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

Abstract

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.

VEP asymmetry with ophthalmological and MRI findings in two achiasmatic children

“…In our previous study we showed that visual electrophysiological screening of infants with congenital nystagmus can establish or exclude retinal and postretinal pathway dysfunction [30]. Children with maldeveloped optic chiasm in this and other studies were electrophysiologically assessed due to their congenital nystagmus [4,6,8,9,31] or were suspected of having albinism [2]. Therefore, from previous studies and ours, we may assume that already in early infancy the distinctive VEP will help in the diagnosis of achiasmia.…”

“…Achiasmia can appear as an isolated abnormality [1,2,5,6,8,9] or as a part of septooptic dysplasia with a constellation of small anterior visual pathways, absence of the septum pelucidum, thinning of the corpus callosum and possible other abnormalities of pituitary gland, hemispheric migration anomalies and perinatal hemispheric injury [3,4,7]. In child 1 the MRI results confirmed lack of connection between both hypoplastic optic nerves and tracts as an isolated abnormality.…”

“…In both children there was no evidence of see-saw nystagmus previously believed to be characteristic in achiasmia [1,2], or associated with other midline abnormalities. Purely horizontal nystagmus or combined horizontal and torsional nystagmus, or other descriptions of nystagmus were reported in achiasmia [4,5,[7][8][9].…”

“…This achiasmatic condition was confirmed by MRI and reported as the non-decussating retinal-fugal fibre syndrome. In further studies, a small number of children with absence or deficiency of optic nerve fibres that cross over at the optic chiasm, have also been presented [2][3][4][5][6][7][8][9].…”

Cranial Nerves N‐VI

“…One study noted an asymmetrical optic chiasm, possibly due to optic nerve aplasia (Taylor 2005). Unilateral or bilateral absence of the optic chiasm presented as one of three types, where nasal-retinal fibers would not decussate to the contralateral hemisphere (Taylor 2005(Taylor , 2007Brown et al 2006;Biega et al 2007). These were classified as the following: type 1: reduced decussation, normal appearing optic nerves; type 2: reduced decussation chiasmal hypoplasia, midline defects (septo-optic dysplasia); and type 3: reduced decussation, chiasmal hypoplasia, clefts, and agenesis of corpus callosum.…”