Microtropia ( Lang, 1966 ), or microsquint, may be found as an apparently primary condition, or may be present as a residual deviation after the treatment of a larger strabismus. It may have inherited characteristics ( Burian & von Noorden, 1974 ). Anisometropia is often a major factor, and a foveal scotoma can result from the confusion of the blurred image with the sharp one in the other eye. The condition has also been called Parks monofixational syndrome ( Parks, 1969 ; Parks & Eustis, 1961 ). Typically, microtropia develops before the age of 3 years, but it may break down into a larger-angle strabismus and give the impression that a strabismus has developed in later childhood. It is usually an eso-deviation, but microhypertropia ( Lang, 1966 ) and microexotropia ( Stidwill, 1998 ) have also been described.
Primary microtropia describes microtropia when there is no prior history of a larger deviation and secondary microtropia , when a primary comitant larger-angle deviation has been reduced as a result of treatment ( Houston, Cleary, Dutton, & McFadzean, 1998 ). Another cause of secondary microtropia is a foveal lesion. It has been said that secondary microtropia is more common than primary microtropia ( Griffin & Grisham, 1995 ).
The terminology surrounding small angle strabismus has been confused, but microtropia is now recognised as having certain characteristics in very many cases. These characteristics are listed below, and they are incorporated into a diagnostic algorithm at the end of the chapter. It is hoped that this will help to standardise the diagnosis of microtropia.
Small angle . The microtropia is less than 6Δ in angle. Some authors say less than 10Δ (Lang; cited by Mallett, 1988a ) and others, less than 5Δ ( Caloroso & Rouse, 1993 ). The deviation may not show on the cover test, not because it is too small, but because it is a fully adapted strabismus (see later). Microtropia is usually constant at all positions of gaze and fixation distances.
Anisometropia . There is usually a difference between the refractive errors in the two eyes ( Hardman Lea et al., 1991 ) of more than 1.50D. Occasionally, microtropia will be found in patients without anisometropia.
Amblyopia . There is reduced acuity in one eye, and as the deviation may not be apparent on the cover test, amblyopia may be the first indication of the microtropia. Usually the acuity is only reduced to 6/9 or 6/12. Very rarely microtropia can be alternating, when there will be no amblyopia.
Eccentric fixation . Central fixation is lost in microtropia and there is likely to be a suppression scotoma in the foveal area of the amblyopic eye. The angle of the eccentric fixation is usually the same as the angle of the strabismus, and this is why the eye does not move on the cover test: the area of the retina on which the image falls in binocular vision is the same as the eccentrically fixating area (the area used for fixation when the other eye is covered). Occasionally in microtropia, the degree of eccentric fixation is less than the angle of the strabismus, and in these cases a very small cover test movement may be seen. Some authors (e.g., Jennings, 1985 ) define microtropia as a strabismus in which no movement is seen on the cover test ( Helveston & von Noorden, 1967 ), and hence would not classify this latter type as microtropia.
Anomalous correspondence . Harmonious anomalous retinal correspondence (HARC) is present in microtropia. Therefore, in most cases there will be identity of the retinal area on which the image falls in the patient’s habitual vision with both the area used for fixation monocularly and the anomalously corresponding area. This has been referred to as microtropia with identity , and most microtropia is of this type. In these cases, the strabismus is said to be fully adapted.
Peripheral fusion . The eyes in microtropia seem to be held in the nearly straight position of the small angle by fusional impulses provided by peripheral vision. A form of ‘pseudofusional reserves’ can be measured. During the cover test it is therefore important to position the cover close to the eye to ensure complete dissociation, otherwise peripheral fusion may reduce the magnitude of any ocular movement and prevent accurate diagnosis.
Pseudoheterophoria . In many cases of microtropia, the angle of the deviation may increase on the alternating cover test or even if one eye is covered for a slightly longer time than normal for the cover test. When the cover is removed, the eye which was last covered will be seen to return to the microtropia position. It is as if a heterophoric movement is superimposed on the microtropia. This ‘pseudoheterophoria’ may be larger and more obvious than the microtropia, which may not show at all on the cover test. This cover test recovery movement can be described as an anomalous fusional movement.
Mallett (1988a) felt that these cases did not have a strabismus but in fact had a heterophoria with normal retinal correspondence (NRC) and a gross fixation disparity. This fixation disparity is much larger than that normally found in heterophoria, but does not cause diplopia because of a large foveal suppression area in the strabismic eye. Pickwell (1981) suggested a sequence of events which linked these features and could explain the development of some cases of microtropia. He argued that a decompensating heterophoria leads to an increasing fixation disparity which in time becomes associated with an enlargement of Panum’s area and an increase in the deviation. This results in microtropia with identity. It is interesting that Pickwell suggested an enlargement of Panum’s area, as had Goersch (1979) , in contrast to the foveal suppression that Mallett proposed, and which is detected with the Mallett foveal suppression test ( Jennings, 1996 ; see Fig. 4.8 ).
Symptoms . There are usually no symptoms and a good cosmesis.
Investigation and Diagnosis
The investigation and diagnosis of microtropia ensue from a full routine eye examination, but the following aspects are particularly useful in detecting the condition. These are summarised in Table 16.2 .
The presence of amblyopia in one eye is usually the first clue that microtropia may be found. The amblyopic eye shows the crowding phenomenon referred to in Chapter 13 , that is, single letter acuity is better than line acuity. The foveal scotoma may also result in the patient missing out letters when reading lines of Snellen letters, or they may read the line more easily backwards ( Fig. 13.1 ).
The patient should be tested for eccentric fixation ( Chapter 13 and Appendix 6 ). Eccentric fixation in microtropia is usually parafoveal and slightly nasal and superior to the fovea in microesotropia. The other diagnostic features of strabismic amblyopia are summarised in Table 13.1 . The depth of amblyopia seems to be the main factor influencing binocularity ( Tomac & Birdal, 2001 ).
For microtropia without identity, the diagnosis is usually made from a strabismic result on the unilateral cover test of between 1Δ and 10Δ. As explained earlier, microtropia with identity is unlikely to be detected as a strabismic movement with the cover test. There may be an apparent heterophoria movement when the cover is removed, and this could result in the microtropia being missed.
Four-Prism Dioptre Test (Irvine Prism Test)
In this test ( Irvine, 1948 ; Jampolsky, 1964 ), a 4Δ base-out prism is placed before one eye and the eye movements are observed. The typical response in normal eyes ( Ciuffreda & Tannen, 1995 ) is a small initial vergence movement (which may not be seen), followed by a conjugate saccade (version movement) and then a symmetric vergence movement. The theory behind the test ( Frantz, Cotter, & Wick, 1992 ) is that, in microtropia, if the prism is placed before the strabismic eye the image will move within the suppression area, and there will be no movement of either eye. If the prism is placed before the nonmicrotropic eye, both eyes will make the initial version movement, but the microtropic eye will fail to make the subsequent vergence movement. When the patient has HARC, the test can still reveal a small suppression area at the fovea which can coexist with HARC ( Chapter 12 ).
Surprisingly, in most descriptions of the test the fixation target is not mentioned. The test should only work if the patient is fixating an angular, isolated, target on a large featureless background. If this is not the case (e.g., if the patient fixates a letter chart), other detail in the field of view will also appear to move, not just the fixation target. The importance of fixation target ( Irvine, 1948 ) was highlighted by Irvine shortly after the initial description of the test ( Irvine, 1944 ), but has since then been omitted from descriptions of the test. This factor probably explains some confusing results from publications that do not specify whether a featureless background was used ( Tomac, Sener, & Sanac, 2002 ). As the strength/depth of suppression is likely to vary with target conditions, it is best to avoid an unusually high contrast target (e.g., spotlight). One study evaluated the effect of target parameters, but only included eight cases of microtropia, all of whom had HARC without central suppression and who exhibited normal responses on the 4Δ base-out test ( Savino, Di Nicola, Bolzani, & Dickmann, 1998 ).
In some cases where there is amblyopia in one eye and no movement on the cover test, it is important to differentiate microtropia from organic amblyopia. It is possible that a central scotoma in organic amblyopia could cause a 4Δ test result similar to that in microtropia. In these cases, it may be useful to occlude the good eye and repeat the 4Δ base-out test monocularly. If there is a large pathological scotoma, as in many cases of organic amblyopia, there will be no monocular response to the prism. Because any monocular suppression area in a microtropic eye is likely to be lighter than the larger suppression area that occurs under binocular viewing, a microtropic eye should make a version movement to a 4Δ lens that is introduced monocularly. The test method is summarised in Table 16.1 .