As the evaluation of strabismus therapy became increasingly critical, attention was focused on a relatively large group of patients who had a small residual deviation. This group attracted particular attention because, in addition to the consistent findings of a deviation measuring 8Δ or less and good fusional vergence amplitudes, there was a scotoma within the deviated eye that prevented diplopia.

Further interest was stimulated when it became apparent that some patients with small deviations had no history of strabismus. Anisometropia was identified early as a frequently associated factor in the nonstrabismic patients. Even more interesting was the discovery that some of the population free of strabismus and anisometropia had this same disorder. After it was noted that the common denominator in all patients with a small deviation was a small facultative macular scotoma within the visual field of one eye, binocular perimetry studies on a control series of patients with straight eyes revealed that some of these patients also had a macular scotoma. It was also recognized that the rare patient with a unilateral macular lesion, having straight eyes and extramacular fusion, has the organic counterpart to the unilateral functional facultative macular scotoma just described.

Consequently, a group of patients, either with or without a small deviation, from varied sources constitutes a specific ophthalmologic entity characterized by monofixation, due to the macular scotoma precluding bifixation and active extramacular binocular vision. The entity is referred to as the monofixation syndrome.

Initial interest in the monofixation syndrome came by way of the small angle deviations. Patients with these small angle deviations were referred to as “flicker cases,” by the British, because the cover test revealed a small “flick” as the deviated eye assumed fixation. This syndrome soon became confused with the normal fixation disparity described by Ogle¹ and others. In addition, this condition has been called fixational disparity and also referred to as subnormal binocular vision due to the lack of central fusion. In 1956 Jampolsky² described how some of these patients have greater alternate cover measurements than cover-uncover measurements, and he emphasized that this is diagnostic of the disorder. He described the suppression within the central retinal area in one eye of these patients, and he used this as an explanation for solving the diplopia caused by the minimal deviation. Jampolsky further reasoned that the peripheral portion of Panum’s fusional space is sufficiently large to permit fusion with normal retinal correspondence (NRC). His opinion regarding the lack of success with orthoptic treatment for these patients to convert them to centrally fusing rather than suppressing the central retinal area is clearly stated. Jampolsky and co-workers³ also noted the paucity of small angle exodeviations as compared to the frequent number of cases of convergent small angle deviations. In 1962 Jampolsky⁴ referred to the monofixation syndrome as “fusion disparity.” He implied that there is normal fusion, except for the absence of bifoveal fixation. He chose the term “fusion disparity” to separate a monofixation syndrome from fixation disparity, which is a normal physiologic entity. There are two obvious dissimilarities between fixation disparity and the monofixation syndrome (Jampolsky’s fusion disparity). The quantity of deviation does not exceed 10 to 14 minutes of arc in fixation disparity, but it may be as large as 8Δ in the monofixation syndrome. In fixation disparity, both macular areas function simultaneously; whereas, in the monofixation syndrome, one or the other macula does not function during binocular vision.

The impossibility of accurately naming this condition in accordance with the established semantic code in common usage for ocular motility and binocular vision was apparent early after the initial interest developed in this large group of patients. Appraised according to one respect, the patient was heterotropic, but in another respect, hetero-phoric. Any term selected to identify these patients was arbitrary. In 1961 the name “monofixational phoria” was applied to those patients with a deviation that was greater by alternate cover than by cover-uncover; it was claimed that the deviation was made partially latent by extramacular fusion while the image projected onto the deviated eye’s macula was not seen.⁵ At that time, interest was directed only to the small angle aspect of the deviations, and physicians were unaware that many patients without a deviation also had the identical sensory finding of a scotoma of one macula. Jampolsky’s concept of NRC peripheral fusion acquired by the normal stretched out peripheral Panum’s space was accepted, and the NRC seemed to be confirmed by the findings from binocular perimetry performed during dissociated conditions. The following significant facts about the monofixation syndrome also added by this report were:

In 1966 Lang criticized monofixational phoria as a name for small angle strabismus since there is a manifest tropia. Burian’s definition of heterophoria is a “deviation of the eyes kept latent by fusion; heterotropia is a patent (manifest) deviation of the eyes in the absence of fusion.”⁶ Lang⁷ proposed that the syndrome be known by the full name of microtropia unilateralis anomalo fusionalis, but he suggested that it be referred to ordinarily as microtropia or microstrabismus. In 1967 Helveston and von Noorden⁸ used the term “microtropia” to describe an inferred small angle strabismus in their amblyopia patients with eccentric fixation whose amblyopic eye did not make a movement to assume fixation and who grossly appeared to have straight eyes. The majority of their patients were anisometropic. Since, by visuscopy, the fixation point was adjacent to the macular borders, they inferred that the strabismus angle was ultra small. Others can confirm these findings in many patients with the monofixation syndrome whose poor sighted eye either has not responded to amblyopia therapy or has never been treated; the syndrome occurs either as a primary condition or secondary to strabismus, anisometropia (or the two combined), or a macular lesion. These patients seem to have monofixational orthophoria since there is no detectable shift in either eye by the cover test. Perhaps Helveston and yon Noorden are correct in their assumption that in some cases there probably would be a discernible shift were it not for the slight eccentric fixation in the amblyopic eye; therefore, the patient is not orthophoric. However, use of the term “microtropia” is not justified to describe the patients without shift to cover-uncover when Lang previously used the term to describe patients having a deviation by cover-uncover. The group described by Helveston and von Noorden probably represents only one of many various groups of patients within the overall monofixation syndrome.

The semantic structure that evolved as a result of many attempts to label various categories of patients that constitute the monofixation syndrome has become a monstrosity. Surely such terms as retinal slip, fixation disparity, esophoria with fixation disparity, fixational disparity, flicker cases, subnormal binocular vision, convergent fixation disparity, pathologic fixation disparity, monofixational phoria, fusion disparity, strabismus spurius, microtropia unilateralis anomalo fusionalis, microtropia, and microstrabismus will vanish from ordinary usage.

There are three principal reasons for the past difficulties encountered in naming this syndrome: (1) an element of both phoria and tropia is present and whichever feature the author chooses to emphasize determines the selection; (2) fixation disparity, as a name for a specific physiologic process in binocular single vision, was plagiarized since the condition under discussion seemed to be a pathologic extension of the same process; and (3) the names selected revealed the lack of a total concept of the syndrome. As the syndrome was gradually put together, the lack of organization in naming each of the facets is now apparent.

Essentially, the patients with this syndrome have straight or almost straight eyes and a form of binocular vision in which their inability to bifixate is proved by a demonstrable scotoma in the visual field of the nonfixating eye during binocular vision. This essential monofixating feature and other associated features are always present, while others may be either present or absent. Fusional vergence amplitudes are always associated with the monofixation syndrome. The variable features associated with this syndrome are a history of strabismus, anisometropia, a unilateral macular lesion, amblyopia, eccentric fixation, orthophoria, phoria, small tropia, and possibly a larger deviation by alternate cover than by cover-uncover. The majority of the patients with the monofixation syndrome have gross stereopsis; occasionally, the only exception is a patient with congenital esotropia who is straight and has sensory and motor fusion. The name that fits all of these features is simply “the monofixation syndrome.”