The decompensated monofixation syndrome (an American Ophthalmological Society thesis).

PURPOSE: To describe the clinical features and response to treatment of patients with decompensated monofixation syndrome (MFS) and to propose a hypothesis for a decompensation mechanism in such patients.

METHODS: Fourteen adults with MFS who had been symptomatically stable for a mean duration of 25 years developed diplopia in the absence of neurologic or orbital disease. After retrospective chart review, they underwent detailed orthoptic testing. Results from this cross-sectional analysis were compared with similar data from 16 control subjects with stable MFS.

RESULTS: Compared to stable MFS patients, decompensated subjects had significantly poorer horizontal fusional amplitudes but greater torsional fusional amplitudes; they were also more likely to have a small vertical strabismus and to have received initial treatment later. Stable subjects, however, also had subnormal horizontal as well as torsional fusional amplitudes. There was no difference between groups with respect to refractive error, amblyopia, type or prior treatment of strabismus, stereoacuity, or angle of deviation. After treatment, all patients regained monofixational alignment, but up to one-third had continued diplopia. Symptoms recurred in two patients whose treatment was initially successful.

CONCLUSIONS: Patients with MFS lose fusional amplitudes over time. In some cases this results in development of sensory torsion with secondary decompensation and diplopia. The rate of decompensation averages 7% per year from ages 20 to 70. Treatment for decompensation offers excellent motor results, but sensory symptoms may persist and recurrent symptoms may develop. Monitoring and maintenance of fusional vergence amplitudes should be part of the routine care for patients with MFS.