Year : 1996 | Volume
: 44 | Issue : 2 | Page : 103--112
Practical approach to diagnosis of strabismus
Ravi Thomas, Andrew Braganza, Thomas George
Schell Eye Hospital, Christian Medical College, Vellore, TamilNadu 632001, India
Schell Eye Hospital, Christian Medical College, Vellore, TamilNadu 632001
|How to cite this article:|
Thomas R, Braganza A, George T. Practical approach to diagnosis of strabismus.Indian J Ophthalmol 1996;44:103-112
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Thomas R, Braganza A, George T. Practical approach to diagnosis of strabismus. Indian J Ophthalmol [serial online] 1996 [cited 2020 May 28 ];44:103-112
Available from: http://www.ijo.in/text.asp?1996/44/2/103/24586
There is a certain mystique surrounding the teaching of traditional (orthoptic procedure dominated) strabismus diagnosis and management. Dark rooms with children peering down tubes, flashing lights, red-green goggles, Hess screens, Lee's screens, afterimage tests etc: all guaranteed to create confusion, if not diplopia in both the patient as well as the examiner. It's no surprise then that (traditional) strabismus diagnosis (and management?) is the average ophthalmic registrar's (and practitioner's) nightmare. This is indeed unfortunate because in reality the teaching of strabismus can be simplified to make it comprehensible (even enjoyable) for the average ophthalmologist.
The simplified and practical approach to strabismus that we follow is unabashedly the approach propagated by Jampolsky, modified slightly by our own experience. According to Jampolsky, the teaching and practice of strabismus is needlessly complicated by procedures that may yield interesting and thought-provoking results, but results which unfortunately have no bearing on practical patient management. The intriguing, often brilliantly conceived tests that are described in most textbooks rarely offer any additional information over the basic clinical sensory-motor examination; on the contrary they may be misleading. Jampolsky's practical approach to strabismus dispenses with such procedures.
AIM OF THE CLINICAL EXAMINATION
What is the aim of our clinical evaluation? It is to determine the normality or abnormality of the motor and sensory system, both in the primary as well as other positions of gaze. Objective testing is preferred to the subjective and in nearly all situations, use of "specialized" equipment such as the major amblyoscope is avoided. In the following text we will assume that routine data for history, refraction and vision already exist and need not be elaborated upon.
What is the information we require from our clinical examination? We need to know:
1. Is fusion present under "real life" conditions?
2. Is there a deviation (phoria-latent squint; or tropia manifest squint); if so, what, if any are the compensatory mechanisms (fusional vergence) that the patient uses to compensate for the deviations?
3. What is the measurement of the deviation (distance as well as near)? Finally, (but equally importantly) we need to know:
4. The existence and nature of the suppression mechanisms if any.
The first three bits of information are obtained from the cover test and the careful observation of ocular movements; the fourth can almost always be inferred from the first three combined with a knowledge of the vision and refraction correction history.
In a sensory test oriented squint world this last statement demands some explanation. The sensory (fusion) and motor (deviations & fusion vergence compensation) systems are strongly interdependent. It is not always appreciated that useful information about the sensory system can be obtained by careful examination of the motor system. A good sensory system is necessary for a good functioning motor system. A 6/6 vision in each eye makes for bifoveal fusion which makes for strong and large amplitudes of fusional vergences; these in turn can mask even large deviations. The observation of a fusional vergence under appropriate testing conditions is therefore evidence of fusion.
EQUIPMENT REQUIRED AND TESTING CONDITIONS
While the tests used are simple, the testing conditions must be appropriate and strictly adhered to.
The equipment needed is simple. A comfortable, straight, highbacked chair for the patient (padded so that the patient's head can be held firmly but comfortably against it), occluders (the examiner's thumb serves as a convenient non-threatening occluder for children), fixation targets, prisms and forceps.
Three factors must be totally under the examiner's control: 1. Fixation; 2. Accommodation and 3. Fusion vergence.
Fixation control: In adults control of fixation is achieved by asking the patient to read a line on the Snellen's chart. The patient must be reading (resolving) letters on the chart when the test is performed. It is insufficient to tell the patient to "look" at a particular line or at the chart.
In the examination of children, remote-controlled toys that make a lot of sound serve as a distant fixation target. A TV and video cassette player are also very useful (but optional). The toys and TV are kept at a six metre distance but controlled from the examiner's table. The volume of the TV set playing an interesting cartoon can initially be increased to attract the child's attention, and then turned down while fixation is observed. This serves to achieve a quick qualitative assessment of the child's vision. For control of near fixation, the adult reads the near vision test card held by the patient in the frontal plane, not in the usual reading downgaze position. The child looks at hand-held mechanical moving toys. Very young children have an extremely short attention span; many toys may be needed to elicit all the desired information in a series of quick looks. With practice, the examiner can elicit most of the information he needs with a couple of "quick looks". The fixation targets used in our clinic are shown in [Figure:1] and [Figure:2].
Accommodation control: Accommodation is more difficult to control and is often the cause of variability in the results of the sensory-motor examination. Accommodation is controlled by performing all tests after a cycloplegic refraction with appropriately prescribed spectacles worn during the test. It has to be stressed that testing has to be done after the cycloplegia has worn off. Refraction under atropine is necessary for many hypermetropes and all children under 10 years of age. (We agree that atropine induced cycloplegia takes some wearing off and is inconvenient but, it is necessary). It is impossible to control the accommodation at distance in an uncorrected hypermetrope - it is necessary to prescribe the full correction under atropine; the glasses may need to be worn for a month or so before an accurate assessment of the deviation is possible. The importance of controlling accommodation cannot be overemphasized. The deviation we detect and measure at distance is the basic deviation we will probably aim to correct surgically. Errors in estimating this will compromise the cosmetic and functional results of our treatment. It is also important to ensure that the normal accommodation-convergence is exercised when testing at near. In young children this may be difficult. Encouraging a child to reach for one of the interesting toys held at near encourages both fixation and accommodation.
Fusional vergence: The third factor to control is fusional vergence - a vergence movement like convergence or divergence made in the interest of fusion. That's all fusional vergence is. The fusional vergence is assessed during the uncover test by observing the character of the refusion movement. This is assessed at the moment the cover is removed. The dissipation of an existing fusional vergence can also be observed by watching the eye under cover. (This is more easily observed using the translucent occludes recommended by Annette Spielmann). Refusion vergence is usually faster and easier to observe than dissipation. Often the observed movement is a combination of a vergence and a fast eye movement (saccade) to re-establish bifoveal fusion. Many patients are able to achieve refusion only by some "trick" movement, often a quick series of saccades, sometimes a head movement. If the patient is not allowed to perform this movement, refusion vergence may not occur and we may misdiagnose an intermittent tropia as a tropia. The testing must simulate real life conditions for the patient. Again, in cases where rapid saccades occur spontaneously on removal of the cover it is necessary to distinguish the true disjunctive movement (vergence) and establish that it does result in return of bifoveal fusion. The presence of bifoveal fusion is deduced from the presence of 6/6 vision, absence of movement on the cover test, and the observation of a real life fusional vergence on the uncover test as described below.
SEQUENCE OF CLINICAL TESTS
The information we require from our clinical examination is obtained in the following sequence. The reason for following this sequence is explained later in the text.
1. The presence of bifoveal fusion, first in the habitual position and then in the primary position; at distance and near.
2. The type and quality of fusional vergences used to compensate for the deviation (phoria, intermittent squint).
3. The deviations that exist for distance and near.
4. The measurements of the above deviations.
5. Ocular movements and tests for incomitance. Any limitation of movement is immediately investigated clinically (including forceps tests) to determine if it is due to a restriction or weakness. A and V patterns are looked for; and a head tilt test is done, preferably for all cases.
DETERMINATION OF FUSION, FUSIONAL VERGENCES AND DEVIATIONS
This is done using the cover test. To understand the cover test, we must understand the very definition of strabismus.
The visual axis is the line joining the object of attention or point of fixation to the fovea. Normally the two visual axes intersect at the point of fixation. If the visual axes do not intersect at the point of fixation, strabismus exists. This misalignment of one visual axis with respect to the point of fixation is easily and most practically determined by the cover test.
The experienced observer usually performs the cover test in one movement. To understand it well however, it is better described as three tests: the cover/uncover test, the cover test and the alternate cover test. These tests are invaluable in that they detect the presence or absence of bifoveal fusion and also characterize fusional vergence movements if present: information that is indispensable in the diagnosis and management of strabismus. The cover tests are performed in the primary position, for distance as well as near; they may need to be performed in other positions of gaze as and when required. The patient must be wearing the prescribed spectacle correction (following a cycloplegic refraction) and fixing an appropriate accommodative target.
Abnormal head posture is noted before any tests are done. Often fusion may be present only with the abnormal head position; the cover test should be performed in that habitual position before forcing the head into the primary position and performing the tests again. This is because some patients have a very tenuous fusion and that too only with the habitual head posture. It is easily disrupted, even by the simple act of bringing the head into the primary position. Once lost it may be impossible to demonstrate fusion during the rest of the examination, and we would have lost a most important bit of information. This is the reason we follow a particular sequence of examination.
The cover test: The cover test detects the presence or absence of a manifest deviation (tropia). The patient must be wearing the prescribed spectacles correction (following a cycloplegic refraction) and fixing an appropriate accommodative target. The question asked during the cover test is - Is the eye (let us take the right eye for example) fixing under bifoveal conditions? This is determined by covering the left eye and observing the right eye when the left is covered. Note that in the cover test, observations are made on the uncovered eye. The patient must be fixing (and resolving, not just "looking at") an appropriate accommodative target at the appropriate distance. If the right eye has good vision, and does not move to take fixation, we deduce that it was already fixating the target. Now the right eye is covered. If the left eye has good vision and does not move to take fixation, we deduce that the left eye too was fixating the target. In this situation, it is obvious that both visual axes intersect at the point of fixation, and no strabismus exists. In the presence of 6/6 vision in each eye, the lack of a movement on cover test implies bifoveal fusion.
Let us now take a different scenario. On covering the left eye, the right makes a movement to fixate the target. A movement of the right eye to take up fixation on the target means that (before applying the cover to the fellow eye), the right eye was not fixating the target, i.e, the visual axis of the right eye did not intersect the point of fixation. The movement of the right eye that we have detected indicates the presence of a manifest squint (tropia). An inward movement would represent an exotropia while an outward movement would be due to an esotropia. Vertical tropias would be represented by the appropriate vertical corrective fixation movements. The other eye is examined in a similar manner.
Sometimes it is difficult to detect a small fixation movement. In such cases, the cover is applied as before.
If no movement of the opposite (uncovered) eye is detected, while the patient is exhorted to fixate the appropriate target, the examiner gently grasps the patients chin and moves the patient's head from side to side. This ensures that the patient is indeed fixating the desired target - i.e, the fovea is locked on to the target. As the cover is removed, the examiner continues to look at the fixating (uncovered) eye. In the presence of a small strabismus, it may be difficult to see a fixation movement in the uncovered eye, but it is much easier to see that eye relinquish fixation. This is a good technique to use in suspected small angle deviations.
It has already been emphasized that fixation and accommodation must be adequately controlled during this test. This is very important and merits repetation. The patient must be fixating an appropriate accommodative target for distance (also for near) and must be wearing the prescribed spectacle correction.
In this context there are two mistakes we may inadvertently commit. Some occluders have fixation targets painted on them. If asked to "look at the target", the patient may look at the target on the occluder covering the eye instead of the one you want him to look at; the results may be quite misleading. Also some patients may have prisms prescribed in their glasses to compensate for the deviation. This will prevent any movement on the cover tests and mask the deviation; in simple terms we will miss the squint.
The uncover test: The uncover test observes and characterizes the fusional vergences; it is used to determine the presence of latent squints (phorias) and intermittent squints. While the patient is fixing an appropriate accommodative target at the appropriate distance using the prescribed spectacle correction, the examiner covers one eye. Observations are made on the eye that has been covered. At this stage we do not look at the uncovered eye. When the cover is removed the examiner looks for refusion movement in the just uncovered eye. This is done at the moment the cover is removed. The importance of this test is that in the presence of 6/6 vision in both eyes the presence of a real life fusional vergence is positive proof of bifoveal fixation. This inference does not need further "confirmation" by sensory tests and can be used for management decisions. The phenomena observed in fusion vergence have already been described while discussing control of these vergences. The difficulty in observing the dissipative movement can partly be overcome by use of a translucent occluder such as Spielman's occluder [Figure:3] and [Figure:4].
Some patients use accommodative convergence in order to straighten the eyes; this is easily confused with fusional vergence. This problem is best avoided by ensuring that the test is performed while the patient is actually resolving (reading or describing features), rather than "looking" at the target. The pupils provide a clue. If the patient is using accommodative convergence to straighten the eyes, the vision will blur and the pupils will constrict. The pupils will not constrict with fusional vergence.
Alternate cover test: In young patients with exophoria the fusion vergence may be so strong that only prolonged occlusion will dissipate it. The alternate cover test is designed to comprehensively dissociate the fusional reflexes and is done by rapidly switching the cover from eye to eye. The testing conditions described for the cover and uncover tests remain. It is important to remember that the alternate cover test on it's own only detects the presence of a deviation; it is impossible to characterize the deviation i.e, it is impossible to say if the deviation is a phoria or a tropia. The test is therefore rarely used on it's own - perhaps only for screening.
In the presence of strong fusion, it may become necessary to bandage the eye for an hour (or even upto 48 hours) to allow the fusional vergence to relax completely and reveal the presence and full extent of a phoria.
MEASUREMENT OF THE DEVIATION
Quantitative assessment of the deviation is performed quite simply by the prism cover tests: the deviation is compensated by an appropriate prism (base out for esodeviations, base in for exo) before the eye and the cover test is performed [Figure:5]. It is necessary to dissociate the eye for some time in order to bring out the full amount of deviation. The cover may need to be placed alternately over each eye a few times in order to achieve this.
The test is performed with the usually fixing eye fixating the target at the appropriate distance. The power of the prism is slowly increased till no movement is seen. As far as possible the prisms are applied before the deviating eye but if a large power of prism is required for neutralization, it is better to split the prisms between the two eyes rather than "stack" them together in front of one eye. Stacked prisms have a power that is different from the two individual prisms.
It is also important to take into account the calibration of different prisms. Glass prisms must be held with the posterior face of the prism perpendicular to the line of sight (Prentice position). On the other hand plastic prisms are calibrated for the frontal plane position and should be positioned parallel to the infraorbital rim. Failure to follow this will result in inaccurate measurements. In some cases the eye may overshoot the target and make a secondary movement to assume fixation. In these cases only an approximate measurement can be obtained. The testing conditions described for the cover tests remain operative.
In order to ensure that the eye we want to fixate the target is indeed fixating it, or if deviations need to be measured with either eye fixating the following procedure may be adopted: once maximum dissociation has been achieved (by alternate cover) one eye is made to fixate while the other is alternately covered and uncovered; a prism is placed before the covered eye to stop the movement of that eye. The process can be repeated with the other eye fixing if required.
If the deviating eye has very poor vision, the deviation can be assessed using the Krimsky technique. Corneal reflections are produced in the two eyes using an appropriately positioned torch. Prisms are placed before the fixing (sighted) eye till the corneal reflex in the deviated eye is centered. In order to avoid parallax errors the examiner observes from the side of the deviated eye. The amount of prism required to center the deviated
eye is the measurement of the amount of squint that needs correction to produce a cosmetically satisfactory result.
When the squinting eye cannot move into the primary position, several options are available. 1: The Hirschberg test estimates the deviation based on the reflex of a light source on the squinting eye. The test is well known and described in standard texts but is quite inaccurate. 2: The Krimsky test is used with the prisms before the squinting eye to center the reflex in this eye. Estimation of centration through the prism makes this test inaccurate. 3: Utilizing Herring's law is the most subtle and probably the best method to use in this situation and whenever the eye being measured has slow saccades. For example, the left eye is esotropic and can adduct normally, but cannot abduct to the primary position. The patient's head is immobilized in the primary position; the prism bar is held apex in before the squinting eye; the patient is forced to fix with this squinting eye by occluding the normal right eye. If the prism used is excessive, the patient must adduct the left eye in order to fix. This causes abduction of the right eye under the cover. The power of the prism is then decreased until the right eye does not have to move to take up fixation. This lack of movement of the right eye is the end point.
OCULAR MOVEMENTS AND TESTS FOR INCOMITANCE
Careful observation of the ocular movements elicits basic information about the motor system and detects any incomitance if present. Actually, all squints are, to a small extent, incomitant. It is any abnormal degree of incomitance that we are looking for. Incomitant factors may be rotational defects (weaknesses or restrictions), A-V patterns or an abnormal head posture.
Rotational defects are sought by testing at arm's length distance without corrective spectacles. The spectacle frames tend to restrict the extremes of gaze and can lead to errors if kept in place during the examination. It is most convenient to perform the examination with one hand on the patient's forehead to steady the head position; the forefinger and thumb can be used to hold the eyelids open or push one of them closed if needed. The other hand moves the fixation target to the extremes of gaze.
For diagnostic purposes we observe the eyes in the extreme positions of gaze: adducted position to determine deficiencies of adduction or abduction as well as overactions or underactions of the obliques. The obliques are also tested in the up-and-in and down-and-in positions. When a deficiency is elicited on testing binocular rotations it should be confirmed by retesting monocularly. Over- and under- actions can be graded on a scale of 0 to 4 or more (+ or -). Grades 1 to 4 represent about 2mm increments in the horizontal plane, and about 10 to 12 degrees in the vertical plane. For example with the left eye fixing a target up and to the left (testing the right inferior oblique - RIO), if the right eye is in the horizontal plane, that is - 4 underaction of the RIO. In the same situation, if the right eye is turned vertically upwards, that is + 4 overaction of the RIO [Figure:6]. Grades 1 and 2 fall in between. Superior oblique actions can be similarly graded [Figure:7]. Similarly, with the left eye fixating a target in extreme adduction, if the right eye does not move past the midline that is minus 4 underaction of the right lateral rectus [Figure:8]. If the right eye cannot reach the midline despite an attempted lateral movement, the minus range is extended, to minus 8 if required. The motility pattern can be conveniently represented diagrammatically in the modified method of White and Brown as shown in [Figure:9].
Esotropia is abbreviated to ET, exotropia to XT, hypertropia to HT and hypotropia to HOT with the appropriate letter for right (R) or left (L) preceding. Thus figure 9 shows a right hypertropia and esotropia in the primary position. The esotropia has a "V" pattern with right inferior oblique overaction of 4+ seen as an upshoot in adduction marked by the arrow. Minus 2 underactions of the lateral recti have been noted bilaterally. As the underaction of the right lateral rectus persists on monocular testing, it is enclosed within brackets. The underaction of the left lateral rectus improves on monocular testing and is therefore left unbracketed. The positive head tilt to the right has been indicated. In this scheme phorias are represented with a "P" (eg. XP for exophoria, EP for esophoria), while intermittent tropias are shown by enclosing the "T" (for tropia) in square brackets; intermittent exotropia would be X[T] and intermittent esotropia would be E[T]. This pictorial representation provides all the qualitative and quantitative data we have elicited from our examination at a single glance.
Many patients have a horizontal deviation with an associated vertical deviation (with the eye in its horizontally deviated position). We need to predict whether our horizontal muscle surgery will take care of the vertical deviation, or whether we need to do something extra for the vertical deviation? Many experts would suggest that the vertical deviation be measured along with the horizontal during the prism bar cover test. This however does not tell us what will happen to the vertical deviation once the eyes are aligned horizontally. Jampolsky has described a simple rotational test that predicts the vertical position of the eyes once the horizontal deviation has been corrected.
Let us assume that a patient has a left divergent squint. In this position, the left eye is hypertropic. What we need to know is what will happen to the hypertropia if we perform horizontal muscle surgery and align the eyes horizontally. Will the vertical deviation disappear with horizontal realignment of the eyes, or do we need to undertake additional vertical surgery to achieve this? We ask the patient to fix with the right eye. The left eye is exotropic and hypertropic. By turning the patient's head, the fixing right eye is moved increasingly into abduction until the exotropic left eye reaches the primary position. The vertical deviation that the (exotropic) left eye manifests in this position is what we expect to find after horizontal alignment (Fig 10). Based on this simple test a decision regarding the additional vertical deviation is made.
A-V patterns are detected (and measured if necessary) at distance only, with the refractive correction in place. The horizontal deviation is measured in the primary position, extreme upgaze and downgaze. In practice, the latter two positions are achieved by a chin-up and chin-down position, all the while having the patient fixing the distance target. It is easy enough to detect an A or V pattern in a patient with a unilateral tropia by "eyeballing" the position of the deviated eye as it moves from upgaze into downgaze. In alternating deviations or phorias, all the deviation is made manifest in one eye by applying a cover over that eye. A translucent cover makes this easy but is not mandatory. The eyes are then moved into up and down gaze by lifting or lowering the chin, and the A or V pattern "eyeballed" under the cover. If an A or V pattern is detected by these manoeuvers, it is measured. If the patient wears glasses they are slightly elevated by the examiner's thumb in the chin-down position. In the chin-up position the thumb is used to move the glasses down slightly. These manoeuvers allow fixation of the distance target in extreme up and down gaze. The A-V phenomena are never checked at near fixation as convergence may interfere and provide false patterns.
Next the head tilt test is performed. It is mandatory in all patients with abnormal head postures, but should be done in routine strabismus examinations also. This test and its interpretations are well described in standard texts; we will not repeat these descriptions here. Practically, it is done by tilting the head towards the shoulder and performing the cover test in this position to note any vertical deviations. Small vertical deviations are better noted by observing the upper lid. This faithfully follows the vertical movements of the globe, in fact exaggerates them and eliminates the effect of associated horizontal movements which may be confusing. The findings on forced tilt towards the right and left shoulders are compared for interpretation of this test. This test is most useful in the diagnosis of superior oblique paresis. There are however other causes for a positive test. According to Jampolsky the commonest cause of a positive head tilt test is a tight superior rectus muscle (from any cause). It is interesting that dissociated vertical deviation (DVD) produces a "wrong way" head tilt (compared to superior oblique palsy). On tilting the head to the left, the right eye elevates; on tilting the head to the right, the left eye elevates. This sign if present is considered to be fairly characteristic of DVD. It cannot however, be demonstrated in all patients with DVD.
When incomitance is noted, often the forceps tests (forced duction and force generation tests) yield useful information. Certainly whenever there is a limitation of ocular movement these tests must be employed to distinguish between a paralysis and a restriction. The presence of a muscle paralysis is suspected by observing the patient's fast voluntary ocular movements. For example in the suspected paralysis of a horizontal muscle, the patient is asked to look to the right and then asked to look to the left. Normal voluntary movements are fast. A paralysis or weakness of a muscle leads to a "floating" movements compared to the usually fast voluntary movements made by towards the nonparalysed side. The use of the forceps tests confirm the diagnosis.
FORCED DUCTION (FDT) AND FORCE GENERATION (FGT) TESTS
These tests recognize the role of mechanical factors (restrictions and weaknesses) in strabismus and seek to assess them objectively. This is done by physically grasping the globe with a forceps and either attempting to rotate it beyond the point of voluntary rotation by the patient (FDT) or by feeling the force generated on attempted voluntary rotation by the patient (FGT).
The anaesthesia for the forceps test is topical. However, pressure on the area of the limbus to be "grabbed" using a Q-tip soaked in 4% Xylocaine will make the patient more comfortable. The FDT is performed by grasping the conjunctiva near the limbus with the patient attempting to look in the direction of limitation of movement. A Hoskins forceps is a suitable instrument for the purpose. The examiner then attempts to rotate the globe further into the area of limitation. Inability to do so indicates restriction; if the globe moves easily it is characteristic of a weakness. Care must be taken to institute a pure rotation and not to press the globe back into the orbit. The latter action will relieve the "leash effect" of a restriction and lead to a misdiagnosis.
The FGT is performed by fixing the globe in the primary position with a forceps at the limbus and then asking the patient to attempt to rotate the globe in the direction of suspected weakness. A subjective assessment of the power of the concerned muscle is made by the examiner based on the "feel" of the tug at the forceps.
INFERENCES ABOUT THE SENSORY SYSTEM
The sensory system is of inestimable importance in the management of strabismus. Good fusion may "lock" an otherwise abnormal motor apparatus and make the difference between surgical success and failure; alternatively, an abnormal sensory system may undo the results of the best surgery. However, all the information that we require about the sensory system can be derived from our examination as well as a knowledge of the patient's refraction correction history and vision. No ancillary tests are necessary to "prove" what we have inferred.
The inferences we make at this point in the examination principally concern the sensory state and how it will affect our management. A few basic tenets must be understood and accepted for us to make valid and useful inferences.
The first of these is that any tropia with onset during the age of visual immaturity will be accompanied by an appropriate sensory adaptation (in the form of a suppression mechanism) to compensate for the deviation and prevent diplopia while the deviation is present. Conversely, a tropia whose onset is after the age of maturity of the visual system will inevitably result in diplopia.
Suppression mechanisms are hemiretinal in nature, involving the entire temporal hemiretina in exotropia and only a limited area of the nasal hemiretina in esotropia, which varies with the degree of the deviation. Intermittent exotropes will manifest the binocular hemiretinal suppression scotoma only when the eyes are deviating. Long-standing esotropia of constant degree may result in a modification of the suppression scotoma to a dual area, namely the fovea and the area nasal to the fovea corresponding to the degree of the deviation. Intervening will be areas of normal retinal sensitivity. These adaptations are inevitably present and need not be re-established by any other methods. In fact they will invariably be present despite what other tests suggest. The tests of sensory function can produce results that differ according to the testing situation. That is not to say we should never do tests to determine their presence. It is necessary to learn the theory behind sensory testing and do these tests a few times in normals and different types of strabismus. But once the basis is understood, and the information can be inferred from easier and more controlled sources, they are probably not necessary for each and every case.
The importance of knowing the sensory status lies in the prognostication of the response of the visual system to the therapy that we are going to institute. It is inevitable that when we move the image of the fixation target out of the suppression scotoma (surgically or with prisms) diplopia will result. If the potential for bifoveal fusion exists, as determined by the cover tests, then this diplopia may well be a stimulus for fusion to lock the eyes on target and give a good functional and cosmetic result. This is the reason why a slight overcorrection is aimed for in exotropes, especially intermittent exotropes. However, in a situation where fusion potential is poor, a purely cosmetic correction may be achieved by undercorrecting so that the retinal image still lies within the suppression scotoma. Diplopia induced by surgical overcorrection is to be avoided in these cases.
Inferences about fusion potential can also be made from the vision. Equal and good vision in both eyes usually implies good fusion potential. The converse is also true; amblyopia or unequal vision with or without eccentric fixation implies a poor potential for bifoveal fusion.
The history is equally important. The age of onset of the strabismus is the single most important factor in our inferences regarding the sensory status, as detailed earlier. In adults, an acquired strabismus is associated with a good fusion potential.
The above tests and inferences are adequate in most cases for a diagnosis of the sensory-motor system to be made and a rational plan of management to be evolved. You will notice that there are no etiological conclusions drawn regarding the causes or the classification of the strabismus, nor are these necessary for a good management plan.
In some situations a few more tests may be found necessary. These are 1) the 4 prism diopter eye movement test 2) the prism adaptation test (PAT).
4 PRISM DIOPTER TEST
This test is used for the determination of bifoveal fixation in cases where the cover test (even with the modification for small deviations) is difficult to interpret. It establishes whether both foveas are fixing under everyday viewing conditions.
The test consists of placing a 4 pd prism base out in front of the fixing eye and observing the other eye. Under normal circumstances, as the fixing eye moves towards the apex of the prism (version movement), the other eye will first abduct (carried along with the fixing eye); then it adducts to regain fusion (vergence movement - fusional vergence). If the eye being observed is not fixing bifoveally this refusion vergence will be absent. If the prism is applied before the eye with the microtropia, there will be no movement of either eye. This test is useful because it concentrates purely on the presence or absence of bifoveal fusion and hence answers only this specific and important question without confusion. It is especially useful in small-angle strabismus (microtropia).
PRISM ADAPTATION TEST (PAT)
Before undertaking a major step in treatment such as surgical realignment of the visual axes, it is important to predict how the sensory status might influence the mechanical (surgical) motor realignment. This is specially true in early-onset long-standing esotropia where an unfavorable response to surgery may take the form of a overconvergence resulting in recurrence of esotropia. The PAT seeks to address this specific issue.
The test is performed by introducing base out prisms in front of the eyes so as to just overcorrect the deviation. It is not enough to just compensate the defect; the retinal image must be made to fall just temporal to the suppression scotoma so as to induce a diplopia. This can be done objectively by increasing the prism power till an exo movement is demonstrated on the cover test. A single prism in front of one eye, or prisms divided between the eyes may be used as necessary. The prisms may need to be used over a period of days.
The responses noted may be 1) no movement of the eyes 2) slight convergence to result in bifoveal fusion (fusion vergence) 3) massive overconvergence resulting in re-establishment of the esotropia. The first two responses imply a favorable prognosis. The third (overconvergence) constitutes rejection of the prism and carries a poor prognosis. If in this situation the prism power is increased, the deviation may become relatively stable. If a stable angle can be obtained and surgery is done for this new "augmented" deviation, the prognosis may be improved. Sometimes if the prism power is increased, the patient just "eats up the prism". Prism orthoptics prior to surgery may alter the strength of the sensory influence on the motor alignment. Surgical correction here will usually aim at a slight undercorrection and if the deviation is large, the mechanical effect of the realignment may prevent a recurrence of the esotropia.
Jampolsky maintains that the PAT is superior to the usual subjective tests for anomalous retinal correspondence and suppression in the prognostication of esotropia. The cost and difficulty in obtaining and modifying prismatic correction prevents us from using this test in a routine manner in our practice.
Jampolsky's approach to the diagnosis of strabismus is simple. Certain tests are to be performed in sequence to determine 1) the presence of fusion in the habitual head position under normal viewing conditions and in the primary position at near and distance; 2) the presence and characteristics of fusion vergences and 3) the deviations that exist at distance and near. These are established by the cover tests and the prism cover test. 4) Incomitance of the strabismus, A-V phenomena and mechanical weaknesses and restrictions are assessed by observations of eye movements and the forceps tests. Finally, 5) inferences about the state of the sensory system are made based on the history, vision and clinical examination. Additional tests performed on indication are the 4 diopter prism test, the prism adaptation test and the forceps muscle tests.
This approach allows an accurate and reproducible examination to be performed in an objective manner making it possible to compare clinical findings over time and allow a rational plan of management to be evolved. Errors in examination arise from poor control of three factors, accommodation, fixation and fusion vergence. These can be controlled by careful attention to the test conditions.
This system of diagnosis is simple to follow and understand, economic on time and altogether less of a strain to examiner (and patient) than the traditional system.
|1||Jampolsky, A. A Simplified Approach to Strabismus Diagnosis. Symposium on Strabismus. Transactions of the New Orleans Academy of Ophthalmology, 1971, C.V.Mosby Company|
|2||Pittar, G. Practical Management of Squint, 1990, Turton & Armstrong.|
|3||von Noorden, GK. Binocular Vision and Ocular Motility in Theory and Management of Strabismus, IV ed. 1990, C.V. Mosby Co.|