Ocular Motor Disorders Associated with Inborn Chiasmal Crossing Defects

The present study briefly reviews the accompanying ocular motor function and dysfunction in two major inborn conditions of naturally occurring aberrant optic pathway projections, i.e., albinism and non-decussating retinal-fugal fibre syndrome (Apkarian et al. 1994; 1995; Apkarian 1996). The latter is a rare, isolated achiasmatic condition in which nasal retinal projections and concomitant visuotopic and retinotopic mapping misroute due to the inborn and isolated absence of the optic chiasm. The achiasmatic ocular motor profile is compared with that of albinos who also present with optic pathway misrouting but in opposite form from that of the achiasmatic condition. In albinism there is a preponderance of erroneously decussating temporal retinal-fugal fibres whilst in achiasmatic syndrome there is a complete absence of decussating nasal retinal-fugal fibres. The simplified sketches of Figure 1, adapted from Polyak (1957), emphasize the normal organisation of retinal-fugal projections compared to the relative majority of contralateral projections pathognomonic to albinism and the nonextant contralateral projections characteristic of the achiasmatic condition.