Introduction

The chiasm is so named because it is shaped like the Greek letter chi.¹ Over 2 million nerve fibers pass through it: most are visual but some non-visual fibers project from the optic chiasm to hypothalamic nuclei, forming the retinohypothalamic tract subserving circadian rhythms.² The ratio of crossed to uncrossed fibers in the human chiasm is about 53 : 47.³

Evolutionary considerations

The chiasm provides the major route for the juxtaposition of corresponding parts of the visual field in each eye. It is important for binocular vision. To establish fusion, it is necessary to have overlapping visual fields, congruence of corresponding retinal elements, and extraocular muscles to maintain alignment of the visual axes.⁴ In lateral-eyed animals, optic fibers from each eye decussate entirely to the contralateral hemisphere. The percentage of uncrossed fibers increases as the orbits rotate anteriorly and the frontal field of single binocular vision increases.⁵ The reason that our visual system is crossed is controversial^5,6 (Fig. 54.1).

Fig. 54.1 Ramón y Cajal’s original diagram of the hypothetical uncrossed chiasm (left). On the right is the completely crossed chiasm in lateral-eyed animals.

(With permission from Polyak S. The Vertebrate Visual System. Edited by Heinrich Kluver. © 1957 University of Chicago Press.)

In humans, there is more retina nasal to the fovea than temporally so the right eye covers more right visual field than left visual field. The primordial nasal retinas are concerned with a phylogenetically older “panoramic” function. The temporal retinas have entered into a phylogenetically younger “binocular” function.

Our foveas are placed where nasal (panoramic) and temporal (binocularity providing) retinas meet.⁶ This allows vision to subserve its elemental functions:

Exploration of the surround is a primordial panoramic function of the nasal retinas, while binocular vision, stereopsis, and convergence concern the temporal retinas.

Anatomy

The optic nerves, chiasm and optic tracts extend posteriorly and upwards 45 degrees from the optic canals in adults and children.⁷ The chiasm lies in the suprasellar cistern several millimeters above the diaphragma sellae. The anterior cerebral arteries and the anterior communicating arteries lie anteriorly and above the chiasm and optic nerves. The carotid arteries lie laterally with the posterior communicating artery passing underneath the optic tracts. The chiasm lies in the floor of the anterior end of the third ventricle. Expansion of the third ventricle may cause chiasmal compression. Posteriorly lies the hypothalamus and the pituitary stalk, the tuber cinereum, and the mamillary bodies. The optic nerves emerge from the optic canals. The length of the intracranial optic nerve varies so the position of the chiasm in relation to other structures also varies. When the optic nerves are short, the chiasm is “prefixed”, when long it is “postfixed”. Von Willebrand’s knee is an artefact.⁸

Embryology

The chiasm appears in the first month of life,⁹ arising from a thickening of the floor of the forebrain. Retinal ganglion cells grow down the optic stalks and enter the floor of the third ventricle where they decussate to form the optic chiasm. The pattern of axon crossing occurs in two phases.¹⁰ The first retinal axons meet in the midline at the ventral diencephalon forming an X-shaped chiasm; subsequent axons grow into either the ipsilateral or contralateral optic tract.¹¹

A morphological specialization of the axon called a growth cone allows the axon to sense and respond to signals in the embryonic brain environment.¹² Neuronal and glial cells provide guidance information to ingrowing RGC axon. Histologic evidence suggests that uncrossed axons are confined within the lateral portion of the optic nerve and do not approach the chiasmal midline.¹³ Neurons and radial glial cells in the ventral diencephalon serve a role in retinal ganglion cell axon guidance during chiasm formation. Growth associated proteins in retinal growth cones enable RGC axons to progress and perform path finding tasks. In the mouse, neurons at the site of the future chiasm are required for its formation by retinal ganglion cell axons. A growth associated protein essential for chiasm formation has been identified.¹⁴ There is substantial overproduction of neurons which later die back by apoptosis.¹⁵ The chiasm reaches its definitive form by the fourth month of gestation.

Transcription factors that pattern the developing chiasm have been identified.^16–19 Foxd1 expressed in the developing temporal retina and its downstream effectors (tyrosine kinase membrane proteins) are involved in patterning of the optic chiasm in chicks and mammals.^17,18 Foxd1 imprints axons ipsilaterally. Foxd1 is expressed in progenitors of Zic2 positive retina ganglion cells and is the determinant of temporal retinal identity.^17,18 Neuropilin molecules, transmembrane proteins, serve as receptors for axonal guidance to regulate axon divergence at the chiasm in mammals.¹⁹ Neuropilin 1 (NRP1) is expressed at the chiasm midline and acts on contralateral retinal ganglion cells to provide growth-promoting and chemo-attractive signals for commissural axon crossing at the chiasm.¹⁹

Signs and symptoms

Developmental defects and suprasellar tumors are common in children (Box 54.1). Most chiasmal syndromes result from neoplastic disorders, developmental derangements, radiation injury, inflammation, infection, demyelination, infarction, transection, or hypoplasia.²⁰ Dominant optic atrophy may present with a bitemporal hemianopia simulating a chiasmal disorder.²⁰

In young children, chiasmal disease often presents late because the child compensates and is not suspected of having poor vision until there is substantial bilateral visual loss.

The hallmark of chiasmal disease is a bitemporal hemianopia. Some form of visual field testing is essential. Inferior lesions have to grow large before signs of chiasmal compression appear. They compress the lower nasal fibers first and tend to give an upper bitemporal field defect. Lesions from above tend to cause an inferior defect. By the time a compressive lesion has caused defects, there is usually thinning of the chiasm and the pattern of the field defect is not always clear-cut.

There is often an acuity defect. Chiasm splitting lesions, such as trauma, do not affect acuity greatly because the nasal field and the nasal half of the fovea is not affected, but involvement of the optic nerve or widespread involvement of both crossed and uncrossed fibers in the chiasm give rise to an acuity defect. Frequently, one eye has a very severe acuity defect and the other is relatively spared, except for a field defect. In chronic lesions, there is often preservation of a high level of acuity despite funduscopic evidence of a profound loss of neurons.

With optic nerve involvement there is significant color vision defect. Stereoacuity tests and Bagolini striated lens are useful tests in patients with suspected chiasmal compression.⁴ Decreased stereoacuity is common with chiasmal lesions, even when no visual field abnormalities are detectable.⁴ Stereopsis can be elicited with complete chiasmal transection by haploscopic stimulation of the intact temporal retinas.²¹ This shows the sensory capacity for stereopsis is retained in chiasmal transection but the absence of motor fusion precludes stereopsis.²² Bagolini striated lens testing reveals a binocular “mountain” pattern in chiasmal lesions.²³ Children with early-onset chiasmal disease may present with nystagmus. The classic form is see-saw nystagmus, but most patients have a compound nystagmus with vertical, horizontal, and rotary components (see Chapter 90).

Optic atrophy frequently occurs in chiasmal disease. There may be a generalized loss of neurons, or a pattern of band atrophy due to the loss of the fibers subserving the temporal fields and the preservation of fibers subserving the intact nasal field (Fig. 54.2). In developmental chiasmal defects and tumors there are often optic disc anomalies (Figs 54.3 and 54.4), e.g. hypoplasia.^24,25 Congenital suprasellar tumors can produce horizontal “bow-tie” cupping with selective loss of the nasal and temporal nerve fiber layer.²⁶ Papilledema in these optic discs occurs predominantly in the superior and inferior poles (Fig. 54.2C).

Fig. 54.2 Craniopharyngioma. (A) The right eye has bare perception of hand movements. The left eye has an absolute temporal hemianopia, normal color vision, and a visual acuity of −0.1logMAR (6/4.8, 20/16, 1.25). The left optic disc shows band atrophy: there is loss of the nerve fibers that subserve the temporal visual field but preservation of those that subserve the nasal field which are inserted into the upper and lower segments of the disc. (B) The origin of the band of atrophy is due to the fact that the horizontal band or bow-tie area is the visible area of atrophy where temporal field fibers alone are inserted into the disc. (C) Craniopharyngioma showing “bi-lobed” papilledema during a period of raised intracranial pressure. Since papilledema occurs only when the retinal ganglion cell axons are swollen and only the superior and inferior (nasal field) axons survive in chiasmal compression, the papilledema occurs only in the upper and lower poles giving bi-lobed or “twin peaks” papilledema.

Fig. 54.3 Craniopharyngioma. Bilateral segmental hypoplasia or “tilted” optic disc. Bitemporal hemianopia with a visual acuity of −0.1logMAR (6/4.8, 20/16, 1.25) right eye (refraction: –4.0 D) and −0.22 (6/3.6. 20/12, 1.67) left eye (–4.50 D).

Fig. 54.4 Midline facial defect. (A) Dysplastic tilted left optic disc in a patient with a midline facial defect. (B) MRI of corpus callosum lipoma in a patient with a midline facial defect.

Because of the proximity of the hypothalamus and pituitary gland, endocrine and growth defects may occur. Infants with hypothalamus-involving tumors may develop Russell’s diencephalic syndrome: emaciation with loss of subcutaneous fat (Fig. 54.5), accelerated growth in length relative to weight (Fig. 54.6), and personality changes with euphoria and hyperactivity. One must examine and measure the infant’s general body habitus and inquire about weight gain.

Fig. 54.5 Chiasmal glioma. (A) This boy presented with poor vision and recent weight loss. Photograph on 12 February 1973. (B) Photograph on 31 January 1974 showing rapid growth in weight and height. Weight and growth rate fluctuation are common in chiasmal glioma. (C) Bilateral band atrophy (same patient).

Fig. 54.6 Height growth record of a child with chiasmal glioma showing height growth rate fluctuation. Pediatric ophthalmology clinics need to have these charts available.

In bitemporal hemianopia the hemifield slide phenomenon may occur (Fig. 54.7). Since only the nasal portion of each visual field is functioning fully, corresponding retinal points between the two eyes no longer exist. Sensory fusion becomes impossible, and motor fusion cannot maintain alignment. A previous heterophoria becomes a manifest deviation. Esodeviations lead to letters or words appearing deleted. Exodeviations produce letters or words appearing duplicated. A vertical hemifield slide causes the child to lose track of which line of text they are reading. These children do not complain of diplopia but rather a duplication of the middle of words or objects. The hemifield slide phenomenon does not require a complete bitemporal hemianopia and can occur as the initial symptom.^27a Signs and symptoms of chiasmal disease are summarized in Table 54.1. Rarely, chiasmal tumors can produce photophobia as their presenting symptom.^27b

Fig. 54.7 Hemifield slide phenomenon.

(Fritz KJ, Brodsky MC. Elusive neuro-ophthalmic reading impairment. American Orthoptic Journal 1992; 42: 159–164. Reprinted by permission of The University of Wisconsin Press.)

Table 54.1 Chiasmal diseases in children

Further investigations

Further investigations consist of endocrine studies, neurophysiological evaluation (see Chapter 8) and neuroimaging. Neurophysiological studies may detect a crossover defect (particularly in a preverbal child) and quantitatively and qualitatively assess the visual defect. Magnetic resonance imaging (MRI)^28,29 provides neuro-anatomical detail of the chiasm and surrounding structures. Computed tomography (CT) scanning may provide important information about tumor and bony changes involving the parasellar area.

Developmental defects

Developmental derangements of the optic chiasm include:

Albinism (see Chapter 40)

Anomalous decussation of chiasmal projections occurs in albinos.²³ Retinogeniculate axons arising from ganglion cells in the portion of the temporal retina within 20 degrees of the vertical meridian decussate abnormally in the optic chiasm to synapse in the contralateral lateral geniculate nucleus.^30–32 The human optic chiasm retains a predominance of crossed fibers. MRI shows smaller optic nerves, chiasm, and tracts, with a wider angle between the two optic nerves and the two optic tracts.³³ The crossed predominance can be diagnosed electrophysiologically by asymmetrical hemispheric visual evoked potentials^34,35 (Fig. 54.8; see Chapters 8 and 40). Pigment around the optic disc plays an important role in axonal guidance, suggesting that loss of retinal epithelial pigment could be the source of chiasmal misrouting in albinism.³⁶ Recently, the role of the transcription factor Zic2 has been implicated in the chiasmal misrouting in albinism.³⁷

Fig. 54.8 Schematic comparing the visual pathway and VEP distribution in albino and achiasmic patients for flash stimulation of the left eye. Stimulation of the right eye produces the mirror image distribution for either condition. In the achiasmic subject, all the visual fibers from the left eye project to the left occipital cortex, and at 80–100 milliseconds a positivity is recorded over the right scalp and a negativity over the left. In contrast, most of the fibers from one eye cross at the chiasm in albinism, and the VEP distribution is the opposite, with a positivity recorded over the left scalp, and negativity over the right.

(Courtesy of Dr. Dorothy Thompson.)

Achiasmia

Belgian sheep dogs with achiasmia have congenital nystagmus and see-saw nystagmus.^38,39 Two unrelated girls with achiasmia had normal visual fields and no stereoacuity.⁴⁰

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