In patients with full monocular visual fields there is a partial overlapping of the nasal fields of one eye and most of the temporal field of the other eye. This permits stable binocular fusion. When there is complete bitemporal hemianopsia or a dense bitemporal hemianopic scotoma from damage to the optic chiasma, this can lead to the hemifield slide phenomenon. In this situation, there is no longer overlap of the corresponding points of the central visual field of both eyes leading to no region in the visual field having binocular representation (Kirkham, 1972). The intact non-corresponding nasal hemifields of both eyes are simply adjacent to each other. This may result in decompensation of preexisting phoria into tropia and can lead to diplopia. A similar phenomenon can take place in patients with complete superior altitudinal defect in one eye and a complete inferior altitudinal defect in the other eye from bilateral retinopathy or more commonly, optic neuropathy (Borchert, 1996). Patients with this phenomenon experience double vision during reading and may note disappearance or duplication of the letters, at the vertical meridian during episodes of eso or exo deviation. The two hemifields will be seen to overlap, in exotropia, separate in esotropia or become vertically separated in patients with vertical tropias.
Another possible explanation of this phenomenon is that the complete separation of the afferents of both eyes into separate hemispheres results in loss of the interocular alignment even in non-phoric patients. The fact that the phenomenon does not occur to all patients supports the suggestion that the condition typically occurs in certain predisposed patients.
1. Kirkham TH. The ocular symptomatology of pituitary tumours. Proc R Soc Med. 1972; (6):517-8.
2. Borchert MS, Lessell S, Hoyt WF. Hemifield slide diplopia from altitudinal visual field defects. J Neuroophthalmol. 1996; (2):107-9.