Microtropia Flashcards
Clinical characteristics
Microtropia is a small-angle heterotropia (usually of ≤ 10 prism dioptres) in which a form of binocular single vision occurs. Due to the small angle its not usually observed and to pick up if s child has microtropia, we often need a screening programme to detect if there is a strabismus present or there’s a reduction in visual acuity which then leads to a referral and discovery of a microtropia. component. We are expecting a manifest deviation of about 10 diopters or less. Some of these patients will also have an underlaying heterophoria so not only they’ll have manifest deviation but when you perform the alternate cover test , you will find an even greater eso movement which indicates to you both the manifest strabismus and the latent strabismus together. The reason these patients can have a combination of manifest or latent is due to they have binocular functions and they do have peripheral fusion therefore they are able to have both manifest or latent component to their deviation.
Clinical charectists
Almost all patients with microtropia will have an anisometropia. Some patients may have foveal suppression. In terms of retinal correspondence, some will be NRC and some will be ARC. Very importantly patients with a microtropia will have BSV. They won’t be able to demonstrate to you foveal fixation but they will demonstrate monocular functions. So this means if you give the patient a dot stereo acuity test, they will fail it but they will pass a contour line acuity test because it doesn’t rely on the patient being foveal. These patients will also demonstrate sensory and motor fusion so despite that you have a small angle strabismus or manifest strabismus, you will be able to measure a fusion range in these patients.
Some patients will have eccentric fixation and/or will develop some level of amblyopia.
Primary microtropia
Is one where we suspect that anisometropia is the cause of the deviation. If a patient doesn’t have an anisometropia, then its relatively unknown why they have that microtropia but in almost all instances, the microtropia will present with some level of anisometropia.
Secondary microtropia
Secondary Microtropia is that develops post operatively where we have as the example of the infantile residual esotropia and then we classify the patient as having a postoperative microtropia.
Clinical characteristics -3
When we diagnose with a microtropia we can further subclassify the patient as either having a microtropia with identity or a microtropia without identity. The key difference between the two is one has eccentric fixation and the other doesn’t. The patient with a microtropia without identity has no eccentric fixation, they have central fixation. A patient with identity has eccentric fixation. What this means is we will have different cover test results for these types of strabismus.
Microtropia without identity
The patient will have central fixation. If we perform a cover test, we expect the eye to move out to take up fixation. In terms of retinal correspondence, the patient may or may not have ARC. If the patient has ARC, it’ll be harmonious, it’ll be at the CIP giving the patient BSV. If they have normal retinal correspondence, you would expect that CP point would give Diplopia but it doesn’t. What we have instead are extensions of panums fusional areas which leads to the patient maintaining BSV despite the fact that they have a small angle esotropia. So in both instances if the patient has NRC or ARC, we should be able to demonstrate stereopsis and sensory and motor fusion. The 4 prism diopter test is utilised in the presence of a microtropia. If preformed on a patient with a microtropia without identity they will have a positive result for microtropia, in other words they will have a failed response however, its not necessary to perform a 4 dioptre prism test on a patient that has a microtropia without identity
Microtropia with identity
A patient with identity has eccentric fixation and that eccentric fixation point will exactly where the contralateral image. This means for the cover test , when you cover the fixing eye, we know that eccentric fixation is a monocular condition, and is a pseudophobia under monocular condition, so when we cover this eye the patient will have no movement to take up fixation that eccentric point will take on the role of the pseudophobia during the cover test. There is no manifest deviation on cover test so another key difference between the two conditions is that microtropia without identity gives you a movement on cover test and a microtropia with identity gives you no movement on cover test. This is where the 4 prism test is essential. With a microtropia with identity , we need to prove that the patient has a manifest strabismus and the cover test is not giving you confirmation of a manifest strabismus and the only way you can confirm that there is a micro strabismus present is by using the full prism diopter test , the patients will have a positive result for microtropia but will fail the 4 diopter prism test and the reason is absolutely necessary to perform the 4 diopter prism test in patients with this subtype of microtropia is that we do not have confirmation on cover test that so 4 diopter prism test is that we do not have confirmation on cover test that the patient has a manifest strabismus. Whereas microtropia without identity, we have already detected we have already detected the micro strabismus or manifest on cover test. The 4 diopter test doesn’t particular assist in diagnosis but it confirms what we see on cover test.
Monocular characteristics
we expect similar outcomes in relation to stereopsis and motor and sensory fusion to patients diagnosed with microtropia without identity in patients with microtropia with identity they tend to have ARC rather than normal retinal correspondence so we will find that there is ARC and this matches the eccentric fixation point and the contralateral image point so we will have harmonious ARC. A patient that has monofixation, who is straight but has central suppression due to anisometropia, will also fail the 4 diopter prism test, so in performing the 4 diopter prism test in a patient, who gives you no movement, that doesn’t guarantee that the patient has a microtropia. It indicates a microtropia or monofixation. It is therefore necessary to then look at visiocopy and conform that there is eccentric fixation, and its then when you have the positive 4 diopter prism test with eccentric fixation, that you have the absolute confirmation that the reason you see no movement on cover test is because the patient wasn’t centrally fixing.
Diagnosis
Diagnosis of microtropia relies on a variety of tests. Cover test which looks out for manifest strabismus and the we need to look at binocular functions. When we see we have a manifest strabismus with binocular functions then we have a microtropia. We should also find the VA to be reduced because amblyopia. If the patient has a normal cover test, we can tell if the patient has a micro strabismus, the cover test will be normal and you will usually find an esophoria, but VA will be indicating there’s likely amblyopia. There will be reduced VA in the eye that has the microtropia. In addition to this , you will find the patient will fail a random dot stereo acuity test so if you have a patient that has reduced vision in one eye who fails a random dot stereopsis test but has an esophoria on cover test and you do not detect a manifest strabismus, we must be thinking this patient must likely have a microtropia to explain the reduction of visual acuity and to explain the lack of passing a bifoveal test. In which case we proceed to do visioscopy and a full prism diopter test and a cyclo ret which will tell us the patient has anisometropia.
Investigations
How to investigate a patient with microtropia?
1. History and Symptoms:
* Usually asymptomatic
* Detected late at school age or undetected
2. Visual acuity:
* Reduction of VA in the deviated eye → Usually the first clue of a microtropia
* The reduction in VA will depend of eccentric fixation
* The amblyopic eye usually shows crowding phenomenon
3. Refraction:
- Anisometropia (up to 80% of cases)
4. Cover test:
With identity
* No deviation seen on unilateral cover test
* There may be an apparent heterophoria movement when the cover is removed and this could result on microtropia being missed
Without identity
Deviation identified on unilateral cover test
What tests to carry out when investigating microtropia
4 PRISM DIOPTRE TEST:
1. Place 4A BOUT before one eye and observe the eye movements
2. Typical response in patients with good BV: small initial version movement, conjugate saccade and a symmetric vergence movement -
3. If the prism is placed in the strabismic eye the image will move within the suppression area and there will be no movement of either eye
4. If the prism is placed in the non-strabismic eye both eyes will make the initial vergence movement but the strabismic eye will fail to make the subsequent vergence movement (due to scotoma)
5. The test only works when the patient is fixing an angular, isolated target on a large featureless background - The 4A BOUT test has been proposed as a diagnostic test for microtropia, but it often gives atypical responses (e.g. in esophoric patients the prism may correct the deviation) - Low test-retest repeatability has been suggested
Management - part 1
Almost all patients will have anisometropia so It is important that one of the first things we do is manage the anisometropia. It is suggested that perhaps if you detect an anisometropia earlier on, you may prevent microtropia from developing however there is not strong evidence to suggest that but given there is a strong link between the two, they’re may be causal relationship it is strongly suggested that we manage the anisometropia as early as possible.
Management - part 2
Occlusion is another treatment method. These children will have amblyopia and tend not to be dense amblyopia, but we will need to manage the reduction in the visual acuity. As the patient has a small angle esotropia, excessive dissociation of a patient can lead to the breakdown in the control of the microtropia. Patching is a dissociative treatment method, so we have to be careful with how many hours of occlusion we prescribe to a patient with microtropia. You do not want to be prescribing all-day occlusion for instance in a patient with a microtropia. If you do this, there is a risk of the patient breaking down and when they break down which means they’ll go from a 10 prism dioptre esotropia to a much larger esotropia and for example this may even be 50 prism dioptres and at this point, they will experience diplopia because they’ve never had a larger strabismus before. Ideally, 2-4 hours maximum daily of occlusion is sufficient with patients with microtropia.
More info
Furthermore, it can be difficult to give patients equal visual acuity, when they have a microtropia. It is not often you can get to 6/6 visual acuity and is highly likely that the best outcome we can get is a one-line difference at the conclusion of the occlusion therapy.
More info - part 2
With regards to Orthoptic, we do not generally prescribe optics in patients with microtropia unless there are issues with the control of the latent component of the deviation. So in other words there is a decompensating. Children tend not to have this issue and its usually adults, who’ve had a micro strabismus as a young child who may present with a decompensating association esophoria. If this is the case, then we would give them divergence exercises such as base in prisms and negative balance
