The Stability of the Monofixation Syndrome


To determine etiology, characteristics, and stability of the monofixation syndrome (MFS) in a chart review of 63 consecutive patients in a private practice of pediatric ophthalmology.


Retrospective, consecutive chart review.


The charts of 63 consecutive patients with MFS, encountered from 2007-2012 followed for a minimum of 3 years were included. Best visual acuity, motor angle deviation at near, fusion, and stereoacuity as recorded on the most recent visit during the follow-up period were examined. Stability of the MFS was determined by comparing the dates of diagnosis with the last examination for those patients who remained stable. Decompensated patients were also studied for characteristics and results of secondary surgery.


The etiology of the MFS was esotropia in 58 (92.1%), anisometropia in 2 (3.2%), and exotropia in 3 patients (4.8%). The majority of patients, 57 of 63 (92%), had stable MFS for a mean of 13.9 years. Six esotropic patients decompensated after a mean of 6 years; 5 of these patients were restored to MFS by secondary surgery and 1 spontaneously recovered. Five patients were found to have improved stereoacuity to 60 arc seconds or better by the end of the study.


The MFS is a relatively stable binocular status during the first 2 decades of follow-up. A small percentage of patients decompensated, requiring secondary surgery to restore the MFS binocular status. A similar small percentage of MFS patients developed a higher grade of stereoacuity when followed for a sufficient interval of time.

The monofixation syndrome (MFS), as described by Parks, is a specific subnormal binocular vision status, either with or without a small deviation (heterotropia) . Parks determined that there are 3 key features to the MFS. These features are (1) monofixation attributable to a central suppression scotoma in the vision of 1 eye when tested for binocular viewing; this central suppression scotoma is not present on monocular testing; (2) patients with MFS, with possible heterotropia up to 8 prism diopters, are able to demonstrate fusion with amplitudes and do not have diplopia; (3) MFS patients are unable to bifixate; therefore, these patients lack bifoveal fusion. MFS patients achieve only gross (60-3000 arc seconds) stereoacuity.

Parks also reported that patients with MFS often presented with elements of both heterotropia and heterophoria. This characteristic can be demonstrated by comparing the simultaneous prism cover test with an alternating cover test in the same patient. The simultaneous prism cover test is performed by placing a base-out prism over the nonfixing eye at the same time as a cover is placed over the fixing eye. Parks considered a simultaneous prism cover test measuring a horizontal deviation up to 8 prism diopters as the maximum that would permit fusion. Invariably, the quantity of deviation found with a simultaneous prism cover test would be less than the quantity of deviation needed to neutralize corrective eye movements on an alternate prism cover test in MFS patients. The larger deviation found on alternating cover tests in MFS demonstrates the heterophoric component of the combined heterotropia and heterophoria binocular status.

The MFS features that have been described above may follow surgical correction for congenital esotropia. Indeed, MFS is, perhaps, the optimum binocular outcome that can be expected following the surgical treatment of this type of strabismus. The MFS may also be found in anisometropia, in treated amblyopia, in accommodative or acquired esotropia, and following surgical correction for exotropia. In addition, MFS was recently described to be the most common form of binocularity following bilateral cataract and intraocular lens placement in children.

Patients with primary MFS, who have no history of strabismus, also have the inability to bifixate and demonstrate only gross stereoacuity. Indeed, these patients with primary MFS remain undisclosed until they are tested with graded stereopsis tests, in which these patients are unable to distinguish high grade (50 arc seconds or better) on stereoacuity testing. Primary MFS has also been found in first-order relatives of patients with congenital esotropia.

The fact that high-grade stereopsis was so rarely found after surgical treatment of congenital esotropia led Parks to speculate that congenital esotropic patients may have “an inherent inability to bifixate.” Worth claimed that congenital esotropic patients had a deficit in their fusion faculty. However, Worth’s concept has been, at least partially, discredited by the extramacular fusion that is demonstrated in the MFS result obtained by early surgical alignment of congenital esotropia. Parks reported that MFS patients had fusional amplitudes that were comparable to normal patients, and he observed that MFS patients were able to maintain alignment as well as if they were bifixating.

The stability of MFS has been examined by few investigators. Decompensated or deteriorated MFS patients have been compared to patients who have not developed MFS by Arthur and associates. In this study, it was found that patients, aligned to within 8 prism diopters of orthophoria, who developed MFS were more stable than those who did not develop MFS. This study reported that, after 17.5 years, 74% of the MFS patients maintained alignment to within 8 prism diopters compared to 45% of the patients without MFS who maintained alignment after 14 years ( P < .005).

Despite the reputation of MFS patients as achieving stable binocularity, Hunt and Keech described a group of 29 MFS patients who deteriorated despite maintaining binocularity for an average of 7.8 years. In this study, 28 of the 29 patients had esotropia prior to the development of MFS and 1 was reported to have intermittent exotropia. After the loss of fusion, diplopia was reported in one third of the patients. The complaints of diplopia persisted in 4 out of 9 patients after they had received realignment surgery.

In addition to the 2 aforementioned investigations, in a recent study by Siatkowksi, 14 patients between the ages of 20 and 70, who had been stable for over 25 years, were found to lose fusion over time. These patients all complained of diplopia and were found to lose their binocularity at a rate averaging 7% per year from ages 20-70. In this latter study, similar to the study by Hunt and Keech, it was reported that complaints of diplopia remained in one third of the patients despite surgical realignment.

The purpose of the present study is to determine the etiology, characteristics, and the stability of the MFS in a retrospective consecutive chart review of 63 patients encountered during a 5-year period (2007-2012). The patients selected had been followed for a minimum of 3 years.


The retrospective chart study protocol received a waiver by the Hawaii Pacific Health Privacy Board (HPH PB) from regulations for category 4 using guidelines set by the Office of Human Research Protection (45 CFR 46.101(b)). The HPH PB determined that the protocol of the present study satisfied the 3 requirements for a waiver of authorization, namely: there was an adequate plan to protect identifiers from disclosure, the research could not be practicably conducted without access to the Protected Health Information, and the research could not be practicably conducted without the waiver. An informed consent had been obtained from the parents for any corrective surgery performed on their children. The study was conducted in compliance with the tenets of the Declaration of Helsinki.

Ninety-four charts were selected from a consecutive series of patients examined during 2007-2012 for which the diagnosis of monofixation syndrome had been entered into the chart. The charts selected were reviewed to confirm the diagnosis of MFS with a documentation of a small heterophoria or heterotropia, and the presence of fusion as demonstrated by a Worth 4 dot test. Any chart in which microtropia was documented without fusion was eliminated. Charts were eliminated from the analysis if insufficient evidence of the syndrome was present in the chart or the patient had not been followed for a minimum of 3 years. There were 63 charts that met the inclusion criteria for the study. Decompensation was designated if there was an increasing heterotropia with loss of fusion or complaints of diplopia.

The charts were studied for the recordings of visual acuity, the etiology of the MFS syndrome, and motor and sensory tests for binocularity. Decompensation was detected by an increasing heterotropia over 8 prism diopters, with the loss of fusion and/or complaints of diplopia. The decompensation rate of the MFS and the result of surgical realignment are reported. The stereoacuity of the MFS patients, recorded over a period of time, was studied to determine any possible changes.

The MFS was classified into 3 subgroups (esotropia, exotropia, and anisometropia). The date of the initial MFS diagnosis, the presence or absence of decompensation, and the date of secondary surgery and recovery were examined. The duration of follow-up for each patient was calculated as the time difference between the initial MFS diagnosis date and final chart entry date with the diagnosis of MFS for all patients. In addition, visual acuity and data concerning motor tests with prisms were tabulated, as well as the results of sensory tests with Worth 4 dots and stereopsis testing with the Titmus stereogram. The change of stereoacuity score from initial entry to last recorded entry in the follow-up period was recorded for each patient.

The SPSS statistical package (IBM SPSS Statistics for Windows, version 20.0, released 2011; IBM Corp, Honolulu, Hawaii, USA) was used to calculate means and standard deviations (SD) for continuous variables and frequencies and percentages for categorical variables. Patients’ demographic and clinical data were summarized by these descriptive statistics. Differences of key variables among the 3 subgroups were evaluated using a 1-way analysis of variance approach. Two-tailed P value of less than .05 was regarded as statistically significant.


The etiology of the MFS was predominantly esotropia in 58 of 63 patients (92.1%). Of the total patients, 24 (38.1%) were diagnosed with congenital esotropia, 18 (28.6%) were diagnosed with accommodative esotropia, 9 (14.3%) were classified as unspecified esotropic origin, and the other 7 patients (11.1%) were classified as mixed (congenital and accommodative esotropia). The etiology of the MFS syndrome was exotropia in 3 patients (4.8%) and anisometropia in 2 patients (3.2%).

Fifty-seven of 63 patients (90.5%) remained without decompensation for a mean (SD) follow-up period of 13.9 (7.5) years. Of these 57 patients, the mean (SD) duration of follow-up, in years, was 12.6 (6.5) for the congenital esotropia patients (n = 22), 15.5 (8.0) for the accommodative esotropia patients (n = 16), 16.5 (11.5) for the unspecified esotropia patients (n = 8), and 15.1 (3.4) for mixed (congenital and accommodative esotropia) patients (n = 6). The duration differences between the subgroups of esotropia were not statistically significant ( P = .27). The overall mean (SD) duration of follow-up for all stable esotropic patients (n = 52) was 14.4 (7.6) years. Patients with the diagnosis of exotropia (n = 3) without decompensation were followed for a mean (SD) of 8.3 (3.7) years and patients with the diagnosis of anisometropia (n = 2) without decompensation were followed for a mean (SD) of 9.4 (4.8) years ( Table 1 ).

Table 1

Etiology, Characteristics, and Duration of Stable vs Decompensated Monofixation Syndrome Patients

Stable Patients Decompensated Patients Total Monofixation Syndrome Patients
Monofixation syndrome patients, n (%) 57 (90.5%) 6 (9.5%) 63 (100%)
Primary diagnosis, n (%)
Total esotropia 52 (82.5%) 6 (9.5%) 58 (92.1%)
Congenital esotropia 22 (38.6%) 2 (3.2%) 24 (38.1%)
Accommodative esotropia 16 (25.4%) 2 (3.2%) 18 (28.6%)
Esotropia (unspecified) 8 (12.7%) 1 (1.6%) 9 (14.3%)
Mixed (accommodative and congenital esotropia) 6 (9.5%) 1 (1.6%) 7 (11.1%)
Exotropia 3 (4.8%) 0 (0.0%) 3 (4.8%)
Anisometropia 2 (3.2%) 0 (0.0%) 2 (3.2%)
Mean (SD) duration of follow-up (y)
All patients 13.9 (7.5) 15.8 (8.8) 14.1 (7.6)
Total esotropia 14.4 (7.6) 15.8 (8.8) 14.5 (7.7)
Congenital esotropia 12.6 (6.5) 12.8 (3.9) 12.6 (6.3)
Accommodative esotropia 15.6 (8.0) 23.6 (11.2) 16.5 (8.4)
Esotropia (unspecified) 16.5 (11.5) 5.4 (n/a) 16.5 (11.5)
Mixed (accommodative and congenital esotropia) 15.1 (3.4) 16.5 (n/a) 15.1 (3.4)
Exotropia 8.3 (3.7) n/a 8.3 (3.7)
Anisometropia 9.4 (4.8) n/a 9.4 (4.8)
Mean (SD) age at time of chart review (y) 23.4 (10.7) 21.7 (8.9) 22.9 (10.4)

Six of 63 patients (9.5%) were found to have decompensated during their follow-up period and 5 of the 6 patients received secondary surgery to reestablish MFS status. All 6 of the decompensated patients had a diagnosis of esotropia. One decompensated patient, with accommodative esotropia, experienced a spontaneous recovery to reestablish a MFS status. Of the 6 decompensated esotropic patients, 2 were congenitally esotropic and the MFS diagnosis had been present for a mean (SD) of 8.9 (2.1) years prior to decompensation. Two accommodative esotropia patients were stable for a mean (SD) of 6.4 (2.3) years prior to decompensation. A single patient with unspecified esotropia was followed for less than 1 year (0.3 years) before decompensation and a single esotropia patient of mixed (accommodative and congenital esotropia) origin was stable for 5.6 years before decompensation. The mean (SD) duration of the MFS for the total group of patients who decompensated was 6.4 (3.7) years before decompensation. Despite the decompensation, none of these patients complained of diplopia and all decompensated patients had reestablishment of the MFS after surgery or spontaneously. The mean (SD) age of the decompensated patients was 11.2 (2.8) years at the time of the decompensation and the mean (SD) age at the last follow-up examination was 20.8 (8.6) years. The mean (SD) duration of follow-up after reestablishment of MFS was 8.7 (9.5) years. There was a difference in visual acuity (amblyopia) in 4 of the 6 patients who experienced decompensation ( Tables 2 and 3 ).

Table 2

A Summary of Characteristics of All Decompensated Monofixation Syndrome Patients

Mean (SD) duration of follow-up prior to decompensation (y)
All decompensated patients 6.0 (3.7)
Congenital esotropia 8.9 (2.1)
Accommodative esotropia 6.4 (2.3)
Esotropia (unspecified) 0.3 (n/a)
Mixed (accommodative and congenital esotropia) 5.6 (n/a)
Exotropia N/A
Anisometropia N/A
Mean (SD) age at time of decompensation (y) 11.2 (2.8)
Mean (SD) age at last follow-up examination (y) 20.8 (8.6)
Mean (SD) duration of follow-up after re-establishment of monofixation syndrome (y) 8.7 (9.5)
Number (%) of decompensated patients with difference in best-corrected visual acuity 4 (66.7%)

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Jan 9, 2017 | Posted by in OPHTHALMOLOGY | Comments Off on The Stability of the Monofixation Syndrome

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