribution to clinical perimetry. The Goldmann perimeter incorporated a projected stimulus on an illuminated bowl, with standardization of background illumination as well as size and intensity of the stimulus, and allowed effective use of both static and kinetic techniques. For these reasons, the Goldmann instrument has remained the clinical standard throughout the world until widespread acceptance of automated perimetry. Harms and Aulhorn later designed the Tübingen perimeter with a bowl-type screen exclusively for the measurement of static threshold fields, using stationary test objects with variable light intensity. While excellent threshold measurements were possible with this instrument, the time and effort involved in such measurements prevented this perimeter from becoming widely used.

Retrogeniculate Visual Field Defects

Automated perimetry has changed visual field testing considerably in recent years. What was considered an art has become an exercise in interpreting a set of data points obtained mechanically. Automated perimetry saves ophthalmologists time, which ideally should allow for more visual fields to be obtained on patients with unexplained vision loss. However, one must still keep in mind that automated perimetry still depends on the subjective responses from the patient. More important, automated perimetry has made interpretation of visual field defects, especially those due to occipital lesions, more difficult. For example, macular sparing may not be reflected, especially with programs limited to the central 24° or 30°. A 10° field may be required to show macular sparing. Also, sparing or involvement of the temporal crescent will not be shown with 24° or 30° visual fields. The limitation of most programs may lead to the appearance of incongruity when in fact the field is indeed congruous. Sometimes, a small homonymous hemianopic scotoma will be detected when one eye is tested but will be completely missed when the other eye is tested, giving the false impression that the visual loss is monocular. This is especially problematic if the patient also falsely interprets his or her homonymous loss of vision as monocular. Such individuals may complain of loss of vision in one eye when in fact it is one half of their visual field that is defective. The strategy of automated testing on either side the vertical and horizontal meridians may lead to the false impression that field defects respect the vertical or horizontal meridian when they do not. Automated perimetry should make it possible to test more patients with unexplained vision loss, but all automated visual fields must be interpreted with caution and, when necessary, substantiated with some other method, such as the tangent screen, which remains the most powerful method of detecting the size, shape, and density of visual field defects. Because most ophthalmologists no longer use tangent screen testing, at least an Amlser grid should be used to qualify the nature of a paracentral visual field defect.

Visual Field Defects in Chorioretinal Disorders

Pathologic processes involving the retina or choroid can present with a wide variety of visual field defects. Usually visual field defects of retinal diseases directly correlate with the fundus findings. Visual field changes are often the result of damage to the retina or scarring but, in conjunction with other clinical findings, they may help narrow the differential diagnosis. Most of the macular lesions result in visual field defects that do not respect the vertical or horizontal midline. Occasionally inflammatory disorders result in visual field defects that do not directly correlate with the retinal findings. For example, patients with multiple evanescent white dot syndrome (MEWDS) may have an enlarged blind spot. Macular disorders can cause central or paracentral scotomas depending on the location of the lesion. Causes of macular pathology include drusen, atrophy from dry age-related macular degeneration (AMD), retinal hemorrhage, choroidal neovascular membrane, macular edema, macular hole, macular scar, pathologic myopia, and macular dystrophies of the retina or choroid. Central serous chorioretinopathy (CSCR) can show a relative defect that is anatomically correlated with the area of subretinal or sub RPE (retinal pigment epithelium) fluid accumulation. Residual pigmentary changes in inactive CSCR can also cause a relative depression in the corresponding visual field. Pathologic myopia can present with a variety of visual field defects depending on the retinal findings, such as posterior staphyloma or choroidal neovascular membrane. AMD may show nonspecific changes in the central or paracentral visual field that correlate with the structural damage to the retina and choroid. Geographic atrophy in dry AMD can cause a dense scotoma correlated with the pattern of the atrophy. Choroidal neovascular membranes can cause a depression in the correlating visual field due to edema or hemorrhage. Disciform scars in endstage AMD can also cause a dense scotoma. Macular holes may cause a small central scotoma. Pattern dystrophies are a family of disorders with a common pathology at the level of the RPE. Butterfly dystrophy, an autosomal dominant disorder, and Sjögren reticular dystrophy, an autosomal recessive disorder, are two examples of pattern dystrophies.

Acquired Optic Nerve Diseases

This chapter focuses on the most frequently acquired optic nerve diseases: their signs and symptoms, visual field findings, and the required basic workup and management. Acquired optic nerve diseases are often vision threatening and sometimes even life threatening. There is a need for accurate and timely diagnosis. Therefore, it is incumbent on the clinician to identify optic neuropathies, separate them from chronic congenital and hereditary problems, and aggressively pursue the diagnosis and treatment as necessary. In the workup of optic neuropathies, the visual field is extremely helpful. All patients with suspected optic neuropathies require careful examination of the visual fields for detection, characterization, and monitoring. Acquired optic neuropathies include inflammatory, ischemic, compressive, metabolic, and central nervous system–reflected pathology (papilledema). Inflammatory optic neuropathies include optic neuritis and its various etiologies such as demyelination, infective, immune-mediated (atypical), and slowly progressive/ chronic. Ischemic optic neuropathies include nonarteritic ischemic optic neuropathy (NAION) and arteritic ischemic optic neuropathy (AAION). Metabolic optic neuropathies include nutritional and/or toxic etiologies. Compressive optic neuropathies can occur due to mass effect on the disc optic, gliomas, and perioptic meningiomas. Papilledema may be primary (pseudotumor cerebri) or secondary to central nervous system mass effect. Optic neuritis is defined as a primary inflammation of the optic nerve. It is characterized by central visual loss that worsens over days and usually peaks about 1 to 2 weeks after the onset. It is usually unilateral but may be bilateral, especially in children, following viral infections like measles, mumps, and chickenpox. It occurs most commonly in adults (18-45 years old). Orbital or periocular pain may be present or precede the visual loss and is exacerbated with eye movements. Etiologies include demyelinating diseases/multiple sclerosis;, idiopathic, viral, or bacterial infections (syphilis); contiguous inflammation of the meninges, orbit, or sinuses,; granulomatous inflammation (tuberculosis, sarcoidosis, and cryptococosis); and autoimmune diseases. It is the most common cause of acute visual loss from optic nerve disease in the young and middle-aged adult group.

Inherited or Congenital Optic Nerve Diseases

In this chapter, we discuss inherited/congenital optic nerve diseases and their related visual fields defects. It is important for the ophthalmologist to establish, on the basis of the visual fields defect whether the optic nerve is involved and, if so, whether this is at the level of the optic disc or further back. In addition, the visual fields defect can help establish whether the etiology is acquired or congenital. If the former is the case, then the visual fields defect may reveal an insult that is rapidly progressive and hence requires immediate and aggressive management. The diseases are divided into the categories of congenital optic disc anomalies and heredodegenerative optic atrophies. Congenital optic disc anomalies include aplasias, dysplasias (hyperplasia and hypoplasia), optic nerve colobomas and pits, anomalous disc elevations: pseudopapilledema with or without hyaline bodies (drusen), and tilted disc and crescents. Absence of the optic disc (aplasia) is extremely rare and only a few case reports have been published in the literature. Optic disc size varies and may be larger (hyperplasia) or smaller (hypoplasia) than normal. Hyperplasia is much less common than hypoplasia and is usually related to ametropias. Optic nerve hypoplasia (ONH) may be unilateral or bilateral and isolated or associated with different syndromes. It may be associated with good or poor visual prognosis. It is the most common congenital optic disc anomaly encountered in pediatric ophthalmic practice. When the nerve head is slightly or segmentally reduced, especially in the presence of normal visual acuity, the diagnosis is based on comparison of disc photographs or calculation of the ratio of the disc center-to-fovea distance to disc diameter. Usually this ratio is increased in hypoplasia and, if higher than 3.0, is almost diagnostic. Sometimes the hypoplastic disc is surrounded by a ring of sclera and a ring of hyperpigmentation, described as “double-ring sign”. Maternal diabetes and use of teratogenic agents such as phenytoin, alcohol, quinine, and cocaine may be associated with ONH.

Overview of Perimetry

Like a painter, the practitioner of perimetry must learn his or her profession from experience. Just as a painting does not spring from the paint and brushes alone, the perimetrist does get his or her diagnosis from just a printout of the field test. Rather, the perimetrist’s experience in interpreting field test results, his clinical skill in examining the validity of the patient’s performance, and his selection of the needed field technique chosen under the appropriate clinical circumstances combine to produce a suitable test and interpretation of results. In this age of computerization, we tend to accept the infallibility of perimetry. It is true that new developments have corrected some of the errors in technique that have been troubling in earlier methods such as the tangent screen and Goldmann perimeter. However, in our rush to embrace these new techniques, we might forget that there is still a place for these older techniques in selected cases. Among other things, these older techniques may allow for a human element to be introduced when the patient is overwhelmed by technology—that is, a well-performed tangent screen is more valuable on a given occasion than a poorly performed computerized field examination. Such circumstances occur almost always with neuro-ophthalmology patients, who are usually ill in other ways than just visually and need more help in performing the test. Most other patients, particularly those with glaucoma, are much more reliable in their responses in using the newer techniques. They frequently start testing at a younger age and do their testing frequently so they become skilled at performing the test. Many neuro-ophthalmologic patients do not have that experience. Interpreting the blind spot remains a standard part of any field examination. Interpreting the blind spot size requires experience. The blind spot may be enlarged because the patient is a slow responder or because a large myopic crescent is present. An important use of measuring the blind spot is to show the patient what a scotoma is and to test his validity of fixation by putting the target in the blind spot from time to time.

Anatomic Basis and Differential Diagnosis of Field Defects

The purpose of any medical test is to confirm or rule out a diagnosis based on the clinical facts. In performing perimetry, the printout of the defect is not the end of the test. For even the most experienced reader, the test results at best tell the location of the defect. The next step is to consider the causes of such a defect in that part of the vision system. The experienced perimetrist will look at the results and suggest a differential list of causes. The primary diagnostic list is frequently aided by adding to the perimetry the medical history and other physical signs. The results of both then lead to the next step: ordering tests to confirm the cause of the field defect. It may require the ordering of a magnetic resonance (MR) image, but that may not be the proper test if the original differential diagnosis is faulty. Sedimentation rate and C-reactive protein may be more appropriate tests if the clinical facts suggest cranial arteritis. If carotid disease is suspected, a computed tomography (CT) angiogram may be more appropriate. In the following discussion of these defects, there has been a melding of a discussion explaining anatomically why these defects occur in certain areas. Because the course and relations of the primary visual sensory pathway have been frequently and well described (including in other chapters of this monograph), this chapter concentrates on the multiple anatomic substrates that may explain each particular pattern of visual field abnormality. Visual field abnormalities are represented by three categories: monocular, binocular, and junctional. Monocular field defects include those that can be caused by lesions of one eye or optic nerve. Binocular field defects include those that may result from single or multiple lesions at one or more points along the visual pathway. Junctional field defects include three types of visual field defects resulting from a lesion at the junction of the optic nerve and optic chiasm or of the optic tract and optic chiasm.

Functional Visual Loss.

Functional loss of vision or visual fields can present some of the most difficult diagnostic challenges, but even more difficult to diagnose are patients who have, in addition to functional visual loss, an organic disease. No matter how functional the symptoms appear, the clinician has to go that extra step to find any true pathology. This is why different field techniques may be more appropriate than the more sophisticated techniques. A tangent screen, for instance, may be better to control the patient’s response than a Humphrey Visual Field Analyzer. Remember the functional patients get sick, too. Patients with functional field loss can be placed into one of three general groups: neurasthenics, hysterics, or malingerers. The neurasthenic patient usually has many complaints, not limited to the visual system or to one set of visual symptoms such as field loss. The complaints, as well as the degree of field or visual defect, frequently vary from one examination to another, as well as during an examination as fatigue increases. The spiral field is often found in neurasthenic patients. The patient with hysteria, on the other hand, usually has a single ocular complaint. This chapter discusses the hysterical loss of acuity or fields. The usual field defect is a severe tubular type of contraction. The spiral field is sometimes seen in these patients. The malingerer may be the most difficult patient with functional loss, particularly if he has previously been examined by another physician and has acquired more experience with field testing than he had when first examined. The spiral field is not limited to functional loss; it can also be seen as a fatigue phenomenon in sick patients when the field testing is unduly prolonged. Its essential feature is that the field becomes progressively smaller as a specific isopter is tested for a second and third time with the same size of test object. shows all the points that were tested with a 3-mm white test object.