Microtropia part 2

Question 1

What is microtropia?

Answer 1

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.

Question 2

Characteristics

Answer 2

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.
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.

Question 3

Types of microtropia

Answer 3

Microtropia can be classified into cases with or without identity. However, the management of these groups is essentially the same

Question 4

With identity microtropia

Answer 4

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.

Question 5

Without identity

Answer 5

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.

Question 6

Microtropia can also be classified as

Answer 6

Primary or secondary

Question 7

Primary

Answer 7

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.

Question 8

Differential diagnosis of primary and secondary

Answer 8

The differential diagnosis of primary and secondary microtropia is made difficult by the frequent presence of anisometropia in apparently secondary cases, suggesting that there was a primary microesotropia that decompensated to a larger deviation and required treatment to restore BSV. It is also possible that the concomitant deviation preceded the microtropia. For the purposes of management it is sufficient to identify the presence of a microtropia.

Question 8

Investigations - aim

Answer 8

The aims of the investigation are to diagnose the microtropia and to assess the quality of BSV by measuring the fusional amplitude and stereoacuity.

Question 9

Secondary

Answer 9

when a microtropia is demonstrated following treatment for a larger angle manifest deviation.

Question 10

Investigation- history

Answer 10

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.

Question 11

Investigation - VA

Answer 11

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). Linear vision testing is necessary to detect small differences in VA, which may appear equal when tested with single optotypes. The speed of reading letters should be noted for near and distance and is often slower when the affected eye is tested.

Question 12

Investigation - Cover Test

Answer 12

A small manifest deviation is usually seen , although non is present in microtropia with identity

Alternate cover test may reveal an associated heterophoria ; the speed of recovery indicates the degree of compensation

Question 13

Investigation - fixation

Answer 13

Examination with a fixation ophthalmoscope where a fixation target is incorporated into the ophthalmoscope beam, usually in the form of a graticule that can be removed when desired, is essential to diagnose the state of fixation. Stable parafoveolar fixation is seen in many cases of microtropia, usually situated nasal and often slightly superior to the fovea in microesotropia. In microexotropia, fixation is still parafoveolar but may be sited temporal, superior or nasal to the fovea.

Question 14

Investigation - Suppression scotoma

Answer 14

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.

Question 15

Investigations - conformation of binocular single vision - subjective testing - bagolini straighted glasses

Answer 15

A symmetrical cross is the usual response in microtropia. The presence of a suppression scotoma may be indicated subjectively as a central or paracentral gap in the line seen by the affected eye, but few patients observe this phenomenon even if its presence is suggested to them. Although a suppression response is almost invariably seen in microtropia, Bagolini (1985) has reported that some patients maintain a perfect cross with Bagolini glasses during the 4 Δ prism test, indicating sensory adaptation rather than suppression.

Question 16

Investigation - Worth’s 4 light test

Answer 16

A patient with microtropia should see four lights unless a large heterophoria or intermittent strabismus becomes decompensated by the test. Parks (1996) reported fusion using the near Worth lights test at 1/3 metre, but if the distance from the patient was increased, the macular scotoma became apparent, usually at approximately 2/3 metre.

Question 17

Investigation- stereotests

Answer 17

The majority of patients with microtropia demonstrate some degree of stereoacuity. Some patients have difficulty in obtaining a response from pure random dot tests and a few are stereoblind. It is advisable to test patients using both a contoured and random dot test. The standard reached is only rarely as good as 40 seconds of arc.

Question 18

Investigation- measurement of the deviation

Answer 18

The manifest part of the deviation in microtropia without identity can be measured using the simulatanious prism cover test .

An alternate prism cover test will measure the amount if deviation i.e the manifest plus latent component.

In making a diagnosis of microtropia , it is essential to relate VA, fixation and the response to the 4 Dioptre prism test and the level of stereo acuity .

It may be that correspondence is abnormal in microtropia but it may also be that
Panams fusion all area is enlarged in microtropia ; Parks (1971) believed that this allows normal correspondence with peripheral fusion.

Reports of a small number of patients with apparently normal correspondence are common.

Question 19

Management of Microtropia

Answer 19

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.

Question 20

Management - VA

Answer 20

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. It may be necessary to use intermittent part-time occlusion or possibly penalisation in young children until stability is obtained.