What Don't You See? - Abstract

This four-year old male presented with congenital see-saw and horizontal nystagmus. The see-saw component of his nystagmus prompted our investigation into para-sellar pathology. His prior neuroimaging was reportedly normal; however, this study was unavailable for direct review. Repeat MRI of the brain demonstrated achiasma. Achiasma is the congenital absence of the optic chiasm, which may be isolated or associated with other midline central nervous system abnormalities. The first description of mammalian achiasma and its association with ocular motor abnormalities, such as see-saw nystagmus and congenital nystagmus, was reported in the 1990s in a family of mutant Belgian sheepdogs. These observations later led to the association of these findings in humans, prompting analyses of the visual pathways in achiasmatic humans. The lack of crossing fibers at the chiasm results in representation of the entire visual field to each ipsilateral occipital cortex, as has been demonstrated by methods such as visual-evoked potential and functional MRI. Achiasmatic patients described in the literature generally have very poor stereoacuity and moderately reduced visual acuity. In the absence of other visual pathway disease, there is no visual field defect because the entire visual field is represented cortically. There may be associated strabismus and ocular torticollis. Idiopathic infantile nystagmus is present and may have a see-saw and/or horizontal waveform.