Ultrasmall angles of hetertropia are usually missed by routine methods of examination and are frequently overlooked. The cover test is negative and the fixation movement of the deviated eye is absent when the sound eye is covered.

A microtropia should always be suspected in unilateral decrease of visual acuity for which no organic cause can be found in patients without apparent strabismus or a history of strabismus and without significant refractive errors or anisometropia. Extensive neuro-opthalmologic investigations and fears of an intracranial lesion can be avoided by making the correct diagnosis.

 When the cover test is negative, special diagnostic procedures are used to differentiate a microtropia from nonstrabismic abnormalities causing decreased visual acuity in one eye. A cycloplegic refraction should be carried out at the beginning of such an examination since microtropia occurs frequently with anisometropic amblyopia. Nonfoveolar fixation in the amblyopic eye clearly establishes the diagnosis of microtropia. Whenever stereoacuity is reduced in the presence of apparent ocular alignment with scotoma response on the 4 diopter base-out prism test, the examiner must search for microtropia.

Microtropia in the older child or adult does not require therapy, treatment in such patients is ill advised, for elimination of the central scotoma may cause diplopia. Such patients have comfortable and nearly normal binocular vision with good peripheral fusional amplitudes. In young children up to 6 years of age, attempts should be made to treat the amblyopia. If significant anisometropia is present, occlude the fixating eye and prescribe the full refractive correction, many patients showed the disappearance of microtropia after successful occlusion therapy. Fixation of the amblyopic eye changed from parafoveal to central and steady, visual acuity reached a level of 6/6, retinal correspondence became normal, and stereoacuity improved.