Monocular fixation with the optic nerve head: a case report

In early acquired strabismus the visual system typically develops sensory adaptations to avoid the symptoms of diplopia and visual confusion (Pickwell, 1984), often reported as “double vision”. If these adaptations did not develop, the patient would experience diplopia, as the image of an object seen with the fovea of the nondeviated eye would also be seen with a peripheral retinal area of the deviated eye (retinal images would not fall on corresponding points); and visual confusion, as the fovea of the deviated eye would receive an image different to that falling on the fovea of the non-deviated eye (two corresponding points would receive different images which would appear superimposed, in the same direction). These phenomena are usually reported, when they occur in central retina, by patients who develop strabismus as adults (at an age when they typically do not develop adaptations). While diplopia and visual confusion occur only under binocular viewing conditions (as a consequence of strabismus), the resultant adapting mechanisms may function under either binocular viewing conditions or may persist under monocular conditions. The sensory adaptive changes that may occur during binocular vision are suppression and abnormal retinal correspondence (ARC) (Von Noorden, 1969; Pickwell, 1984). A suppression scotoma may appear in the central retina of the deviated eye to avoid visual confusion and may extend (nasally in ET) to avoid diplopia (Pickwell, 1984). ARC is a binocular adaptation of the visual direction system, measurable only under binocular conditions, where objects are perceived as localized in the same direction in spite of their images falling on normally non-corresponding retinal locations (Pickwell, 1984). These adaptation mechanisms may persist monocularly in the form of amblyopia or eccentric fixation. Amblyopia is the developmental impaired vision (of no organic cause) typified by reduced visual acuity, distorted space perception and incorrect localization of objects in space (Bedell et al., 1985; Skottun et al., 1986; Fronius et al., 2004). It may be a consequence of long term binocular suppression that remains under monocular conditions - although some advocate that it may precede the strabismus (Schor, 1978). Strabismic amblyopia is most commonly associated with constant ET (Quah et al., 1991). Eccentric fixation refers to abnormal monocular visual fixation with an area other than the fovea (Von Noorden, 1970; Kirschen and Flom, 1978; Pickwell, 1984). The eccentric fixation locus typically occurs within 5deg (~10Δ) from the fovea (Flom et al., 1980; Pickwell, 1981; Bedell et al., 1990; Matilla et al., 1995; Cleary and Thompson, 2001) and it may be stable or unstable. The perceived direction of an object in space is determined by the retinal location it stimulates in conjunction with extraretinal information (e.g. eye position in orbit). Each retinal locus has a retinal visual direction associated with it. In a normal eye, the fovea has the principal visual direction (PVD) that is associated with the straight-ahead egocentric visual direction when the eye is in the primary position of gaze. Likewise, each retinal locus has a motor value, which means that it induces a fixed magnitude eye movement (saccadic) during the fixation reflex that brings the fovea to the direction of the object that stimulated that retinal location. The zero retinomotor value (ZRMV) (normally at the fovea) is the retinal point to which the eye makes a reflex saccadic re-fixation movement when presented with a peripheral stimulus (Ciuffreda, 1991; Steinman et al., 2000). Usually the new monocular fixation locus in eccentric fixation becomes associated with the PVD and the ZRMV (Von Noorden, 1970; Ciuffreda et al., 1979; Bedell and Flom, 1981), although some authors argue that the PVD may remain unchanged (Pickwell, 1984). ET with an angle of deviation of about 15deg (~28Δ) is not rare (Swan, 1948; De Muelenaere and Hambresin, 1956; Guzzinati, 1956; Prizner, 1975; Olivier and Von Noorden, 1981; Birch et al., 2004). In these cases, the fovea of the deviated eye is directed at an object whose image falls within the physiological blind spot of the non-deviated eye and vice versa. It has been suggested that this phenomenon may be a mechanism to avoid diplopia (Swan, 1947) and visual confusion. Swan (1948) defined the Blind Spot Syndrome as a type of adaptation to ET with angle of deviation of about 15deg, with occasional diplopia, normal retinal correspondence, and normal visual acuity. Others have reported cases of Blind Spot Syndromes (De Muelenaere and Hambresin, 1956; Guzzinati, 1956; Uemura, 1964; McKenzie et al., 1970; Prizner, 1975; Harrer, 1984). However, controversy surrounds the nature of these cases. The case we report here is not a case of Blind Spot Syndrome (see Discussion section). We have documented, using an objective method of assessing visual direction, a case of left ET wherein (under left monocular viewing) the left PVD and ZRMV are located within the left physiological blind spot.