- Microtropia is a small-angle, usually unilateral, strabismus of 10Δ or less. It is the most common form of abnormal binocular single vision. - Characteristics include a foveal suppression scotoma in the deviating eye, reduced visual acuity in that eye, and anisometropia in nearly all cases. Microtropia can be classified as primary or secondary. - Management aims to obtain the best possible visual acuity in each eye through refractive correction, occlusion therapy, and restoration of binocular single vision if needed for an associated strabismus.

1. D R A Z M A T K H A N
MICROTROPIA

2. Microtropia
 The term Microtropia was first used by Lang (1968)
to describe small-angle unilateral strabismus with
BSV in which the manifest deviation did not exceed 10
Δ. This is the most common form of abnormal BSV.

3. Incidence
 Lang (1976) reviewed 33644 patients with ocular
defects and reported an incidence of microtropia of
2.84%; the large majority had microesotropia.
 Cobb et al. (2002) reported only three
miroexotropias in a series of 37.

4. Characteristics
The features of microtropia are:
 A monocular manifest strabismus of 10 Δ or less,
often with an associated heterophoria.
 A foveal suppression scotoma in the affected eye.
 Abnormal BSV with sensory and motor fusion.

5.  Reduced visual acuity (VA) in the deviating eye.
 Anisometropia in nearly all cases, commonly with
hypermetropia or hypermetropic astigmatism.
 Parafoveal fixation in the affected eye in many cases.
 Reduced or, more rarely absent stereopsis.

6. Types of Microtropia
 Microtropia can be classified into:
 With identity
 without identity.

 However, the management of these groups is
essentially the same.

7. Microtropia with identity
 Identity is present when the deviation is associated
with eccentric fixation, which is coincident with the angle
of deviation.
 No manifest deviation is detected.
 Factors that suggest the presence of a microtropia are
reduced unilateral VA and demonstrable BSV.

8. Microtropia without identity
 Microtropia without identity is present when a very
small manifest deviation is seen on the cover test. It
may be associated with central fixation or with
eccentric fixation, which is not coincident with the
angle of deviation.

9.  Microtropia may also be classified as
 primary or
 secondary.
 Primary: when microtropia is the initial defect and
there is no history of a larger angle of strabismus.
It may also accompany other concomitant intermittent
deviations, for example microtropia with fully
accommodative characteristics.
 Secondary or residual:
when a microtropia is demonstrated following
treatment for a larger angle manifest deviation.

10. Etiology
 Anisometropia:
patients with microtropia have some
degree of anisometropia, resulting in a defocused
image to the more ametropic eye.
This could explain:
 The foveal suppression scotoma;
 Amblyopia;
 Eccentric fixation on the border of the scotoma;
 Peripheral BSV with defective stereopsis.

11.  Hereditary factors:
It has been shown that members of different
generations as well as siblings of microtropic patients
have other types of strabismus and/or microtropia
(Lang 1984). However, parents and siblings of
microtropic children were found to have an increased
incidence of refractive error as well as
squint, suggesting that a genetically determined
refractive error may be the fundamental anomaly.

12. Investigations
 The aims of the investigation are to diagnose the
microtropia and to assess the quality of BSV by
measuring the fusional amplitude and stereoacuity.
 History
 Visual acuity
 Cover test
 Fixation
 Suppression scotoma
 Confirmation of BSV

13.  History:
Patients may present with constant or intermittent
strabismus or because defective vision in one eye has
been discovered by chance or by routine visual
screening.
 Visual acuity:
The difference in VA between the two eyes can vary
from part of a logMAR line to a much larger difference.
The optimum vision after amblyopia treatment is
equal VA, which is shown to be achievable in these
patients (Cleary et al. 1998).

14.  Cover test:
A small manifest deviation is usually seen, although
none is present in microtropia with identity. The alternate
cover test may reveal an associated heterophoria; the speed
of recovery indicates the degree of compensation.
 Fixation:
Stable parafoveolar fixation is seen in many cases of
microtropia, usually situated nasal to fovea in
microesotropia. In microexotropia, fixation is still
parafoveolar but may be sited temporal to thefovea
(Johnson et al. 1981).

15.  Suppression scotoma
For objective testing, the scotoma is most easily detected
using the 4 Δ prism test. The test is best performed using a
detailed target whenever possible. If a suppression scotoma
is present there will be no movement of either eye when the
prism is placed in front of the eye with the suspected
scotoma. When the prism is placed in front of the other eye
that eye will move to fixate the target and the other eye will
make a simultaneous conjugate movement, but there will
be no corrective vergence movement to achieve bifoveal
fusion.

16.  Confirmation of BSV:
This may be obtained by using the following
techniques.
 Bagolini striated glasses.
 Worth’s four light test
 Fusion amplitude
 Stereotest

17. Management of Microtropia
 Management is directed to obtaining and
maintaining the best possible VA.
 Restoration of BSV may be necessary if the
microtropia coexists with other types of strabismus

18.  Visual acuity:
The first stage is the correction of any significant
refractive error and the constant wearing of spectacles.
• Occlusion: Part-time total occlusion is the treatment
of choice. Occlusion is continued until there is no
further improvement over two or three visits. Fixation
should be checked at intervals as this may indicate the
prognosis for complete recovery.

19.  Restoration of constant binocular single
vision:
If a microtropia is associated with a concomitant
deviation, for example a fully accommodative
esotropia,treatment follows conventional lines
depending on the characteristics of any intermittent
deviation present.