Tenacious Proximal Fusion: The Scobee Phenomenon

When patients with intermittent exotropia show an increase in their near deviation after prolonged monocular occlusion, they have been said to have tenacious proximal fusion (TPF). That term is not adequately descriptive, since this finding can occur without the patient having been allowed to fuse. The purpose of this study is to investigate the possibility that this phenomenon is mediated by the preponderance of binasal retinal disparity and uncrossed localization that occurs with near fixation.

Ten patients with intermittent exotropia who manifested TPF were measured at 6 m, 1/3 m, again at 1/3 m after 1 hour of monocular occlusion, and at 1/3 m with a peripheral crossed localization stimulating device (PCLSD) that simulated the retinal bitemporal disparity and peripheral crossed localization usually found with distance fixation.

For the ten patients, the mean measurement at distance was 28.3Δ±3.1, initially at near was 4Δ±3.9, at near after prolonged monocular occlusion was 25.3Δ±5.3, and at near with the PCLSD was 18.5Δ±4.1. The differences between the initial near measurement and the measurement with the PCLSD, and between the PCLSD and post-prolonged monocular occlusion were significant with P<0.001 and P=0.0049, respectively.

What has been previously referred to as TPF in fact appears to be convergence induced by the preponderance of binasal retinal disparity and/or peripheral uncrossed localization typically found with near fixation. For simplicity, I propose the term TPF should be replaced with the "Scobee Phenomenon."