Recovery in microtropia: implications for aetiology and neurophysiology

Abstract

Aims/background-The basis of binocular single vision in microtropia remains a matter of contention. This paper discusses the implications of recovery, in a group of primary microtropes following treatment, in relation to current concepts on the aetiology of the condition and proposed retinocortical correspondence. Methods-Nine previously untreated primary microtropes whose condition resolved, were reviewed in detail to assess the patterns of change in retinal correspondence, uniocular fixation, stereoacuity, and visual acuity in response to treatment. Results-Anomalous retinal correspondence (ARC), a central scotoma, reduced or absent stereoacuity, amblyopia, and uniocular eccentric fixation were present in all patients before treatment. Following treatment, all cases attained normal retinal correspondence (NRC), elimination of the central scotoma, stereo acuity levels of between 20 and 40 seconds of arc, 6/5 Snellen visual acuity in both eyes, and foveal fixation in both eyes. Conclusion-A hypothesis is put forward that for the patients described, a period of normal development is likely to have preceded the onset of microtropia, during which the retinocortical "foundations" for NRC were established. Precise pairing of foveo-foveal receptive fields was not abolished by the presence of amblyopia and a central scotoma, but this relation was temporarily suspended and binocular single vision was sustained via the neural substrate of paired receptive fields over a wide retinocortical area. (Br J Ophthalmol 1998;82:225-231) Microtropia, first described by Lang, 1 is a unique form of small angled strabismus with a highly developed degree of binocular cooperation compared with other forms of heterotropia, but stereoacuity is invariably reduced or absent with particularly poor responses demonstrated to random dot stereo tests.2 3 Microtropia can be classified as being "with" or "without identity" 2 according to the cover test findings. The term "identity" refers to the relation between the binocular angle of anomaly of anomalous retinal correspondence (ARC) and the uniocular angle of eccentric fixation: where these are coincident there is no movement on the cover-uncover test, as the same retinal area is stimulated uniocularly and binocularly. The microtropia is said to be without identity when the angle of anomaly exceeds the angle of eccentricity, so a manifest movement is seen on cover test. Primary microtropia is considered essentially sensorial in origin, unlike other forms of strabismus in which suppression and ARC develop in response to a motor anomaly in order to avoid diplopia and confusion. Proposed aetiological factors include an inherent inability or loss of ability for bifoveal fusion, 4-6 anisometropia producing a habitually defocused image in one eye and a subsequent foveal scotoma, 7 a "statistical variant" in the fixation reflex feedback mechanism, 2 8 and a genetic/hereditary component with a particular familial disposition to ARC. 9 10This paper appraises the clinical features before and af...

Changes in binocular function in anisometropic nonstrabismic children with optical correction and occlusion therapy

“…In addition, they used the Titmus test, which has monocular clues that may reduce the accuracy of the results. Cleary and colleagues 15 reported improvement of binocular function in a prospective study, which, however, was mainly 2.9 AE 0.9 (1-6.5)…”

“…12 Occlusion therapy and atropine further improved the outcome when refractive correction alone was not satisfactory. 13,14 Several studies have reported that binocular function can be improved with improvement of visual acuity [15][16][17] ; however, most of these were retrospective studies, [17][18][19] and they included a variety of patients including those with strabismus. [15][16][17] This study aimed to evaluate the degree of improvement of binocular function among previously untreated anisometropic children and to identify the factors that affect the degree of improvement.…”

Two-year follow-up of a randomized trial of spectacles plus alternate-day patching to treat strabismic amblyopia

“…Cleary et al (1998) reported recovery in microtropia with improved stereopsis. The current study was not designed with a phase of refractive adaptation followed by a phase of occlusion.…”

“…Recently the PEDIG (Cotter et al 2012) completed a clinical trial that evaluated optical treatment of strabismic and combined strabismic-anisometropic amblyopia; after 18 weeks, they found resolution of amblyopia in about a quarter of the children before initiating patching. However, Cleary et al (1998) reported that occlusion combined with spectacle correction in strabismic amblyopia seems more effective than spectacle correction alone.…”

Monofixation with Eso-, Exo-, or Hypertropia: Is There a Difference?

“…[1][2][3] The prevalence of micro-esotropia in several large series of primary and secondary microtropia has been reported from 61% to 90%. [4][5][6][7] None of these studies reported a single case of monofixational hypertropia, suggesting an extremely low prevalence of this form of small angle strabismus.…”