Letter to editor: “is microtropia a reliable indicator for the presence of amblyopia in anisometropic patients?” by Lysons and Tapley

Dear Editor:

I read with interest the recent article by Lysons and Tapley,¹ “Is microtropia a reliable indicator of amblyopia in anisometropic patients?” The authors conclude that the presence of microtropia with identity is a reliable indicator for the presence of amblyopia and possible need for occlusion therapy following optical treatment in “straight-eyed” anisometropic children. Children responding positively with the 4-diopter prism test were diagnosed as having microtropia with identity and did not achieve equal visual acuity with either optical correction or with optical correction and occlusion. Children responding negatively with the 4-diopter prism test and not having microtopia with identity achieved equal visual acuity following 2–6 months of optical therapy alone.

I agree with Lysons and Tapley¹ that a full period of optical treatment is imperative prior to prescribing occlusion, particularly if visual acuity is improving. However, they misused the term microtropia with identity. According to most authorities, microtropia with identity not only includes straight eyes with unilateral cover test, amblyopia, suppression, anisometropia, and deficient stereopsis, but also parafoveal or eccentric fixation in the poorer seeing and more ametropic eye.^2,3 Helveston and von Noorden,² who described this unique and rare condition in 1967, indicated that the area of eccentric fixation replaces the fovea for binocular vision as well as monocular vision, resulting in no refixation movement by the poorer seeing eye when the other eye is occluded. Absence of a shift on the unilateral cover test in the presence of eccentric fixation is interpreted as an identity between the degree of eccentric fixation, the angle of anomaly, and the angle of manifest strabismus.^2,3

The purpose of the 4diopter prism test is to objectively detect foveal suppression in patients having “straight eyes” and unexplained reduction in visual acuity and/or stereopsis.⁴ It is not specific for microtropia with identity and does not differentiate between foveal suppression due to other conditions such as anisometropia or organic factors. The diagnosis of microtropia with identity requires visuoscopic assessment of the fixation pattern in the poorer seeing eye.^2,3,5

A more appropriate conclusion by Lysons and Tapley¹ would be the following: the presence of foveal suppression as determined with the 4-diopter prism test appears to be a good predictor of which “straight-eyed,” anisometropic children will develop amblyopia and possibly require occlusion as part of their therapy.

Response to Robert Rutstein’s Letter to Editor

We agree that non-foveal fixation determined by visuoscopy is a feature of microtropia with identity along with other features including reduced stereo-acuity and central suppression using the four dioptre prism test. Our study was retrospective and visuoscopy had not been carried out in the majority of cases. Our paper is a transcript of a clinical presentation that was originally given to a predominantly orthoptic audience where central suppression using a four dioptre prism test is accepted as diagnostic for the presence of microtropia with identity when visuoscopy is unavailable, hence our use of this term. The finer points of microtropia differentiation, diagnosis and terminology were beyond the remit of our paper. Whether central suppression is present due to microtropia with identity or some other foveal abnormality the implications remain the same. In the absence of a suppression response our cohort did not require occlusion treatment and this fact enables us to avoid rushing into unnecessary occlusion treatment which is beneficial for both the patient and departmental patch budgets.

Debbie Lysons & Jane Tapley 9th September 2018