Non-decussating retinal-fugal fibre syndrome: An inborn achiasmatic malformation associated with visuotopic misrouting, visual evoked potential ipsilateral asymmetry and nystagmus

Summary: We report a newly identified syndrome in which nasal retinal fibres fail to decussate due to the inborn absence of an optic chiasm. Visual evoked potential (VEP) assessment and neuroophthalmic evaluation in two unrelated, non-albino children revealed the unusual visual pathway anomaly in the form of misrouted retinal-fugal projections. Monocular VEP responses across the occiput, regardless of stimulus mode (full- or partial-field pattern onset, pattern reversal, luminance flash or high temporal frequency luminance flicker) showed unequivocal evidence of pathological VEP ipsilateral asymmetry. Marked attenuation of primary visual evoked responses from the occiput contralateral to the eye of stimulation, indicative of aberrant contralateral retinal-fugal projections, was confirmed by MRI which depicted the remarkable achiasmatic condition. MRIs and neurological evaluation also confirmed the absence of accompanying congenital or acquired brain malformations or anomalies. Ophthalmic evaluation revealed that both achiasmatic children had reduced distance acuity for age, alternating esotropia, torticollis, head tremor and ocular motor instability; visual fields were normal. Eye movements were also monitored and indicated congenital nystagmus waveforms in the horizontal plane; see-saw nystagmus was observed in the vertical and torsional planes. The age range of the two children during evaluation and follow-up, over a 6-year period, was about 4-15 years. Comparisons of VEP responses from age-matched normal, albino and idiopathic congenital nystagmus controls, recorded under the same VEP test conditions, were also performed. In contrast to the achiasmatic ipsilateral inter-ocular asymmetry, the albinos showed the expected monocular VEP topography pattern of contralateral asymmetry. Also as expected, VEP profiles from the normal controls and those with congenital nystagmus, evinced no aberrant asymmetry patterns. In general, the results indicate that the VEP misrouting protocol is indispensable for the non-invasive electrophysiological detection and differential diagnosis of optic pathway mutations and may well identify individuals with purported idiopathic congenital nystagmus or albinism that are, in fact, achiasmatic.