Description, Causes and Risk Factors:
Microtropia is defined as a manifest deviation of less than 5° in which ARC (giving rise to abnormal binocular single vision (ABSV)), normal motor fusion, and reduced or absent stereoacuity are found. In addition, amblyopia, a foveal suppression scotoma, and uniocular eccentric fixation are present and there is a close association with anisometropia. The term microtropia “with identity” is used by most authorities to describe patients with no manifest movement on cover test, the eccentric fixation point coinciding with the angle of ARC. Microtropia “without identity” describes patients in whom the manifest movement is demonstrated on the cover-uncover test. Microtropia may be primary, when there is no history of previous large angled strabismus, or secondary following surgical or optical correction of a larger strabismus or associated with other ocular pathology.
The most feasible etiology of microtropia seems to be the statistical theory of Goldmann (I967), who postulated that there must be a statistical variation in the interaction between the feed-back of uniocular fixation and the feed-back of binocular fusion. From this variation primary microtropia would result. We have only to add that convergence and also heredity have their place in the development of this condition.
Patients with microtropia usually presents with diminished visual acuity in one eye not improving with refractive correction. Fundus examination of these patients reveals no abnormality. Acocrding to Havelston and Von Noorden, it is assumed that even small degree of anisometropia if left uncorrected in early childhood, may lead to the establishment of foveal supression scotoma, anisometroopic amblyopia and finally to microstrabismus.
Uncorrected anisometropia seems to be the principal cause of central scotoma. The cover test for tropia, the cover-uncover test for phoria and the alternate cover test to demonstrate the maximum deviation should be carried out meticulously and repeatedly before diagnosis case of microtropia.
Inaccurate/inconsistent visual attention.
Inaccurate eye-hand coordination
Transient blurred vision.
The diagnosis of microtropia is made by the unilateral cover test, by the investigation of fixation, and by the examination of anomalous correspondence, which striated lenses show to be harmonious. The alternate prism-cover test shows additional heterophoria. Among other helpful tests the most important is bifoveal visuscopy whereby the angle of anomaly and the centre of anomalous correspondence can be determined with the aid of a periscope-like double mirror.
The doctor of optometry determines appropriate diagnostic and therapeutic modalities, and frequency of evaluation and follow-up, based on the urgency and nature of the patient's conditions and unique needs. The management of the case and duration of treatment would be affected by:
In patients below 6 years of age if anisometropia is present, refractive error should be corrected and fixating eye should be occluded. The results are encouraging if the treatment is started at an early age. In patients above 6 years of age, no active treatment is needed since they have all the three grade of binocularity.
Successful treatment of microtropia must address the defective performance of the amblyopic visual system and the accompanying strabismus and associated conditions. Orthoptics/vision therapy (including prism/lens therapy) is usually required to achieve maximum improvement in patients with microtropia. Optometric orthoptics/vision therapy usually incorporates the prescription of specific treatments in order to:
Normalize and equalize fixation accuracy.
Normalize and equalize oculomotor control.
Normalize and equalize accommodative accuracy and responses.
Normalize visual discrimination.
Normalize spatial judgments and visual information processing.
Eliminate abnormal suppression.
Reestablish normal retinal correspondence.
Eliminate the strabismus and associated conditions.
Provide a clear optical image.
NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
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