A unique achiasmatic anomaly detected in non-albinos with misrouted retinal-fugal projections

Clinical features and imaging characteristics in achiasmia

Achiasmia is a rare visual pathway maldevelopment with reduced decussation of the axons in the optic chiasm. Our aim was to investigate clinical characteristics, macular, optic nerve and brain morphology in achiasmia. A prospective, cross-sectional, observational study of 12 participants with achiasmia [8 males and 4 females; 29.6 ± 18.4 years (mean ± standard deviation)] and 24 gender-, age-, ethnicity- and refraction-matched healthy controls was done. Full ophthalmology assessment, eye movement recording, a high-resolution spectral-domain optical coherence tomography of the macular and optic disc, five-channel visual-evoked responses, eye movement recordings and MRI scans of the brain and orbits were acquired. Achiasmia was confirmed in all 12 clinical participants by visual-evoked responses. Visual acuity in this group was 0.63 ± 0.19 and 0.53 ± 0.19 for the right and left eyes, respectively; most participants had mild refractive errors. All participants with achiasmia had see-saw nystagmus and no measurable stereo vision. Strabismus and abnormal head position were noted in 58% of participants. Optical coherence tomography showed optic nerve hypoplasia with associated foveal hypoplasia in four participants. In the remaining achiasmia participants, macular changes with significantly thinner paracentral inner segment (P = 0.002), wider pit (P = 0.04) and visual flattening of the ellipsoid line were found. MRI demonstrated chiasmatic aplasia in 3/12 (25%), chiasmatic hypoplasia in 7/12 (58%) and a subjectively normal chiasm in 2/12 (17%). Septo-optic dysplasia and severe bilateral optic nerve hypoplasia were found in three patients with chiasmic aplasia/hypoplasia on MRI. In this largest series of achiasmia patients to date, we found for the first time that neuronal abnormalities occur already at the retinal level. Foveal changes, optic nerve hypoplasia and the midline brain anomaly suggest that these abnormalities could be part of the same spectrum, with different manifestations of events during foetal development occurring with varying severity.

Triple visual hemifield maps in a case of optic chiasm hypoplasia

In humans, each hemisphere comprises an overlay of two visuotopic maps of the contralateral visual field, one from each eye. Is the capacity of the visual cortex limited to these two maps or are plastic mechanisms available to host more maps? We determined the cortical organization of the visual field maps in a rare individual with chiasma hypoplasia, where visual cortex plasticity is challenged to accommodate three hemifield maps. Using high-resolution fMRI at 7T and diffusion-weighted MRI at 3T, we found three hemiretinal inputs, instead of the normal two, to converge onto the left hemisphere. fMRI-based population receptive field mapping of the left V1-V3 at 3T revealed three superimposed hemifield representations in the left visual cortex, i.e. two representations of opposing visual hemifields from the left eye and one right hemifield representation from the right eye. We conclude that developmental plasticity including the re-wiring of local intra- and cortico-cortical connections is pivotal to support the coexistence and functioning of three hemifield maps within one hemisphere.

Commissural axon guidance in the developing spinal cord: from Cajal to the present day

During neuronal development, the formation of neural circuits requires developing axons to traverse a diverse cellular and molecular environment to establish synaptic contacts with the appropriate postsynaptic partners. Essential to this process is the ability of developing axons to navigate guidance molecules presented by specialized populations of cells. These cells partition the distance traveled by growing axons into shorter intervals by serving as intermediate targets, orchestrating the arrival and departure of axons by providing attractive and repulsive guidance cues. The floor plate in the central nervous system (CNS) is a critical intermediate target during neuronal development, required for the extension of commissural axons across the ventral midline. In this review, we begin by giving a historical overview of the ventral commissure and the evolutionary purpose of decussation. We then review the axon guidance studies that have revealed a diverse assortment of midline guidance cues, as well as genetic and molecular regulatory mechanisms required for coordinating the commissural axon response to these cues. Finally, we examine the contribution of dysfunctional axon guidance to neurological diseases.

Altered organization of the visual cortex in FHONDA syndrome

A fundamental scheme in the organization of the early visual cortex is the retinotopic representation of the contralateral visual hemifield on each hemisphere. We determined the cortical organization in a novel congenital visual pathway disorder, FHONDA-syndrome, where the axons from the temporal retina abnormally cross to the contralateral hemisphere. Using ultra-high field fMRI at 7 T, the population receptive field (pRF) properties of the primary visual cortex were modeled for two affected individuals and two controls. The cortical activation in FHONDA was confined to the hemisphere contralateral to the stimulated eye. Each cortical location was found to contain a pRF in each visual hemifeld and opposing hemifields were represented as retinotopic cortical overlays of mirror-symmetrical locations across the vertical meridian. Since, the enhanced crossing of the retinal fibers at the optic chiasm observed in FHONDA has been previously assumed to be exclusive to the pigment-deficiency in albinism, our direct evidence of abnormal mapping in FHONDA highlights the independence of pigmentation and development of the visual cortex. These findings thus provide fundamental insights into the developmental mechanisms of the human visual system and underline the general relevance of the interplay of subcortical stability and cortical plasticity.

Electrophysiology of Olfactory and Optic Nerve in Outpatient and Intraoperative Settings

Evoked potentials are time-locked electrophysiologic potentials recorded in response to standardized stimuli using scalp electrodes. These responses provide good temporal resolution and have been used in various clinical and intraoperative settings. Olfactory evoked potentials (OEPs) may be used as an adjunct tool in identifying patients of Parkinson disease and Alzheimer dementia. In clinical practice, visual evoked potentials (VEPs) are particularly useful in identifying subclinical cases of optic neuritis and in treatment surveillance. In recent times, pattern electroretinograms and photopic negative response have been gaining attention in identifying glaucoma suspects. During surgical manipulation, there is a risk of damage to optic or olfactory nerve. Intraoperative neurophysiologic monitoring can provide information regarding the integrity of olfactory or visual pathway. OEPs and VEPs, however, show high degree of variability and are not reliable tools because the responses are extremely susceptible to volatile anesthetic agents. Newer techniques that could possibly circumvent these drawbacks have been developed but are not used extensively. In this article, we briefly review the available techniques to obtain OEPs and VEPs, diagnostic applications, the utility of intraoperative monitoring, the limitations of the current techniques, and the future directions for research.

Bilateral population receptive fields in congenital hemihydranencephaly

PURPOSE

Congenital hemihydranencephaly (HH) is a very rare disorder characterised by prenatal near-complete unilateral loss of the cerebral cortex. We investigated a patient affected by congenital right HH whose visual field extended significantly into the both visual hemifields, suggesting a reorganisation of the remaining left visual hemisphere. We examined the early visual cortex reorganisation using functional MRI (7T) and population receptive field (pRF) modelling.

METHODS

Data were acquired by means of a 7T MRI while the patient affected by HH viewed conventional population receptive field mapping stimuli. Two possible pRF reorganisation schemes were evaluated: where every cortical location processed information from either (i) a single region of the visual field or (ii) from two bilateral regions of the visual field.

RESULTS

In the patient affected by HH, bilateral pRFs in single cortical locations of the remaining hemisphere were found. In addition, using this specific pRF reorganisation scheme, the biologically known relationship between pRF size and eccentricity was found.

CONCLUSIONS

Bilateral pRFs were found in the remaining left hemisphere of the patient affected by HH, indicating reorganisation of intra-cortical wiring of the early visual cortex and confirming brain plasticity and reorganisation after an early cerebral damage in humans.

Retinal axon guidance at the midline: Chiasmatic misrouting and consequences

The visual representation of the outside world relies on the appropriate connectivity between the eyes and the brain. Retinal ganglion cells are the sole neurons that send an axon from the retina to the brain, and thus the guidance decisions of retinal axons en route to their targets in the brain shape the neural circuitry that forms the basis of vision. Here, we focus on the choice made by retinal axons to cross or avoid the midline at the optic chiasm. This decision allows each brain hemisphere to receive inputs from both eyes corresponding to the same visual hemifield, and is thus crucial for binocular vision. In achiasmatic conditions, all retinal axons from one eye project to the ipsilateral brain hemisphere. In albinism, abnormal guidance of retinal axons at the optic chiasm leads to a change in the ratio of contralateral and ipsilateral projections with the consequence that each brain hemisphere receives inputs primarily from the contralateral eye instead of an almost equal distribution from both eyes in humans. In both cases, this misrouting of retinal axons leads to reduced visual acuity and poor depth perception. While this defect has been known for decades, mouse genetics have led to a better understanding of the molecular mechanisms at play in retinal axon guidance and at the origin of the guidance defect in albinism. In addition, fMRI studies on humans have now confirmed the anatomical and functional consequences of axonal misrouting at the chiasm that were previously only assumed from animal models. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 844-860, 2017.

Hemifield columns co-opt ocular dominance column structure in human achiasma

In the absence of an optic chiasm, visual input to the right eye is represented in primary visual cortex (V1) in the right hemisphere, while visual input to the left eye activates V1 in the left hemisphere. Retinotopic mapping In V1 reveals that in each hemisphere left and right visual hemifield representations are overlaid (Hoffmann et al., 2012). To explain how overlapping hemifield representations in V1 do not impair vision, we tested the hypothesis that visual projections from nasal and temporal retina create interdigitated left and right visual hemifield representations in V1, similar to the ocular dominance columns observed in neurotypical subjects (Victor et al., 2000). We used high-resolution fMRI at 7T to measure the spatial distribution of responses to left- and right-hemifield stimulation in one achiasmic subject. T₂-weighted 2D Spin Echo images were acquired at 0.8mm isotropic resolution. The left eye was occluded. To the right eye, a presentation of flickering checkerboards alternated between the left and right visual fields in a blocked stimulus design. The participant performed a demanding orientation-discrimination task at fixation. A general linear model was used to estimate the preference of voxels in V1 to left- and right-hemifield stimulation. The spatial distribution of voxels with significant preference for each hemifield showed interdigitated clusters which densely packed V1 in the right hemisphere. The spatial distribution of hemifield-preference voxels in the achiasmic subject was stable between two days of testing and comparable in scale to that of human ocular dominance columns. These results are the first in vivo evidence showing that visual hemifield representations interdigitate in achiasmic V1 following a similar developmental course to that of ocular dominance columns in V1 with intact optic chiasm.

Nondecussating retinal-fugal fiber syndrome: Clinical and neuroimaging clues to diagnosis

We report the clinical details and imaging findings for a case of nondecussating retinal-fugal fiber syndrome or isolated achiasma in a 4-year-old female child. Findings included the isolated absence of optic chiasm with unremarkable rest of the optic pathway and midline structures in a child presenting clinically with see-saw nystagmus. Clinically congenital see-saw nystagmus, “mirror reversal” of visual field representation and interocular ipsilateral asymmetry on monocular visual evoked potential point toward achiasma and warrant further evaluation with magnetic resonance imaging (MRI). Isolated achiasma is a rare condition that may remain undiagnosed unless MRI is done.

Using an achiasmic human visual system to quantify the relationship between the fMRI BOLD signal and neural response

Achiasma in humans causes gross mis-wiring of the retinal-fugal projection, resulting in overlapped cortical representations of left and right visual hemifields. We show that in areas V1-V3 this overlap is due to two co-located but non-interacting populations of neurons, each with a receptive field serving only one hemifield. Importantly, the two populations share the same local vascular control, resulting in a unique organization useful for quantifying the relationship between neural and fMRI BOLD responses without direct measurement of neural activity. Specifically, we can non-invasively double local neural responses by stimulating both neuronal populations with identical stimuli presented symmetrically across the vertical meridian to both visual hemifields, versus one population by stimulating in one hemifield. Measurements from a series of such doubling experiments show that the amplitude of BOLD response is proportional to approximately 0.5 power of the underlying neural response. Reanalyzing published data shows that this inferred relationship is general.

Connecting the retina to the brain

The visual system is beautifully crafted to transmit information of the external world to visual processing and cognitive centers in the brain. For visual information to be relayed to the brain, a series of axon pathfinding events must take place to ensure that the axons of retinal ganglion cells, the only neuronal cell type in the retina that sends axons out of the retina, find their way out of the eye to connect with targets in the brain. In the past few decades, the power of molecular and genetic tools, including the generation of genetically manipulated mouse lines, have multiplied our knowledge about the molecular mechanisms involved in the sculpting of the visual system. Here, we review major advances in our understanding of the mechanisms controlling the differentiation of RGCs, guidance of their axons from the retina to the primary visual centers, and the refinement processes essential for the establishment of topographic maps and eye-specific axon segregation. Human disorders, such as albinism and achiasmia, that impair RGC axon growth and guidance and, thus, the establishment of a fully functioning visual system will also be discussed.

Congenital visual pathway abnormalities: a window onto cortical stability and plasticity

Sensory systems project information in a highly organized manner to the brain, where it is preserved in maps of the sensory structures. These sensory projections are altered in congenital abnormalities, such as anophthalmia, albinism, achiasma, and hemihydranencephaly. Consequently, these abnormalities, profoundly affect the organization of the visual system. Surprisingly, visual perception remains largely intact, except for anophthalmia. Recent brain imaging advances shed light on the mechanisms that underlie this phenomenon. In contrast to animal models, in humans the plasticity of thalamocortical connections appears limited, thus demonstrating the importance of cortical adaptations. We suggest that congenital visual pathway abnormalities provide a valuable model to investigate the principles of plasticity that make visual representations available for perception and behavior in humans.

Impact of chiasma opticum malformations on the organization of the human ventral visual cortex

Congenital malformations of the optic chiasm, such as enhanced and reduced crossing of the optic nerve fibers, are evident in albinism and achiasma, respectively. In early visual cortex the resulting additional visual input from the ipsilateral visual hemifield is superimposed onto the normal retinotopic representation of the contralateral visual field, which is likely due to conservative geniculo-striate projections. Counterintuitively, this organization in early visual cortex does not have profound consequences on visual function. Here we ask, whether higher stages of visual processing provide a correction to the abnormal representation allowing for largely normal perception. To this end we assessed the organization patterns of early and ventral visual cortex in five albinotic, one achiasmic, and five control participants. In albinism and achiasma the mirror-symmetrical superposition of the ipsilateral and contalateral visual fields was evident not only in early visual cortex, but also in the higher areas of the ventral processing stream. Specifically, in the visual areas VO1/2 and PHC1/2 no differences in the extent, the degree of superposition, and the magnitude of the responses were evident in comparison to the early visual areas. Consequently, the highly atypical organization of the primary visual cortex was propagated downstream to highly specialized processing stages in an undiminished and unchanged manner. This indicates largely unaltered cortico-cortical connections in both types of misrouting, i.e., enhanced and reduced crossing of the optic nerves. It is concluded that main aspects of visual function are preserved despite sizable representation abnormalities in the ventral visual processing stream.

Novel brain imaging approaches to understand acquired and congenital neuro-ophthalmological conditions

PURPOSE OF REVIEW

The arrival of large datasets and the on-going refinement of neuroimaging technology have led to a number of recent advances in our understanding of visual pathway disorders. This work can broadly be classified into two areas, both of which are important when considering the optimal management strategies. The first looks at the delineation of damage, teasing out subtle changes to (specific components of) the visual pathway, which may help evaluate the severity and extent of disease. The second uses neuroimaging to investigate neuroplasticity, via changes in connectivity, cortical thickness, and retinotopic maps within the visual cortex.

RECENT FINDINGS

Here, we give consideration to both acquired and congenital patients with damage to the visual pathway, and how they differ. Congenital disorders of the peripheral visual system can provide insight into the large-scale reorganization of the visual cortex: these are investigated with reference to disorders of the optic chiasm and anophthalmia (absence of the eyes). In acquired conditions, we consider the recent work describing patterns of degeneration, both following single insult and in neurodegenerative conditions. We also discuss the developments in functional neuroimaging, with particular reference to work on hemianopia and the controversial suggestion of cortical reorganization following acquired retinal injury.

SUMMARY

Techniques for comparing neuro-ophthalmological conditions with healthy visual systems provide sensitive metrics to uncover subtle differences in grey and white matter structure of the brain. It is now possible to compare the massive reorganization present in congenital conditions with the apparent lack of plasticity following acquired damage.

Functional organisation of visual pathways in a patient with no optic chiasm

Congenital achiasma offers a rare opportunity to study reorganization and inter-hemispheric communication in the face of anomalous inputs to striate cortex. We report neuroimaging studies of a patient with seesaw nystagmus, achiasma, and full visual fields. The subject underwent structural magnetic resonance imaging (MRI), diffusion tensor imaging (DTI) studies, and functional MRI (fMRI) using monocular stimulation with checkerboards, motion, objects and faces, as well as retinotopic quadrantic mapping. Structural MRI confirmed the absence of an optic chiasm, which was corroborated by DTI tractography. Lack of a functioning decussation was confirmed by fMRI that showed activation of only ipsilateral medial occipital cortex by monocular stimulation. The corpus callosum was normal in size and anterior and posterior commissures were identifiable. In terms of the hierarchy of visual areas, V5 was the lowest level region to be activated binocularly, as were regions in the fusiform gyri responding to faces and objects. The retinotopic organization of striate cortex was studied with quadrantic stimulation. This showed that, in support of recent findings, rather than projecting to an ectopic location contiguous with the normal retinotopic map of the ipsilateral temporal hemi-retina, the nasal hemi-retina's representation overlapped that of the temporal hemi-retina. These findings show that congenital achiasma can be an isolated midline crossing defect, that information transfer does not occur in early occipital cortex but at intermediate and higher levels of the visual hierarchy, and that the functional reorganisation of striate cortex in this condition is consistent with normal axon guidance by a chemoaffinity gradient.

Plasticity and stability of the visual system in human achiasma

The absence of the optic chiasm is an extraordinary and extreme abnormality in the nervous system. The abnormality produces highly atypical functional responses in the cortex, including overlapping hemifield representations and bilateral population receptive fields in both striate and extrastriate visual cortex. Even in the presence of these large functional abnormalities, the effect on visual perception and daily life is not easily detected. Here, we demonstrate that in two achiasmic humans the gross topography of the geniculostriate and occipital callosal connections remains largely unaltered. We conclude that visual function is preserved by reorganization of intracortical connections instead of large-scale reorganizations of the visual cortex. Thus, developmental mechanisms of local wiring within cortical maps compensate for the improper gross wiring to preserve function in human achiasma.

Superimposed hemifields in primary visual cortex of achiasmic individuals

In the rare condition of achiasma, the visual cortex in each hemisphere receives information from both halves of the visual field. How is this "doubling" of information accommodated in V1? In this issue of Neuron, Hoffmann et al. (2012) investigate the cortical consequences of this anomaly.

Congenital aplasia of the optic chiasm and esophageal atresia: a case report

INTRODUCTION

The complete absence of the chiasm (chiasmal aplasia) is a rare clinical condition. Hypoplasia of the optic nerve and congenital nystagmus are almost invariably associated characteristics. Microphthalmos or anophthalmos are common features in chiasmal aplasia, while central nervous system abnormalities are less frequent. Esophageal atresia can be isolated or syndromic. In syndromic cases, it is frequently associated with cardiac, limb, renal or vertebral malformations and anal atresia. More rarely, esophageal atresia can be part of anophthalmia-esophageal-genital syndrome, which comprises anophthalmia or microphthalmia, genital abnormalities, vertebral defects and cerebral malformations. Here, a previously unreported case of chiasmal aplasia presenting without microphthalmos and associated with esophageal atresia is described.

CASE PRESENTATION

Aplasia of the optic chiasm was identified in a Caucasian Italian 8-month-old boy with esophageal atresia. An ultrasound examination carried out at 21 weeks' gestation revealed polyhydramnios. Intrauterine growth retardation, esophageal atresia and a small atrial-septal defect were subsequently detected at 28 weeks' gestation. Repair of the esophageal atresia was carried out shortly after birth. A jejunostomy was carried out at four months to facilitate enteral feeding. The child was subsequently noted to be visually inattentive and to be neurodevelopmentally delayed. Magnetic resonance imaging revealed chiasmal aplasia. No other midline brain defects were found. His karyotype was normal.

CONCLUSION

If achiasmia is a spectrum, our patient seems to depict the most severe form, since he appears to have an extremely severe visual impairment. This is in contrast to most of the cases described in the literature, where patients maintain good--or at least useful-- visual function. To the best of our knowledge, the association of optic nerve hypoplasia, complete chiasmal aplasia, esophageal atresia and atrial-septal defect, choanal atresia, hypertelorism and psychomotor retardation has never been described before.

Congenital achiasma and see-saw nystagmus in VACTERL syndrome

A 29-year-old man with vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal defects, and limb defects (VACTERL) presented with headache, photophobia, and worsening nystagmus. He had near-normal visual acuity and visual fields, absent stereopsis, and see-saw nystagmus. Brain MRI revealed a thin remnant of the optic chiasm but normal-sized optic nerves. Functional MRI during monocular visual stimulation demonstrated non-crossing of the visual evoked responses in the occipital cortex, confirming achiasma. These findings have not previously been reported in VACTERL.

Bilateral Optic Nerve Aplasia in a Child

An 3-month-old female infant was admitted to the authors' clinic because of abnormal gaze position. On examination, the eyes were in the down gaze position. The left eye had a partial iris and lens coloboma. On funduscopic examination, the optic nerve and retinal vessels could not be detected. Optic nerve and optic chiasm were not observed on magnetic resonance imaging.

Developmental abnormalities of the optic nerve and chiasm

Developmental anomalies of the optic nerve are an important and growing cause of lifelong visual handicap and they are often associated with systemic abnormalities. This review focuses on the ocular and systemic aspects of developmental anomalies arising from defects of fetal fissure closure and retinal ganglion cell development, and covers some other optic-disc anomalies that have systemic significance.

VEP asymmetry with ophthalmological and MRI findings in two achiasmatic children

Achiasmia is a rarely diagnosed visual pathway maldevelopment where all or the majority of nasal retinal fibres fail to decussate at the optic chiasm. It has been identified by neuroimaging and also by visual evoked potential (VEP) asymmetry. VEP asymmetry has not been defined consistently in previous studies. The aim was to study VEP asymmetry to flash stimulation in two children with maldevelopment of the optic chiasm in comparison to control children. Both children had congenital nystagmus, optic nerve hypoplasia with a bilateral small double ring, bitemporal visual field defect and normal colour vision. In child 1 visual acuity in both eyes was 0.1, in child 2 it was 0.2. MRI showed reduced chiasmal size in child 1, while in child 2 it was combined with other midline abnormalities. VEP to monocular flash stimulation showed in both children distinctive occipital distribution, which was not observed in control children. The N2 wave was distributed asymmetrically over the ipsilateral hemisphere to the stimulated eye, while the P2 wave was distributed over both hemispheres. The P2 wave was however better defined over the ipsilateral hemisphere. Flash VEP occipital distribution remained similar in child 1, who was followed from 10 months to 9 years. These cases of achiasmia demonstrate a distinctive VEP asymmetry in the distribution of the flash VEP N2 wave, as well as the expected structural defect determined by neuroimaging.

Oculomotor instabilities in zebrafish mutant belladonna: a behavioral model for congenital nystagmus caused by axonal misrouting

A large fraction of homozygous zebrafish mutant belladonna (bel) larvae display a reversed optokinetic response (OKR) that correlates with failure of the retinal ganglion cells to cross the midline and form the optic chiasm. Some of these achiasmatic mutants display strong spontaneous eye oscillations (SOs) in the absence of motion in the surround. The presentation of a stationary grating was necessary and sufficient to evoke SO. Both OKR reversal and SO depend on vision and are contrast sensitive. We built a quantitative model derived from bel fwd (forward) eye behaviors. To mimic the achiasmatic condition, we reversed the sign of the retinal slip velocity in the model, thereby successfully reproducing both reversed OKR and SO. On the basis of the OKR data, and with the support of the quantitative model, we hypothesize that the reversed OKR and the SO can be completely attributed to RGC misrouting. The strong resemblance between the SO and congenital nystagmus (CN) seen in humans with defective retinotectal projections implies that CN, of so far unknown etiology, may be directly caused by a projection defect.

Congenital absence of optic chiasm: demonstration of an uncrossed visual pathway using monocular flash visual evoked potentials

A 35 month old child was referred for electrophysiology testing with pendular nystagmus, corresponding head oscillations and reduced vision. Flash visual evoked potential (VEP) revealed large responses at the right occiput (but not the left occiput) from the right eye and similar large responses at only the left occiput from the left eye, indicating absent/deficient crossover at the chiasm. A magnetic resonance imaging (MRI) scan subsequently confirmed absence of the optic chiasm. There was no other evidence of midline brain defects. Her subsequent development to age 11 has been followed. The nystagmus has remained mainly horizontal but a torsional component was noted from age 5 years and described as see-saw at age 6 years. A small right esotropia was noted at 6 years and spectacles prescribed for low hypermetropic refractive error. Bilateral superior rectus recessions at age 7 years produced an improved head posture. Her visual acuity has remained stable at around 6/24 from age 4 years. No binocularity nor stereopsis has been demonstrated over subsequent visits.

The achiasmia spectrum: congenitally reduced chiasmal decussation

AIM

To describe the clinical spectrum of achiasmia, a congenital disorder of reduced relative decussation at the optic chiasm.

METHODS

A retrospective case note and patient review of nine children (four boys). Achiasmia was defined by the combination of a characteristic asymmetry of the monocular visual evoked potential (VEP) response to flash and neuroimaging showing reduced chiasmal size.

RESULTS

Three of the children had an associated skull base encephalocele with agenesis of the corpus callosum. In two patients achiasmia was associated with septo-optic dysplasia. Three patients had no neuroimaging abnormalities other than reduced chiasmal size and have no known pituitary dysfunction. One child had multiple physical deformities but the only brain imaging abnormality was reduced chiasmal size.

CONCLUSIONS

Some children with disorders of midline central nervous system development, including septo-optic dysplasia and skull base encephaloceles, have congenitally reduced chiasmal decussation. Reduced relative decussation may co-exist with overall chiasmal hypoplasia. Children with an apparently isolated chiasmal decussation deficit may have other subtle neurological findings, but our clinical impression is that most of these children function well.

Achiasmia and unilateral optic nerve hypoplasia in an otherwise healthy infant

An 18-month-old white boy, observed by his parents at 1-2 months age to have poor visual attentiveness and nystagmus, underwent an ophthalmological evaluation. The patient also underwent unsedated 5-channel flash visual evoked potentials (VEP) and sedated electroretinogram (ERG) testing as well as magnetic resonance imaging (MRI) of the brain and orbits. The VEP in response to monocular stimulation demonstrated occipital asymmetry and was clearly suggestive of crossed asymmetry and also showed right optic nerve hypoplasia. The MRI and fundoscopic examinations supported the findings of achiasmia and probable optic nerve hypoplasia. The patient also had decreased Teller card visual acuity, nystagmus and a variable right esotropia. Neurological examination was normal. The ophthalmological and MRI findings in this 18-month-old male patient support the diagnosis of isolated non-decussating retinal-fugal fibre syndrome as well as hypoplasia of the optic nerve.

Optic nerve axons: life and death before birth

In the 2004 Bowman Lecture, I give a panegyric for Sir William Bowman, an estimate of the importance and the epidemiology of anterior visual pathway developmental disorders, followed by a history of the anterior visual system. I review the normal development of the optic nerve and chiasm and the main developmental disorders: Optic Nerve Aplasia, Optic Nerve Hypoplasia and Achiasmia.

Chiasmal coefficient of flash and pattern visual evoked potentials for detection of chiasmal misrouting in albinism

The diagnosis of albinism can be confirmed by electrophysiological examination, when chiasmal misrouting can be demonstrated. The present study describes a quantitative analysis method for this purpose. A chiasmal coefficient (CC) was calculated by correlating the differential potential over left and right hemisphere, when stimulating left versus right eye. This CC will be negative in albinism and positive in normal individuals. VEPs were recorded in 20 control subjects, four children with congenital motor nystagmus and six children with albinism. In up to 25% of the controls the CC was negative, when using flash VEP. However, with pattern VEP all had a positive CC. All children with albinism had a negative CC. Three of the four patients with congenital motor nystagmus had a positive CC, and one child had a small negative value with flash stimulation. In conclusion, determination of CC is a valuable and objective analysis method for electrophysiological determination of chiasmal misrouting. The method is relatively simple and only needs two electrode tracings. One should be aware of false-positive results when using flash stimulation. Whenever possible pattern stimulation should be used.

Effects of extraocular muscle tenotomy on congenital nystagmus in macaque monkeys

INTRODUCTION

Extraocular muscle tenotomy has been reported to damp congenital nystagmus in an achiasmatic sheepdog. We performed extraocular muscle tenotomy to evaluate its effects on congenital nystagmus in primates.

METHODS

Magnetic search coil eye movement recordings were used to document the presence of horizontal congenital nystagmus in 2 adult macaque monkeys that also had naturally occurring infantile strabismus. Extraocular muscle tenotomy was performed by operating on all 4 horizontal recti, surgically detaching the muscles from the globe and suturing them back to their original insertions without resection or recession. Eye movement recordings were repeated 4 months after the procedure, comparing the waveform, amplitude, retinal slip velocity, and intensity (frequency x amplitude) of the nystagmus before and after tenotomy. Visual acuity was also measured before and after surgery in 1 animal.

RESULTS

Preoperatively, a disconjugate, pendular nystagmus was evident in 1 monkey, and the other had a conjugate pendular-jerk nystagmus damped by convergence. After tenotomy, nystagmus mean amplitude decreased 18% to 52% in 1 monkey but increased 14% in the other (t test, P <.002). Retinal slip velocity and nystagmus intensity increased in both monkeys. After tenotomy, mean velocity increased 22% to 218%, while mean intensity increased 40% to 208% (t test, P <.002). Visual acuity measured after tenotomy decreased an average of 20% ( approximately 2.0 cycles per degree) in each eye. Tenotomy had no noteworthy effects on eye alignment or other aspects of visual behavior other than the congenital nystagmus.

CONCLUSION

Nystagmus velocity and intensity increased after extraocular muscle tenotomy in 2 monkeys. Further studies are required to establish the clinical value of this procedure as a treatment for various subtypes of congenital nystagmus in humans.

A girl without a chiasm: electrophysiologic and MRI evidence for the absence of crossing optic nerve fibers in a girl with a congenital nystagmus

An otherwise healthy 15-year-old girl with a congenital nystagmus was evaluated at our department using visual evoked potential recording and magnetic resonance imaging. She appears to have the unique isolated inborn absence of the optic chiasm, described only once before in two unrelated girls. Unlike these previously described cases, our patient does not seem to display a see-saw nystagmus.

Optokinetic behavior is reversed in achiasmatic mutant zebrafish larvae

The vertebrate optokinetic nystagmus (OKN) is a compensatory oculomotor behavior that is evoked by movement of the visual environment. It functions to stabilize visual images on the retina. The OKN can be experimentally evoked by rotating a drum fitted with stripes around the animal and has been studied extensively in many vertebrate species, including teleosts. This simple behavior has earlier been used to screen for mutations affecting visual system development in the vertebrate model organism zebrafish. In such a screen, we have found a significant number of homozygous belladonna (bel) mutant larvae to be defective in the correct execution of the OKN [1]. We now show that about 40% of homozygous bel larvae display a curious reversal of the OKN upon visual stimulation. Monocular stimulation leads to primary activation of ipsilateral eye movements in larvae that behave like the wild type. In contrast, affected larvae display contralateral activation of eye movements upon monocular stimulation. Anatomical analysis of retinal ganglion cell axon projections reveal a morphological basis for the observed behavioral defect. All animals with OKN reversal are achiasmatic. Further behavioral examination of affected larvae show that OKN-reversed animals execute this behavior in a stimulus-velocity-independent manner. Our data support a parsimonious model of optokinetic reversal by the opening of a controlling feedback loop at the level of the optic chiasm that is solely responsible for the observed behavioral abnormality in mutant belladonna larvae.

Visual-evoked potential evidence of chiasmal hypoplasia

PURPOSE

To show that chiasmal hypoplasia or aplasia need not be an isolated developmental anomaly and to examine the spectrum of associated clinical findings to explore the possibility that these patients may represent a phenotypic manifestation of a developmental gene anomaly.

DESIGN

An observational case series.

PARTICIPANTS

Five infants, between several weeks and 7 months of age, in whom the electrophysiologic characteristic of chiasmal hypoplasia had been noted were included.

METHODS

Flash electroretinography and flash and pattern visual-evoked potentials (VEPs) were elicited from all patients. Clinical ophthalmologic examinations, including funduscopy, were performed, and all patients had magnetic resonance imaging (MRI) brain scans.

MAIN OUTCOME MEASURES

The occipital distribution of monocular VEP response peaks was studied. The symmetry of lateral channel responses was compared for monocular stimulation.

RESULTS

All five patients had a crossed asymmetry in the monocular VEP occipital distribution, which is consistent with a paucity of fibers crossing at the chiasm. The MRI findings supported this electrophysiologic observation, illustrating degrees of chiasmal hypoplasia and variable coincidence of other midline abnormalities of the brain. Optic disc appearances varied from normal to hypoplastic and colobomatous.

CONCLUSIONS

The ophthalmologic and MRI findings of five patients who showed a crossed asymmetry in monocular flash VEPs are consistent with a paucity of axons crossing at the chiasm. The similarities between achiasmia in humans and mice due to a Pax2 gene anomaly are discussed.

Randomized retinal ganglion cell axon routing at the optic chiasm of GAP-43-deficient mice: association with midline recrossing and lack of normal ipsilateral axon turning

During mammalian development, retinal ganglion cell (RGC) axons from nasal retina cross the optic chiasm midline, whereas temporal retina axons do not and grow ipsilaterally, resulting in a projection of part of the visual world onto one side of the brain while the remaining part is represented on the opposite side. Previous studies have shown that RGC axons in GAP-43-deficient mice initially fail to grow from the optic chiasm to form optic tracts and are delayed temporarily in the midline region. Here we show that this delayed RGC axon exit from the chiasm is characterized by abnormal randomized axon routing into the ipsilateral and contralateral optic tracts, leading to duplicated representations of the visual world in both sides of the brain. Within the chiasm, individual contralaterally projecting axons grow in unusual semicircular trajectories, and the normal ipsilateral turning of ventral temporal axons is absent. These effects on both axon populations suggest that GAP-43 does not mediate pathfinding specifically for one or the other axon population but is more consistent with a model in which the initial pathfinding defect at the chiasm/tract transition zone leads to axons backing up into the chiasm, resulting in circular trajectories and eventual random axon exit into one or the other optic tract. Unusual RGC axon trajectories include chiasm midline recrossing similar to abnormal CNS midline recrossing in invertebrate "roundabout" mutants and Drosophila with altered calmodulin function. This resemblance and the fact that GAP-43 also has been proposed to regulate calmodulin availability raise the possibility that calmodulin function is involved in CNS midline axon guidance in both vertebrates and invertebrates.

The congenital and see-saw nystagmus in the prototypical achiasma of canines: comparison to the human achiasmatic prototype

We applied new methods for canine eye-movement recording to the study of achiasmatic mutant Belgian Sheepdogs, documenting their nystagmus waveforms and comparing them to humans with either congenital nystagmus (CN) alone or in conjunction with achiasma. A sling apparatus with head restraints and infrared reflection with either earth- or head-mounted sensors were used. Data were digitized for later evaluation. The horizontal nystagmus (1-6 Hz) was similar to that of human CN. Uniocular and disconjugate nystagmus and saccades were recorded. See-saw nystagmus (SSN), not normally seen with human CN, was present in all mutants (0.5-6 Hz) and in the one human achiasmat studied thus far. This pedigree is an animal model of CN and the SSN caused by achiasma or uniocular decussation. Given the finding of SSN in all mutant dogs and in a human, achiasma may be sufficient for the development of congenital SSN and, in human infants, SSN should alert the clinician to the possibility of either achiasma or uniocular decussation. Finally, the interplay of conjugacy and disconjugacy suggests independent ocular motor control of each eye with variable yoking in the dog.

Achiasmia in a case of midline craniofacial cleft with seesaw nystagmus

Images

Analysis of the retinas and optic nerves of achiasmatic Belgian sheepdogs

An autosomal recessive mutation carried in a family of black Belgian sheepdogs eliminates the optic chiasm--all retinal ganglion cell axons extend directly into the ipsilateral optic tract. One key issue we are trying to resolve is whether the retina or the chiasm is the principal site of mutant gene action. In this study, we have examined retinas of mutants to discover any associated changes in retinal structure. Retinas of mutant animals are relatively normal. Inner and outer nuclear layers are qualitatively indistinguishable from those of normal dogs. The principal difference is that the area centralis of mutants is smaller and had a lower peak ganglion cell density than that of normal dogs (8,100 vs 10,500/mm2, P < 0.05). This mutant phenotype is similar to that seen in retinas of Siamese cats and albino ferrets. Beyond area centralis, the central-to-peripheral gradient in ganglion cell density is normal in mutants. The size of the optic nerves, density of axons, and total number of axons do not differ between mutant and normal dogs. One of three mutant dogs had a small abnormal optic chiasm. Retrograde labeling of ganglion cells demonstrated that the residual crossed projection originated from cells in a widespread region in nasal retina and not solely from the peripheral nasal region, as might be expected of an anti-albino. Although our analysis does not rule out the retina as a site of mutant gene action, the modest differences between mutant and normal retinas suggest that the mutation either acts outside the retina or exerts a highly specific effect on ganglion cell trajectories alone.